Publication No. FHWA-HIF-19-059 

January 2020

Hydraulic Engineering Circular No. 25

Highways in the Coastal Environment

Third Edition
Page Intentionally Left Blank
Technical Report Documentation Page

1. Report No. 2. Government Accession No. 3. Recipient’s Catalog No.

FHWA HIF-19-059

4. Title and Subtitle 5. Report Date

HIGHWAYS IN THE COASTAL ENVIRONMENT

Hydraulic Engineering Circular Number 25

Third Edition

January 2020

6. Performing Organization Code

7. Author(s) 8. Performing Organization Report No.

Scott L. Douglass and Bret M. Webb

9. Performing Organization Name and Address 10. Work Unit No. (TRAIS)

South Coast Engineers

PO Box 72

Fairhope, AL 36533 11. Contract or Grant No.

DTFH61-16-D-00033

12. Sponsoring Agency Name and Address 13. Type of Report and Period Covered

Office of Bridges and Structures

Federal Highway Administration

1200 New Jersey Avenue, S.E.

Washington, D.C. 20590

Technical Report

November 2017-December 2019

14. Sponsoring Agency Code

15. Supplementary Notes

Project Manager: Brian Beucler, FHWA Office of Bridges and Structures

Technical Reviewers: Joe Krolak, Tina Hodges, Rob Kafalenos, Kornel Kerenyi, Rob Hyman, Paul Sharp,

Eric Brown, Jia-Dzan (Jerry) Shen, Stephanie Smallegan, Thomas (Beau) Buhring

Technical Editor: Valarie Arms

16. Abstract

This manual presents tools for the planning, design, and operation of highways in the coastal environment. It

focuses on roads near the coast that are influenced by coastal tides and waves constantly, or occasionally during

storms. FHWA estimates that there are more than 60,000 miles of these “coastal highways” in the US. A primary

goal is the integration of coastal engineering principles and practices in the planning and design of these roads

and bridges to make them more resilient.

The manual summarizes coastal science concepts and modeling tools that have been developed by the coastal

engineering community, and are applicable to highways. This includes engineering tools for estimating water

levels, waves, and sand movement.

Coastal highway applications include coastal revetment design; planning and alternatives for highways that are

threatened by coastal erosion, including nature-based solutions; roads that overwash in storms; roads that are

experiencing increased flooding due to sea level rise; coastal bridge issues including wave loads on bridge decks;

and coastal scour. The manual presents technical information and methods for assessing the vulnerability of

coastal transportation facilities to extreme events with techniques to improve resilience.

Some coastal transportation infrastructure is highly exposed to extreme events and that exposure will increase as

sea levels rise. Many of the adaptations needed for future sea levels are the same engineering approaches needed

for improving infrastructure resilience today.

17. Key Words 18. Distribution Statement

Coast, Coastal Engineering, Highways, Hydraulics, Hydrodynamics,

Tides, Water Levels, Waves, Storm Surge, Erosion, Extreme Events,

Sea Level Rise, Shorelines, Protection, Modeling, Nature Based &

Natural Infrastructure, Scour, Resilience

Unlimited distribution

19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of Pages 22. Price

Unclassified Unclassified 434 N/A

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized
 

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HEC-25 3rd ed. Table of Contents

i 

Table of Contents

Table of Contents........................................................................................................................... i

List of Figures .............................................................................................................................. ix

List of Tables............................................................................................................................... xx

List of Symbols........................................................................................................................... xxi

Acknowledgements...................................................................................................................xxiv

Notice........................................................................................................................................xxiv

Non-Binding Contents...............................................................................................................xxiv

Quality Assurance Statement ................................................................................................... xxv

Supporting Purpose .................................................................................................................. xxv

Glossary....................................................................................................................................xxvi

List of Acronyms ........................................................................................................................xliv

Part 1 – Background & Context .................................................................................................... 1

Chapter 1 - Introduction ................................................................................................................ 3

1.1 Purpose and Scope..................................................................................................... 3

1.2 Organization................................................................................................................ 4

1.3 Target Audience.......................................................................................................... 6

1.4 Units in this Manual..................................................................................................... 6

Chapter 2 - Federal Policy for Coastal Highways ......................................................................... 7

2.1 Federal Highway and the Coast: National Overview................................................... 7

2.2 FHWA Statutes & Regulations .................................................................................... 7

2.2.1 FHWA Statute.................................................................................................. 7

2.2.2 Appropriation Language .................................................................................. 9

2.2.3 FHWA Regulations .......................................................................................... 9

2.3 Other Federal Agency Coastal Statutes & Regulations ............................................11

2.3.1 Rivers and Harbors Act of 1899 [33 U.S.C. § 401 and § 403].......................11

2.3.2 General Bridge Act of 1946 [33 U.S.C. § 525 through 533] ..........................11

2.3.3 Transportation Act of 1966 [Public Law 89-670]............................................11

2.3.4 Coastal Zone Management Act [16 U.S.C. § 1451-1466] .............................12

2.3.5 National Environmental Policy Act [42 U.S.C. § 4321, et seq.] .....................12

2.3.6 Clean Water Act [33 U.S.C. § 1251-1387].....................................................12

2.3.7 Endangered Species Act [16 U.S.C. § 1531-1544] .......................................13

2.3.8 Coastal Barrier Resources Act [16 U.S.C. § 3501 through 3510] .................13
Table of Contents HEC-25 3rd ed.

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2.3.9 Magnuson-Stevens Fishery Conservation and Management Act [16 U.S.C. § 

1801-1891d] ..................................................................................................13

2.3.10 Marine Mammal Protection Act [16 USC §§ 1361-1407]...............................13

2.3.11 Migratory Bird Treaty Act [16 U.S.C. §§ 703-712.]. .......................................14

2.3.12 National Historic Preservation Act [54 U.S.C. 300101 et seq.]......................14

2.3.13 National Flood Insurance Act of 1968 [42 U.S.C. § 4001 et seq.] .................14

Chapter 3 - Coastal Highways: An Overview..............................................................................17

3.1 What are Coastal Highways? ....................................................................................17

3.2 The Extent of Coastal Roads and Bridges ................................................................18

3.3 Societal Demand for Coastal Highways ....................................................................18

3.4 Natural Processes Impacting Coastal Highways.......................................................19

3.4.1 Water Level Change......................................................................................19

3.4.2 Sea Level Rise ..............................................................................................19

3.4.3 Storm Surge ..................................................................................................19

3.4.4 Major Weather Events ...................................................................................19

3.4.5 Waves............................................................................................................22

3.4.6 Shoreline Erosion and Barrier Island Breaches.............................................22

3.4.7 Littoral Drift ....................................................................................................22

3.4.8 Shoreline Recession......................................................................................23

3.4.9 Tsunamis .......................................................................................................23

3.4.10 Upland Runoff................................................................................................24

3.4.11 High-Velocity Flows .......................................................................................24

3.4.12 Other Processes............................................................................................24

3.5 Coastal Highway Planning and Design .....................................................................24

3.6 Coastal Engineering as a Specialty Area..................................................................25

3.6.1 Education.......................................................................................................25

3.6.2 State-of-Practice ............................................................................................26

3.6.3 Professional Specialty Certification as a Coastal Engineer...........................26

3.6.4 Resources .....................................................................................................26

3.6.5 Coastal Engineering in the Highway Community ..........................................27

Part 2 – Principles of Coastal Science for Highway Engineering................................................29

Chapter 4 - Water Levels ............................................................................................................31

4.1 Sea Level Rise ..........................................................................................................31

4.1.1 Causes of SLR and Terminology...................................................................31

4.1.2 Historical RSLR Data: Tide Gages ................................................................32
HEC-25 3rd ed. Table of Contents

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4.1.3 Historical SLR Data: Satellites.......................................................................38

4.1.4 Historical SLR Data: Geologic Record ..........................................................38

4.1.5 Discussion of Historical and Present-Day GMSLR........................................39

4.1.6 Projections of Future GMSLR........................................................................40

4.1.7 SLR Values for Planning and Design aligned with NCHRP 15-61 ................44

4.1.8 Example Future RSLR Calculation: Atlantic City...........................................50

4.2 Astronomical Tides....................................................................................................51

4.2.1 Characteristics of Astronomical Tides ...........................................................52

4.2.2 Tidal and Survey Datums ..............................................................................53

4.3 Storm Surge ..............................................................................................................55

4.3.1 Characteristics of Storm Surge......................................................................55

4.3.2 Storm Surge Modeling...................................................................................56

4.3.3 Wave Setup ...................................................................................................63

4.3.4 Impacts of SLR on Storm Surge....................................................................63

4.4 Coastal-Riverine Flooding.........................................................................................64

4.5 Lake Water Level Fluctuations ..................................................................................66

4.6 Design Water Levels ................................................................................................. 68

Chapter 5 - Waves ......................................................................................................................73

5.1 Definitions, Theories, and Properties of Waves ........................................................73

5.2 Wave Transformation and Breaking..........................................................................80

5.3 Irregular Waves .........................................................................................................88

5.4 Wave Generation and Modeling................................................................................91

5.5 Wave Runup..............................................................................................................94

5.6 Ship Wakes ...............................................................................................................94

5.7 Tsunamis...................................................................................................................95

Chapter 6 - Coastal Sediment Processes...................................................................................97

6.1 Overview of Coastal Geomorphology........................................................................97

6.2 Beach Terminology ................................................................................................. 100

6.3 Coastal Sediment Characteristics ...........................................................................106

6.4 Cross-Shore Sand Transport and Dune Erosion Modeling .....................................108

6.5 Longshore Sand Transport and Shoreline Change Modeling ................................. 111

6.6 Tidal Inlets...............................................................................................................113

6.7 Future Sea Level Rise Effects on Coastal Processes.............................................118

6.8 Physical Models in Coastal Engineering ................................................................. 119
Table of Contents HEC-25 3rd ed.

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Part 3 – Issues and Applications in Coastal Highway Design...................................................123

Chapter 7 - Coastal Revetments...............................................................................................125

7.1 Types of Revetments and Seawalls ........................................................................125

7.2 Hudson’s Equation for Armor Stone Size................................................................130

7.3 Design Wave Heights for Revetment Design ..........................................................136

7.4 Other Issues for Coastal Revetment Design ...........................................................138

7.4.1 Common Failure Mechanisms of Coastal Revetments ...............................138

7.4.2 Alternative Revetment Concepts and Materials ..........................................141

7.5 Wave Runup and Overtopping – Coastal Structures ..............................................143

7.6 Design Example: Coastal Revetment......................................................................146

7.6.1 Example Situation........................................................................................146

7.6.2 Design Steps ...............................................................................................146

Chapter 8 - Roads in Areas of Receding Shorelines ................................................................153

8.1 Examples of Issues ................................................................................................. 153

8.2 Quantifying Shoreline Change Rates ......................................................................156

8.3 Estimating Future Shoreline Positions ....................................................................158

8.3.1 Shortcomings of Shoreline Change Assumptions .......................................158

8.3.2 Sediment Budgets .......................................................................................161

8.4 Relocation/Abandonment Considerations ...............................................................161

8.5 Shoreline Stabilization Options ...............................................................................161

8.6 The Functional Design of Coastal Structures..........................................................162

8.7 Beach Nourishment................................................................................................. 164

8.8 Combining Beach Nourishment with Structures......................................................167

8.9 Marsh Restoration/Creation ....................................................................................172

8.10 Unproven “Experimental” Shoreline Stabilization Approaches................................176

Chapter 9 - Increased Flooding due to Relative Sea Level Rise ..............................................177

9.1 Description of the Problem......................................................................................177

9.2 Assessing the Problem............................................................................................182

9.2.1 Sweet et al. (2014)/Sweet and Park (2014).................................................182

9.2.2 Kriebel and Henderson (2018) ....................................................................183

9.2.3 Jacobs et al. (2018a)...................................................................................188

9.3 Raising Roadway Elevation.....................................................................................188

Chapter 10 - Highway Overwashing .........................................................................................193

10.1 Description of Overwashing ....................................................................................193
HEC-25 3rd ed. Table of Contents

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10.2 The Coastal Weir-Flow Damage Mechanism..........................................................194

10.2.1 Coastal Weir-Flow Damage Mechanism Investigations ..............................198

10.3 Strategies for Roads that Overwash .......................................................................202

10.3.1 Road Location Considerations ....................................................................202

10.3.2 Road Elevation Considerations ...................................................................203

10.3.3 Construction of Sand Dunes........................................................................203

10.3.4 Armoring of Shoulders.................................................................................205

Chapter 11 - Coastal Bridges....................................................................................................209

11.1 Coastal Bridge Settings...........................................................................................209

11.1.1 Bridges at Inlets...........................................................................................209

11.1.2 Bridges over Bays and Sounds ...................................................................210

11.1.3 Bridges Spanning Tidal Embayments .........................................................210

11.1.4 Bridges Crossing Rivers ..............................................................................212

11.2 Wave Loads on Bridge Decks: Examples and Description .....................................213

11.2.1 I-10 Escambia Bay (Hurricane Ivan)............................................................214

11.2.2 US 90 Biloxi Bay (Hurricane Katrina) ..........................................................217

11.2.3 Description of the Wave Load Damage Mechanism ...................................219

11.3 Wave Loads on Bridge Decks: Literature................................................................221

11.4 Wave Loads on Bridge Decks: Method for Estimating ............................................223

11.4.1 Equations for Estimating Wave Loads on Bridge Decks .............................223

11.4.2 Example Application of Wave Load Equations............................................225

11.4.3 Discussion of the Method ............................................................................226

11.5 Wave Loads on Bridge Decks: Countermeasures ..................................................229

11.6 Bridge Deck Elevation to Avoid Wave Loads ..........................................................229

11.6.1 Nominal Maximum Wave Height Approach.................................................230

11.6.2 Depth-Limited Maximum Wave Height Approach........................................231

11.6.3 Estimating the Maximum Wave Crest Elevation..........................................231

11.6.4 Freeboard Considerations ...........................................................................231

11.7 Other Coastal Bridge Issues ...................................................................................232

11.8 Selection of Design Storm Surge & Design Wave Heights .....................................233

11.8.1 Design Storm Surge SWL ...........................................................................234

11.8.2 Design Wave Heights ..................................................................................234

11.8.3 Coastal Engineer Involvement.....................................................................234
Table of Contents HEC-25 3rd ed.

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Chapter 12 - Coastal Scour ......................................................................................................235

12.1 Introduction..............................................................................................................235

12.1.1 Bridge Scour Policy, Guidance, and Reference Materials...........................236

12.2 Hydraulics for Coastal Bridge Locations & Crossings .............................................237

12.2.1 Tidally-Influenced River Crossings ..............................................................237

12.2.2 Tidally-Dominated River Crossings .............................................................239

12.2.3 Tidal Crossings............................................................................................240

12.2.4 Non-Tidal Waters.........................................................................................240

12.2.5 Other Considerations...................................................................................240

12.3 Types of Coastal Scour...........................................................................................241

12.3.1 Long-term Changes.....................................................................................241

12.3.2 Bedforms .....................................................................................................244

12.3.3 Contraction Scour........................................................................................245

12.3.4 Local Scour at Bridge Piers .........................................................................247

12.3.5 Local Scour at Bridge Abutments ................................................................254

12.3.6 Wave-Induced Scour...................................................................................256

12.3.7 Tsunami-Induced Scour ..............................................................................260

12.3.8 Time-Dependent Scour ...............................................................................262

12.3.9 Other Types of Coastal Scour .....................................................................263

12.4 Bridge Scour Evaluation Methods ...........................................................................263

12.4.1 Tidally-Influenced Bridge Scour Evaluation.................................................263

12.4.2 Tidally-Dominated Bridge Scour Evaluation ................................................264

12.4.3 Tidal Bridge Scour Evaluation .....................................................................265

12.4.4 Example of Tidal Bridge Scour Calculations ...............................................266

12.5 Scour Appraisal Methodologies...............................................................................270

12.5.1 Low-Risk Crossings.....................................................................................270

12.5.2 Typical Crossings ........................................................................................270

12.5.3 Special Cases..............................................................................................270

12.6 Scour Countermeasures .........................................................................................271

Part 4 – Coastal Highway Vulnerability Assessment ................................................................275

Chapter 13 - Engineering Risk at the Coast .............................................................................277

13.1 Return Period and Probability of Occurrence..........................................................277

13.2 Coastal Storm Flood Frequencies...........................................................................279

13.2.1 Atlantic and Gulf Coast Surge Frequencies ................................................279
HEC-25 3rd ed. Table of Contents

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13.3 Available Estimates of Storm Surge: SSIMs and FIRMs.........................................281

13.4 Other Approaches to Quantifying Coastal Risk.......................................................282

Chapter 14 - Analysis Methods for Assessing Vulnerability to Extreme Coastal Events ..........285

14.1 Literature on Assessing the Impacts of Future Environmental Conditions..............287

14.2 Levels of Effort in Assessments ..............................................................................290

14.2.1 Level of Effort 1: Use of Existing Data and Resources................................290

14.2.2 Level of Effort 2: Modeling of Storm Surge and Waves...............................291

14.2.3 Level of Effort 3: Modeling in a Probabilistic Risk Framework.....................292

14.3 Gulf of Mexico and South Atlantic Coast................................................................. 293

14.3.1 Level of Effort 1: Gulf of Mexico and South Atlantic Coast..........................293

14.3.2 Level of Effort 2: Gulf of Mexico and South Atlantic Coast..........................297

14.3.3 Level of Effort 3: Gulf of Mexico and South Atlantic Coast..........................300

14.4 Mid-Atlantic and New England Coast......................................................................302

14.4.1 Level of Effort 1: Mid-Atlantic and New England Coast...............................302

14.4.2 Level of Effort 2: Mid-Atlantic and New England Coast...............................303

14.4.3 Level of Effort 3: Mid-Atlantic and New England Coast...............................306

14.5 Great Lakes.............................................................................................................307

14.5.1 Level of Effort 1: Great Lakes Coast ...........................................................307

14.5.2 Level of Effort 2: Great Lakes Coast ...........................................................310

14.5.3 Level of Effort 3: Great Lakes Coast ...........................................................312

14.6 Pacific Coast & Islands – Storms ............................................................................313

14.6.1 Level of Effort 1: Pacific Coast – Storms .....................................................314

14.6.2 Level of Effort 2: Pacific Coast – Storms .....................................................316

14.6.3 Level of Effort 3: Pacific Coast – Storms .....................................................317

14.7 Pacific Coast & Islands – Tsunamis ........................................................................318

14.7.1 Level of Effort 1: Pacific Coast – Tsunamis.................................................318

14.7.2 Level of Effort 2: Pacific Coast – Tsunamis.................................................319

14.7.3 Level of Effort 3: Pacific Coast - Tsunamis..................................................321

14.8 Vulnerability Assessment Case Studies..................................................................322

14.8.1 Level 1 Case Study: Adapting to Rising Tides - San Francisco Bay ...........323

14.8.2 Level 2 Case Study: The Gulf Coast 2 Study - Mobile, Alabama ................328

14.8.3 Level 3 Case Study: Central Artery Project, Boston, Massachusetts..........335

Chapter 15 - Adaptation Strategies for Coastal Highways........................................................345

15.1 Typical Coastal Damage Mechanisms ....................................................................345
Table of Contents HEC-25 3rd ed.

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15.1.1 Roadway Damage by Wave Attack .............................................................345

15.1.2 Roadway and Railway Damage by Coastal “Weir-Flow”.............................347

15.1.3 Roadway Damage by Bluff Erosion and Shoreline Recession....................348

15.1.4 Bridge Deck Damage by Waves on Surge ..................................................349

15.1.5 Structure Damage by Wave Runup.............................................................351

15.1.6 Tunnel and Road Damage by Overtopping .................................................351

15.1.7 Damage by Tsunamis..................................................................................351

15.2 Adaptation Strategies for Coastal Highway Infrastructure.......................................352

15.2.1 Manage and Maintain ..................................................................................352

15.2.2 Increase Redundancy..................................................................................352

15.2.3 Protect .........................................................................................................353

15.2.4 Accommodate..............................................................................................354

15.2.5 Relocate ......................................................................................................355

References................................................................................................................................359

Appendix – Workshop Participants ...........................................................................................385
HEC-25 3rd ed. List of Figures

ix

List of Figures

Figure 1.1. A US coastal highway (Florida SH A1A in Ft. Lauderdale 2018)................................ 3

Figure 3.1. A local coastal road along an estuary (Shore Road along Mt. Sinai Harbor, 

Town of Brookhaven, New York)................................................................................17

Figure 3.2. Big Creek Bridge on the Pacific Coast Highway (California SH 1) ...........................18

Figure 3.3. Damage caused by surge and waves in Hurricane Michael (October 14, 2018; 

permission to use photograph provided by John Cleary) ........................................... 21

Figure 3.4. US Highway 90 bridge across Biloxi Bay, Mississippi, after Hurricane Katrina. 

(photograph looking northeast from Biloxi September 21, 2005) ...............................23

Figure 4.1. Relative sea level rise (RSLR) trend for New York City based on tide gage data 

(image downloaded from NOAA Tides and Currents website September 6, 2018) ...33

Figure 4.2. Annual average sea level at Dauphin Island, Alabama ............................................34

Figure 4.3. Sea levels along the US Atlantic and Gulf Coasts for the past century ....................35

Figure 4.4. Sea levels along the US Pacific Coast for the past century......................................36

Figure 4.5. Relative sea level rise (RSLR) rates measured at tide gages around the US 

(image downloaded from NOAA Tides and Currents website, Sept 5, 2018) ............37

Figure 4.6. Map of sea level change as measured by satellites 1992-2017 (permission for 

use provided by CNES/LEGOS/CLS).........................................................................38

Figure 4.7. Global Mean Sea Level Rise (GMSLR) 1993-2018 as measured by satellite 

altimetry (data from CNES/LEGOS/CLS 2018)..........................................................39

Figure 4.8. Projections of GMSLR in this century (relative to 2000) from Kopp et al. (2014) 

for a) RCP 8.5, b) RCP 4.5, and c) RCP 2.6 ..............................................................42

Figure 4.9. GMSLR scenarios and process-based projections. The scenarios (dashed 

curves) are the SLR Interagency Task Force (Sweet et al. 2017b) scenarios and 

the projections (blue line for median and shading for 90% confidence interval) are 

the Kopp et al. (2014) scientific, process-based projections for RCP 8.5 (upper 

panel) and for RCP 4.5/6.0 (lower panel)...................................................................43

Figure 4.10. RSLR projections for four US cities corresponding to the GMSLR scenarios of 

the Interagency report (adapted from Sweet et al. 2017b).........................................45

Figure 4.11. Future SLR values for planning and design as per NCHRP 15-61: the RSLR 

corresponding with the GMSLR values shown by the circles on the lower panel 

are minimum values (see Table 4.1). The circles on the upper panel are higher 

values for more sensitive, or fragile, asset planning (see Table 4.2) (see Figure 

4.9 caption for explanation of the other curves) .........................................................47

Figure 4.12. Relative sea level rise (RSLR) trend for Atlantic City, New Jersey (image 

downloaded from NOAA Tides and Currents website September 10, 2018) .............50

Figure 4.13. Basic definitions of tides (image downloaded from 

http://tidesandcurrents.noaa.gov) ...............................................................................53

Figure 4.14. An example of the relationship between the survey and tidal datums....................54
List of Figures HEC-25 3rd ed.

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Figure 4.15. Approximate predicted change from NAVD 88 to the new vertical datum which 

will replace it in 2022 (adapted from NGS 2018)........................................................55

Figure 4.16. Storm surge at Galveston, Texas, from Hurricane Alicia in 1983 ...........................56

Figure 4.17. When and where to apply riverine hydraulic and coastal hydrodynamic models 

as a function of distance from the coast and importance of infrastructure (Webb 

2017) ..........................................................................................................................58

Figure 4.18. Example of numerical mesh used for a storm surge model....................................60

Figure 4.19. Detail of a seamless topographic-bathymetric mesh surface used for modeling 

coastal storm surge flooding.......................................................................................61

Figure 4.20. Example of storm surge model output (with input winds) along the Alabama 

coast during Hurricane Katrina (2005)........................................................................62

Figure 4.21. Water level measurements near Sabine, Texas (upper panel) and the path of 

Hurricane Harvey (lower panel)..................................................................................65

Figure 4.22. Example of seasonal lake level fluctuations (USACE-Detroit District 2018)...........67

Figure 4.23. Annual lake level fluctuations – Lakes Michigan and Huron (dashed line is the 

long-term average) .....................................................................................................68

Figure 4.24. An example of water level, wave height and wave period values near San Luis 

Pass, Texas obtained from the USACE Coastal Hazards System database .............70

Figure 5.1. Wave parameter definitions ......................................................................................74

Figure 5.2. Water particle paths under waves in deep water......................................................77

Figure 5.3. Water particle paths under waves in shallow and intermediate water depths ..........77

Figure 5.4. Stokes 2nd order wave theory water surface profile ..................................................78

Figure 5.5. Bending of wave crests as they approach the shore due to refraction (adapted 

from USACE 2002).....................................................................................................81

Figure 5.6. Wave energy focused on headland by a wave refraction and diffraction (adapted 

from USACE 2002).....................................................................................................82

Figure 5.7. Waves refracting, diffracting and breaking on the shallow-water reefs south of 

the Diamond Head Lighthouse, Honolulu, Hawaii......................................................83

Figure 5.8. Depth-limited wave breaking in shallow water..........................................................84

Figure 5.9. Depth-limited wave breaking during storm surge (permission to use photograph 

provided by Happy Partridge).....................................................................................85

Figure 5.10. Wave breaker types ................................................................................................86

Figure 5.11. Example of a plunging breaker (from Douglass 2002) ...........................................87

Figure 5.12. Example of a spilling breaker (Huntington Beach, California, 2017) ......................87

Figure 5.13. Example of a collapsing breaker (Morro Bay, California) .......................................88

Figure 5.14. A train of long-period swell approaching the Oregon coast ....................................89

Figure 5.15. Irregular waves on San Francisco Bay, California..................................................89

Figure 5.16. A storm-driven, irregular, sea state in the Gulf of Mexico (Florida SH 30E) ...........90
HEC-25 3rd ed. List of Figures

xi

Figure 5.17. ADCIRC+SWAN wave model hindcast of maximum significant wave heights 

generated during Hurricane Ivan (2004) at the I-10 bridge across Escambia Bay, 

Pensacola, Florida......................................................................................................94

Figure 6.1. Sea cliff and pocket beach on the Oregon coast (Oswald West State Park)............98

Figure 6.2. An example of a barrier island. Dauphin Island, Alabama (circa 2001, looking 

east with Gulf of Mexico to the right and Mississippi Sound to the left)......................99

Figure 6.3. Sea level rise curve for the past 20,000 years........................................................100

Figure 6.4. Terminology used to describe processes of waves and currents in the surf zone..101

Figure 6.5. Terminology used to describe the beach profile .....................................................102

Figure 6.6. Sand bar and trough exposed at low tide (Garden City Beach, South Carolina)....102

Figure 6.7. A beach scarp (Los Angeles County, California)....................................................103

Figure 6.8. A sand dune (St. George Island, Florida) ...............................................................103

Figure 6.9. Beach profile surveying crew using a traditional level and rod ...............................104

Figure 6.10. A CRAB (Coastal Research Amphibious Buggy) used to measure beach 

profiles during beach nourishment ...........................................................................105

Figure 6.11. Dean’s equilibrium beach profile shape definition sketch .....................................106

Figure 6.12. Examples of colors of US beach sands (from Douglass 2002, photograph used 

with permission of the author) ..................................................................................107

Figure 6.13. Example grain size distribution based on a sieve analysis for beach sand ..........108

Figure 6.14. Typical beach profile changes in response to cross-shore transport of sand.......109

Figure 6.15. Dune erosion model results example ...................................................................110

Figure 6.16. Definition sketch of wave angle at breaking .........................................................112

Figure 6.17. The CERC equation model for longshore sand transport rate plotted with field 

data (adapted from USACE 1984)............................................................................113

Figure 6.18. Two tidal inlets on the southwest Florida coast (New Pass and Big Sarasota 

Pass, Lido Key is the barrier island between the two inlets) ....................................115

Figure 6.19. Typical inlet morphology .......................................................................................116

Figure 6.20. Idealized ocean-inlet-bay system (adapted from USACE 1984) ..........................116

Figure 6.21. Tidal prism versus minimum inlet throat area for all major inlets on the Atlantic, 

Gulf, and Pacific Coasts (USACE 1984) ..................................................................117

Figure 6.22. Physical model experiment on wave loads on a bridge deck in a wave flume 

(University of South Alabama wave flume)...............................................................121

Figure 6.23. A small-scale wave flume being used to model wave runup for professional 

development training in NHI Course #135082 Highways in the Coastal 

Environment .............................................................................................................121

Figure 6.24. A movable-bed, physical model experiment on beach salient formation in a 

wave basin (University of South Alabama wave basin)............................................122

Figure 7.1. A revetment protecting a coastal highway (Bayfront Road, Mobile, Alabama, 

2019) ........................................................................................................................126
List of Figures HEC-25 3rd ed.

xii

Figure 7.2. Types of shore protection walls ..............................................................................126

Figure 7.3. Galveston Seawall (Seawall Boulevard, 2018).......................................................127

Figure 7.4. San Francisco’s Great Highway Seawall (California SH 35, 1991) ........................127

Figure 7.5. Seawall protecting a coastal highway (Venice, Florida, 2001) ...............................128

Figure 7.6. Seawall protecting a coastal highway (Pacific Coast Highway, Pacific Palisades, 

California, 2003) .......................................................................................................128

Figure 7.7. Seawall protecting a coastal highway (Florida SH A1A, Flagler Beach, 2002) ......129

Figure 7.8. Seawall protecting a coastal highway (US 101, Curry County, Oregon, 2001) ......129

Figure 7.9. Wave striking a seawall protecting Kaumuali’i Highway on Kauai (Hawaii SH 50, 

2018) ........................................................................................................................130

Figure 7.10. Revetment protecting a highway along a bay shoreline (Florida SH 60 crossing 

Tampa Bay, 2003)....................................................................................................131

Figure 7.11. Revetment protecting a highway along a bay shoreline (Washington SH 105, 

Willapa Bay, 2003) ...................................................................................................132

Figure 7.12. Seawall protecting a local road (West Cliff Drive, Santa Cruz, California, 2001)..132

Figure 7.13. Concrete seawall designed to look like the natural rock formation built on an 

eroding sea cliff to protect a local road (East Cliff Drive, Santa Cruz, California, 

2001) ........................................................................................................................133

Figure 7.14. Typical coastal revetment design cross-section ...................................................133

Figure 7.15. Storm waves breaking on a rock, rubble-mound breakwater in a bay (Mobile 

Bay, Alabama)..........................................................................................................135

Figure 7.16. A revetment with rocks too small to withstand the wave attack at this location....139

Figure 7.17. Example of a failed attempt at embankment protection (USACE archives 

photographs) ............................................................................................................139

Figure 7.18. An example of splash damage behind seawall (circa 2004)................................. 140

Figure 7.19. Example of a splash apron (center of photograph) at the top of a coastal rock 

revetment protecting a local road along a bluff on Chesapeake Bay (Shore Road, 

Anne Arundel County, Maryland, 2017) ...................................................................141

Figure 7.20. Example of rigid concrete-block revetment failure (Florida SH A1A, Delray 

Beach, circa 1972; University of Florida and USACE archive photos).....................142

Figure 7.21. Example of failed block revetment (Louisiana SH 82, circa 1980, USACE 

archives photos) .......................................................................................................143

Figure 7.22. Wave runup on a structure definition sketch.........................................................144

Figure 7.23. Wave overtopping a seawall (Lihue, Hawaii, 2018)..............................................145

Figure 7.24. Critical values of average overtopping discharges (adapted from USACE 2002) 147

Figure 7.25. Eroding profile input to coastal revetment design example ..................................148

Figure 7.26. Coastal revetment design example cross-section ................................................151

Figure 8.1. A highway initially built inland now threatened by long-term shoreline erosion. 

Cape Shoalwater area of Washington SH 105 (2003) .............................................154
HEC-25 3rd ed. List of Figures

xiii

Figure 8.2. A local road threatened by long-term shoreline recession (Ocean Drive, Whale 

Beach area of Cape May County Road 619, Ludlam Island, New Jersey 2003) .....155

Figure 8.3. A local road being undermined by bluff erosion and long-term shoreline 

recession on the Great Lakes (Painesville, Ohio, 2001) ..........................................155

Figure 8.4. Road destroyed by shoreline recession: a) broken pavement on the beach at 

the old location; b) south end of the closed section; c) location map. (Texas SH 

87, Jefferson County, circa 2002).............................................................................156

Figure 8.5. An example of shoreline position changes through time. Stump Hole area of St. 

Joseph’s Peninsula, Florida......................................................................................158

Figure 8.6. An example of shoreline change rates along 30,000 ft of coast showing 

temporal and spatial variations but a significant recessional trend. Western-facing 

shoreline of St. Joseph’s Peninsula, Florida.............................................................159

Figure 8.7. Example of projected future shoreline positions at Stump Hole .............................160

Figure 8.8. Groin field in Long Branch, New Jersey (2006)......................................................163

Figure 8.9. A beach nourishment project under construction in Gulf Shores, Alabama (2001) 165

Figure 8.10. Beach nourishment project with constructed dune on top of old, failed 

revetment protecting Florida SH A1A (Delray Beach, 2001)....................................166

Figure 8.11. Beach nourishment seaward of a seawall protecting a road. (New Jersey SH 

35, Sea Bright, 2001)................................................................................................166

Figure 8.12. Offshore segmented breakwaters with salients in beach nourishment 

protecting a highway (Louisiana SH 82, Holly Beach)..............................................167

Figure 8.13. Offshore segmented breakwaters with salients in beach nourishment (Presque 

Isle State Park, Pennsylvania, circa 1980, USACE archive photograph).................168

Figure 8.14. Offshore segmented breakwater system at Presque Isle, Pennsylvania..............169

Figure 8.15. Empirical relationships for shoreline effect of offshore segmented breakwaters. 

(after Pope and Dean 1986, and USACE 2002).......................................................170

Figure 8.16. Offshore segmented breakwaters with groins and beach nourishment on 

Corpus Christi Bay (Ocean Drive, Corpus Christi, Texas)........................................170

Figure 8.17. Constructed pocket beach stabilized with a T-head groin breakwater system 

(Point Clear, Alabama, circa 2006)...........................................................................171

Figure 8.18. Beach nourishment project stabilized as pocket beaches with a headland 

breakwater system protecting a road (Water Street, Yorktown, Virginia, circa 

2006) ........................................................................................................................172

Figure 8.19. Example of a constructed marsh stabilized with a nearshore breakwater (Little 

Bay, Bayou LaBatre, Alabama) ................................................................................174

Figure 8.20. Example of a constructed marsh protected by nearshore rock breakwaters 

(Holts Landing State Park, Indian River Bay, Delaware, 2009)................................175

Figure 9.1. Flooding of the Tidal Basin in Washington, District of Columbia (June 2018; 

permission for use of photograph provided by Loic Pritchett) ..................................178

Figure 9.2. Example of coastal flooding on an urban road (Honolulu, March 30, 2017)...........179
List of Figures HEC-25 3rd ed.

xiv

Figure 9.3. Example of coastal flooding on a primary road (Florida SH A1A, Hollywood 

Beach, October 21, 2017) ........................................................................................179

Figure 9.4. Example of coastal flooding on a local road that crosses a developed barrier 

island (Hollywood Beach, Florida, October 21, 2017) ..............................................180

Figure 9.5. Example of flooding of a coastal road which is affecting traffic and parking (Dock 

Street, Annapolis, Maryland, 2017 photograph provided by Joe Krolak) .................180

Figure 9.6. The increased frequency of flooding occurs because the full distribution of tides 

rises with RSLR (from Sweet et al. 2014)................................................................. 183

Figure 9.7. Increased flooding due to RSLR including historical exceedances (orange), 

future projections through 2100 based upon the continuation of the historical 

trend (blue), and future projections under median RCP2.6, 4.5 and 8.5 

conditions, for Charleston, South Carolina and San Francisco, California (from 

Sweet et al. 2017a adapted from Sweet and Park 2014).........................................184

Figure 9.8. Flooding of McNair Road, US Naval Academy, Annapolis, Maryland (permission 

for use of photograph provided by David Kriebel) ....................................................184

Figure 9.9. Probability distribution for high tides at Annapolis approximately follows a 

normal distribution (from Kriebel and Henderson 2018)...........................................185

Figure 9.10. Average annual flood events above threshold water levels at Annapolis, 2014-

2017 (from Kriebel and Henderson 2018). ...............................................................186

Figure 9.11. The effect of RSLR on annual flood events at Annapolis (from Kriebel and 

Henderson 2018)......................................................................................................187

Figure 9.12. Applying the annual flood events curves to McNair Road, Annapolis (from 

Kriebel and Henderson 2018)...................................................................................188

Figure 9.13. Annual vehicle-hours of delay projected for major roads (principal arterials, 

minor arterials, and major collectors) due to increased flooding due to RSLR by 

state; year (2020, 2060, 2100), and RSLR scenario (Intermediate-Low, 

Intermediate, and Extreme) (from Jacobs et al. 2018a) ...........................................189

Figure 9.14. Example of a road which has been raised to reduce flooding due to RSLR 

(Sunset Harbour Drive, Miami Beach, 2017)............................................................190

Figure 9.15. Flooding of McNair Road due to RSLR without (left panel) and with (right 

panel) a 1 ft increase in road elevation (from Kriebel and Henderson 2018) ...........191

Figure 10.1. Example of pavement damage due to storm surge (Florida SH 292 on Perdido 

Key after Hurricane Ivan, September 2004).............................................................193

Figure 10.2. Example of pavement damaged by hurricane (photograph looking west on 

Florida SH 399, Gulf Islands National Seashore, 2005)...........................................194

Figure 10.3. Flow regimes leading to failure of embankments in riverine flooding situations 

(after Clopper and Chen 1988).................................................................................195

Figure 10.4. Embankment failure mechanisms (after Clopper and Chen 1988).......................196

Figure 10.5. Pavement destroyed by the weir-flow mechanism (Fort Pickens Road, Gulf 

Islands National Seashore, near Pensacola, Florida) ..............................................196

Figure 10.6. Weir-flow damage beginning (Fort Pickens Road, Gulf Islands National 

Seashore, Florida, July 2005; FHWA photograph)...................................................197
HEC-25 3rd ed. List of Figures

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Figure 10.7. Weir-flow damage occurring. (Fort Pickens Road, Gulf Islands National 

Seashore, Florida, July 2005; FHWA photograph)...................................................197

Figure 10.8. USA/TAMU laboratory experiment model setup schematic..................................198

Figure 10.9. Schematic of results from USA/TAMU laboratory experiments test run one ........199

Figure 10.10. Schematic of results from USA/TAMU laboratory experiments test run two.......199

Figure 10.11. Laboratory tests of the weir-flow damage mechanism showing scour 

destroying the downstream shoulder and beginning to undermine the edge of 

pavement. (USA/TAMU flume tests, June 2005)......................................................200

Figure 10.12. Laboratory tests of the weir-flow damage mechanism showing scour has 

continued to the point of undermining failure of 3 sections (6 ft) of roadway 

surface. (USA/TAMU flume tests, June 2005)..........................................................200

Figure 10.13. Pavement moved landward by overwash processes. (Florida SH 399, Gulf 

Islands National Seashore, Florida after Hurricane Ivan, 2004)...............................201

Figure 10.14. Evidence of weir-flow damage to the seaward edge of pavement due to return 

flow late in the storm (Alabama SH 182, after Hurricane Ivan, Gulf Shores, 2004) . 202

Figure 10.15. Schematic of sand erosion and deposition on a barrier island resulting from 

overwash ..................................................................................................................203

Figure 10.16. Example of road buried by overwash sand after it was opened by plows ..........204

Figure 10.17. Artificial sand dune constructed seaward of a highway to protect the highway 

(North Carolina SH 12; looking north, the Atlantic Ocean is to the right and 

Pamlico Sound is to the left, 2002)...........................................................................204

Figure 10.18. Schematic summarizing three approaches (bayward location, low elevation, 

constructed sand dunes near road) to minimize damage to roads that overwash ...205

Figure 10.19. Sheet pile, with buried gabions for scour protection, at edge of pavement to 

resist pavement damage due to coastal storm surge overwash. (Florida DOT 

figure) .......................................................................................................................206

Figure 10.20. Gabions at edge of pavement to resist pavement damage due to coastal 

storm surge overwash. (Florida DOT figure) ............................................................206

Figure 10.21. Highway shoulder which has been designed to withstand the weir-flow 

damage during overwashing events (US 98, Okaloosa Island, Florida, 2016).........207

Figure 10.22. Conceptual design to resist pavement damage due to coastal storm surge 

overwash ..................................................................................................................208

Figure 11.1. Conceptual schematic of four typical bridge settings within the coastal 

environment..............................................................................................................210

Figure 11.2. Bridge spanning a small inlet (Alabama SH 182 in Gulf Shores) .........................211

Figure 11.3. The F.J. Torras Causeway bridges between Brunswick and Saint Simons 

Island, Georgia .........................................................................................................211

Figure 11.4. Interstate 10 bridge crossing Mobile Bay, Alabama .............................................212

Figure 11.5. Pacific Coast river mouth crossings (clockwise from top left: Big Creek, 

Oregon; Pistol River, Oregon; Yachats River, Oregon, and Redwood Creek, 

California) .................................................................................................................213
List of Figures HEC-25 3rd ed.

xvi

Figure 11.6. Location map of some of the highway bridges damaged by wave loads in 

hurricanes along the north-central Gulf Coast (from Douglass et al. 2006a) ...........214

Figure 11.7. Location map and damage overview for the I-10 bridge across Escambia Bay, 

Florida in Hurricane Katrina......................................................................................215

Figure 11.8. Interstate-10 bridge across Escambia Bay, Florida, after Hurricane Ivan. 

Photograph looking northeast from Pensacola September 16, 2004 (photograph 

used with a license from AP Images) .......................................................................216

Figure 11.9. US 90 bridge over Biloxi Bay, Mississippi showing the spans below a critical 

elevation were removed by Hurricane Katrina (photograph looking southwest 

from Ocean Springs, February 19, 2016)................................................................. 218

Figure 11.10. Details of damage to US 90 bridge over Biloxi Bay caused by Hurricane 

Katrina ......................................................................................................................218

Figure 11.11. Schematic of resultant wave-induced load, with both uplift and lateral 

components, on a bridge deck .................................................................................220

Figure 11.12. Schematic of typical time-history of wave loads on rigid structures....................220

Figure 11.13. Horizontal and vertical wave-induced loads on bridge decks .............................224

Figure 11.14. Definition sketch for Δzh, Δzv, Ah, Av, and ηmax in Equations 11.1 and 11.2 for 

estimating wave loads on elevated bridge decks) ....................................................225

Figure 11.15. Example of concave seaward face of a coastal bridge (I-10 bridge over 

Mobile Bay, Alabama) ..............................................................................................228

Figure 11.16. Definition sketch of wave parameters and water levels for determining 

elevation of bridge deck for clearance from wave crests..........................................230

Figure 12.1. Discharge record for a tidally-influenced location (image downloaded from 

USGS website May 2019) ........................................................................................238

Figure 12.2. Discharge record for a tidally-dominated location (image downloaded from 

USGS website May 2019) ........................................................................................239

Figure 12.3. Representation of recession-dominated flow following Hurricane Harvey............241

Figure 12.4. Aerial imagery of Oregon Inlet, North Carolina from 1993, March 2011, and 

August 2011 .............................................................................................................243

Figure 12.5. Aerial imagery of Matanzas Inlet, Florida from 1995, 2012, and 2016 .................243

Figure 12.6. Bathymetric elevations showing scour holes inside Indian River Inlet (circa 

1999) ........................................................................................................................244

Figure 12.7. Definition sketch showing the representative variables used to estimate the 

height and length of orbital ripples............................................................................245

Figure 12.8. Examples of possible horizontal contractions at coastal bridges (thin lines 

denote possible streamlines, double-ended arrows denote the potential for flow 

reversals)..................................................................................................................246

Figure 12.9. Vertical contraction or pressure scour schematic (Arneson et al. 2012)...............247

Figure 12.10. Local scour depression at the terminus of a coastal structure............................249

Figure 12.11. Complex pier components for Sheppard pier scour equations (FDOT 2005).....250
HEC-25 3rd ed. List of Figures

xvii

Figure 12.12. Individual elements for Sheppard pier scour equations (FDOT 2005)................251

Figure 12.13. Concept of effective diameter for complex piers in the Sheppard pier scour 

equations (FDOT 2005)............................................................................................251

Figure 12.14. Three cases for Sheppard pier scour equations (FDOT 2005)...........................252

Figure 12.15. Johns Pass, Florida (2002).................................................................................253

Figure 12.16. Location overview for Johns Pass, Florida .........................................................253

Figure 12.17. Typical scour profiles due to waves at the toe of a vertical wall (from USACE 

2002) ........................................................................................................................ 254

Figure 12.18. Scour due to breaking waves at a vertical wall (USACE 2002) ..........................255

Figure 12.19. Damage to bridge approach behind abutment due to wave action in a 

hurricane...................................................................................................................256

Figure 12.20. Wave-induced local pier scour (Webb and Matthews 2014)...............................258

Figure 12.21. Definition sketch for effective diameter of pile groups for wave-induced pier 

scour (Webb and Matthews 2014)............................................................................259

Figure 12.22. Scour hole formed by Hurricane Katrina (permission to use photograph 

provided by Joe Krolak)............................................................................................259

Figure 12.23. Jensen Beach Causeway bridge ........................................................................260

Figure 12.24. Jensen Beach Causeway bridge post-event scour bathymetry (2005)...............261

Figure 12.25. Proposed design scour analysis workflow for a tidally-influenced bridge 

crossing ....................................................................................................................264

Figure 12.26. Proposed design scour analyses workflow for a tidally-dominated bridge 

crossing ....................................................................................................................265

Figure 12.27. Proposed design scour analyses workflow for a tidal bridge crossing................266

Figure 12.28. Location overview for the Pensacola Bay Bridge scour example .......................267

Figure 12.29. Return period still water levels and corresponding event velocity magnitudes 

on Neches River, TX ................................................................................................267

Figure 12.30. Definition sketch for scour protection at the base of vertical walls......................273

Figure 13.1. Probability of occurrence as a function of return period, T, and years of service, 

n................................................................................................................................278

Figure 14.1. Vulnerability of highways in the coastal environment ...........................................285

Figure 14.2. FHWA’s Vulnerability Assessment and Resilience “Framework” (FHWA 2017d).286

Figure 14.3. Deerfield Beach, Florida (2018)............................................................................294

Figure 14.4. Portland Head Lighthouse, Maine (2016).............................................................304

Figure 14.5. Chicago waterfront (2018) ....................................................................................309

Figure 14.6. Laguna Beach, California .....................................................................................315

Figure 14.7. Wainiha River, on the north shore of the island of Kauai, Hawaii, is an area 

with an extremely high tsunami hazard and a history of tsunami-induced bridge 

damage.....................................................................................................................320
List of Figures HEC-25 3rd ed.

xviii

Figure 14.8. Shoreline Drive, Alameda, California on San Francisco Bay (2018) ....................324

Figure 14.9. Inundation map of Alameda County for 100-year storm flood with 16 inches of 

sea level rise and additional elevation for wave effects (from ART 2011b)..............326

Figure 14.10. Detailed inundation map of the northern portion of Alameda County for the 

100-year storm flood with 16 inches of sea level rise and additional elevation for 

wave effects (from ART 2011b)................................................................................327

Figure 14.11. Modeled storm surge depths in Mobile County, Alabama for the scenario of 

Hurricane Georges conditions with 2.5 ft (75 cm) of future sea level rise (from 

Choate et al. 2012)...................................................................................................332

Figure 14.12. Validation of ADCIRC surge estimates by comparison with a tide gage at 

Dauphin Island, Alabama .........................................................................................333

Figure 14.13. Validation of ADCIRC storm surge estimates by comparison with measured 

high water marks ......................................................................................................334

Figure 14.14. Modeled wave heights for the scenario of Hurricane Georges conditions with 

2.5 ft (75 cm) of future sea level rise (from Choate et al. 2012) ...............................336

Figure 14.15. Example of the fine, highly-resolved hydrodynamic model mesh used in the 

Boston Central Artery/Tunnel vulnerability assessment study (from Douglas 

2015; permission to use image provided by MassDOT)...........................................339

Figure 14.16. Example of the inundation map products from the Boston Central 

Artery/Tunnel Vulnerability Study. The exceedance probability of coastal flooding 

in 2070/2100 (from Bosma et al. 2015; permission to use image provided by 

MassDOT) ................................................................................................................341

Figure 14.17. Example of coastal flood mapping results from the Boston Central 

Artery/Tunnel vulnerability study focused on the area around Long Wharf and 

Christopher Columbus Park (from Bosma 2019; permission to use image 

provided by MassDOT).............................................................................................341

Figure 14.18. Model results showing depth for a 1% flooding probability in 2013 at the 93 

Granite Ave. location, as well as residence time and local flood pathways (from 

Bosma et al. 2015; permission to use image provided by MassDOT)......................343

Figure 14.19. Model results showing depth for a 1% flooding probability in 2030 at the 93 

Granite Ave. location, as well as residence time and local flood pathways (from 

Bosma et al. 2015; permission to use image provided by MassDOT)......................344

Figure 15.1. Partial embankment damage caused by waves on storm surge during 

Hurricane Ivan (Pensacola, Florida, 2004)...............................................................346

Figure 15.2. Example of the coastal weir-flow damage mechanism as it occurs (photograph 

provided by FDOT, circa 2005) ................................................................................347

Figure 15.3. Railway embankment damage caused primarily by the coastal weir-flow 

damage mechanism during (Mississippi, 2005) .......................................................348

Figure 15.4. Pavement damage due to waves and surge in an extreme event (North 

Carolina SH 12, 2009; permission to use photograph provided by Dave 

Henderson)...............................................................................................................349

Figure 15.5. Two bridges destroyed by wave loads in Hurricane Katrina (Biloxi Bay, 

Mississippi)...............................................................................................................350
HEC-25 3rd ed. List of Figures

xix

Figure 15.6. New, much higher, US 90 bridge across Bay St. Louis, Mississippi built after 

Hurricane Katrina (June 14, 2007) ...........................................................................355

Figure 15.7. Example of coastal highway relocation in Florida (SH 399: the dashed line 

shows the previous highway location: permission to use image provided by the 

Gulf Islands National Seashore)...............................................................................357
List of Tables HEC-25 3rd ed.

xx

List of Tables

Table 4.1 Minimum GMSLR estimates for use in planning and design as per NCHRP 15-61 

(values are relative to 2000).......................................................................................46

Table 4.2 Higher projections of GMSLR for consideration in the planning and design of 

high-value assets which are sensitive to SLR as per NCHRP 15-61 (See circles 

on upper panel of Figure 4.11, values are relative to 2000).......................................49

Table 4.3. Numerical coastal models cited in this manual ..........................................................58

Table 5.1. Wave height statistics in irregular seas......................................................................91

Table 12.1. Tsunami-induced scour guidance from Dames and Moore (1980) and FEMA 

(2012). ......................................................................................................................262

Table 13.1. Probability of extreme event occurrence for various periods of time ..................... 279

Table 14.1. Exposure assessment steps for level of effort 1: Gulf of Mexico/South Atlantic 

Coast ........................................................................................................................295

Table 14.2. Exposure assessment steps for level of effort 2: Gulf of Mexico/South Atlantic 

Coast ........................................................................................................................298

Table 14.3. Exposure assessment steps for level of effort 3: Gulf of Mexico/South Atlantic 

Coast. .......................................................................................................................301

Table 14.4. Exposure assessment steps for level of effort 1: Mid-Atlantic/New England 

Coast ........................................................................................................................304

Table 14.5. Exposure assessment steps for level of effort 2: Mid-Atlantic/New England 

Coast. .......................................................................................................................305

Table 14.6. Exposure assessment steps for level of effort 3: Mid-Atlantic/New England 

Coast ........................................................................................................................307

Table 14.7. Exposure assessment steps for level of effort 1: Great Lakes Coast ....................308

Table 14.8. Exposure assessment steps for level of effort 2: Great Lakes Coast ....................310

Table 14.9. Exposure assessment steps for level of effort 3: Great Lakes Coast ....................312

Table 14.10. Exposure assessment steps for level of effort 1: Pacific Coast ...........................314

Table 14.11. Exposure assessment steps for level of effort 2: Pacific Coast. ..........................316

Table 14.12. Exposure assessment steps for level of effort 3: Pacific Coast. ..........................317

Table 14.13. Exposure assessment steps for level of effort 1: tsunamis.................................. 319

Table 14.14. Exposure assessment steps for level of effort 2: tsunamis..................................321

Table 14.15. Exposure assessment steps for level of effort 3: tsunamis..................................322

Table 14.16. Scenarios modeled in GC2 (modified from Choate et al. 2012) ..........................330

Table 14.17. Validation analysis of ADCIRC model storm surge estimates .............................334
HEC-25 3rd ed. List of Symbols

xxi

List of Symbols

A = horizontal orbital semi-displacement

A = parameter in Dean’s equilibrium profile shape

Ah = area of the projection of the bridge deck onto the vertical plane

Av = area of the projection of the bridge deck onto the horizontal plane

ax = horizontal component of water particle acceleration

az = vertical component of water particle acceleration

cr = coefficient to account for horizontal loads due to bridge girders

C = wave celerity

C0 = wave celerity in deep water

CD = drag coefficient

Cg = wave group velocity

Cgb = wave celerity at breaking

Ch = empirical coefficient for the horizontal, wave-induced loads on bridge decks

CM = coefficient of mass or inertia

Cr = reflection coefficient

Cv = empirical coefficient for the vertical, wave-induced loads on bridge decks

d = water depth

d50 = median diameter of sand grains

de = depth of wall penetration below seabed

ds = design flood depth

D = diameter

D*

 = effective diameter of the complex pier

De = effective diameter of a pile group

Dcol

∗ = effective diameter of the column

Dpc

∗ = effective diameter of the pile cap

Dpg

∗ = effective diameter of the pile group

 � = total energy in a wave train per unit area of sea averaged over one wavelength

F = Coriolis parameter

F = fetch length

fD = drag force per unit length of pile in Morison’s equation

fi = inertial force per unit length of pile in Morison’s equation

fp = horizontal force per unit length of a vertical pile in Morison’s equation 

(Fh)max = maximum of the horizontal wave-induced load

Fs = factor of safety 

(Fv)max = maximum of the vertical wave-induced load

g = gravitational constant having a value of 32.2 ft/s2 (9.81 m/s2) 

H = wave height 

H1% = height exceeded by 1% of waves

H10% = height exceeded by 10% of waves

 ���� = average of the highest 1% of waves

 1 ����� = average of the highest 10% of waves

 ���� = average of the highest 5% of waves

Hb = maximum breaking wave height

 = maximum ratio of wave height to water depth

Hi = incident wave height

H/L = incident wave steepness
List of Symbols HEC-25 3rd ed.

xxii

Hmax = maximum wave height

Hmo = spectral-based significant wave height

Ho’ = unrefracted deepwater wave height

Hr = reflected wave height

Hs = significant wave height

K = an empirical coefficient in CERC equation for longshore sand transport rate

k = wave number = 2π/L

KC = Keulegan-Carpenter number

KD = empirical coefficient in Hudson’s equation for revetment stone size for wave 

attack

Ks = shoaling coefficient

L = wavelength

Lo = deepwater wavelength

Lp = wavelength associated with the peak wave period

n = ratio of wave group velocity to wave celerity

N = the number of girders supporting the bridge span deck

P = probability

 ̄ = wave power, wave energy flux

Pls = wave energy flux factor

Q = longshore sand transport rate

r = a roughness coefficient (r = 0.55 for the stone revetments)

R = roughness coefficient

Ru = vertical extent of wave runup

Ru,2% = runup level exceeded by 2% of runups in an irregular sea

S = equilibrium scour depth

Sm = maximum scour depth

Sr = specific gravity of stone

T = return period of storm in years

t = time

T = wave period

Tp = wave period corresponding with the peak of the energy density spectrum

u = horizontal component of water particle velocity

Um = maximum nearbed wave orbital velocity

urms = the root-mean-square of the horizontal nearbed wave orbital velocity

w = vertical component of water particle velocity

W = weight

W = pile group width

W50 = median weight of armor stone

wr = unit weight of stone

x = horizontal position

Ymax = difference between the SWL elevation and wave crest elevation for the 

maximum wave in the design sea-state

z = vertical direction (coordinate measured from SWL) 

α = angle of the breaking wave crest with the shoreline

γ = specific weight of water

Δzh = difference between the elevation of the crest of the maximum wave and the 

elevation of the centroid of Ah

Δzv = difference between the elevation of the crest of the maximum wave and the 

elevation of the underside of the bridge deck

ξop = surf similarity parameter
HEC-25 3rd ed. List of Symbols

xxiii

η = water surface elevation

 � = water surface elevation with waves removed (used in hydrodynamic modeling)

ηmax = maximum elevation of wave crest

θ = slope

λ = ripple length

ρ = density

σ = wave frequency = 2π/T

φ = sediment angle of internal friction, sediment angle of repose
Acknowledgements HEC-25 3rd ed.

xxiv

Acknowledgements

The cover image is a mosaic showing (from the top): 

• a coastal marsh in New York protecting a road (from Figure 13b of FHWA 2017c), 

• global mean sea level rise projections for planning and design (see Figure 4.11, lower 

panel),

• an interstate highway bridge over coastal waters (see Figure 11.4), 

• output from a numerical, hydrodynamic model of coastal storm surge in a hurricane (see

Figure 4.20),

• storm waves striking a rubble-mound seawall protecting a coastal highway (see Figure 

5.16), and

• the definition sketch for wave height, H (see Figure 5.1). 

Unless noted otherwise, all photographs and graphics in this document were developed by the 

authors, who give permission for their use in this document. 

In Figure 4.21, Figure 5.17, Figure 6.18, Figure 8.12, Figure 8.14, Figure 11.3, Figure 11.5, Figure 

11.7, Figure 12.4, Figure 12.5, Figure 12.6, Figure 12.16, Figure 12.23, Figure 12.28, and Figure 

14.15, the original maps are the copyright property of Google® EarthTM and can be accessed 

from https://www.google.com/earth (Google 2019, 2020). The authors developed the map 

overlays for this document. 

Notice

This document is disseminated under the sponsorship of the US Department of Transportation 

(USDOT) in the interest of information exchange. The US Government assumes no liability for 

the use of the information contained in this manual. This manual does not constitute a standard, 

specification, or policy. 

The US Government does not endorse products or manufacturers. Trademarks or manufacturer’s 

names appear in this manual only because the Government considers them essential to the 

objective of the document. The manual includes them for informational purposes only and the 

Government does not intend them to reflect a preference, approval, or endorsement of any one 

product or entity. 

Non-Binding Contents

The contents of this document do not have the force and effect of law and are not meant to bind 

the public in any way. This document is intended only to provide clarity to the public regarding 

existing requirements under the law or agency policies. 
HEC-25 3rd ed. Quality Assurance Statement

xxv

Quality Assurance Statement

The Federal Highway Administration (FHWA) provides high-quality information to serve 

Government, industry, and the public in a manner that promotes public understanding. The FHWA 

uses standards and policies to ensure and maximize the quality, objectivity, utility, and integrity 

of its information. The FHWA periodically reviews quality issues and adjusts its programs and 

processes to ensure continuous quality improvement. 

Supporting Purpose

The FHWA developed this manual, in part, to support 23 U.S.C. § 503(b)(3)(B)(viii), that directs 

the Department of Transportation to “... carry out research and development activities ... to study 

the vulnerabilities of the transportation system to ... extreme events and methods to reduce those 

vulnerabilities” and the July 27, 2017 US Senate Report 115-138 for the “Transportation, and 

Housing and Urban Development, and Related Agencies Appropriations Bill, 2018” that directed 

FHWA to “... expand its technical assistance ... to help coastal States, MPOs, and cities to revise 

their practices in all phases of transportation planning and asset management, project planning 

and development, and operations with the goal of improving the resiliency of our coastal highways 

and reducing the life-cycle costs for these natural disaster-prone roadways.” 
Glossary HEC-25 3rd ed.

xxvi

Glossary

0.2-PERCENT-ANNUAL-CHANCE FLOOD: The flood that has a 0.2-percent chance of being 

equaled or exceeded in any given year. See 500-year flood.

1-PERCENT-ANNUAL-CHANCE FLOOD: The flood that has a 1-percent chance of being 

equaled or exceeded in any given year. See 100-year flood.

2-PERCENT-ANNUAL-CHANCE FLOOD: The flood that has a 2-percent chance of being 

equaled or exceeded in any given year. See 50-year flood.

10-PERCENT-ANNUAL-CHANCE FLOOD: The flood that has a 10-percent chance of being 

equaled or exceeded in any given year. See 10-year flood.

10-YEAR FLOOD: Flood level which will recur on average once every 10 years. See 10-PercentAnnual-Chance Flood. 

50-YEAR FLOOD: Flood level which will recur on average once every 50 years. See 2-PercentAnnual-Chance Flood. 

100-YEAR FLOOD: Flood level which will recur on average once every 100 years. See 1-PercentAnnual-Chance Flood. 

500-YEAR FLOOD: Flood level which will recur on average once every 500 years. See 0.2-

Percent-Annual-Chance Flood.

ACCRETION: The extension of a beach out into the water by deposition of sand. Accretion is 

often used to refer to a net seaward movement of the shoreline over a specified time.

ADAPTATION – Adjustment in natural or human systems in anticipation of or response to a 

changing environment in a way that effectively uses beneficial opportunities or reduces negative 

effects.

ADAPTIVE CAPACITY: The degree to which the system containing the asset (road, bridge, etc.) 

can adjust or mitigate the potential for damage or service interruption by the hazards.

ALONGSHORE: Parallel to and near the shoreline; longshore.

ARMOR LAYER: Protective layer on the outside or top of a revetment or seawall composed of 

armor units.

ASTRONOMICAL TIDE: The tidal levels and character which would result from gravitational 

effects, e.g. of the Earth, Sun, and Moon, without any atmospheric influences.

ATTENUATION: A lessening of the height or amplitude of a wave with distance. 

A-ZONE: FEMA zones which are estimated to be subject to inundation by the 1-percent-annualchance flood event (the 100-yr storm). In a coastal setting, A-zones landward of a V-zone are 

considered to be subject to storm-related waves of up to 3 feet in wave height.

BACKSHORE: The zone of the shore or beach lying between the foreshore and the coastline 

comprising the berm or berms and acted upon by waves only during severe storms, especially 

when combined with exceptionally high water.

BAR: A submerged or emerged embankment of sand, gravel, or other unconsolidated material 

built on the sea floor in shallow water by waves and currents. 

BARRIER ISLAND: An unconsolidated, elongated body of sand or gravel lying above the hightide level and separated from the mainland by a lagoon or marsh. It is commonly between two 
HEC-25 3rd ed. Glossary

xxvii

inlets, has dunes, vegetated areas, and swampy terrains extending from the beach into the 

lagoon.

BATHYMETRY: The depths of water in oceans, seas, and lakes.

BAY: 1) a body of water almost completely surrounded by land but open to some tidal flow 

communications with the sea. 2) a recess in the shore or an inlet of a sea between two capes or 

headlands, not so large as a gulf but larger than a cove.

BEACH: The zone of unconsolidated material, typically sand, that extends landward from closure 

depths where sand is moved by waves to the place where there is marked change in material or 

physiographic form, or to the line of permanent vegetation (usually the effective limit of storm 

waves). 

BEACH FILL: Sand placed on a beach; beach nourishment

BEACH BERM: A nearly horizontal part of the beach or backshore formed by the deposit of 

material by wave action. Some beaches have no berms, others have one or several. 

BEACH EROSION: The carrying away of beach materials by wave action, tidal currents, littoral 

currents, or wind.

BEACH FACE The section of the beach normally exposed to the action of the wave uprush. The 

foreshore of a beach. (Not synonymous with shoreface.)

BEACH NOURISHMENT: The direct placement of large amounts of good quality sand on the 

beach to widen the beach.

BEACH PROFILE: A cross-section taken perpendicular to a given beach contour; the profile may 

include the face of a dune or sea wall; extend over the backshore, across the foreshore, and 

seaward underwater into the nearshore zone.

BEACH SCARP: An almost vertical slope along the beach caused by erosion by wave action. It 

may vary in height from a few cm to a meter or so, depending on wave action and the nature and 

composition of the beach.

BERM: 1) On a beach: a nearly horizontal plateau on the beach face or backshore, formed by the 

deposition of beach material by wave action or by means of a mechanical plant as part of a beach 

renourishment scheme. Some natural beaches have no berm, others have several. 2) On a 

structure: a nearly horizontal area, often built to support or key-in an armor layer.

BERM BREAKWATER: Rubble mound structure with horizontal berm of armor stones at about 

sea level, which can be (re)shaped by the waves.

BLUFF: A high, steep bank or cliff.

BORE: A broken wave propagating across the surf zone characterized by turbulent white water.

BOUNDARY CONDITIONS: Environmental conditions, e.g. water levels, waves, currents, drifts, 

etc. used as boundary input to numerical models.

BREACH: Gap in a barrier island or spit or dune caused by a storm.

BREAKER: A wave breaking on a shore, over a reef, etc. Breakers may be classified into four 

types: collapsing, plunging, spilling, and surging.

BREAKER ZONE: The zone within which waves approaching the coastline commence breaking 

caused by the reduced depths.

BREAKING: Reduction in wave energy and height. In the surf zone breaking is due to limited 

water depth.
Glossary HEC-25 3rd ed.

xxviii

BREAKWATER: A structure protecting a shore area, harbor, anchorage, or basin from waves.

BULKHEAD: A structure or partition to retain or prevent sliding of the land. A secondary purpose 

is to protect the upland against damage from wave action.

CAUSEWAY: A raised road across wet or marshy ground, or across water.

CAUSTIC: In refraction of waves, the name given to a region of crossed orthogonals and high 

wave convergence.

CELERITY: Wave speed.

CHANNEL: 1) A natural or artificial waterway of perceptible extent which either periodically or 

continuously contains moving water, or which forms a connecting link between two bodies of 

water. 2) The part of a body of water deep enough to be used for navigation through an area 

otherwise too shallow for navigation. 3) A large strait, as the English Channel. 4) The deepest 

part of a stream, bay, or strait through which the main volume or current of water flows.

CHART: A special-purpose map, esp. one designed for navigation such as a bathymetric chart.

CLIFF: A high, steep face of rock; a precipice.

CNOIDAL WAVE: A type of wave in shallow water (i.e. where the depth of water is less than 1/8 

to 1/10 the wavelength). 

COASTAL AREA: The land and sea area bordering the shoreline.

COASTAL ENGINEERING: The planning, design, construction and operation of infrastructure in 

the wave, tide and sand environment that is unique to the coast. A well-established specialty area 

of civil engineering that focuses on the coastal zone and coastal processes.

COASTAL PROCESSES: Collective term covering the action of natural forces on the shoreline 

and nearshore seabed.

COASTAL ZONE: The transition zone where the land meets water, the region that is directly 

influenced by marine and lacustrine hydrodynamic processes. Extends offshore to the continental 

shelf break and onshore to the first major change in topography above the reach of major storm 

waves. On barrier coasts, includes the bays and lagoons between the barrier and the mainland.

COASTLINE: Commonly, the line that forms the boundary between the land and the water, esp. 

the water of a sea or ocean.

COBBLE: A rock fragment between 64 and 256 mm (2 to 10 inches) in diameter, usually rounded. 

Also called a cobblestone.

COLLAPSING BREAKER: A descriptive type of wave breaking characterized by the wave 

peaking up as if to break and then moving rapidly onto the structure or beach face including the

lower half of wave, with no air pocket but with bubbles and foam present.

CONTINENTAL SHELF: 1) The zone bordering a continent extending from the line of permanent 

immersion to the depth, usually about 100 m to 200 m (300 ft to 600 ft), where there is a marked 

or rather steep descent toward the great depths of the ocean. 2) The area under active littoral 

processes during the Holocene period. 3) The region of the oceanic bottom that extends outward 

from the shoreline with an average slope of less than 1:100, to a line where the gradient begins 

to exceed 1:40 (the continental slope).

CONTOUR: A line on a map or chart representing points of equal elevation with relation to a 

datum. Also called depth contour.
HEC-25 3rd ed. Glossary

xxix

CORIOLIS: Force due to the Earth's rotation, capable of generating currents. It causes moving 

bodies to be deflected to the right in the Northern Hemisphere and to the left in the Southern 

Hemisphere. The "force" is proportional to the speed and latitude of the moving object. It is zero 

at the equator and maximum at the poles.

CRITICAL FLOW: The flow condition where the specific energy of flow is at a minimum and the 

Froude number for the flow is one; term from open-channel flow hydraulics. Related terms are 

sub-critical flow and super-critical flow.

CROSS-SHORE: Perpendicular to the shoreline.

CURRENT: 1) The flowing of water, 2) That portion of a stream of water which is moving with a 

velocity much greater than the average or in which the progress of the water is principally 

concentrated. 3) Ocean currents can be classified in several different ways. Some important types 

include the following: A) Periodic – a result of the effect of the tides; such Currents may be rotating 

rather than having a simple back and forth motion. The currents accompanying tides are known 

as tidal currents; B) Temporary - due to seasonal winds; C) Permanent or ocean - constitute a 

part of the general ocean circulation. The term drift current is often applied to a slow broad 

movement of the oceanic water; D) Nearshore - caused principally by waves breaking along a 

shore. 

CYCLONE: A system of winds that rotates about a center of low atmospheric pressure. Rotation 

is clockwise in the Southern Hemisphere and anti-clockwise in the Northern Hemisphere. In the 

Indian Ocean, the term refers to the powerful storms called hurricanes in the Atlantic. 

DATUM: Any permanent line, plane, or surface used as a reference datum to which elevations 

are referred. 

DEEPWATER: Water so deep that surface waves are little affected by the ocean bottom. 

Generally, water deeper than one-half the surface wavelength is considered deep water. 

DENSITY: Mass per unit of volume of a substance. For pure water, the density is 1,000 kg/m3

(62.4 lb/ft3). For seawater, the density is usually more. Density increases with increasing salinity, 

and decreases with increasing temperature. 

DEPTH-LIMITED: Wave height is limited by the local depth of water. 

DESIGN STORM: A hypothetical extreme storm whose wave’s coastal protection structures will 

often be designed to withstand. The severity of the storm (i.e. return period) is chosen in view of 

the acceptable level of risk of damage or failure. A design storm consists of a design wave

condition, a design water level and a duration. 

DESIGN WAVE: In the design of harbors, harbor works, etc., the type or types of waves selected 

as having the characteristics against which protection is desired. 

DESIGN WAVE CONDITION: Usually an extreme wave condition with a specified return period 

used in the design of coastal works. 

DIFFRACTION: The phenomenon by which energy is transmitted laterally along a wave crest. 

When a part of a train of waves is interrupted by a barrier, such as a breakwater, the effect of 

diffraction is manifested by propagation of waves into the sheltered region within the barrier's 

geometric shadow. 

DIURNAL: Having a period or cycle of approximately one tidal day. 

DIURNAL INEQUALITY: The difference in height of the two high waters or of the two low waters 

of each day. Also the difference in velocity between the two daily flood or ebb currents of each 

day. 
Glossary HEC-25 3rd ed.

xxx

DIURNAL TIDE: A tide with one high water and one low water in a tidal day. 

DOWNDRIFT: The direction of predominant movement of littoral materials. 

DREDGING: Excavation or displacement of the bottom or shoreline of a water body. Dredging 

can be accomplished with mechanical or hydraulic machines. Most is done to maintain channel 

depths or berths for navigational purposes; other dredging is for shellfish harvesting, for cleanup 

of polluted sediments, and for placement of sand on beaches. 

DRIFT: 1) Sometimes used as a short form for littoral drift. 2) The speed at which a current runs. 

DUNES: 1) Ridges or mounds of loose, wind-blown material, usually sand. 

DURATION: In wave forecasting, the length of time the wind blows in nearly the same direction 

over the fetch (generating area). 

DYNAMIC EQUILIBRIUM: Short term morphological changes that do not affect the morphology 

over a long period. 

EBB: Period when tide level is falling; often taken to mean the ebb current which occurs during 

this period. 

EBB TIDAL DELTA: The sand shoals formed at the seaward mouth of tidal inlets as a result of 

interaction between tidal currents and waves. Also called outer bar. 

EBB TIDE: The period of tide between high water and the succeeding low water; a falling tide. 

EL NIÑO: A large scale weakening of the trade winds and warming of the surface layers in the 

eastern and central equatorial Pacific Ocean. El Niño events occur irregularly at intervals of 2-7 

years, although the average is about once every 3-4 years. They typically last 12-18 months, and 

are accompanied by swings in the Southern Oscillation (SO), an interannual see-saw in tropical 

sea level pressure between the eastern and western hemispheres. During El Niño, unusually high 

atmospheric sea level pressures develop in the western tropical Pacific and Indian Ocean regions, 

and unusually low sea level pressures develop in the southeastern tropical Pacific. The SO 

tendencies for unusually low pressures west of the date line and high pressures east of the date 

line have also been linked to periods of anomalously cold equatorial Pacific sea surface 

temperatures (SSTs) sometimes referred to as La Niña. 

EL NIÑO SOUTHERN OSCILLATION (ENSO): The atmospheric component of El Niño. See 

definition of El Niño.

EMBAYMENT: An indentation in the shoreline forming an open bay.

EPOCH: An extended period time with some consistent characteristic. A tidal epoch is about 19 

years (18.6). A geological epoch, a subdivision of a period, can be many thousands of years

EROSION: The wearing away of land by the action of natural forces. On a beach, the carrying 

away of beach material by wave action, tidal currents, littoral currents, or by deflation.

ESTUARY: 1) The region near a river mouth in which the fresh water of the river mixes with the 

salt water of the sea and which received both fluvial and littoral sediment influx. 2) The part of a 

river that is affected by tides.

EUSTATIC SEA LEVEL RISE: Change in sea level due to change in the volume of the world's 

ocean basins and the total amount of ocean water. Vertical Land Movement (VLM) is not included. 

See Global Mean Sea Level Rise.

EXPOSURE: The degree to which a transportation asset (road, bridge, etc.) experiences a 

hazard.
HEC-25 3rd ed. Glossary

xxxi

EXTRATROPICAL: A term used in advisories and tropical summaries to indicate that a cyclone 

has lost its "tropical" characteristics. The term implies both poleward displacement of the cyclone 

and the conversion of the cyclone's primary energy source from the release of latent heat of 

condensation to baroclinic (the temperature contrast between warm and cold air masses) 

processes. It is important to note that cyclones can become extratropical and still retain winds of 

hurricane or tropical storm force.

EXTREME EVENT: Severe, rarely occurring event that usually causes damage, destruction or 

severe economic losses. Such events may include unseasonable weather, heavy precipitation, a 

storm surge, flooding, drought, windstorms (including hurricanes, tornadoes, and associated 

storm surges), extreme heat, extreme cold, earthquakes and tsunamis. 

FEMA 540 RULE: Guidance FEMA uses to determine if a sand dune is large enough to survive 

a 100-year storm. If the cross-sectional area of the dune above the storm SWL surge level is 

greater than 540 square feet, the dune is assumed to be large enough to survive.

FETCH: The distance or area in which wind blows across the water forming waves. Sometimes 

used synonymously with fetch length and generating area.

FETCH-LIMITED: Situation in which wave energy (or wave height) is limited by the size of the 

wave generation area (fetch).

FINITE-DIFFERENCE: A general type of numerical method for approximating the solutions to 

boundary value problems with differential equations using finite-difference equations, algebraic 

equations across small distances, to approximate derivatives.

FINITE ELEMENT: a general type of numerical method for finding approximate solutions to 

boundary value problems of differential equations which discretizes the area, or domain, of 

interest into small, variable-sized, usually triangular, mesh elements.

FLOOD: A general and temporary condition of partial or complete inundation of normally dry land 

areas from 1) the overflow of inland or tidal waters or 2) the unusual and rapid accumulation or 

runoff of surface waters from any source. 

FLOOD CURRENT: The tidal current toward shore or up a tidal stream. Usually associated with 

the increase in the height of the tide.

FLOOD TIDAL DELTA: The shoal of sand formed at the landward mouth of tidal inlets as a result 

of flow expansion.

FLOOD TIDE: The period of tide between low water and the succeeding high water; a rising tide; 

an incoming tide.

FORESHORE: The part of the shore, lying between the crest of the seaward berm (or upper limit 

of wave wash at high tide) and the ordinary low-water mark, that is ordinarily traversed by the 

uprush and backrush of the waves as the tides rise and fall.

FREEBOARD: 1) The vertical distance between the water level and the top of a coastal levee or 

dike. 2) The distance from the waterline to the low-chord of the bottom of a suspended deck such 

as a bridge deck or offshore platform. 3) The distance from the crest of the design wave to the 

low-chord of the bottom of a suspended deck such as a bridge deck or offshore platform.

FROUDE NUMBER: The dimensionless ratio of the inertial force to the force of gravity for a given 

fluid flow situation. It may be given as Fr = V/Lg where V is a characteristic velocity, L is a 

characteristic length, and g the acceleration of gravity - or as the square root of this number.
Glossary HEC-25 3rd ed.

xxxii

FULLY-ARISEN SEA: The waves that form when wind blows for a sufficient period of time across 

water. The waves of a fully developed sea have the maximum height possible for a given wind 

speed, fetch and duration of wind.

GABION: 1) Steel wire-mesh basket to hold stones or crushed rock to protect a bank or bottom 

from erosion. 2) Structures composed of masses of rocks, rubble or masonry held tightly together 

usually by wire mesh so as to form blocks or walls. Sometimes used on heavy erosion areas to 

retard wave action or as a foundation for breakwaters or jetties.

GEOMORPHOLOGY: 1) That branch of physical geography which deals with the form of the 

Earth, the general configuration of its surface, the distribution of the land, water, etc. 2) The 

investigation of the history of geologic changes through the interpretation of topographic forms.

GEOTEXTILE: A synthetic fabric which may be woven or non-woven used as a filter.

GLACIER: A large body of ice moving slowly down a slope of valley or spreading outward on a 

land surface (e.g. Greenland, Antarctica) and surviving from year to year.

GLOBAL MEAN SEA LEVEL RISE: The sea level rise averaged across the world’s oceans. This 

is the average change in sea level due to a change in the volume of the world's ocean basins and 

the total amount of ocean water. Vertical land movement (VLM) is not included. See Eustatic Sea 

Level Change. 

GLOBAL POSITIONING SYSTEM: Commonly called GPS. A navigational and positioning system 

by which the location of a position on or above the Earth can be determined by a special receiver 

at that point interpreting signals received simultaneously from several of a constellation of special 

satellites.

GORGE: 1) The deepest portion of an inlet, the throat. 2) A narrow, deep valley with nearly vertical 

rock walls.

GRAVITY WAVE: A wave whose velocity of propagation is controlled primarily by gravity because 

gravity is the restoring force. Most waves of importance in coastal engineering are gravity waves. 

Water waves more than 5 cm long are considered gravity waves. Waves longer than 2.5 cm and 

shorter than 5 cm are in an indeterminate zone between capillary and gravity waves.

GRID POINT: Location specified in the domain of a numerical model solution.

GROIN: Narrow, roughly shore-normal structure built to reduce longshore currents, and/or to trap 

and retain littoral material. Most groins are of timber or rock and extend from a seawall, or the 

backshore, well onto the foreshore and rarely even further offshore. 

GULF: 1) A relatively large portion of the ocean or sea extending far into land; the largest of 

various forms of inlets of the sea. 2) The Gulf of Mexico.

HEADLAND: A promontory extending out into a body of water 

HEADLAND BREAKWATER: A rock breakwater constructed to function as a headland by 

retaining an adjacent sandy pocket beach.

HIGH TIDE: The maximum elevation reached by each rising tide. 

HIGH WATER: Maximum height reached by a rising tide. The height may be solely due to the 

periodic tidal forces or it may have superimposed upon it the effects of prevailing meteorological 

conditions. Nontechnically, also called the high tide.

HIGHER HIGH WATER: The higher of the two high waters of any tidal day. The single high water 

occurring daily during periods when the tide is diurnal is considered to be a higher high water.
HEC-25 3rd ed. Glossary

xxxiii

HINDCAST: Application of a numerical model to simulate a past event. Often used in model 

validation to see how well the output matches known events.

HOLOCENE: An epoch of the quaternary period, from the end of the Pleistocene, about 12,000 

to 20,000 years ago, to the present time. This is the geologic time period of the most recent rise 

in eustatic sea level.

HURRICANE: An intense tropical cyclone in which winds tend to spiral inward toward a core of 

low pressure, with maximum surface wind velocities that equal or exceed 33.5 m/sec (75 mph or 

65 knots) for several minutes or longer at some points. Tropical storm is the term applied if 

maximum winds are less than 33.5 m/sec but greater than a whole gale (63 mph or 55 knots). 

Term is used in the Atlantic, Gulf of Mexico, and eastern Pacific.

HYDRODYNAMIC: Having to do with the science of moving water.

HYDROGRAPH: 1) a graph of the variation of SWL with time. 2) a graph of discharge with time.

ICE AGE: A loosely-used synonym of glacial epoch, or time of extensive glacial activity; 

specifically, of the latest period of widespread continental glaciers, the Pleistocene Epoch.

INLET: 1) A short, narrow waterway connecting a bay, lagoon, or similar body of water with a 

large parent body of water. 2) An arm of the sea (or other body of water) that is long compared to 

its width and may extend a considerable distance inland. 

IRREGULAR WAVES: Waves with random wave periods (and in practice, also heights), which 

are typical for natural wind-induced waves.

JETTY: 1) (US coastal engineering usage) On open seacoasts, a structure extending into a body 

of water, which is designed to prevent shoaling of a channel by littoral materials and to direct and 

confine the stream or tidal flow. Jetties are built at the mouths of rivers or tidal inlets to help 

deepen and stabilize a channel. 2) (British usage) Wharf or pier.

KEY: Also “cay,” one of the low, insular banks of sand, coral, and limestone off the southern coast 

of Florida or in the Caribbean Sea

KINETIC ENERGY (OF WAVES): In a progressive oscillatory wave, a summation of the energy 

of motion of the particles within the wave.

KING TIDE: Non-technical term for an extremely high tide level due to regular astronomical 

(interactions of the moon/earth/sun system) fluctuations. Photos of these high tide levels have 

been used to help visualize and understand the impacts of sea level rise along the Pacific coast.

KNOT: The unit of speed used in navigation equal to 1 nautical mile (6,076.115 ft or 1,852 m) per 

hour.

LA NINA: Is characterized by periods of below-average sea surface temperatures across the eastcentral Equatorial Pacific. Global La Niña impacts tend to be opposite those of El Niño impacts. 

In the tropics, ocean temperature variations in La Niña also tend to be opposite those of El Niño. 

During a La Niña year, winter temperatures are warmer than normal in the Southeast and cooler 

than normal in the Northwest.

LAGOON: A shallow body of water, like a pond or sound, partly or completely separated from the 

sea by a barrier island or reef. Sometimes connected to the sea via an inlet.

LIFE CYCLE COST ANALYSIS: An engineering economic analysis that allows officials to quantify 

the differential costs of alternative investment options for a given project.

LITTORAL: Of or pertaining to a shore, especially of the sea.
Glossary HEC-25 3rd ed.

xxxiv

LITTORAL ZONE: In beach terminology, an indefinite zone extending seaward from the shoreline 

to just beyond the breaker zone. The zone of wave-induced sand movement.

LONGSHORE: Parallel to and near the shoreline; alongshore.

LONGSHORE BAR: A sand ridge or ridges, running roughly parallel to the shoreline and 

extending along the shore outside the trough, that may be exposed at low tide or may occur below 

the water level in the offshore.

LONGSHORE CURRENT: The littoral current in the breaker zone moving essentially parallel to 

the shore, usually generated by waves breaking at an angle to the shoreline.

LONGSHORE SAND TRANSPORT: Littoral drift; the movement of sand down the beach driven 

by breaking waves and longshore current.

LOW TIDE: The minimum elevation reached by each falling tide. 

LOW WATER: The minimum height reached by each falling tide. Nontechnically, also called low 

tide.

LOWER LOW WATER: The lower of the two low waters of any tidal day. The single low water 

occurring daily during periods when the tide is diurnal is considered to be a lower low water. 

MARSH: 1) A tract of soft, wet land, usually vegetated by reeds, grasses and occasionally small 

shrubs. 2) Soft, wet area periodically or continuously flooded to a shallow depth, usually 

characterized by a particular subclass of grasses, cattails and other low plants.

MEAN HIGH WATER: The average height of the high waters over a 19-year tidal epoch. For 

shorter periods of observations, corrections are applied to eliminate known variations and reduce 

the results to the equivalent of a mean 19-year value. All high water heights are included in the 

average where the type of tide is either semidiurnal or mixed. Only the higher high water heights 

are included in the average where the type of tide is diurnal. So determined, mean high water in 

the latter case is the same as mean higher high water.

MEAN HIGHER HIGH WATER: The average height of the higher high waters over a 19-year 

period. For shorter periods of observation, corrections are applied to eliminate known variations 

and reduce the result to the equivalent of a mean 19-year value.

MEAN LOW WATER: The average height of the low waters over a 19-year period. For shorter 

periods of observations, corrections are applied to eliminate known variations and reduce the 

results to the equivalent of a mean 19-year value. All low water heights are included in the average 

where the type of tide is either semidiurnal or mixed. Only lower low water heights are included in 

the average where the type of tide is diurnal. So determined, mean low water in the latter case is 

the same as mean lower low water. 

MEAN LOWER LOW WATER: The average height of the lower low waters over a 19-year period. 

For shorter periods of observations, corrections are applied to eliminate known variations and 

reduce the results to the equivalent of a mean 19-year value. Frequently abbreviated to lower low 

water.

MEAN SEA LEVEL: The average height of the surface of the sea for all stages of the tide over a 

19-year period, usually determined from hourly height readings. 

METEOTSUNAMI: Long-wave motions principally caused by meteorologically-induced 

disturbances, including those associated with pressure jumps, frontal passages, and squalls. 

MIXED TIDE: A type of tide in which the presence of a diurnal wave is conspicuous by a large 

inequality in either the high or low water heights, with two high waters and two low waters usually 

occurring each tidal day. In strictness, all tides are mixed, but the name is usually applied without 
HEC-25 3rd ed. Glossary

xxxv

definite limits to the tide intermediate to those predominantly semidiurnal and those predominantly 

diurnal.

MONOCHROMATIC WAVES: A series of waves generated in a laboratory, each of which has the 

same length and period.

MONTE CARLO: A class of computational algorithms that use repeated random sampling to 

obtain risk estimates.

MORPHOLOGY: The form and structure, and the changes of form and structure of the earth’s 

surface. Same as geomorphology.

NATURE-BASED SOLUTIONS: Options which mimic characteristics of natural features, but are 

created by human design, engineering, and construction. They include a spectrum of natural and 

nature-based features that serve as alternatives to or enhancements of traditional shoreline 

stabilization and infrastructure protection techniques.

NEAP TIDE: Tide of decreased range occurring semimonthly as the result of the moon being in 

quadrature. The neap range of the tide is the average semidiurnal range occurring at the time of 

neap tides and is most conveniently computed from the harmonic constants. The neap range is 

typically 10 to 30 percent smaller than the mean range where the type of tide is either semidiurnal 

or mixed. Where the type of tide is diurnal, the term is commonly used to refer to the portion of 

the lunar month with reduced tide range. 

NEARSHORE: 1) In beach terminology, an indefinite zone extending seaward from the shoreline 

well beyond the breaker zone. 2) The zone which extends from the swash zone to the position 

marking the start of the offshore zone, typically at water depths of the order of 50 ft. 

NEARSHORE CURRENT: A current in the nearshore zone. 

NONLINEAR: Occurring as a result of a mathematical operation that is not linear.

NOR'EASTER or NORTHEASTER: Common storm type in the North Atlantic Ocean which 

produces northeast winds along the US Atlantic seaboard.

NUMERICAL MODELING: The analysis of natural processes including storm surge, tidal 

circulation, and wave generation using computational methods with computers.

OCEANOGRAPHY: The study of the sea, embracing and indicating all knowledge pertaining to 

the sea's physical boundaries, the chemistry and physics of seawater, marine biology, and marine 

geology.

OFFSHORE: 1) In beach terminology, the comparatively flat zone of variable width, extending 

from the shoreface to the edge of the continental shelf. It is continually submerged. 2) The 

direction seaward from the shore. 3) The zone beyond the nearshore zone where sediment 

motion induced by waves alone effectively ceases and where the influence of the sea bed on 

wave action is small in comparison with the effect of wind. 4) The breaker zone directly seaward 

of the low tide line.

ONSHORE: A direction landward from the sea.

ORBITAL VELOCITY: The flow of water accompanying the orbital movement, the back and forth 

movement, of the water particles in a wave. Not to be confused with wave-generated littoral 

currents. 

OSCILLATORY WAVE: A wave in which each individual particle oscillates about a point with little 

or no permanent change in mean position. The term is commonly applied to progressive 

oscillatory waves in which only the form advances, the individual particles moving in closed or 

nearly closed orbits. 
Glossary HEC-25 3rd ed.

xxxvi

OUTCROPPING: A surface exposure of bare rock in the sea not covered by soil or vegetation.

OVERTOPPING: Passing of water over the top of a structure as a result of wave runup or surge 

action.

OVERWASH: 1) The part of the uprush that runs over the crest of a berm or structure or barrier 

island and does not flow directly back to the ocean or lake. 2) The effect of waves overtopping a 

coastal defense, often carrying sediment landwards which is then lost to the beach system.

PACIFIC DECADAL OSCILLATION: A long-lived El Niño-like pattern of Pacific weather variability.

PARTICLE VELOCITY: The velocity induced by wave motion with which a specific water particle 

moves within a wave.

PASS: In hydrographic usage, a navigable channel through a bar, reef, or shoal, or between 

closely adjacent islands. On the Gulf of Mexico Coast, inlets are often known as passes.

PEAK WAVE PERIOD: The wave period determined by the inverse of the frequency at which the 

wave energy spectrum reaches its maximum. Most of the energy in an irregular sea state is at 

this wave period.

PEBBLES: Beach material usually well-rounded and between about 4 mm to 64 mm (1/8 in to 2 

in) diameter.

PENINSULA: An elongated body of land nearly surrounded by water and connected to a larger 

body of land by a neck or isthmus.

PHASE: In surface wave motion, a point in the period to which the wave motion has advanced 

with respect to a given initial reference point, e.g. the crest of the wave is a phase of the wave.

PHYSICAL MODELING: Refers to the investigation of coastal or hydraulic processes using a 

scaled model.

PIER: A structure, usually of open construction, extending out into the water from the shore, to 

serve as a landing place, recreational facility, etc., rather than to afford coastal protection. In the 

Great Lakes, a term sometimes applied to jetties.

PILE: A long, heavy timber or section of concrete or metal that is driven or jetted into the earth or 

seabed to serve as a support or protection.

PINEAPPLE EXPRESS: A weather system characterized by a jet stream dip into the vicinity of 

Hawaii (thus the "pineapple") which carries a moisture-laden storm system to Washington, 

Oregon, or California. Unlike tropical events, these winter storms do not behave as cyclonic 

systems; instead they are characterized by high winds that drive waves onto coastal areas.

PLEISTOCENE: An epoch of the Quaternary Period characterized by several glacial ages. 

PLUNGING BREAKER: A descriptive type of wave breaking characterized by the crest of the 

wave curling over an air pocket and forming a tube of air under a jet, or “lip,” of water which falls, 

or “plunges,” down on the face of the wave. This type of breaking can create extreme turbulent 

eddies at that location. 

POCKET BEACH: A beach, usually small and curved, in a coastal embayment between two 

headland littoral barriers.

POTENTIAL ENERGY OF WAVES: In a progressive oscillatory wave, the energy resulting from 

the elevation or depression of the water surface from the undisturbed level.

PROTOTYPE: In laboratory usage, the full-scale structure, concept, or phenomenon used as a 

basis for constructing a scale model or copy. 
HEC-25 3rd ed. Glossary

xxxvii

QUATERNARY PERIOD: The most recent, current, period in the geologic time scale. 

RANDOM WAVES: The irregular, non-monochromatic, sea states that occur in nature. 

RAYLEIGH DISTRIBUTION: A model probability distribution, commonly used in analysis of waves

because individual wave heights in naturally occurring, irregular, sea fits this distribution. 

RECESSION: Landward movement of the shoreline. A net landward movement of the shoreline 

over a specified time. 

REEF: Offshore consolidated rock. Often refers to coral fringing reefs in tropical waters. 

REFLECTED WAVE: That part of an incident wave that is returned seaward when a wave 

impinges on a steep beach, barrier, or other reflecting surface. 

REFLECTION: The process by which the energy of the wave is returned seaward. 

REFRACTION: The process by which the direction of a wave moving in shallow water at an angle 

to the contours is changed: the part of the wave advancing in shallower water moves more slowly 

than that part still advancing in deeper water, causing the wave crest to bend toward alignment 

with the underwater contours. 

REFRACTION COEFFICIENT: The ratio of the refracted wave height at any point to the 

deepwater wave height. 

REFRACTION DIAGRAM: A drawing showing positions of wave crests and/or orthogonals in a 

given area for a specific deepwater wave period and direction. 

REGULAR WAVES: Waves with a consistent height, period, and direction; monochromatic 

waves. 

RELATIVE SEA LEVEL RISE: Sea level change at a coastal location relative to the land. This 

includes both the eustatic sea level rise component and the vertical land movement (VLM) 

component. This is the sea level change measured by long-term tide gages. 

RETURN PERIOD: Average period of time between occurrences of a given event. 

REVETMENT: A layer or layers of stone, concrete, etc., to protect an embankment, or shore 

structure, against erosion by wave action or currents. 

RIP CURRENT: A strong surface current flowing seaward from the shore that is part of a 

nearshore circulation cell driven by incident wave energy. A rip current is often miscalled a rip 

tide. 

RIPRAP: A protective layer or facing of quarrystone, usually well graded within a wide size limit, 

randomly placed to prevent erosion, scour, or sloughing of an embankment or bluff; also the stone 

so used. 

RISK: Chance or probability of failure due to all possible environmental inputs and all possible 

mechanisms. The concept of flood risk typically captures both the probability of the flood event 

and the consequences of the flood event.

ROCK: An aggregate of one or more minerals. 

RUBBLE: Rough, irregular fragments of broken rock. 

RUBBLE-MOUND STRUCTURE: A mound of random-shaped and random-placed stones 

protected with a cover layer of selected stones. 

RUNUP: The upper level reached by a wave on a beach or coastal structure, relative to still water 

level. 
Glossary HEC-25 3rd ed.

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SAFFIR-SIMPSON HURRICANE WIND SCALE: A 1 to 5 rating based on a hurricane’s sustained 

wind speed. Named for the originators of the scale. 

SALIENT: Coastal formation of beach material developed by wave refraction and diffraction and 

littoral drift comprising of a bulge in the coastline towards an offshore island or breakwater, but 

not connected to it as in the case of a tombolo. 

SALINITY: Number of grams of salt per thousand grams of sea water, usually expressed in parts 

per thousand. 

SAND: Sediment particles, often largely composed of quartz, with a diameter of between 0.0024 

inches and 0.079 inches (0.062 mm and 2 mm), generally classified as fine, medium, coarse or 

very coarse. Beach sand may sometimes be composed of organic sediments such as calcareous 

reef debris or shell fragments. 

SAND BAR: A submerged or emerged embankment of sand built on the sea floor in shallow water 

by waves and currents. 

SAND BYPASSING: Hydraulic or mechanical movement of sand from the accreting updrift side 

to the eroding downdrift side of an inlet or harbor entrance. The hydraulic movement may include 

natural movement as well as movement by man (which is sometimes referred to as artificial sand 

bypassing). 

SAND DUNE: A hill formed of sand through wind-blown processes landward of the active beach 

and berm. 

SAND SPIT: A narrow sand embankment, created by an excess of deposition at its seaward 

terminus, with its distal end (the end away from the point of origin) terminating in open water. 

SCOUR: Removal of underwater material by waves and currents, especially at the base or toe of 

a structure. 

SCOUR PROTECTION: Protection against erosion of the seabed. 

SEA: 1) Waves caused by wind at the place and time of observation. 2) State of the ocean or lake 

surface, in regard to waves. 

SEA CLIFF: A cliff situated at the seaward edge of the coast. 

SEA LEVEL RISE: The long-term upward trend in mean sea level. 

SEA STATE: Description of the sea surface with regard to wave action. 

SEAS: Waves caused by wind at the place and time of observation. 

SEAWALL: A structure, often concrete or stone, built along a portion of a coast to prevent erosion 

and other damage by wave action. Often it retains earth against its shoreward face. A seawall is 

typically more massive and capable of resisting greater wave forces than a bulkhead. 

SEDIMENT: 1) Loose, fragments of rocks, minerals or organic material which are transported 

from their source for varying distances and deposited by air, wind, ice and water. Other sediments 

are precipitated from the overlying water or form chemically, in place. Sediment includes all the 

unconsolidated materials on the sea floor. 2) The fine-grained material deposited by water or 

wind. 

SEDIMENT TRANSPORT: The main agencies by which sedimentary materials are moved are: 

gravity (gravity transport); running water (rivers and streams); ice (glaciers); wind; the sea 

(currents and littoral drift). 
HEC-25 3rd ed. Glossary

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SEICHING: Wave oscillation of an enclosed or semi enclosed waterbody that continues, 

pendulum fashion, after the cessation of the originating force, which may have been seismic, 

atmospheric, or vessel-generated. 

SEMIDIURNAL: Having a period or cycle of approximately one-half of a tidal day (12.4 hours). 

The predominating type of tide throughout the world is semidiurnal, with two high waters and two 

low waters each tidal day. The tidal current is said to be semidiurnal when there are two flood and 

two ebb periods each day. 

SENSITIVITY: The degree to which an asset (road, bridge, etc.) is damaged or service is 

interrupted by the hazards. 

SHALLOW WATER: 1) Commonly, water of such a depth that surface waves are noticeably 

affected by bottom topography. 2) More strictly, in hydrodynamics with regard to progressive 

gravity waves, water in which the depth is less than 1/25 the wavelength. 

SHALLOW WATER WAVE: A progressive wave which is in water less than 1/25 the wave length 

in depth. 

SHINGLE: flat or flattish pebbles common on some beaches. 

SHOAL: 1) (noun) A detached area of any material like a sand deposit: The depths over it are a 

danger to surface navigation. 2) (verb) To become shallow gradually. 3) To cause to become 

shallow. 4) To proceed from a greater to a lesser depth of water. 

SHOALING: Decrease in water depth. The transformation of wave profile as they propagate 

toward shore. 

SHOALING COEFFICIENT: The ratio of the height of a wave in water of any depth to its height 

in deep water with the effects of refraction, friction, and percolation eliminated. 

SHORE: The narrow strip of land in immediate contact with the sea, including the zone between 

high and low water lines. A shore of unconsolidated material is usually called a beach. Also used 

in a general sense to mean the coastal area (e.g. to live at the shore). 

SHOREFACE: The narrow zone seaward from the low tide shoreline, covered by water, over 

which the beach sands and gravels actively oscillate with changing wave conditions.

SHORELINE: The intersection of a specified plane of water with the shore or beach (e.g. the high 

water shoreline would be the intersection of the plane of mean high water with the shore or beach). 

The line delineating the shoreline on National Ocean Service nautical charts and surveys 

approximates the mean high water line. 

SIGNIFICANT WAVE HEIGHT: The primary measure of energy in a sea state. that is calculated 

either as the average height of the one-third highest waves or via energy density spectral analysis 

methods. 

SOLITARY WAVE: A wave consisting of a single elevation (above the original water surface), 

whose height is not necessarily small compared to the depth, and neither followed nor proceeded 

by another elevation or depression of the water surfaces. 

SORTING: Process of selection and separation of sediment grains according to their grain size 

(or grain shape or specific gravity). 

SPILLING BREAKER: A descriptive type of wave breaking characterized by the presence of air 

bubbles and turbulent white-water falling, or “spilling,” down front face of the wave. Breaking 

generally occurs relatively slowly over a distance across the surf zone. 

SPIT: A small point of land or a narrow shoal projecting into a body of water from the shore. 
Glossary HEC-25 3rd ed.

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SPRING TIDE: A tide that occurs at or near the time of new or full moon (syzygy) and which rises 

highest and falls lowest from the mean sea level. 

STACK: An isolated, pillar-like rocky island isolated from a nearby headland by wave erosion; a 

needle or chimney rock. 

STILLWATER LEVEL: Commonly abbreviated to SWL. The surface of the water if all wave and 

wind action were to cease. 

STONE: Quarried or artificially-broken rock for use in construction. 

STORM SURGE: A rise in average (typically over several minutes) water level above the normal 

astronomical tide level due to the action of a storm. Storm surge results from wind stress, 

atmospheric pressure reduction, and wave setup. 

STORM SURGE HYDROGRAPH: Graph of the variation in the rise in SWL with time due to a 

storm. 

SUBSIDENCE: Sinking or downwarping of a part of the earth's surface. 

SURF: 1) Collective term for breakers. 2) The wave activity in the area between the shoreline and 

the outermost limit of breakers. 3) In literature, the term surf usually refers to the breaking waves 

on shore and on reefs when accompanied by a roaring noise caused by the larger waves 

breaking. 4) the recreational riding of waves. 

SURF ZONE: The zone of wave action extending from the water line (which varies with tide, 

surge, set-up, etc.) out to the most seaward point of the zone (breaker zone) at which waves 

approaching the coastline commence breaking. 

SURGING BREAKER: A descriptive type of wave breaking characterized by the wave peaking

up as if to break, but bottom rushes forward from under wave, and wave slides up beach or 

structure face with little or no bubble production. 

SWASH: The rush of water up onto the beach face following the breaking of a wave. 

SWASH ZONE: The zone of wave action on the beach, which moves as water levels vary, 

extending from the limit of run-down to the limit of runup. 

SWELL: Wind-generated waves that have traveled out of their generating area. Swell 

characteristically exhibits a more regular and longer period and has flatter crests than waves 

within their generating fetch (seas). 

T-HEAD GROIN: A groin built in the general shape of a letter "T" with the trunk section connected 

to land. 

TECTONICS: Forces generated from within the earth that result in uplift, movement, or 

deformation of part of the earth's crust. 

TIDAL CURRENT: The alternating horizontal movement of water associated with the rise and fall 

of the tide caused by the astronomical tide-producing forces. 

TIDAL EPOCH: An 18.6-year time period used to calculate sea levels. 

TIDALLY-INFLUENCED: A river location where the discharge and stage are modulated by the 

astronomical tides but where the flow does not reverse under normal tidal conditions. 

TIDAL INLET: 1) An inlet maintained by tidal flow. 2) Loosely, any inlet in which the tide ebbs and 

flows. 

TIDAL PERIOD: The interval of time between two consecutive, similar, phases of the tide. 
HEC-25 3rd ed. Glossary

xli

TIDAL PRISM: 1) The total amount of water that flows into a bay or out again with movement of 

the tide, excluding any fresh water flow. 2) The volume of water between mean low and mean 

high tide. 

TIDAL RANGE: The difference in height between consecutive high and low (or higher high and 

lower low) waters. The mean range is the difference between mean high water and mean low 

water. The great diurnal range or diurnal range is the difference in height between mean higher 

high water and mean lower low water. Where the type of tide is diurnal, the mean range is the 

same as the diurnal range. 

TIDAL SHOALS: Sand shoals that accumulate near inlets due to the transport of sediments by 

tidal currents associated with the inlet. 

TIDAL WAVE: 1) The wave motion of the tides. 2) In popular usage, any unusually high and 

destructive water level along a shore. It usually refers to storm surge or tsunamis. 

TIDE: The periodic rising and falling of the water that results from gravitational attraction of the 

Moon and Sun and other astronomical bodies acting upon the rotating Earth. Although the 

accompanying horizontal movement of the water resulting from the same cause is also sometimes 

called the tide, it is preferable to designate the latter as tidal current, reserving the name tide for 

the vertical movement. 

TOE: Lowest part of a revetment or seawall slope, generally forming the transition to the seabed. 

TOMBOLO: A bar or spit that connects or "ties" an island to the mainland or to another island. 

Also applied to sand accumulation between land and a detached breakwater. 

TROPICAL STORM: A tropical cyclone with maximum winds less than 34 m/sec (75 miles per 

hour). Less strength when compared with hurricane or typhoon (winds greater than 75 miles per 

hour). 

TROUGH: A long and broad submarine depression with gently sloping sides. Often used to 

describe the deeper water between the beach face and an offshore sand bar. 

TSUNAMI: A long-period wave caused by an underwater disturbance such as a volcanic eruption 

or earthquake. Commonly miscalled "tidal wave."

UPDRIFT: The direction opposite that of the predominant movement of littoral materials. 

VALIDATION: The process of determining the degree to which a model is an accurate 

representation of the real world from the perspective of the intended uses of the model. 

VULNERABILITY: The extent to which a transportation asset or system is susceptible to 

sustaining damage from hazards during extreme events. Vulnerability is a function of the extent 

to which an asset or system is exposed to damaging forces; its sensitivity to those forces; and its 

adaptive capacity. 

V-ZONE/VE-ZONE: FEMA zones subject to inundation by the 1-percent-annual-chance flood 

event (the 100-yr storm) with additional hazards due to storm-induced velocity wave action (waves 

higher than 3 feet). 

WAVE: A ridge, deformation, or undulation of the surface of a liquid. 

WAVE AMPLITUDE: The magnitude of the displacement of a wave from a mean value. An ocean 

wave has an amplitude equal to the vertical distance from still water level to wave crest. For a 

sinusoidal wave, the amplitude is one-half the wave height. 

WAVE CELERITY: The speed of individual wave propagation. 

WAVE CLIMATE: The seasonal and annual distribution of wave height, period and direction. 
Glossary HEC-25 3rd ed.

xlii

WAVE CREST: 1) The highest part of a wave. 2) That part of the wave above still water level. 

WAVE DIRECTION: The direction toward, or from, which a wave is moving. 

WAVE FORECASTING: The theoretical determination of future wave characteristics, usually from 

observed or predicted meteorological phenomena (winds). 

WAVE FREQUENCY: The inverse of wave period. 

WAVE GROUP: A series of waves in which the wave direction, wavelength, and wave height vary 

only slightly. 

WAVE HEIGHT: The vertical distance between a crest and the preceding trough. 

WAVE PERIOD: The time for a wave crest to traverse a distance equal to one wavelength. The 

time for two successive wave crests to pass a fixed point. 

WAVE RAY: On a wave-refraction diagram, a line drawn perpendicularly to the wave crests; also 

known as orthogonals. 

WAVE RUNUP: The upper level, elevation, reached by a wave on a beach or coastal structure, 

relative to still water level. 

WAVE SETUP: Superelevation of the water surface over normal surge elevation due to onshore 

mass transport of the water by wave action alone. 

WAVE SPECTRUM: In ocean wave studies, a graph, table, or mathematical equation showing 

the distribution of wave energy as a function of wave frequency. The spectrum may be based on 

observations or theoretical considerations. 

WAVE STEEPNESS: The ratio of wave height to wavelength. 

WAVE TRAIN: A series of waves from the same direction. 

WAVE TRANSFORMATION: Change in wave energy due to the action of physical processes. 

WAVE TROUGH: The lowest part of a wave form between successive crests. Also that part of a 

wave below still water level. 

WAVE VELOCITY: The speed at which an individual wave advances. See also wave celerity. 

WAVELENGTH: The horizontal distance between similar points on two successive waves 

measured perpendicular to the crest. 

WEIR: A low dam or wall across a stream to raise the upstream water level. 

WELL-SORTED: Clastic sediment or rock that consists of particles all having approximately the 

same size. Example: sand dunes. 

WETLANDS: Lands whose saturation with water is a dominant factor determining the nature of 

soil development and the types of plant and animal communities that live in the soil and on its 

surface (e.g. mangrove forests). 

WHITECAP: On the crest of a wave, the white froth caused by wind-induced spilling breaking. 

WIND SEA: Wave conditions directly attributable to recent winds, as opposed to swell. 

WIND SETUP: On reservoirs and smaller bodies of water: 1) the vertical rise in the still water level 

on the leeward side of a body of water caused by wind stresses on the surface of the water; 2) 

the difference in still water levels on the windward and the leeward sides of a body of water caused 

by wind stresses on the surface of the water. 
HEC-25 3rd ed. Glossary

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WIND STRESS: The way in which wind transfers energy to the sea surface. 

WIND WAVES: 1) Waves being formed and built up by the wind. 2) Loosely, any wave generated

by wind. 
List of Acronyms HEC-25 3rd ed.

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List of Acronyms

AASHTO American Association of State Highway and Transportation Officials

ABFE Advisory Base Flood Elevation (FEMA acronym)

ACOPNE Academy of Coastal, Ocean, Ports and Navigation Engineers

ADCIRC ADvanced CIRCulation model

AEP annual exceedance probability

ASCE American Society of Civil Engineers

BFE Base Flood Elevation (FEMA acronym)

CFD computational fluid dynamics

CEM Coastal Engineering Manual

CERA Coastal Emergency Risks Assessment

CERC Coastal Engineering Research Center

CH3D Curvilinear Hydrodynamics Three Dimensional

CMS Coastal Modeling System

COMCOT Cornell Multi-grid Coupled Tsunami

CO-OPS Center for Operational Oceanographic Products and Services

CRAB Coastal Research Amphibious Buggy

CSSR Climate Science Special Report

CU US Customary Units

DCE Diplomate of Coastal Engineering. A specialty certification by ACOPNE

DOT Department of Transportation

EST Empirical Simulation Technique

FDEP Florida Department of Environmental Protection

FDOT Florida Department of Transportation

FEMA Federal Emergency Management Agency

FHWA Federal Highway Administration

FIS Flood Insurance Study (FEMA acronym)

FIRM Flood Insurance Rate Map (FEMA acronym)

FVCOM Finite Volume Coastal Ocean Model

GIS geographical information system

GPS global positioning system

GMSLR global mean sea level rise

HEC-RAS Hydrologic Engineering Center's River Analysis System

HICE Highways in the Coastal Environment
HEC-25 3rd ed. List of Acronyms

xlv

HURDAT HURricane DATabases

IPCC Intergovernmental Panel on Climate Change

JONSWAP Joint North Sea Wave Project

JPM joint probability method

MassDOT Massachusetts Department of Transportation

MEOW maximum envelope of water

MHHW mean higher high water

MHW mean high water

MLLW mean lower low water

MLW mean low water

MOST Method of Splitting Tsunami

MPO metropolitan planning organization

MSL mean sea level

NAVD 88 North American Vertical Datum of 1988

NAVFAC Naval Facilities Engineering Command

NCA National Climate Assessment

NCDOT North Carolina Department of Transportation

NCEP National Center for Environmental Prediction

NGVD 92 National Geodetic Vertical Datum of 1929

NHC National Hurricane Center

NOAA National Oceanic and Atmospheric Administration

NOS National Ocean Service. 

NRC National Research Council

NTHMP National Tsunami Hazard Mitigation Program

NTR non-tidal residual

RCP representative concentration pathway

REF/DIF REFraction/DIFfraction model

Risk MAP Risk Management, Assessment, and Planning

ROW right of way

RSLR relative sea level rise

SH State Highway

SI International System of Units

SDOT State Departments of Transportation

SLOSH Sea, Lake, and Overland Surges from Hurricanes
List of Acronyms HEC-25 3rd ed.

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SLR sea level rise

SMB Sverdrup, Munk and Bretschneider 

SPM Shore Protection Manual

SRH-2D Sedimentation and River Hydraulics - Two-Dimensional

SSIM Storm Surge Inundation Map (FEMA acronym)

STWAVE Steady State Spectral Wave

SWAN Simulating WAves Nearshore

SWL still water level

TAMU Texas A&M University

TxDOT Texas Department of Transportation

US United States

USA University of South Alabama

USACE US Army Corps of Engineers 

USC&GS US Coast and Geodetic Survey

USEPA US Environmental Protection Agency

USGS US Geological Survey

USDOT US Department of Transportation

VLM vertical land movement

WAM Wave Analysis Model

WIS Wave Information Study
HEC-25 3rd ed. Part 1 – Background & Context

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Part 1 – Background & Context
Part 1 – Background & Context HEC-25 3rd ed.

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HEC-25 3rd ed. Part 1 – Background & Context

3 

Chapter 1 - Introduction

Two of the United States’ (US) most storied love affairs are with the coast and with roads. Where 

these two intersect, coastal highways, there is a need for wise engineering planning and design. 

Americans began migrating to the coast decades ago and sea levels are rising. Thus, coastal 

highways are experiencing both increasing societal pressures and increasing physical stresses. 

Coastal roads and bridges are exposed to unique extreme events and are in unique ecosystems. 

The Federal Highway Administration (FHWA) has developed this manual, HEC-25: Highways in 

the Coastal Environment (HICE), to assist the transportation professional working in the coastal 

environment.

Figure 1.1. A US coastal highway (Florida SH A1A in Ft. Lauderdale 2018) 

This introductory chapter describes the purpose and scope, organization, target audience, and 

the units of this manual on highways in the coastal environment. 

1.1 Purpose and Scope

This 3rd edition of HEC-25: Highways in the Coastal Environment creates one comprehensive 

document including the latest research for the analysis, planning, design and operation of these 

highways. The focus is on roads and bridges (highways) near the coast that are always, or 

occasionally during storms, influenced by coastal tides and waves. 
Part 1 – Background & Context HEC-25 3rd ed.

4 

The 1st edition of HEC-25 summarized a pooled-fund study of the hydraulics of coastal bridge 

scour. The 2nd edition of HEC-25 was a much more comprehensive discussion of issues with 

HICE (Douglass and Krolak 2008). A supplemental edition, HEC-25: Volume 2, was released in 

2014 with methods for assessing the vulnerability of HICE to extreme events (Douglass et al.

2014). This 3rd edition combines the material in those two latter HICE documents. 

This new edition also includes:

• improvements in our understanding of the relevant coastal sciences, 

• improvements in accepted, related coastal engineering practice tools,

• updated analysis on sea level rise projections and how to incorporate these projections in 

highway engineering planning and design (Section 4.1), 

• new information on increased flooding due to sea level rise (Chapter 9),

• a revision to the method for estimating wave loads on bridge decks (Section 11.4), 

• new information on coastal scour (Chapter 12), and

• new information on nature-based solutions; such as restoration or construction of 

wetlands, beaches, dunes, and reefs; that reduce coastal damage as an alternative to, or 

in conjunction with, more traditional engineering approaches. This new material draws 

from FHWA’s “Nature-based Solutions for Coastal Highways Resilience: An 

Implementation Guide” (Webb, et al. 2019) (Chapter 8)). 

This edition of HEC-25 supersedes the previous editions. This edition provides the best available 

and actionable engineering and science on coastal resilience within a framework that is adaptable 

to future improvements. The pace of research and application in some of these areas (e.g. sea 

level rise projections and response) will rapidly evolve. This document is intended as a reference 

document for the FHWA, State Departments of Transportation (SDOT), the American Association 

of State Highway and Transportation Officials (AASHTO), consultants to these organizations, and 

others. 

This document focuses on tools directly applicable to coastal highways and does not address 

other aspects of coastal engineering more closely related to the maritime industry. For information 

on assessing the vulnerability of highways in the riverine environment, see HEC-17 “Highways in 

the River Environment,” 2nd ed. (Kilgore et al. 2016). 

1.2 Organization 

This HICE document is organized into four major parts with 15 chapters: 

Part 1 discusses the background and context of highways in the coastal environment: 

• Chapter 1 introduces this document and explains the rationale for its necessity,

• Chapter 2 outlines federal requirements and policies that may affect HICE, and

• Chapter 3 very briefly introduces some of the societal and natural processes that make 

the planning, design and operation of HICE unique and challenging. It also includes a 

description and explanation of the specialty field of coastal engineering. 

Part 2 briefly summarizes some of the science that is unique to the coast and is used in 

engineering of HICE: 
HEC-25 3rd ed. Part 1 – Background & Context

5 

• Chapter 4 discusses tides and water levels including tidal datums, storm surge, and sea 

level rise. Water level controls the location of wave attack on the shoreline. Sea level rise, 

which is already causing problems for HICE, is projected to increase significantly, 

• Chapter 5 discusses water waves and engineering models of water waves. Waves are 

often the primary hydraulic force of interest in coastal engineering and clearly have the 

potential to cause significant damage to HICE, and 

• Chapter 6 introduces some of the coastal sediment processes including an overview of 

coastal geology, coastal sediment characteristics and transport, tidal inlet dynamics, and 

the role of physical models in coastal engineering. 

Part 3 discusses some of the common planning and design issues for highways and bridges that 

are unique to the coastal environment: 

• Chapter 7 addresses one of the most common HICE issues - the design of revetments to 

resist wave attack. Coastal rubble-mound revetment design principles are different than 

those of typical rock revetments in the riverine environment, 

• Chapter 8 describes HICE threatened by shoreline recession; providing examples on how 

the HICE community has evaluated such issues and have developed a spectrum of 

mitigation plans and options ranging from walls to nature-based solutions like beaches 

and marshes, 

• Chapter 9 describes more frequent and more extensive flooding of coastal areas including 

roads. Long-term sea level rise has been recently recognized as a significant contributing 

factor to flooding problems in almost every coastal state, 

• Chapter 10 presents engineering strategies for coastal roads that are occasionally 

overwashed by storms because of their location and elevation, 

• Chapter 11 discusses bridges near the coast and the hydrodynamic loads on them due 

to coastal storms, and 

• Chapter 12 summarizes coastal scour information of value to highway engineers including 

scour at coastal bridges. 

Part 4 presents methods for assessing the vulnerability of HICE to extreme events with future sea 

level rise: 

• Chapter 13 summarizes tools available for the quantitative evaluation of probability of 

flooding of HICE. This includes fundamental concepts like the risk of a 100-year storm 

occurring, 

• Chapter 14 presents some existing approaches and methods for assessing the 

vulnerability, particularly the exposure component of vulnerability, of HICE to extreme 

events. The methodologies include the effects of future sea level rise and outlines how 

others have engaged in varying levels of effort for each region of the US, and 

• Chapter 15 briefly summarizes typical damage mechanisms and corresponding strategies 

to improve the resilience of HICE. 

Other materials in this document include a glossary of terms, list of acronyms, references cited, 

and an appendix acknowledging workshop participants. 
Part 1 – Background & Context HEC-25 3rd ed.

6 

1.3 Target Audience

This manual is written for a wide cross-section of users with varying backgrounds and expertise. 

The target audience is civil engineers, hydraulic engineers, roadway designers, planners, 

environmental staff, field inspectors, construction supervisors, scientists, coastal engineers, and 

other personnel involved in the analysis, planning, design and operation of HICE. 

This manual should help those with little experience in coastal engineering to understand and, as 

appropriate, to apply scientific methods and engineering approaches that are unique to the coast. 

For experienced coastal engineers, this manual should serve as a reference document in 

providing specific highway-oriented assistance and consultation for FHWA and SDOT projects. 

The coastal engineering design environment is complex and unique. Staff in a typical FHWA or 

SDOT engineering or planning unit often do not have a formal education in the processes and 

principles of coastal science and engineering. Part 2 of this HICE document is not meant as a 

substitute for more in-depth study of these sciences but rather as a basic, entry-level primer for 

someone with a general civil engineering background. Also some tools outlined throughout this 

manual are presently undergoing significant research and development resulting in changes to 

accepted practices. 

This manual does not attempt to “simplify” this complex practice into mechanistic, “one-size-fitsall” approaches. Rather it provides the transportation community with an overview and awareness 

of good practice in coastal engineering for HICE. The result of this awareness will allow 

practitioners to seek appropriate technical documentation and expertise for specific projects.

Other references, summary manuals, textbooks, and original sources in the coastal sciences and 

engineering fields, are cited for further details. 

Within this framework, this manual does not have the force and effect of law and it is not meant 

to bind the public in any way. The FHWA intends any descriptions of processes and approaches 

to provide illustrative insights into the underlying scientific and engineering concepts and practices 

rather than any proscribed guidance or requirements. 

1.4 Units in this Manual

US Customary (English) units consistent with FHWA policy are used with a few exceptions. In 

limited situations, both customary (English) units and SI units are used, or only SI units are used 

because these are the predominant metrics nationwide and globally for these topics. In these 

situations, this manual provides the rationale for the use of units. 
HEC-25 3rd ed. Part 1 – Background & Context

7 

Chapter 2 - Federal Policy for Coastal Highways

Federal policy for coastal highways is at the nexus, essentially the geographic overlap, of two 

broad federal policy arenas: 

• highway engineering and

• coastal management. 

Each of these has its own evolving history which influences the road system along our coasts. 

This Chapter provides some background on these policy arenas, provides some FHWA specific 

statutes and regulations applicable to the coastal environment, and then provides an overview of 

other Federal statutes and regulations that may affect a coastal highway project. 

2.1 Federal Highway and the Coast: National Overview

The FHWA has the primary responsibility for US federal policy on highways. Legislation for the 

federal road system dates back over a century. The Federal-Aid Road Act of 1916 created the 

Federal-Aid Highway Program which funded state highway agencies so they could make road 

improvements “to get the farmers out of the mud.” This 1916 Act charged the Bureau of Public 

Roads with implementing the program. The growth of the Federal highway system, including the 

addition of the Interstate Highway System and concerns about how these highways affected the 

environment, city development, and the ability to provide public mass transit, led to the 1966 

establishment of the US Department of Transportation (USDOT). The same enabling legislation 

renamed the Bureau of Public Roads to the FHWA. Currently, the FHWA continues to administer 

US federal policy on highways, but also coordinates extensively with other federal agencies on

environmental policies and permits, floodplains, and other compliance issues related to highway 

program and project delivery.

In contrast, US federal policy on coastal management is not entirely concentrated in any one 

agency but dispersed over several according to their historical missions. Agency policy 

responsibilities often began as scientific responsibilities. For example, President Thomas 

Jefferson established the Survey of the Coast in 1807. That agency evolved to become the US 

Coast and Geodetic Survey and today is the National Geodetic Service within the National 

Oceanic and Atmospheric Administration (NOAA) within the US Department of Commerce. The 

NOAA also administers the most comprehensive, modern coastal management legislation, the 

Coastal Zone Management Act of 1972. Similarly, the US Army Corps of Engineers (USACE) has 

had a beach erosion research mission dating back to the Beach Erosion Board established in 

1930. The USACE has both coastal regulatory and civil works missions today. 

2.2 FHWA Statutes & Regulations

The FHWA provides financial and technical assistance to State and local governments to ensure 

that US roads and highways continue to be among the safest and most technologically sound in 

the world. The FHWA authority for the subject matter of this manual on coastal highways includes 

the following statutes and regulations. The section below provides a synopsis of these various 

authorities as well as pertinent Congressional findings and statements, policy, and guidance. 

2.2.1 FHWA Statute

The FHWA operates under the statutory authority of Title 23 (Highways) of the United States 

Code (U.S.C.). For the purposes of this manual, relevant sections include: 
Part 1 – Background & Context HEC-25 3rd ed.

8 

• Standards [23 U.S.C. § 109]. It is the intent of Congress that federally funded projects for 

resurfacing, restoring, rehabilitating highways shall “be constructed in accordance with 

standards to preserve and extend the service life of highways and enhance highway 

safety.” [23 U.S.C. § 109(n)]. Designs for new, reconstructed, resurfaced, restored, or 

rehabilitated highways on the National Highway System must consider, among other 

criteria, the “constructed and natural environment of the area.” [Id. at (c)(1)(a)].

• Maintenance [23 U.S.C. § 116]. Preventive maintenance is eligible for Federal assistance 

under Title 23 if a State Departments of Transportation (SDOT) can demonstrate that it is

a “cost-effective means of extending the useful life of a Federal-aid highway.” [23 U.S.C. 

§ 116(e).] 

• National highway performance program [NHPP] [23 U.S.C. § 119]. The NHPP allows 

FHWA to provide Federal-aid funds for “[c]onstruction, replacement ..., rehabilitation, 

preservation, and protection (including ... protection against extreme events) of bridges 

on the National Highway System.” [23 USC § 119(d)(2)(B)]. The NHPP also allows 

Federal-aid funds for “[c]onstruction, replacement ..., rehabilitation, preservation, and 

protection (including ... protection against extreme events) of tunnels on the National 

Highway System.” [Id. at (d)(2)(C)]. 

• Surface transportation block grant [STBG] program [23 U.S.C. § 133]. The STBG 

program allows FHWA to provide Federal-aid funds for protection of “bridges (including 

approaches to bridges and other elevated structures) and tunnels on public roads” 

including “painting, scour countermeasures, seismic retrofits, impact protection measures, 

security countermeasures, and protection against extreme events.” [23 U.S.C. § 

133(b)(9)]. The STBG program also allows Federal-aid funds for “inspection and 

evaluation of bridges and tunnels and other highway assets.” [Id.] 

• Metropolitan transportation planning [23 U.S.C. § 134]. In the context of metropolitan 

transportation planning, Congress has found that it “is in the national interest ... to 

encourage and promote the safe and efficient management, operation, and development 

of surface transportation systems ... within and between States and urbanized areas” 

including taking “resiliency needs” into consideration. [23 U.S.C. § 134(a)(1)]. 

• National bridge and tunnel inventory and inspection standards [23 U.S.C. § 144]. 

Congress has found that “continued improvement to bridge conditions is essential to 

protect the safety of the traveling public.” [23 U.S.C. § 144(a)(1)(A)]. Congress has further 

found that “the systematic preventative maintenance of bridges, and replacement and 

rehabilitation of deficient bridges, should be undertaken.” [Id. at (a)(1)(B)]. In addition, 

Congress has also declared that “it is in the vital national interest” to use a “data-driven, 

risk-based approach” toward meeting these ends.” [Id. at (a)(2)(B)]. Considering these 

findings and declarations, Section 144 requires FHWA to maintain an inventory of bridges 

and tunnels on public roads both “on and off Federal-aid highways.” [Id. at (b)]. The FHWA 

is also required to “establish and maintain inspection standards for the proper inspection 

and evaluation of all highway bridges and tunnels for safety and serviceability.” [Id. at 

(h)(1)(A).] Section 144 also provides an exception to the requirement to obtain a bridge 

permit from the US Coast Guard for certain bridges over a limited subset of navigable 

waters. [Id. at (c)(2)]. 

• National goals and performance management measures [23 U.S.C. § 150]. Congress 

has declared that it is “in the interest” of the US to focus the Federal-aid highway program 

on certain national transportation goals including Infrastructure Condition, or the objective 

to “maintain ... highway infrastructure in a state of good repair;” and System Reliability, or 
HEC-25 3rd ed. Part 1 – Background & Context

9 

the objective to “improve the efficiency of the surface transportation system.” [23 U.S.C. § 

144(b)]. 

• Research and technology development and deployment [23 U.S.C. § 503]. In carrying 

out certain highway and bridge infrastructure and research and development activities, 

FHWA must “study vulnerabilities of the transportation system to ... extreme events and 

methods to reduce those vulnerabilities.” [23 U.S.C. § 503(b)(3)(B)(viii)]. 

2.2.2 Appropriation Language

The July 27, 2017 US Senate Report 115-138 for the “Transportation, and Housing and Urban 

Development, and Related Agencies Appropriations Bill, 2018” included direction from the 

Committee on Appropriations to the FHWA on “Resilient Infrastructure.” The Committee directed 

FHWA to “expand its technical assistance and training workshops to help coastal States, MPOs, 

and cities to revise their practices in all phases of transportation planning and asset management, 

project planning and development, and operations with the goal of improving the resiliency of our 

coastal highways and reducing the life-cycle costs for these natural disaster-prone roadways.”

[US Senate Report 115-138, at 52-53] 

The FHWA considers this HEC-25 “Highways in the Coastal Environment (3rd edition) to support 

the FHWA’s technical assistance to help coastal States. 

2.2.3 FHWA Regulations

The FHWA’s regulations are found within the Code of Federal Regulations (CFR), Title 23, 

Highways (23 CFR). The FHWA requires compliance with Federal law and the regulations in 

Chapter I, Subchapter A, Part 1 of 23 CFR for a project to be eligible for Federal-aid or other 

FHWA participation or assistance. 23 CFR § 1.36. 

The following FHWA regulations apply to highway projects and actions interacting with and within 

coastal waterways and floodplains (paraphrased for brevity): 

Scope of the statewide and nonmetropolitan transportation planning process [23 CFR § 

450.206]. SDOTs must “carry out a continuing, cooperative, and comprehensive statewide 

transportation planning process that provides for consideration and implementation of projects,

strategies, and services that will ... improve the resiliency and reliability of the transportation 

system. [23 CFR § 450.206(a)]. 

Asset Management Plans [23 CFR § 515]. Part 515 establishes processes that a SDOT must 

use to develop an asset management plan. Two notable sections include: 

a. Section 515.7(b). “A State DOT shall establish a process for conducting life-cycle 

planning for an asset class or asset sub-group at the network level (network to be defined 

by the State DOT). As a State DOT develops its life-cycle planning process, the State 

DOT should include future changes in demand; information on current and future 

environmental conditions including extreme weather events, climate change, and seismic 

activity; and other factors that could impact whole of life costs of assets.” 

b. Section 515.7(c). “A State DOT shall establish a process for developing a risk 

management plan. This process shall, at a minimum, produce the following information: 

Congress directs FHWA to expand its technical assistance to help coastal 

SDOTs, MPOs and cities “with the goal of improving the resiliency of US coastal 

highways.”
Part 1 – Background & Context HEC-25 3rd ed.

10

(1) Identification of risks that can affect condition of NHS pavements and bridges and the 

performance of the NHS, including risks associated with current and future environmental 

conditions, such as extreme weather events, climate change, seismic activity, and risks 

related to recurring damage and costs as identified through the evaluation of facilities 

repeated damaged by emergency events carried out under part 667 of this title.” 

Design Standards [23 CFR § 625]. Part 625 describes structural and geometric design 

standards. 

a. Section 625.3(a)(1) and § 625.4(b)(3). The FHWA, in cooperation with SDOTs, has 

approved the American Association of State Highway and Transportation Officials 

(AASHTO) Load and Resistance Factor Design (LRFD) Bridge Design Specifications. 

Based on FHWA’s approval, certain National Highway System (NHS) projects must follow 

those Specifications including sections related to hydrology, hydraulics, and bridge scour. 

b. Section 625.3(a)(2). Non-NHS projects must follow SDOT standard(s) and specifications 

on drainage, bridges, and other topics. 

Location and Hydraulic Design of Encroachments on Flood Plains [23 CFR Part 650, 

Subpart A]. One of the FHWA’s most important coastal related regulations, 23 CFR Part 650, 

Subpart A sets forth policies and procedures for location and hydraulic design of highway 

encroachments in base (1-percent chance) floodplains. Section 650.111 sets forth requirements 

for location hydraulic studies to identify the potential impact of the highway alternatives on the 

base floodplain; these studies are commonly used during NEPA. The regulations prohibit 

significant encroachments on base floodplains unless FHWA determines that such impacts are 

the only practicable alternative. [23 CFR § 650.113(a)]. This finding must be included in the NEPA 

documents for a project and supported information including the reasons for the finding and 

considered alternatives. [Id]. The procedures also provide minimum standards for Interstate 

Highways, set freeboard requirements to account for debris and scour, and require highway 

encroachments to be consistent with certain established design flood standards for hydraulic 

structures, including standards from FEMA and State and local governments related to 

administration of the National Flood Insurance Program (NFIP). [23 CFR § 650.115(a)]. Notably, 

the policies and procedures in this Subpart apply to encroachments in all base floodplains, not 

just the floodplains regulated by the Federal Emergency Management Agency (FEMA) in the 

NFIP. [23 CFR § 650.107]. Additionally, the Subpart incorporates a requirement for project-byproject risk assessments or analyses. [23 CFR § 650.115(a)(1)]. Two notable sections include: 

a. Section 650.115 [Hydraulic Design Standards]. This regulation applies to all Federalaid projects, whether on the NHS or Non-NHS. Federal, State, local, and AASHTO 

standards may not change or override the design standards set forth under § 650.115 — 

although certain State and local standards must also be satisfied under that section. That 

section requires development of a “Design Study” for each highway project involving an 

encroachment on a floodplain. [23 CFR § 650.115(a)]. 

b. Section 650.117 [Content of Design Studies]. This regulation requires studies to 

contain the “hydrologic and hydraulic data and design computations.” [23 CFR § 

650.117(b)]. As both hydrologic and hydraulic factors and characteristics lead to scour 

formation, data and computations applicable to scour should be provided as well. Project 
HEC-25 3rd ed. Part 1 – Background & Context

11

plans must show the water surface elevations of the overtopping flood and base flood (i.e., 

100-year flood) if larger than the overtopping flood. [23 CFR §650.117(c)]. 

National Bridge Inspection Standards [23 CFR § 650 Subpart C]. This regulation implements 

requirements of 23 U.S.C. § 144. In addition to the inspection and inventory requirements, the 

regulation specifically focuses on scour at bridges. 

Mitigation of Impacts to Wetlands and Natural Habitat [23 CFR § 777]. This regulation 

provides policy and procedures for the evaluation and mitigation of adverse environmental 

impacts to wetlands and natural habitat resulting from Federal-aid funded projects. 

2.3 Other Federal Agency Coastal Statutes & Regulations

Civil engineering projects in the coastal environment are subject to numerous federal laws, 

policies, and regulations. This section describes some of the most common federal statutes, 

regulations, and other authoritative guidance that may govern coastal highway projects. 

2.3.1 Rivers and Harbors Act of 1899 [33 U.S.C. § 401 and § 403]

Coastal highway engineering projects are subject to Section 9 [33 U.S.C. § 401] and Section 10

[33 U.S.C. § 403] of the Rivers and Harbors Act of 1899. Section 9 of this act restricts the 

construction of any bridge, dam, dike, or causeway over or in US navigable waterways. Section 

10 of this act restricts the building of any wharf, pier, jetty, breakwater, bulkhead, or other 

structure, as well as excavation or fill within navigable waterways. With the exception of bridges 

and causeways under Section 9 [33 U.S.C. § 401], the USACE is responsible for maintaining the 

standards set by and for issuing permits under the Rivers and Harbors Act. 

Authority to administer Section 9, applying to bridges and causeways, was redelegated to the US 

Coast Guard under the provisions of the Department of Transportation Act of 1966 (as discussed 

below). 

2.3.2 General Bridge Act of 1946 [33 U.S.C. § 525 through 533]

The General Bridge Act of 1946 requires the location and plans of bridges and causeways across 

the navigable waters of the United States be submitted to and approved by the US Coast Guard 

prior to construction. [33 U.S.C. § 525]. The USACE may also impose conditions relating to 

maintenance and operation of the structure. [Id]. The General Bridge Act of 1946 is cited as the 

legislative authority for bridge construction in most cases. Although the General Bridge Act of 

1946 originally provided authority for issuing bridge permits to the USACE, subsequent legislation 

transferred these responsibilities from the USACE to the US Coast Guard. 

2.3.3 Transportation Act of 1966 [Public Law 89-670] 

The Transportation Act of 1966 transferred the US Coast Guard (USCG) to USDOT. One of

USCG’s newly assigned duties was to issue bridge permits. This, along with the Rivers and 

Harbors Act and General Bridge Act, made the USCG responsible for ensuring that bridges and 

other waterway obstructions do not interfere with the navigability of waters of the United States 

without express permission of the United States Government. Subsequent legislation amended 

23 U.S.C. § 144 to provide certain exceptions to USGC’s authority under 33 U.S.C. § 401 and 33 

U.S.C. § 525 for bridges constructed, reconstructed, rehabilitated, or replaced using Federal-aid 

funds. [23 U.S.C. § 144(c)(2)]. 
Part 1 – Background & Context HEC-25 3rd ed.

12

2.3.4 Coastal Zone Management Act [16 U.S.C. § 1451-1466]

Coastal highway engineering projects are often subject to the Coastal Zone Management Act 

(CZMA) of 1972. The NOAA is responsible for programmatic administration of the CZMA at the 

national level but relies on its 34 Coastal Zone Management Programs in participating coastal 

states for ensuring project compliance with CZMA consistency requirements at the state level. [16 

U.S.C. § 1456(c)]. The CZMA consistency requirements incorporate certain relevant state and 

local requirements related to coastal management into the coastal construction permit process 

for federal agency actions and provides for participation by state and local agencies. Id. The 

CZMA encourages coastal states to develop and implement coastal zone management plans, 

which are used in the “consistency determination” process. This Act was established as a US 

national policy to preserve, protect, develop, and where possible, restore or enhance, the 

resources of the nation's coastal zone for “this and succeeding generations.” [16 U.S.C. § 1452]. 

The Office for Coastal Management within NOAA administers the CZMA and its amendments

(https://coast.noaa.gov). NOAA administers some grant programs that distribute limited funds to 

the states with approved plans. 

2.3.5 National Environmental Policy Act [42 U.S.C. § 4321, et seq.]

The National Environmental Policy Act of 1969 (NEPA) establishes the continuing policy of the 

Federal government to use all practicable means and measures “to foster and promote the 

general welfare, ... create and maintain conditions under which man and nature can exist in 

productive harmony, and fulfill the social, economic, and other requirements of present and future 

generations of Americans. [42 U.S.C. § 4331]. To achieve this goal, NEPA creates a requirement 

for Federal agencies to consider the environmental impacts of their actions before undertaking 

them. [42 U.S.C. § 4332(C)]. 

Section 102(2)(C) of NEPA requires Federal agencies develop a detailed statement on proposals 

for major Federal actions significantly affecting the quality of the human environment. [42 U.S.C. 

§ 4332(C)]. Environmental impact statements address items including “the environmental impact 

of” and “alternatives to” the proposed action.” [Id.] FHWA implements NEPA according to the 

Council on Environmental Quality (CEQ) NEPA regulations at 40 CFR Part 1500 et seq. and the 

FHWA-FRA-FTA joint regulations at 23 CFR Part 771. 

2.3.6 Clean Water Act [33 U.S.C. § 1251-1387]

Almost every project involving work or activities in coastal areas is subject to the Clean Water Act 

(CWA) of 1972, which is administered by the US Environmental Protection Agency (USEPA) in 

coordination with state governments. The CWA is the primary federal statute governing protection 

of the Nation’s surface waters. Engineering of highways in the coastal environment is often 

subject to Section 404 of the CWA, which regulates the discharge of dredged or fill material in 

waters of the US, including wetlands. [33 U.S.C. § 1344]. This includes the use of dredged or fill 

material for development, water resource projects, and infrastructure development (e.g., roads, 

bridges, etc.). The USACE handles the day-to-day administration and enforcement of the Section 

404 program, including issuing permits. In circumstances where Section 404 is triggered, permit 

applicants must also obtain a Section 401 certification from the state in which the discharge of 

dredged or fill material originates. [13 U.S.C. § 1341]. The Section 401 certification assures that 

The “federal consistency” provision of the CZMA brings relevant state and local 

agencies into the coastal permitting process for federal agency actions.
HEC-25 3rd ed. Part 1 – Background & Context

13

materials discharged to waters of the US will comply with relevant provisions of the CWA, 

including water quality standards. 

2.3.7 Endangered Species Act [16 U.S.C. § 1531-1544]

Coastal highway engineering projects have the potential to impact federally listed fish, wildlife, 

plants, and marine mammals. The purposes of the Endangered Species Act of 1973 (ESA) 

include conserving “the ecosystems upon which endangered species and threatened species 

depend may be conserved” and providing “a program for the conservation of such endangered 

species and threatened species.” [16 U.S.C. § 1531]. It is the policy of Congress that all Federal 

agencies shall seek to conserve endangered and threatened species and shall utilize their 

authorities in furtherance of the purposes of the ESA. [16 U.S.C. § 1531]. The US Fish and Wildlife 

Service (USFWS) and the NOAA National Marine Fisheries Service (NMFS) administer the ESA. 

The USFWS and NMFS conduct consultations with the lead federal agency when a proposed 

project may affect federally endangered or threatened species. USFWS or NMFS involvement in 

a project depends on the affected species and the nature and extent of anticipated impacts (direct 

and indirect) to that species and its designated critical habitat. If anticipating a “take” of a federallylisted species, USFWS or NMFS will issue a biological opinion, the terms and conditions of which 

are binding on the lead federal agency. [16 U.S.C. § 1536.] 

2.3.8 Coastal Barrier Resources Act [16 U.S.C. § 3501 through 3510]

The Coastal Barrier Resources Act (CBRA) of 1982, and subsequent amendments, designated

specific portions of relatively undeveloped coastal barriers along the Atlantic, Gulf of Mexico, 

Great Lakes, U.S. Virgin Islands, and Puerto Rico coasts as part of the John H. Chafee Coastal 

Barrier Resources System (CBRS). These undeveloped areas became ineligible for most new 

federal expenditures and financial assistance. The primary reason for the landmark legislation 

was to protect and conserve the fish, wildlife, and other resources unique to the coastal barrier 

islands. [16 U.S.C. § 3501]. One of the original exceptions in the CBRA is the maintenance, 

replacement, reconstruction, or repair, but not the expansion, of publicly-owned or publiclyoperated roads, structures, or facilities that are essential links in a larger network or system. [16 

U.S.C. § 3505]. 

2.3.9 Magnuson-Stevens Fishery Conservation and Management Act [16 U.S.C. § 

1801-1891d]

The Magnuson-Stevens Fishery Conservation and Management Act of 1976, or more simply the 

Magnuson-Stevens Act (MSA), governs commercial and recreational fisheries in US territorial

waters. The MSA may apply to coastal highway engineering projects because they have the 

potential to negatively affect Essential Fish Habitat (EFH). Similar to administration of the ESA, 

NMFS reviews public notices and environmental documents for compliance with the MSA and 

conducts consultations with the lead federal agency when a proposed project may adversely 

affect EFH. [16 U.S.C. § 1855(b)(2)]. 

2.3.10 Marine Mammal Protection Act [16 USC §§ 1361-1407] 

The Marine Mammal Protection Act (MMPA) may apply to coastal engineering projects that have 

the potential to harm or impair marine mammal species. [16 USC §§ 1361-1407]. The MMPA 

prohibits, with certain exceptions, the “take” of marine mammals in US waters and by US citizens 

on the high seas, and the importation of marine mammals and marine mammal products into the 

United States. [16 U.S.C.A. § 1372]. This means people may not harass, hunt, capture, or kill any 

marine mammal unless authorized or exempted. [16 U.S.C. § 1362]. The MMPA also include 
Part 1 – Background & Context HEC-25 3rd ed.

14

other prohibitions related to marine mammals. [16 U.S.C.A. § 1372]. Implementation of MMPA is 

jointly shared by NMFS, USFWS, and the Marine Mammal Commission, which provides 

independent oversight of federal agencies under the MMPA (https://www.mmc.gov/about-thecommission/our-mission/#duties-under-the-mmpa). 

2.3.11 Migratory Bird Treaty Act [16 U.S.C. §§ 703-712.]. 

The protection of all migratory birds is governed by the Migratory Bird Treaty Act (MBTA). [16 

U.S.C. 703-712]. The MBTA generally prohibits the “take” of any migratory bird or any part, nest, 

or eggs of any such bird. [16 U.S.C. § 703(a)]. Under the MBTA, it is illegal to “take, kill, possess, 

transport, or import migratory birds or any part, nest, or egg of any such bird" unless authorized 

by a valid permit from the USFWS. [Id.]. Regulation 50 CFR § 10.13A includes a list of migratory 

birds protected by the Migratory Bird Treaty Act (MBTA). 

2.3.12 National Historic Preservation Act [54 U.S.C. 300101 et seq.] 

Coastal highway engineering projects are commonly subject to the National Historic Preservation 

Act of 1966 (NHPA). Section 106 of the National Historic Preservation Act (NHPA) (commonly 

called “Section 106”) requires Federal agencies to consider the impacts on historic properties of 

projects that they carry out, approve, or fund. [54 U.S.C. § 306108]. The implementing regulations 

for the Section 106 process are found in 36 CFR part 800. Those regulations provide that Federal 

agencies, in consultation with the Advisory Council on Historic Preservation, the State Historic 

Preservation Officers (SHPO) and certain other interested parties, must identify and assess 

adverse effects to historic properties and seek ways to avoid, minimize, or mitigate those effects. 

[36 C.F.R. § 800.4-800.6]. Under Section 106, “historic property” is defined as any prehistoric or 

historic district, site, building, structure, or object included in, or eligible to be included in, the 

National Register of Historic Places [36 CFR 800.16(l)(1); see also 54 U.S.C. 300311 and 

302102]. The responsibilities of SHPOs are set forth at 54 U.S.C. § 302303. 

In addition to Section 106, Section 4(f) of the U.S. Department of Transportation Act of 1966 [23 

U.S.C. 138 and 49 U.S.C. 303] requires that FHWA not approve the use of historic sites for a 

project unless there is no prudent and feasible alternative and the project incorporates all possible 

planning to minimize harm, or any impacts to historic sites are determined to be de minimis. The 

FHWA’s regulations for implementation of Section 4(f) are found at 23 CFR part 774. 

2.3.13 National Flood Insurance Act of 1968 [42 U.S.C. § 4001 et seq.]

The National Flood Insurance Act of 1968 instituted the National Flood Insurance Program (NFIP) 

to help indemnify and reduce impacts associated with floods. The NFIP adopted the area subject 

to a one percent chance or greater of being flooded on any given year (also known as the 100-

year flood) as the standard, or base flood, for mapping US floodplains. See, e.g., 44 CFR § 9.4). 

The area inundated by the 100-year flood determines the Special Flood Hazard Area (SFHA) on 

Flood Insurance Rate Maps (FIRMs) developed by FEMA and used to determine flood insurance 

rates for structures. See, e.g. 44 CFR § 59.1 (defining “area of special flood hazard). See also 

Section 13.1 below for a discussion of the 100-year flood. FEMA implements the NFIP using its 

regulations found in 44 CFR. 

The FHWA’s policies require projects to be consistent with the Standards and Criteria in the NFIP, 

where appropriate. 23 CFR § 650.115(a)(5). To assist SDOTs in complying with this policy, FHWA 

developed coordination procedures for Federal-aid highway projects with encroachments in NFIP 

regulated floodplains. FEMA agreed to these procedures by signing a 1982 Memorandum of 

Understanding with FHWA. Coastal floodplain practices delineate the 100-year coastal floodplain 

using very different engineering analysis tools than the riverine floodplain. And, unlike the riverine 
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system, encroachments in coastal floodplains often have little or no negative impacts due to the 

nature of coastal flooding. See HICE Section 4.6, Chapter 13, and Chapter 14 below for 

discussions of coastal flooding and mapping. 
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Chapter 3 - Coastal Highways: An Overview

Thousands of miles of US highways are influenced by the unique challenges in the coastal 

environment. This chapter is a brief overview of the extent of these highways, the natural 

processes affecting these highways, and the unique issues faced in planning and engineering 

these highways. The specialty area of civil engineering called “coastal engineering” is described 

in Section 3.6.

3.1 What are Coastal Highways?

Coastal highways are those roads influenced by their presence in or near the water level, wave, 

and sand transport environment unique to a coast. While normally associated with the oceans, 

the coastal environment includes the Great Lakes and any other water bodies affected by coastal 

storms and waves. Every coastal state has highways that are flooded and damaged in coastal 

storms. Some of these roads are perpendicular to the coast and serve as access and evacuation 

routes. Some of these roads are parallel to the coast either right along (Figure 3.1) or inland from 

the shore. Some of these roads are major highways that run across or along bays or estuaries. 

Figure 3.1. A local coastal road along an estuary (Shore Road along Mt. Sinai Harbor, Town of 

Brookhaven, New York)

Many of our coastal roads are literally part of the culture. Examples include Florida’s A1A (see 

Figure 1.1) and California’s Pacific Coast Highway (Figure 3.2). Americans drive their cars to the 

beach on coastal highways, and these same highways can influence the quality of the beach 
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itself. Highways along receding shoreline are often protected by walls resulting in the loss of 

intertidal beach or wetland habitat. The full spectrum of possible engineering solutions, including 

nature-based solutions that mimic natural ecosystems, should be considered along the coast. 

Figure 3.2. Big Creek Bridge on the Pacific Coast Highway (California SH 1) 

3.2 The Extent of Coastal Roads and Bridges

Roughly 60,000 road miles in the US are occasionally exposed to coastal waves and surge. Road 

miles in flood zones near the coast were measured to generate this rough national estimate

(Douglass and Krolak 2008). After Hurricane Katrina, the FHWA assessed coastal bridges 

potentially vulnerable to failure from coastal storm events. Using very broad criteria, over 36,000 

bridges are within 15 nautical miles of coasts. Of these, over 1,000 bridges may be vulnerable to 

the same failure modes as those associated with damaging coastal storms (FHWA 2005/2007). 

Some coastal states have assessed the vulnerability of their highways and bridges to coastal 

storms (e.g. Mertz and Hayes 2009, Sheppard and Dompe 2013; Bosma et al. 2015) and some 

of those results are included in asset management and GIS systems (Snead 2018).

3.3 Societal Demand for Coastal Highways

Millions of Americans live near the coast and millions more want to vacation there. The primary 

way that we get to the beach is by car. In 2010, about 39% of the US population lived in the 452 

coastal counties, those with shorelines on the major coasts and the Great Lakes (NOAA 2013). 

Population density in the coastal counties is six times greater than the inland counties. Because 

much of the actual "beachfront" property is already developed in the US, much of the growth and 

new development is in the area near the coast but some miles inland from the water. The 
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implication is that these people use the roads to get to the shoreline. Only a few beaches are 

regularly reached by transit in a few major urban centers (e.g. Coney Island, New York). 

One of the largest industries in the US, travel and tourism, lists beaches as the leading tourist 

destination (Houston 2018). The travel and tourism industry employs over 15 million people (US 

Department of Labor 2017, US Travel Association 2017). 

3.4 Natural Processes Impacting Coastal Highways

Many different natural processes and forces impact roads and bridges near the coast. The natural 

stresses on the coast are challenging today and are increasing in a number of ways. This 

document focuses on the natural processes that are not typically experienced by inland roads. 

The unique challenges for coastal roads are briefly listed here with expanded discussions in Part 

2: Principles of Coastal Science for Highway Engineering.

3.4.1 Water Level Change

Water levels are constantly changing along the coast. Tides rise and fall daily along all the ocean 

coasts. The range of tides varies dramatically along the US coast. Near Anchorage, Alaska, the 

tide range is often over 20 ft. Near Pensacola, Florida, the tide range can be essentially zero 

during some days of the month. The Great Lakes have fluctuations in average water level 

throughout the year in response to seasonal rainfall differences that can approach 2 ft. Multi-year 

fluctuations in response to drought cycles can approach 5 ft in elevation. And the Great Lakes

can have fluctuations, called seiches, that raise and lower the water level several feet in several 

hours during major storms.

3.4.2 Sea Level Rise

Sea level has been rising along most US ocean coasts (relative sea level rise) for the past century. 

Projections suggest that the rate of this rise will increase significantly this century. Sea level rise 

(SLR) is progressively making coastal roads and bridges more vulnerable and less reliable 

(Jacobs et al. 2018b). 

3.4.3 Storm Surge

Storm surge can cause significant changes in the water level along the coast in addition to the 

tides. Storm surge is an increase in water level along the coast in response to the storm winds 

and pressures. Storm surge in Hurricane Katrina (2005) along the Mississippi coast exceeded 27 

ft (Douglass 2005). The Great Lakes can experience water level changes of up to 8 ft in response 

to a severe storm. 

3.4.4 Major Weather Events

Major weather patterns cause high storm surge and waves. The great coastal storms of the 

southeast include tropical storms and hurricanes. The major coastal storms of the northeast 

include those as well as extra-tropical storms including “Nor'easters.” The great coastal storms of 

the West Coast include the El Niño related storms and the “pineapple expresses.” The major 

storms of the Great Lakes include the winter north winds associated with arctic high-pressure

systems and their related weather fronts. 
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3.4.4.1 Tropical Storms and Hurricanes

A hurricane is a type of tropical cyclone, the general term for all rotating weather systems 

(counterclockwise in the Northern Hemisphere) over tropical waters. Tropical cyclones are 

classified as follows: 

• Tropical Depression - an organized system of clouds and thunderstorms with a defined 

circulation and maximum sustained winds of up to 38 miles per hour (mph) 

• Tropical Storm - an organized system of strong thunderstorms with a defined circulation 

and maximum sustained winds of 39 to 73 mph

• Hurricane - an intense tropical weather system with a well-defined circulation and 

maximum sustained winds of greater than 74 mph

The term "sustained wind" refers to surface wind speeds (10 m above the surface) that persist for 

durations of one minute. 

Hurricanes form in the tropical oceans, frequently in the eastern Atlantic Ocean (or the eastern 

Pacific) and are powered by heat from the sea. Many hurricanes are steered westward by easterly 

trade winds. The Coriolis effect provides the characteristic cyclonic spin of these storms. Around 

their core, winds grow with great velocity, generating violent seas. As the fierce winds 

accompanied by the low pressure move ashore, the storm surge grows and creates extensive 

flooding. In addition, the hurricane carries with it torrential rains and can produce tornadoes.

The well-known Saffir-Simpson scale classifies hurricanes according to wind speed:

• Category 1 - winds 74-95 mph

• Category 2 - winds 96-110 mph

• Category 3 - winds 111-129 mph

• Category 4 - winds 130-156 mph

• Category 5 - winds > 156 mph 

Use of the Saffir-Simpson scale as the basis for coastal engineering design decisions about 

infrastructure can be problematic because the scale ignores the important, damaging phenomena 

of storm surge and waves (e.g. see Figure 3.3). There is little correlation of the scale with surge 

and waves: two Category 2 events may result in quite different storm surge still water elevations 

and wave conditions at different locations. 

Damage from hurricanes extends well inland. Frequently, the most noted or newsworthy aspect 

of hurricane damage results from flooding along the coastal area. This is particularly important in 

low-lying areas such as the coastal barrier islands. The coastal flooding will continue upstream 

through every inlet open to the ocean including urban drainage systems. 

The Saffir-Simpson Hurricane Scale is not usually used for planning and 

design in coastal engineering because it is a wind scale... and waves on 

storm surge (not wind) cause most of the damage to roads and bridges in 

hurricanes. 
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Figure 3.3. Damage caused by surge and waves in Hurricane Michael (October 14, 2018; 

permission to use photograph provided by John Cleary) 

3.4.4.2 Extratropical and Nor’easter Events

Cyclonic events such as extratropical storms form when unstable air produces significant 

temperature and pressure differences. At times, such systems may stall off the coast and produce 

long (i.e. several days) periods of high winds and inland rainfall. Although these events rarely 

obtain hurricane level wind speeds, they can cause significant coastal surge effects and wave 

damage. 

Many historically significant coastal flood events on the Mid-Atlantic and New England coasts are 

northeasters, or nor’easters. The “Ash Wednesday” storm of March 1962 was formed by the 

combination of several slow moving coastal low-pressure systems along the Atlantic seaboard. 

This storm resulted in hurricane force winds and water levels 9 ft or more above mean low water 

level in areas of Maryland and Delaware over a period of several days. Likewise, the popularized 

“All Hallows Eve” or “Perfect Storm” of 1991 produced 5 days of high wave action, coastal erosion,

and washover (USGS 2003). A northeaster called a “bomb cyclone” developed and moved up the 

eastern seaboard in early January 2018 which generated wave heights similar to, and in some 

cases exceeding, the largest hurricane waves generated the previous year (Behrens et al. 2018). 

That previous year of 2017 included three major hurricanes in September: Irma, Jose, and Maria. 

These extratropical events extend beyond the Atlantic Coast. For example, Florida’s west coast, 

its Gulf of Mexico Coast, experiences severe flooding from such events. 
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During one March 1993 event, at a location north of Tampa Bay, the resulting inundation and 

damage was similar to those predicted to occur from a Category 1 hurricane (Citrus County 2000, 

Good 2018). Likewise, the Great Lakes coastal regions endure wave damage during winter 

extratropical events (USGS 2003). 

3.4.4.3 Long-term Fluctuations

Mean sea level along the coast also fluctuates on longer timescales in response to weather 

systems. The frequency and intensity of damaging coastal storms are influenced by El Niño 

episodes, the Pacific Decadal Oscillation (PDO), the Atlantic Multidecadal Oscillation (AMO), and 

El Niño Southern Oscillation (ENSO). 

El Niño is a weather pattern characterized by a large-scale weakening of the trade winds and 

warming of the surface layers in the eastern and central equatorial Pacific Ocean. El Niño events 

occur irregularly at intervals of 2 to 7 years, although the average is about once every 3 to 4 years. 

They typically last 12 to 18 months and are accompanied by swings in the Southern Oscillation 

(SO); an inter-annual see-saw in tropical sea level pressure between the eastern and western 

hemispheres. 

During El Niño, high sea levels and very high waves can strike the Pacific Coast of the US. Wavedriven processes of importance include wave runup and wave setup. The mean sea level along 

the Pacific Coast can be over six inches higher, when averaged over an entire year, during El 

Niño years. 

3.4.5 Waves

Waves are one of the major forces affecting coastal systems including roads and bridges. Large, 

damaging waves occur during the great coastal storms previously mentioned. Waves can 

generate tremendous forces and cause considerable damage when they are riding on top of storm 

surge and are able to strike roads and bridges that are not typically designed for such forces. For 

example, the waves in the Gulf Coast hurricanes of 2004 and 2005 caused billions in damage to 

bridges, even moving bridge deck spans that weighed over 340,000 lb each (see Figure 3.4). 

3.4.6 Shoreline Erosion and Barrier Island Breaches

Storm waves can erode coastal dunes and bluffs, potentially damaging roads in the process. 

Storm surge contributes greatly to this erosion by allowing the waves to attack the dunes or bluff 

at higher elevations than normal. The combination of storm surge and waves can cause 

overtopping and overwash on some low elevation roads. Overwash in Hurricane Isabel (2002) 

caused portions of the barrier island west of Cape Hatteras, North Carolina (Pea Island), to breach 

and form a new inlet. This overwash and breach completely removed a stretch of road, North 

Carolina SH 12, which could not be repaired until the barrier island was artificially rebuilt by beach 

nourishment techniques. Similar vulnerable areas exist on other barrier islands. 

3.4.7 Littoral Drift 

Waves can also move tremendous amounts of sand down the coast in longshore sand transport, 

also called “littoral drift.” Thus, our shorelines are always changing location in response to 

changes in wave conditions and local sand supplies. Barrier spits, islands, and inlets migrate. 

Shorelines accrete or recede over the long-term in response to changes in the longshore sand 

transport. 
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Figure 3.4. US Highway 90 bridge across Biloxi Bay, Mississippi, after Hurricane Katrina. 

(photograph looking northeast from Biloxi September 21, 2005) 

3.4.8 Shoreline Recession

Most of the US coast is experiencing long-term shoreline recession. Causes are both natural, e.g. 

responding to sea level rise, and man-made, e.g. interruptions in sand movement along the coast 

by ship channels (Douglass 2002). This coastal erosion can often threaten or even destroy roads 

located near the shoreline. For example, a 20-mile long portion of Texas SH 87 has remained 

closed for decades due to coastal erosion. 

3.4.9 Tsunamis

Tsunamis (“tidal waves”) normally result from an underwater disturbance (usually an earthquake) 

that triggers a series of water waves that can travel many hundreds or thousands of miles. The 

“Boxing Day” tsunami of December 26, 2004 in the Indian Ocean was one of the worst natural 

disasters of the past century. The tsunami waves reached up to 100 ft high on land, killed over 

220,000 people and destroyed entire cities and villages. The 2011 Japanese tsunami generated 

waves which reached even higher, up to 130 ft, and damaged a nuclear power plant. In the US,

tsunamis in 1946 and 1957 damaged bridges on the north coast of the Hawaiian island of Kauai.

The 1958 Lituya Bay tsunami in southeast Alaska caused wave runup over 1,700 ft above sea 

level (Griggs 2011). The 1964 Alaskan earthquake sent tsunami waves between 10 and 20 ft high 

along the coasts of Washington, Oregon, and California. Tsunamis can hit any US coast but 95% 

of them hit the Pacific states and territories (NTHMP 2018). Many US coastal communities in 

these states have tsunami evacuation routes with signs that have been coordinated with the 

SDOT. 
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3.4.10 Upland Runoff

Upland runoff can affect storm surge heights and flow conditions in tidal waterways if significant 

runoff discharges occur during the surge. Hurricanes can produce significant amounts of rainfall 

and extreme flooding in river systems much farther inland than the flooding caused by the surge. 

Runoff from watersheds near the coast can interact with coastal storm surge to exacerbate this 

flooding (e.g. Hurricane Harvey in 2017). During Harvey, rainfall drained more slowly from the 

coastal watersheds from Houston to Sabine because the coastal Gulf of Mexico storm surge was 

high in the bays and acting as a very high tailwater condition in the river systems that drain to 

those bays.

3.4.11 High-Velocity Flows

Floodwaters moving at high velocities can lead to hydrodynamic forces on structural elements in 

the water column, including drag forces in the direction of flow and lift forces perpendicular to the 

direction of flow. Oscillations in lift forces correspond to the repeated shedding of vortices from 

alternate sides of the structural element (for example, these vortices are often visible in the wakes 

behind bridge pilings in rapidly moving water). High-velocity flows can also move large quantities 

of sediment and debris. 

3.4.12 Other Processes

Other coastal processes that can affect coastal roads include common coastal ice problems in 

northern climates, wave overtopping, flooding (Chapter 9), and wave spray (e.g. salt water on 

reinforced concrete). 

Ice can prohibit wave generation. Increases in the “ice-free” durations in northern latitudes, 

including the Arctic Ocean and the Great Lakes, are causing more erosion, recession and 

damage. Native American communities on the north slope of Alaska have suffered losses of 

historical and cultural sites due to increased waves (Jones et al. 2008).

Ice can also remove sediment from beaches as it moves offshore into deeper water since beach

ice can include sediment (Hampton et al. 1999). Similarly, ice flows can remove armor stones 

from rock revetments through a process called “ice picking.” Ice picking occurs when a layer of 

thick ice collects on large armor stones and then cantilevered portions of the ice pull the stones

down the revetment slope when they thaw (Smith and Carter 2011).

3.5 Coastal Highway Planning and Design

Some highway planning and design issues are unique to the coast. For example, the design of 

revetments exposed to wave attack bears additional considerations beyond those in non-coastal 

situations. These revetments can provide embankment protection along roads or at approaches 

to bridges. 

The possible relocation of roads in response to erosion is another common coastal planning issue: 

Historically, some coastal roads have been abandoned or relocated landward due to shoreline 

migrations. Coastal engineering options can stabilize shorelines when a road is threatened by 

erosion. Manipulating the elevation of coastal roads and bridges can avoid some of the unique 

coastal forces. For example, the bridges destroyed by Hurricanes Ivan and Katrina are now at 

much higher elevations. A related issue is the vulnerability of existing bridges that might be 

exposed to similar conditions. Also increasing flooding referred to as “nuisance flooding” has 

become a problem in almost every coastal state. Later chapters in this document discuss each of 

these issues. 
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The coast presents many challenges for roads including some environmental and aesthetic 

issues. Coastal roads traverse bays, estuaries, beaches, dunes and bluffs. These are some of 

the most unique and treasured habitats for humans as well as a variety of plants and animals. 

The list of endangered species requiring these coastal habitats for survival includes numerous 

sea turtles, birds, mammals, rodents, amphibians and fishes. The preservation of natural 

resources has value in itself; natural and nature-based features can be valuable tools for 

transportation engineers and planners. Careful application of nature-based solutions can 

accomplish many of the goals traditionally associated with hard engineering solutions, such as 

shoreline stabilization and erosion control. 

3.6 Coastal Engineering as a Specialty Area

This document provides State Departments of Transportation (SDOT) and FHWA hydraulics units 

with sufficient information for them to understand issues in the coastal environment. However, 

coastal engineering projects – those in the wave, tide, and sand transport environment unique to 

the coast - should be done by trained, experienced coastal engineers. 

Coastal engineering is a well-established specialty area of civil engineering. It is the 

planning, design, construction and operation of infrastructure in the unique wave, water 

level and sand transport climate along the coast. Coastal engineering makes extensive use 

of the sciences of nearshore oceanography and coastal geomorphology as well as geotechnical, 

environmental, structural and hydraulic engineering principles. Traditional coastal engineering 

projects involve solving maritime navigation or shoreline erosion problems. Over time, the scope 

of coastal engineering projects has broadened to include new beaches for recreational and 

resilience purposes and many projects to improve coastal water quality and coastal habitats. The 

focus of this manual is coastal engineering related to highways. 

The design environment - the coastal water level, wave and sand environment - is the primary 

distinguishing factor of coastal engineering from other civil engineering disciplines. The coastal 

design environment is very challenging since design conditions are often affected by storms that 

contain much more energy and induce very different loadings from those normally experienced. 

3.6.1 Education

Coastal engineering is primarily taught at the graduate level in the US. A few US universities have 

undergraduate coastal engineering programs. A formal education in coastal engineering, like any 

other specialty area of civil engineering, is unique and extensive. In other words, the formal 

education of coastal engineers differs significantly from the education of most civil engineers. 

Most coastal engineering graduate programs include: 

• two or more graduate courses in wave mechanics, 

• two or more courses in other coastal hydrodynamics such as tidal circulation and 

modeling, 

• two or more courses in coastal sediment transport, and 

• several courses in the functional and structural design of infrastructure in this environment. 

Many coastal engineering programs also include coastal ecology, geology, and oceanography 

science courses. 

Roughly two dozen US universities have some formal graduate level coastal engineering program 

with a few faculty members teaching in the field. Most of these programs are affiliated with civil 

engineering departments. The American Shore and Beach Preservation Association (ASBPA) 
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periodically surveys and summarizes the breadth of the coastal engineering university community 

(e.g. ASBPA 2018). 

3.6.2 State-of-Practice

The practice of coastal engineering is still much of an art because the physical processes are 

often too complex for adequate theoretical description and the design level of risk is often high. 

Consequently, practitioners should have a broad base of practical coastal engineering experience 

and should exercise sound judgment based on that experience. There is no substitute for the 

judgment that comes from coastal engineering experience. 

Some distinction can be drawn between coastal engineers and coastal modelers. Coastal 

numerical models simulate the complex physical processes along the coast (see Webb 2017). 

This necessitates a distinct, highly-specialized, set of computer skills. But numerical models are 

only one tool available to coastal engineers and are not a substitute for engineering judgment. 

Experienced modelers often perform the modeling and experienced coastal engineering design 

professionals oversee the application of the modeling results. 

3.6.3 Professional Specialty Certification as a Coastal Engineer

Coastal engineers can obtain a specialty certification from the Academy of Coastal, Ocean, Ports 

and Navigation Engineers (ACOPNE) as Diplomates of Coastal Engineering (DCE). ACOPNE is 

affiliated with the American Society of Civil Engineers (ASCE). The ASCE is the US’s oldest 

engineering society and represents more than 150,000 members of the civil engineering 

profession in 177 countries. A DCE is a board certified professional with at least twelve years of 

progressive coastal engineering experience after earning their first engineering degree, has 

achieved a master's degree in the field, and has passed a specialized board examination 

(ACOPNE 2018). The inclusion of a DCE on the professional team is encouraged for complex 

coastal projects.

3.6.4 Resources

The field of coastal engineering is summarized in textbooks such as Kamphuis (2010) and 

Sorensen (2006). Other summary references are mentioned throughout this manual. The 

USACE’s Coastal Engineering Manual (CEM) attempts to summarize the aspects of the field that 

are of most importance to that agency’s mission. Over 2,500 pages long, the CEM was originally 

published in 2002. It replaced the Shore Protection Manual (SPM) which was often called the 

“bible of coastal engineering” (USACE 1984). Other coastal engineering manuals and handbooks 

include Herbich (2000) and Kim (2018). 

The breadth and the changes in the field of coastal engineering are best captured by professional 

specialty conferences and journals. The International Conference on Coastal Engineering takes 

place every two years and typically has hundreds of highly-technical presentations. Most of these 

proceedings are available as open access, on-line resources with a keyword searchable database 

(Coastal Engineering Research Council 2018). 

Several series of ASCE sponsored specialty conferences include the “Coastal Sediments” 

conferences, the “Coastal Structures” conferences, the “Solutions to Coastal Disasters” 

conferences, the “Ports” conferences, and the “Coastal Zone” conferences. Each of these 

A board-certified Diplomate, Coastal Engineer can be included on the 

planning/design team for major coastal planning and engineering projects.
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conference series has a longer, formal title that is more explanatory, but these are the commonly 

used names. These conference series have hundreds of presentations and most publish written 

proceedings. The American Shore and Beach Preservation Association has an annual National 

Beach Conference which is held at different venues around the US. The Florida Shore and Beach 

Preservation Association has an annual National Beach Preservation Technology Conference 

(that is usually held in Florida). The ASCE Journal of Waterway, Port, Coastal and Ocean 

Engineering is published six times per year. Other journals with coastal engineering research 

results include the Journal of Coastal Engineering, Shore and Beach, and the Journal of Coastal 

Research. 

3.6.5 Coastal Engineering in the Highway Community

A goal of this document is to encourage the better integration of coastal engineering 

principles and practices in the planning and design of roads along the coast. As society 

continues its great migration to the coasts in the face of changing natural stresses on the coast, 

opportunities for fruitful integration of coastal engineering in the transportation engineering 

process will continue to arise. 

AASHTO (2008) recommends early input from a coastal engineer to clarify issues and scope for 

any bridge vulnerable to coastal storms and specifies criteria for qualification as a coastal 

engineer. At the time of the development of AASHTO (2008), no specialty certification for coastal 

engineers existed. Now however (see Section 3.6.3), the DCE specialty certification by ACOPNE 

has similar but more quantified criteria as those specified in AASHTO (2008). 

Some coastal states are already encouraging, and even requiring, the inclusion of coastal 

engineers in multi-disciplinary teams addressing highway and bridge projects near the coast.

Florida DOT has required coastal engineers be involved in coastal engineering projects for years.

Texas DOT has developed a new coastal precertification requirement for consultants along these 

lines (TxDOT 2019). This document should aid the transportation professional in understanding 

when input from a trained coastal engineer would be helpful to the planning or design team. 

Most transportation agencies do not have any coastal engineers on staff. However, since these 

agencies are planning, designing, constructing and operating projects that are, in reality, coastal 

engineering projects, they often hire consulting coastal engineering expertise. Coastal engineers 

are vital to transportation planning and decision-making processes in coastal environments. 
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Chapter 4 - Water Levels

Coastal water levels fluctuate because of astronomical tides, storm surges, riverine discharges, 

and long-term sea level rise (SLR). Water level determines where waves act and still water level 

(SWL) is an important input assumption for coastal infrastructure design. The “still” in SWL is 

essentially a convenient construct for engineering design that includes all the contributions to 

coastal water level except the waves themselves. 

4.1 Sea Level Rise

Sea levels (the long-term average ocean levels) are slowly rising along most of the US coast and 

the rate of this rise is projected to increase significantly this century. The effects of SLR on coastal 

highways includes increased flooding (Chapter 9) and more vulnerability during storms (Chapter 

14). 

This section discusses the causes of SLR, historical measurements of it, and projections for its 

future rates. Section 4.1.7 presents specific values of SLR for engineering design. Section 4.2.2 

discusses the impact of ongoing SLR on the relationship between surveying and tidal datums. 

4.1.1 Causes of SLR and Terminology

Causes of global SLR include thermal expansion, melting glaciers and to some extent, reservoir 

storage and groundwater manipulations. Increased heat in the ocean causes thermal expansion 

of the water. Sea level is also rising as glaciers melt. The projected increasing rate of SLR is 

primarily due to increasing contributions of water from major land-based glacial ice sheets 

including those of Antarctica and Greenland. Changes in reservoir storage and groundwater 

manipulations contribute minimally. 

“Relative” sea level rise (RSLR) and “global mean” sea level rise (GMSLR) are important, 

but distinct, terms. 

RSLR is the combined effect of ocean water elevation and land elevation change at a location. 

The RSLR is the sea level rise any coastal engineering project location is experiencing. RSLR 

includes any vertical land movement (VLM). Three common types of VLM are subsidence, postglacial rebound, and tectonics. Some of the US land-mass near the coast is subsiding due to a 

variety of natural geologic factors including compaction and man-induced factors such as 

groundwater or oil and gas extraction (e.g. coastal Louisiana). Some of the US, particularly at 

northern latitudes, however, is rebounding or emerging, due to glacial retreat (post-glacial 

rebound). A significant VLM on parts of the US Pacific Coast is regional tectonic movement. 

Global mean sea level rise (GMSLR), or "eustatic" sea level rise, is the average SLR across all 

the ocean basins. Coastal engineers have traditionally assumed that the difference between the 

RSLR and the GMSLR is due exclusively to the VLM at a location. However, other physical 

processes contribute to this difference because GMSLR is not evenly distributed across all the 

ocean basins. The oceans are not a static bathtub; they have ever-changing winds, tides, and 

currents. The long-term sea level at any location is a function of those currents and the related 

Coriolis forces and this is one reason GMSLR is not evenly distributed across the oceans. Also, 

sea level is a function of the overall gravitational field of the earth which includes the mass of the

major ice sheets. As those major ice sheets melt, their effect on the overall gravitational field 

SLR values for design are presented in Section 4.1.7 (see Figure 4.11).
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changes. Interestingly, the loss of ice sheet mass in Antarctica leads to increased sea levels in 

the northern latitudes of the northern hemisphere as the gravitational attraction decreases. This 

idea that the loss of ice sheet mass leads to greater SLR on the other side of the earth is 

sometimes termed “fingerprinting” in the scientific literature. See Sweet, et al. (2017a) for more 

discussion on the physics of the causes of GMSLR and RSLR.

4.1.2 Historical RSLR Data: Tide Gages

The historical RSLR data are summarized in this section and the next two sections. Sea levels 

have been rising not only in the past century but for the past 20,000 years. US tide gage 

measurements vary in length by location but generally started in the late 1800’s or early 1900’s. 

Satellites started measuring worldwide sea levels in 1993. Geologic records can estimate 

prehistoric sea levels.

US tide gage records provide a wealth of data and analyses which can be readily obtained 

online at the NOAA “Tides and Currents” website (NOAA 2019). An example of long-term tide 

gage data for The Battery in lower Manhattan, New York City is shown in Figure 4.1. Monthly 

average sea levels at that tide gage are shown. The vertical axis, the y-axis, is the sea level where 

the zero value is the average sea level from 1983 to 2001. This is the most recent National Tidal 

Datum Epoch (see Section 4.2 for discussion of tidal epochs). This New York City gage record 

extends back to the 1850’s (with a gap in the data in the 1870-1890’s). Seasonal fluctuations have 

been removed. 

Figure 4.1 shows an obvious long-term trend in the data that is positive, i.e. sea level has been 

rising. The average annual rate of rise is 2.84 mm/yr which is equivalent to 0.93 ft per century. 

The 95% confidence interval about that linear trend rate is 0.09 mm. 

The plot also shows the monthly-average “noise” about that long-term trend. Significant 

fluctuations in average sea level month to month appear as the “spikiness” of the data plot. Yearly 

and multi-year periods have visibly different trends, but the long-term trend is one of RSLR at a 

fairly constant rate. 

Similar plots of the tide gage records are available at the NOAA website for many other locations 

around the US. Engineers can obtain the NOAA plot and analysis from the tide gages 

nearest to their project site to begin to understand the historical RSLR rates.

RSLR is the sea level rise that coastal highways will experience and should be 

accounted for in planning and design.
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Figure 4.1. Relative sea level rise (RSLR) trend for New York City based on tide gage data 

(image downloaded from NOAA Tides and Currents website September 6, 2018) 

Other data visualizations such as smoothing and the use of different averaging periods can 

improve the understanding of these tide gage RSLR data. This is a time-honored way to 

understand a variety of engineering time series data (Bendat and Piersol 2010). The monthly 

average sea level data are available for download for further analysis from the same NOAA 

website (NOAA 2019). 

As an example, Figure 4.2 shows annual average sea levels for Dauphin Island, Alabama. Similar 

to Figure 4.1, the vertical axis in Figure 4.2 is the average sea level and the zero value is the 

average sea level from 1983 to 2001. Thus, in 1967, the average sea level was about 3 inches 

lower than the 1983-2001 average but in 2017 it was about 6 inches higher than that average and 

9 inches higher than the average in 1967. The Dauphin Island tide gage has only been operational 

since 1967 and the plot excludes years with significant data gaps. The important point is that 

viewing the data this way provides some interesting observations for that location:

• The annual average sea levels along the Alabama coast in 2016 and 2017 were the 

highest ever on record, 

• The average Alabama sea levels in 2016 and 2017 were roughly 6 inches higher than the 

average in the 1980’s and 1990’s and 8 inches higher than the average in the 1960’s and 

1970’s,

• The 2016 Alabama record was 2.5 inches higher than the previous record high year, and 

• Seven of the highest years in history for average sea level along the Alabama coast have 

occurred in the last 9 years. 

Although the data do not change with the averaging period, these characterizations of the data 

are not as obvious with NOAA’s monthly average plot. The relatively high sea level shown for 

Alabama in 2016 and 2017 is generally consistent with other gages along this portion of the 

northern Gulf of Mexico Coast and is partly the result of the regional weather patterns those two 

years. Engineers can download these data for the tide gages nearest to their project site 

and plot the historical statistics with a variety of techniques to better understand the 

historical RSLR rates.
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Figure 4.2. Annual average sea level at Dauphin Island, Alabama

Tide gages have measured RSLR around the US for the last century or so depending on location. 

Figure 4.3 (Atlantic and Gulf Coasts) and Figure 4.4 (Pacific Coast) show some of the variation in 

the average annual mean sea level (MSL). The values shown are relative to the latest local MSL 

datum (see explanation of tidal datums in Section 4.2). The relative sea level change rates shown 

in Figure 4.3 and Figure 4.4 indicate a clear upward trend, i.e. RSLR, along much of the US coast.

The rate of RSLR shown is fairly constant at the locations in Figure 4.3 and Figure 4.4. Sea levels 

have risen roughly 1 ft since the 1920’s at the Pensacola, Florida gage. But, at the Galveston, 

Texas gage the RSLR this century is approximately 3 ft. The difference is due to the unique rates 

of VLM at those two locations. In much of southeastern coastal Texas, land is subsiding due to 

groundwater and fossil fuel extraction (Kafalenos et al. 2008).

Some places in the US exhibit lower rates or a negative trend. For example, Juneau, Alaska has 

a negative rate of RSLR. In much of Alaska, the sea level is falling: the vertical land movement 

(VLM) is an upward post-glacial rebound, in response to glacial retreat in the past millennia, at a 

rate that exceeds the GMSLR rate. 

Sea levels have been slowly rising around most of the US for the past century.
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Figure 4.3. Sea levels along the US Atlantic and Gulf Coasts for the past century
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Figure 4.4. Sea levels along the US Pacific Coast for the past century
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Figure 4.5 summarizes the long-term RSLR trends, as measured by tide gages, around the US. 

The relative sea level is rising along most of the US coast. The rates of rise vary by location 

primarily due to regional variability in VLM and other physical phenomenon including oceanic 

circulation patterns. 

Along most of the Atlantic and Gulf Coasts, the rate of rise is between 0 and 1 ft/century or 

between 1 and 2 ft/century. More precise values are available on-line for each location as shown 

in Figure 4.1 for New York City. The largest positive rates of RSLR in the US shown on Figure 

4.5 are those near the mouth of the Mississippi River in Louisiana where the VLM rates are very 

high due to marsh subsidence (note: these are the red, upward-pointing arrows on the northern 

Gulf of Mexico Coast). Chapter 9 - Increased Flooding Due to Relative Sea Level Rise discusses 

problems in areas with large regional subsidence rates.

Figure 4.5. Relative sea level rise (RSLR) rates measured at tide gages around the US (image 

downloaded from NOAA Tides and Currents website, Sept 5, 2018) 

For the past century, the worldwide average rate of SLR based on tide gage records, the 

GMSLR rate, is most commonly estimated to be 1.7 mm/yr (0.067 in/yr) (IPCC 2007). This 

corresponds to about a 0.6 ft rise in GMSLR during the past century. The US Atlantic and Gulf 

coasts have experienced more RSLR in general.
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4.1.3 Historical SLR Data: Satellites

Satellite altimetry can also measure sea levels (Leuliette et al. 2004; JPL 2013). Satellites have 

mapped the sea surface several times per month since 1993, measuring sea level across large 

portions of the earth’s oceans. The measurements are not limited to the locations of the long-term 

tide gages that are all on the coast and mostly in the northern hemisphere. Figure 4.6 shows the 

sea level trends as measured by satellites (CNES/LEGOS/CLS 2018). One interesting aspect of 

this sea level change data measured by satellites is its spatial, or regional, variability across the 

oceans. The values shown range from much higher rates in portions of the western tropical Pacific 

to near zero values in portions of the north Pacific to negative values (i.e. sea level has fallen) in 

a few portions of the south Pacific. Based on satellite data, the average rate of GMSLR has 

been 3.32 mm/yr (0.13 in/yr) since 1993 (see Figure 4.7). 

Figure 4.6. Map of sea level change as measured by satellites 1992-2017 (permission for use 

provided by CNES/LEGOS/CLS) 

4.1.4 Historical SLR Data: Geologic Record

The geologic record predates tide gages and satellites. Coastal geologists estimate past sea 

levels using a variety of sediment analysis techniques including radiocarbon dating. These socalled “proxy” data, combined with other geological evidence, are useful for estimating ancient 

sea levels. Twenty thousand (20,000) years ago, the global eustatic sea level was probably 300 

ft to 400 ft lower than it is today (see Section 6.1 - Coastal Geomorphology). The past 12,000 to 

20,000 years is the Holocene Epoch, characterized by the rise of global sea level in response to 

the melting of the last of the Wisconsin ice-age glaciers (Davis and Fitzgerald 2009). The 

Holocene SLR is divisible into two distinct time periods (see Figure 6.3). Prior to about 6,000 

years ago, sea level rose at a much faster rate of 10 mm/yr (0.39 in/yr) or about 3 ft per century. 

The rate of rise slowed significantly about 6,000 years ago. The SLR rate may have been only 

0.1 to 0.2 mm/yr or about 0.5 ft per century when averaged over the last 3,000 years (Church et 

al. 2001). 
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Figure 4.7. Global Mean Sea Level Rise (GMSLR) 1993-2018 as measured by satellite altimetry 

(data from CNES/LEGOS/CLS 2018) 

4.1.5 Discussion of Historical and Present-Day GMSLR 

Sea levels are rising in all the data discussed. Whether GMSLR rates are accelerating is a subject 

of debate in the scientific literature. A number of investigations have suggested that SLR has been 

accelerating in recent decades (Merrifield et al. 2009; Church and White 2006; Church and White 

2011; Hamlington et al. 2011). Comparing the GMSLR rate as measured by tide gages over the 

past century (1.7 mm/yr) with the satellite rate since 1993 (3.3 mm/yr) suggests acceleration. 

Sweet et al. (2017b) show acceleration by considering an amalgamation of all three types of 

historical SLR data with future SLR scenarios. 

Confidence in a present-day acceleration conclusion would be higher if accelerations were 

measurable within the same data set, either the tide gage data or the satellite data, or within both. 

Church and White (2011) found a very small acceleration rate in tide gage data, 0.009 mm/yr2

(0.00035 in/yr2), which would add less than 100 mm (3 inches) of sea level change in the next 

century. This is significantly below most of the future SLR projections/scenarios discussed in 

Section 4.1.6 and less than the contribution from the historic, global, eustatic, “linear” rate (1.7 

mm/yr). Boon et al. (2018) find a rate acceleration by only considering the most recent decades 

in the tide gage data. Nerem et al. (2018) find an acceleration rate of 0.084 mm/yr2 in an adjusted 

form of the satellite data, which implies 2.2 ft of GMSLR by 2100. That level of rise is consistent 

with the projections for future SLR projections discussed in Section 4.1.6. 

Other investigators have found that historical SLR rates measured by tide gage data have not 

been accelerating (Houston and Dean 2011, Douglas 1992). This finding is consistent with casual 

observation of the full record as shown in Figure 4.1, Figure 4.3, Figure 4.4, and any figure from 

NOAA’s Tides and Currents web-site for tide gage data around the US. An upturn in RSLR rate 

is not clearly obvious in the last several decades. Some of the gage plots shown in these figures 

may show an upturn for the most recent few years, perhaps indicating an upward, accelerating 

trend that is just not yet statistically significant. 

The implication is that while sea level is rising, the rate of rise is not increasing at the locations of 

US tide gages, at least not yet at a statistically significant level. A synthesis report that focused 
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on sea levels, prepared by a large, interagency science group specifically for input to the 3rd

National Climate Assessment (NCA), discussed the Houston and Dean (2011) findings and 

reasonably concluded that, “the presence or absence of accelerations in sea level rise and the 

causal mechanisms remain an area of scientific debate (Parris et al. 2012).” 

The tide gages, of course, only measure past rates and do not forecast future RSLR rates. And 

for design, planning and operation of coastal transportation infrastructure, the future sea levels 

will be important. 

4.1.6 Projections of Future GMSLR

All of the projections of future GMSLR in the scientific literature are for increasing rates this 

century. A range of GMSLR projections is available and the science of these projections will likely 

continue to evolve and improve rapidly. 

The National Cooperative Highway Research Project (NCHRP)1 15-61 “Applying Climate Change 

Information to Hydrologic and Hydraulic Design of Transportation Infrastructure” investigated 

these GMSLR and RSLR future projections. The objectives of NCHRP 15-61 were to review the 

literature for tools and techniques that represent the state of the art for actionable hydrologic 

design of transportation infrastructure accounting for the potential effects of climate change. In 

addition to the Final Report (Kilgore et al. 2019a), the study produced a Design Practices manual 

(Kilgore et al. 2019b) to provide practitioners with information to consider GMSLR. Aligned with 

other literature, both the “Design Practices” and “Final Report” describe two broad types of SLR 

projections: 

• process-based, scientific estimates of the physical processes controlling SLR such as 

IPCC (2013) and Kopp et al. (2014) and

• “scenarios” for planning that are based on the range of scientifically possible sea levels, 

but are arbitrarily selected values within that range, such as Sweet et al. (2017b). 

Both are valuable for practical coastal planning and engineering. Section 4.1.6.1 discusses the 

process-based, scientific estimates, Section 4.1.6.2 discusses the “scenarios” of SLR. 

4.1.6.1 Process-Based, Scientific Estimates

The first broad type is exemplified in Figure 4.8, a process-based, scientific estimate of future 

GMSLR following Kopp et al. (2014). These projections are based on a sum of the best available 

estimates of each of the physical components contributing to sea level rise including thermal 

expansion of the ocean water and ice-sheet melt contributions to the volume of ocean water.

The SLR projections in Figure 4.8 are developed for each of the common “representative 

concentration pathways,” RCPs, following the Intergovernmental Panel on Climate Change 

(IPCC). The RCPs are a set of standard emission scenarios that are the basis for most of the 

atmosphere-ocean global climate model numerical experiments. They are based on a range of 

projections of future population growth, technological development, and societal responses. RCP 

8.5 is a “high” global emissions scenario, RCP 4.5 and RCP 6.0 are “intermediate” scenarios 

 

1 NCHRP operates under the auspices of the Transportation Research Board of the National Academies of 

Sciences, Engineering and Medicine. 

Sea levels are rising along most US coasts and the rate of rise is projected to 

increase significantly.
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consistent with midrange mitigation of future emissions, and RCP 2.6 is a scenario consistent with 

a high level of future emissions reduction. The reader is referred to Fletcher (2013), Kilgore et al. 

(2016), Taylor et al. (2012), or IPCC (2013) for more description of the RCPs and to USGCRP 

(2017) and USGCRP (2018) for discussion about how present-day emissions compare with the 

RCPs. 

Figure 4.8 depicts three plots of the GMSLR projections corresponding to RCP’s with a) RCP 8.5, 

b) RCP 4.5 (and RCP 6.0), and, c) RCP 2.6, respectively. Kopp, et al. (2014) did not model RCP 

6.0, because the GMSLR projections of RCP 6.0 are nearly identical to those of RCP 4.5 through 

2100. The plots show the median values of GMSLR from 2000 as well as estimates of the 

probability distribution about those median values. The solid blue line is the median projection, 

and the blue shaded areas represent the 90% confidence interval about that median projection. 

The RCP 8.5 values shown in Figure 4.8 are higher than those of RCP 4.5/6.0, and the RCP 2.6 

values are lower, as expected. 

For example, for RCP 4.5 as shown in Figure 4.8b, the GMSLR projection by 2050 is 0.85 ft at 

the median probability, with 90% confidence that it will be between 0.6 ft and 1.15 ft (relative to 

the 2000 mean sea level). In other words, the GMSLR is expected to be 0.85 ft. More specifically, 

there is a 50% chance that the increase in GMSLR between 2000 and 2050 will be less than 0.85 

ft and a 95% chance that it will be less 1.15 ft. 

4.1.6.2 Planning “Scenarios” of Future GMSLR and RSLR

The literature describes the second broad type of informative future SLR projections as the use 

of “scenarios” (e.g., Sweet et al. 2017b). Because of the uncertainty ranges for the process-based 

scientific projections, including the significant overlap within the probabilistic ranges of those 

projections, “scenarios” of SLR are often used for planning. 

Scenarios are recommended by the NOAA Technical Report from the Sea Level Rise and Coastal 

Flood Hazard Scenarios and Tools Interagency Task Force jointly convened by the US Global 

Change Research Program (USGCRP) and the National Ocean Council (Sweet et al. 2017b). 

The “SLR Interagency Task Force” charge was to develop and disseminate, through interagency 

coordination and collaboration, future RSLR and associated coastal flood hazard scenarios and 

tools for the entire US. 

The effort developed RSLR projections for six GMSLR “scenarios” of 0.3, 0.5, 1.0, 1.5, 2.0, and 

2.5 m of GMSLR between 2000 and 2100 using the same process models of Kopp et al. 2014. 

These scenarios are named Low, Intermediate-Low, Intermediate, Intermediate-High, High, and 

Extreme respectively. The SLR Interagency Task Force increased the lower bound suggested by 

Parris et al. (2012) and used in the 3rd NCA by 0.1 m to 0.3 m (to 1 ft) and increased the upper 

bound by 0.5 m to 2.5 m (to 8 ft). Thus, the range of scientifically plausible projections for GMSLR 

is 1 to 8 ft by 2100. The half meter increments of the four mid-range scenarios are arbitrary within 

that range. 

Figure 4.9 shows how the “scenarios” compare with the projections of Figure 4.8. This is 

how the full range of scientifically-plausible planning scenarios from the SLR Interagency Task 

Force (Sweet, et al. 2017b) compare with the process-based scientific projections (Kopp, et al. 

2014). The figure has two panels and each panel has both types of curves of GMSLR projections:

• The dashed curves are the scenarios from the SLR Interagency Task Force (Sweet, et al.

2017b). These six scenarios are labeled Low, Intermediate-Low, Intermediate, 

Intermediate-High, High, and Extreme. 
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• The blue curves are the process-based science projections from Kopp et al. (2014). The 

upper panel shows the Kopp projections for RCP 8.5 (the most extreme RCP). The lower 

panel shows the Kopp projections for RCP 4.5/6.0. 

Figure 4.8. Projections of GMSLR in this century (relative to 2000) from Kopp et al. (2014) for a) 

RCP 8.5, b) RCP 4.5, and c) RCP 2.6
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Figure 4.9. GMSLR scenarios and process-based projections. The scenarios (dashed curves) 

are the SLR Interagency Task Force (Sweet et al. 2017b) scenarios and the projections (blue 

line for median and shading for 90% confidence interval) are the Kopp et al. (2014) scientific, 

process-based projections for RCP 8.5 (upper panel) and for RCP 4.5/6.0 (lower panel)
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These two panels show essentially “how bad GMSLR could be” (the higher scenarios shown with 

the dashed curves) and “what we think GMSLR likely will be” (the blue curves). The median 

process-based GMSLR projection curves are between the Intermediate-Low and the Intermediate 

scenarios. The full range of the 90% confidence intervals for the process-based GMSLR 

projection curves are between the Low and the Intermediate-High scenarios.

The SLR Interagency Task Force report provides detailed projections of RSLR at many 

coastal locations, including at all the tide gages around the US coast (Sweet et al. 2017b). These 

projections are powerful new tools for the coastal engineer and planner. The report essentially 

answers the question, “if the GMSLR is 0.5 m (or 1.0 m, 1.5 m, etc.), what will the RSLR be at my 

specific location?” 

A companion table to the report with all the detailed projection outputs is available at the NOAA 

website for the report (Sweet, et al. 2017c). The projections include the RSLR that corresponds 

with the GMSLR scenarios. All the physical process components discussed in Section 4.1.1 are 

included in these RSLR projections (VLM, thermal expansion, ocean circulation, ice sheet 

contributions including the regional variations due to fingerprinting, etc.). Engineers can evaluate 

these scenario-based data, for the locations nearest to their project site, to develop site-specific 

RSLR projections for planning and design of coastal transportation projects.

Figure 4.10 shows RSLR at four US locations for each of the SLR Interagency Task Force GMSLR 

scenarios. For example, at New York City, the Low scenario projects a RSLR of about 1 ft by 

2060 and the Intermediate-High scenario projects a RSLR of about 3 ft by 2060. Corresponding 

values are higher for Galveston and lower for San Francisco due to the other factors effecting 

RSLR (primarily VLM).

4.1.7 SLR Values for Planning and Design aligned with NCHRP 15-61

The NCHRP 15-61 report recommended that the planning and design of successful coastal 

transportation infrastructure include three considerations: 

1. Include future RSLR projections in planning and design.

2. Use minimum projections of RSLR throughout the remainder of this century, 

corresponding to the GMSLR values of Table 4.1, for design 

3. Encourage engineers to be aware of the uncertainty in future RSLR projections and 

account for it appropriately in design. To illustrate, consider higher projections of RSLR 

when overall project performance is very sensitive (i.e. fragile) to design sea levels and/or 

when designing long-lived or expensive infrastructure (see Table 4.2).

The site-specific RSLR estimates from the SLR Interagency Task Force are 

powerful new tools for the coastal engineer and planner.
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Figure 4.10. RSLR projections for four US cities corresponding to the GMSLR scenarios of the 

Interagency report (adapted from Sweet et al. 2017b)

In providing these three considerations within this document, it is important to accept that these 

are not legally binding to HICEs and compliance is voluntary. Additionally, there are many 

possible RSLR choices; nothing in this section should be interpreted as countervailing existing 

Federal, State or (as appropriate) local requirements or guidance. For example, many coastal 

states and local governments may have already developed their own regulations, ordinances, or 

guidance on incorporating RSLR for use in planning (e.g. California, California 2018; Washington, 

Miller et al. 2018; and Broward and Miami-Dade Counties, Southeast Florida 2015). Much of this

existing State and local planning material is in terms of the probability of different levels of RSLR 

occurring for varying planning horizons. Therefore, with a proper understanding of this context, 

the next paragraphs will more fully describe these three considerations. 

4.1.7.1 Consideration 1: Include Future RSLR projection in planning and design

The first consideration recognizes that prudent coastal transportation infrastructure planning 

and design should account for future RSLR projections. Coastal engineering practice has 

evolved over the past several decades toward inclusion of RSLR in design and planning as the 

long-term effects of the process have become more obvious and the scientific projections have 

improved. RSLR projections now are frequently used in both government and private practice for 

coastal engineering. This is true for both the design of traditional infrastructure (seawalls, jetties, 

etc.) as well as the nature-based solutions (e.g. beach nourishment and living shorelines). USACE 

guidance has evolved from limited recommendations to use RSLR based on projections of the 

historical linear rate (USACE 1984, USACE 2002) to explicit guidance to include RSLR for 

planning and design reflecting the best available science (e.g. USACE 2014, Hall et al. 2016). 
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Within FHWA literature the consideration of SLR has similarly evolved. HEC-25 2nd edition 

discussed SLR but did not provide specific guidance (Douglass and Krolak 2008). HEC-25 

Volume 2 recommended that SLR projections be used in vulnerability assessments of coastal 

transportation infrastructure (Douglass et al. 2014). That recommendation was to consider a 

range of RLSR corresponding to GMSLR of between 1 and 4 ft this century, following the 3rd NCA 

(Melillo et al. 2014). HEC-25: Volume 2 did not provide more specific values within that range 

because of the uncertainties in the science and the need to allow the engineer/decision maker 

flexibility in determining the appropriate level of risk tolerance for any specific project. 

4.1.7.2 Consideration 2: Apply minimum projected RSLR aligned to asset service life

The second consideration is that the design of coastal infrastructure apply some minimum

RSLR, commensurate to the service life of the asset. Table 4.1 depicts NCHRP 15-61 aligned 

GMSLR estimates with corresponding decadal values. For example, for an asset with a service 

life extending to the end of this century, the design RSLR corresponds to a GMSLR of about 2 ft. 

Figure 4.11 plots these GMSLR versus decadal values as well (circles in lower panel). 

Table 4.1 Minimum GMSLR estimates for use in planning and design as per NCHRP 15-61 

(values are relative to 2000) 

Units 2020 2030 2040 2050 2060 2070 2080 2090 2100

m 0.09 0.14 0.20 0.26 0.33 0.39 0.46 0.52 0.58

ft 0.31 0.46 0.66 0.85 1.07 1.29 1.50 1.72 1.94

The estimates in Table 4.1 are the 19-year averages centered on the decade relative to that of 

2000. In order to obtain a RSLR estimate, one would adjust these values for local factors (e.g. 

VLM and other processes). In refining the NCHRP 15-61 information, the Table 4.1 values used

a second-order polynomial equation (following NRC 1987 and USACE 2014) to model the 

GMSLR throughout the century to reach the target GMSLR by 2100. The second-order polynomial 

equation is: 

 (4.1)

where: 

R(t) = GMSLR since 2000 

 t = time in years after 2000

 a = 0.0109 ft/yr

 b = 0.000085 ft/yr2 

The initial rate parameter, a, matches the satellite GMSLR estimate since 1993 (3.32 mm/yr = 

0.0109 ft/yr) and the acceleration term, b, is set to match the 2000 to 2100 projected median 

GMSLR value of 1.94 ft of Kopp et al. (2014) for RCP 4.5/6.0 (see Figure 4.11). The polynomial 

equation produces results that are within 0.07 ft (2 cm) of the median RCP 4.5 projection of Kopp 

et al. (2014) throughout the century. 

Projections of future RSLR should be considered in design of coastal 

highways.
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Figure 4.11. Future SLR values for planning and design as per NCHRP 15-61: the RSLR 

corresponding with the GMSLR values shown by the circles on the lower panel are minimum 

values (see Table 4.1). The circles on the upper panel are higher values for more sensitive, or 

fragile, asset planning (see Table 4.2) (see Figure 4.9 caption for explanation of the other 

curves)
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The estimated values in Table 4.1 are consistent with the literature, including NCHRP 15-61 

recommendations and USACE guidance, and provide a simple, reasonable estimate of GMSLR 

for the design of most transportation facilities in the coastal environment. Use of such values 

aligns with asset management life-cycle planning and risk management planning requirements of 

23 CFR § 515.7(b) and 23 CFR § 515.7(c). Projects for which this minimum approach are 

appropriate may include parking lots and roadways for which the consequences of flooding are 

low or moderate as determined by the design team. 

Using the GMSLR values in Table 4.1 to estimate RSLR will result in higher values than the 

present-day rates from historical data. This assumption of a significant increase in RSLR this 

century is justified as use of the “best available science” at this time. The Kopp et al. (2014) 

projections are based on models that are similar to those of the IPCC which have shown some 

skill in modeling the SLR of the past century (Church et al. 2013). This science is expected to 

continue to evolve and improve rapidly over the next decade. Thus, the numerical values in this 

recommendation may change within this general logical framework. 

4.1.7.3 Consideration 3: Scientific and Engineering Uncertainties 

The third consideration is that practitioners should be aware of, and account for 

appropriately, the magnitude of the overall uncertainty in SLR projections. Figure 4.8 shows 

the uncertainty within all the Kopp et al. (2014) projections, at the 90% confidence level, to be 

roughly between 1 and 4 ft of GMSLR by 2100. This range, 1 to 4 ft, corresponds with the 

recommendations of Douglass et al. (2014) and USGCRP (2017). The significant spread of all 

the GMSLR curves shown in Figure 4.11 indicates the uncertainty range that should be 

considered in planning and design. This uncertainty is both scenario uncertainty, e.g. unknown

future emissions levels, and scientific uncertainty, e.g. the variation shown by the extent of the 

scenarios shown by the dashed lines on Figure 4.11. 

Engineers who fully appreciate this uncertainty should be able to account for it appropriately for 

their particular decision and situation. Civil engineers commonly deal with uncertainty in methods 

and models in a variety of other situations (e.g. Manning’s equation, the rational method, flood 

regression equations) and have developed a level of understanding of the scientific principles and 

the engineering science models of those processes. 

Traditionally, water resources engineers have used median estimates of flood statistics and 

developed appropriately conservative designs through an awareness of the level of uncertainty in 

those values. Also, coastal engineers typically use analysis tools and inputs which are not 

“conservative” by themselves. It is incumbent on the engineer to address the appropriate factor 

of safety at some point in the design process. 

Specific approaches for handling the uncertainty in RSLR projection include robust decision 

frameworks and/or adaptive management frameworks appropriate for deeply uncertain contexts: 

• evaluating the impacts of a full range of SLR scenarios, 

• adopting a “minimize regret” viewpoint that uses higher SLR values such as the 95th

percentile for more expensive facilities that might be sensitive (i.e. fragile) to SLRdependent process (e.g. depth-limited waves), 

• analyzing the full life cycle cost, 

RSLR values that correspond to the GMSLR values in Table 4.1 (and the lower 

panel of Figure 4.11) are the minimum suggested values for design.
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• using higher scenarios similar to the use of larger storms as design “check events” are 

used for bridge scour calculations, and 

• designing systems that can be adapted in the future to higher sea levels. 

Projections which reach about 4 ft of GMSLR this century are recommended as higher values to 

be considered for planning and design of high-value assets which are sensitive (i.e. fragile) to 

RSLR. The corresponding decadal values are shown in Table 4.2 and are plotted in Figure 4.11

(circles in upper panel). The values in Table 4.2 are developed using Equation 4.1 with the 

acceleration term, b, is set to match the 2000 to 2100 projected 95th-percentile GMSLR value of 

3.97 ft of Kopp et al. (2014) for RCP 8.5 (see Figure 4.11). There is a 95% chance that GMSLR 

will be less than these values even if society follows the more extreme emissions scenario of RCP 

8.5. 

Table 4.2 Higher projections of GMSLR for consideration in the planning and design of highvalue assets which are sensitive to SLR as per NCHRP 15-61 (See circles on upper panel of 

Figure 4.11, values are relative to 2000). 

Units 2020 2030 2040 2050 2060 2070 2080 2090 2100

m 0.10 0.18 0.27 0.38 0.51 0.66 0.83 1.01 1.21

ft 0.33 0.58 0.89 1.26 1.69 2.17 2.71 3.31 3.97

In alignment with establishing a risk management plan under 23 CFR § 515.7(c)(1) [i.e., risk to 

bridges on the National Highway System (NHS) from current and future conditions], a SDOT may 

consider this higher range of GMSLR estimates as a logical element of their life-cycle analyses. 

For example, the design team may consider NHS bridges that will be damaged by wave loads or 

NHS pavement damages from storm surge and overtopping. However, as described earlier, the 

use of these Table 4.2 values are voluntary for purposes of compliance. 

“A rise of as much as 8 ft by 2100 cannot be ruled out” (USGCRP 2017). Investigators have long 

recognized the possibility of higher rates (Church et al. 2013, Parris et al. 2012, Melillo et al. 

2014). A variety of other projections suggest higher upper limits to the projected GMSLR by 2100 

(e.g. Vermeer and Rahmstorf 2009). In particular, DeConto and Pollard (2016) added a new 

model of the physics of ice sheet melting processes to the process-based approach of Kopp et 

al. (2014). They found the potential for increases in GMSLR over the next century. Some of the 

assumptions inherent in these ice sheet dynamics models and their use in determining “best 

estimates” are an open scientific question (see Kopp et al. 2017). 

The engineer and planner should consider all the higher, scientifically plausible, GMSLR rates 

but this may raise other related infrastructure issues which may be beyond the typical 

responsibility of the transportation engineer. These planning issues may include changes in land 

use, inhabitability, and the overall need for transportation services. Elevations of the surrounding 

area and infrastructure should also be considered in the context of RSLR. It may do limited good 

to elevate a highway asset beyond some threshold unless the adjacent system of roads is also 

elevated or otherwise adapted. Planning decisions often consider the region or corridor. 

Therefore, a prudent approach may be to consider RSLR early in the planning process, well 

before project development or final design. 

All investigators conclude that the values of SLR projections through the mid-century, 2050, are 

very similar. For example, Figure 4.8 shows that the median expected value by 2050 for the three 

RCPs only varies from 0.82 ft to 0.95 ft, a total of 11⁄2 inches. Even the higher projections of 

DeConto and Pollard (2016) only add up to another 11⁄2 inches to those 2050 values. Thus, the 
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engineer and planner tasked with making assumptions about future SLR can have 

confidence that the uncertainty in projections is minimal through the mid-century. 

Likewise, these projections for design past mid-century should be used with care since the 

scientific uncertainty is very large. 

The FHWA expects that future editions of HICE will update these SLR considerations over time 

in accordance with the best available science. See Kilgore et al. (2019a and 2019b) for more 

background on these SLR design recommendations. 

4.1.8 Example Future RSLR Calculation: Atlantic City

This section presents an example calculation of projected future sea level for a specific project 

location near Atlantic City, New Jersey. The planning horizon for the project is mid-century (2050) 

and the asset is assumed to be somewhat insensitive to water level, e.g. exceedance of the RSLR 

estimate will not destroy the asset. 

Two similar methods for this RSLR example calculation are presented here. The first uses Table 

4.1 with location-specific data available from the NOAA Tides and Currents website to account 

for the difference between GMSLR and RSLR. The second uses one of the SLR Interagency Task 

Force scenarios and the corresponding RSLR listed directly in that data report (these values are 

also available in the online USACE 2017 SLR Calculator).

For the example location, the historical RSLR data are inspected first. Figure 4.12 shows the 

monthly average sea levels at the Atlantic City tide gage. The vertical axis, the y-axis, on the 

NOAA plot is the sea level with the zero value the average sea level from 1983 to 2001. This is 

the most recent National Tidal Datum Epoch (see Section 4.2 for discussion of tidal epochs). 

Figure 4.12 shows the long-term, historical linear RSLR rate at the Atlantic City tide gage has 

been 4.08 mm/yr (1.34 ft/century). 

Figure 4.12. Relative sea level rise (RSLR) trend for Atlantic City, New Jersey (image 

downloaded from NOAA Tides and Currents website September 10, 2018)

Since GMSLR is projected to increase this century, the RSLR rates will increase from this value. 

The commonly assumed GMSLR for the period of record of tide gages in the US is 1.7 mm/yr 

(IPCC 2007). This value can be used to estimate the “local’ component of SLR that includes the 

VLM at Atlantic City (and all “local” or regional oceanic SLR components) as: 

4.08 mm/yr - 1.7 mm/yr = 2.38 mm/yr (7.8 x 10-3 ft/yr)
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Using this value gives a total of the “local” component of RSLR for the 50-year planning horizon 

(from 2000) of: 

(50 yr) (7.8 x 10-3 ft/yr) = 0.39 ft

Adding the projected GMSLR from Table 4.1, which is 0.85 ft for 2050, the total RSLR projection 

is: 

0.39 ft + 0.85 ft = 1.24 ft. 

This final result is the projected RSLR at a location near Atlantic City between 2000 and 2050. In 

summary, the first method in this RSLR example projects the average annual sea level near 

Atlantic City will be 1.2 ft higher in 2050 than it was in 2000 (rounding off to 2 significant digits). 

The SLR Interagency Task Force scenarios can be used as an alternative method. Since the 

facility is assumed not to be particularly sensitive to water level, the RSLR corresponding to the 

Intermediate-Low scenario will be used. This Intermediate-Low scenario has GMSLR values close 

to those of the Kopp RCP 4.5/6.0 median projection values, and close to the values of Table 4.1 

(see Figure 4.11). There are two equivalent ways to find the Intermediate-Low scenario RSLR 

values for Atlantic City. 

One way is to use the tabular data released with the SLR Interagency Task Force report (Sweet 

et al. 2017c). The output data table from that report can be downloaded into a spreadsheet for 

easy viewing. It presents RSLR for every tide gage location in the US with the “MED,” “LOW,” and 

“HIGH” values corresponding to each of the six GMSLR scenarios (0.3, 0.5, 1.0, 1.5, 2.0, and 2.5 

m by 2100) where the “MED” value is the median of the projection. The “LOW” and “HIGH” values 

are the +/- one standard deviation or 17th and 83rd percentiles of the spread about the “MED” 

value. The “0.5 – MED” value for Atlantic City for RSLR in 2050 in that data table is 36 cm (1.18 

ft). 

A second but equivalent way to find this value is with the USACE Sea-Level Change Curve 

Calculator (USACE 2019). Selecting the Atlantic City, NJ gage and the “NOAA et al. 2017” as the 

Scenarios Source, produces a table with the RSLR at that location for all the SLR Interagency 

Task Force scenarios for every 10 years in this century. For 2050, the “NOAA-2017 Int-Low” value 

is 1.18 ft. 

This result, 1.18 ft, is the projected RSLR at a location near Atlantic City between 2000 and 2050 

using the second approach for this example calculation. 

The difference between the results of the two alternative methods, 0.06 ft, less than 3⁄4 of an inch, 

is well within the uncertainty of the methods and well within the natural variability of sea level from 

year to year. The difference is primarily because the values in Table 4.1 are slightly higher than 

the Intermediate-Low scenario (see Figure 4.11). Section 4.2 discusses how to relate these RSLR 

estimates to survey datums and how to relate them to different MSL time periods. 

This RSLR by 2050, 1.2 ft, may cause significant increases in frequency of flooding as well as 

depth of flooding in Atlantic City (see Chapter 9). Note that this value should not be considered a 

“high” or “conservative” projection since it is based on Table 4.1. It is the median RSLR estimate 

of the mid-range scenarios. 

4.2 Astronomical Tides

The tide is the slow rise and fall of the ocean waters in response to the gravitational pull of the 

moon and the sun. The tide is essentially a very long ocean wave with a wave period (or tidal 

period) of 12.4 hours. The usual interval between successive high tides is 12.4 hours as the arrival 
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of the crests of these waves represent high tide. The moon exerts a greater influence on the tides 

than does the sun. 

The astronomical tide can be predicted for any time at many locations. Tidal predictions are well 

understood even by most coastal residents and are often included in local daily newspapers and 

weather forecasts. NOAA provides on-line tidal forecasts as well as other information sea levels 

around the nation on their Tides and Currents web-site (NOAA 2019). Along most coasts the 

astronomical tide forecasts are within 1 ft of the actual tide elevation 90% of the time. The 

difference between the forecasts and actual water elevation measurements is normally a result 

of weather-related phenomena (e.g. wind blowing from same direction over some period, i.e. a 

storm surge). Understanding some of the characteristics of tides is helpful in understanding some 

of the terminology used to define tides and tidal datums. 

4.2.1 Characteristics of Astronomical Tides

In most locations in the US, two high tides and two low tides occur every “tidal day” (24.8 hours). 

These are called “semidiurnal” tides (see Figure 4.13). At many locations, the two high tides 

that occur each day are roughly of the same elevation. But at many other locations, there is a 

“mixed tide” with a clear “diurnal inequality” in the high tides as one is significantly higher than the 

other. Some places, like portions of the Gulf of Mexico, have only one high tide and one low tide 

per day. These tides are called “diurnal” tides. 

Large differences in tide range occur at the same location throughout the month. The highest 

tides which occur at intervals of half a lunar month are called “spring tides.” A lunar month is 

roughly 29.5 days. They occur at or near the time when the moon is new or full, i.e. when the sun, 

moon and earth fall in-line, and the tide generation forces of the moon and sun are additive. When 

the tide range is at its lowest during the lunar month, the “neap tides” occur. 

Large differences in the magnitude of the daily tide range occur at different locations in the US. 

Interactions of the oceanic tidal motions with the continental land mass, as well as the depths and 

shape of coastal bays and shelves, cause these differences. At Anchorage, Alaska, the tide range 

can vary up to almost 30 ft between high and low tide. At Pensacola, Florida, however the range 

can be less than 2 ft throughout a day. These differences in tidal range can occur within short 

distances along the coast and up bays. For example, the average tide range at Sandy Hook, New 

Jersey is about 5 ft but just 125 miles away at Montauk Point, New York, it is only 2 ft. 

The basic astronomical tide producing forces go through a “tidal epoch,” a cycle that lasts 

approximately 18.6 years. Thus, water level statistics related to tides, such as mean sea level, 

are computed by averaging over a complete epoch. 

Most of the US coast has “semi-diurnal” tides... two highs and two lows a day.
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Figure 4.13. Basic definitions of tides (image downloaded from http://tidesandcurrents.noaa.gov) 

4.2.2 Tidal and Survey Datums

The distinction between tidal datums and surveying datums can be important in the design, 

construction, and operation of engineering works near the coast. Tidal datums are vertical datums 

based on the epoch-averaged tide levels at a specific location. Tidal datums are based on actual 

measurements at a specific tide gage. Since sea level is changing over the long-term, the tidal 

datums are re-established after every tidal epoch. The most recent, complete tidal epoch ended

in 2001 and NOAA’s National Ocean Survey established the tidal datums for most of the US tide 

gage locations for that 1983-2001 tidal epoch (called the National Tidal Datum Epoch).

Presumably, they will be revising the tidal datums sometime after 2020. 

There are a number of tidal datums. The mean high water datum (MHW) is the average, over an 

18.6-year tidal epoch, of the high water elevations at a specific location. The mean higher high 

water datum (MHHW) is the average of the higher high water elevations. The difference between 

these two datums, MHW and MHHW, is greatest at locations with the greatest “diurnal inequality” 

in high tides during a typical day. Likewise, the mean low water datum (MLW) is an average of 

the low tide elevations and the mean lower low water datum (MLLW) is an average of the lower 

low tide elevations. MLLW is the basis for most navigation charts because it provides mariners 

with a consistent, somewhat conservative, estimate of the depth. The local mean sea level datum 

(MSL) is the average of all the observations of water level over a tidal epoch. 

Survey datums are specified for geodetic surveying and set by the NOAA’s National Geodetic 

Survey. The National Geodetic Vertical Datum of 1929 (NGVD 29) and the North American 
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Vertical Datum of 1988 (NAVD 88) are two primary vertical survey datums used in the US. The 

older NGVD 29 geodetic datum was originally established using estimates of mean sea level at 

26 tide gages around the nation. Thus, it was often referred to as just “mean sea level.” 

However, it has long been recognized that it was not actually the MSL because MSL changes 

gradually through time and survey datums do not. The National Geodetic Survey has not 

called NGVD 29 the “mean sea level” for decades. NAVD 88 was an improvement to the NVGD 

29 and has now replaced it as the primary vertical datum for surveying. It normally will be near

the MSL at the open coast but it is not the MSL. 

The relationship between the survey datum, NAVD 88, and the tidal datums, e.g. MSL or MLLW, 

has been calculated by the NOAA National Ocean Service for many of the tide gages around the 

US. An example is shown in Figure 4.14 using the values for Charleston, South Carolina. The 

mean sea level (1983 to 2001) at Charleston, SC is -0.22 ft NAVD 88. This relationship is not the 

same at other locations around the nation. NOAA provides and supports VDATUM, software for 

converting between all datums at locations around the US.2 

Figure 4.14. An example of the relationship between the survey and tidal datums 

Beginning in 2022 a new vertical datum will replace NAVD88 (NGS 2018). NOAA’s National 

Geodetic Survey plans on replacing NAVD88 with the North American-Pacific Geopotential 

 

2 https://vdatum.noaa.gov/ (accessed September 12, 2018)

NAVD 88 is a fixed survey datum that is not MSL. It is generally near MSL, and 

the relationship is always changing because of RSLR.
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Datum of 2022, NAPGD2022, to address shortcomings in the existing datum system. The new 

datum will be easier to access with modern Global Positioning Systems (GPS). Figure 4.15

summarizes the numerical differences between the two vertical datums. They will vary 

significantly across the US, with only a slight difference in elevation in portions of the Southern 

US and up to 4 ft of difference in some areas of the Pacific Northwest. More information about 

the progress toward replacing switching this datum can be found on the National Geodetic Survey 

website (NGS 2018). 

Figure 4.15. Approximate predicted change from NAVD 88 to the new vertical datum which will 

replace it in 2022 (adapted from NGS 2018)

4.3 Storm Surge

Storm surge is the rise of the water level above the astronomical tide as a result of weather. This 

is primarily wind but also includes the barometric pressure and rainfall runoff. Storm surge, also 

sometimes called storm tide, can be negative, i.e. winds can decrease water levels (but that is 

typically not a problem for highways in the coastal environment). Storm surge is highly influenced 

by geography including the shape of the coast and its bays, the nearshore bathymetry, and the 

flooded topography. High storm surges occur along the coast where the landmass stops the 

hydrodynamic movements in areas with broad shallow nearshore bathymetry. The highest storm 

surge can occur in bays. Wind affects storm surge by placing a stress on the water surface, by 

generating oceanic currents and by generating waves. Breaking waves can contribute to storm 

surge by adding a component of mean water surface elevation called wave setup. Storm surge is 

an important coastal process for the design of coastal infrastructure primarily because it increases 

the design still water level (SWL) and allows waves to attack higher elevations. Surge also can 

be an important component in tidal inlet hydrodynamics. 

4.3.1 Characteristics of Storm Surge

Figure 4.16 is an example of hurricane storm surge. The predicted tide is plotted along with 

measurements from a tide gage located on a pier in the Gulf of Mexico during passage of a 

hurricane. The surge, the difference between the predicted and actual water level, extends for 

several days with a very dramatic peak of over 7 ft above the predicted high tide early on August 
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18. That high peak corresponds with the time that the hurricane made landfall with its eye just to 

the southwest of the tide gage. The tide gage was on the right side of the hurricane’s circulation 

where winds are highest and onshore.

Figure 4.16. Storm surge at Galveston, Texas, from Hurricane Alicia in 1983

The hydrograph of a coastal storm surge is usually considered as the time variation of water 

surface elevation at a specific location (Figure 4.16). Both the magnitude and duration of a coastal 

storm surge can be important. During Hurricane Katrina in 2005, storm surge along much of the 

coast near Bay St. Louis, Mississippi resulted in a rise of water level 27 ft higher than the predicted 

tide elevation (Douglass et al. 2006a; FEMA 2006). This storm surge was unprecedented in US 

history. This storm surge water level exceeded the previous record level of 21 ft, a result of 

Hurricane Camille in 1969 along this same stretch of coast. 

Another of the most destructive storms in US history, the Nor'easter Ash Wednesday Storm of 

1962, caused much of its damage due to its relatively long duration. The storm surge lasted for 

21⁄2 days over five semi-diurnal high tides, or “five high-tides.” This long duration allowed beach 

storm erosional processes to act that long and cause extensive property damage along the 

Atlantic Coast. 

4.3.2 Storm Surge Modeling

Storm surge can be modeled well with modern coastal hydrodynamic modeling techniques. Webb 

(2017) is an introductory “primer” on the broader field of coastal hydrodynamic modeling 

specifically written for transportation engineering professionals. It explains when, why, and 

at what level use of coastal models is beneficial in the planning and design of coastal highways 

as well as when it is appropriate to solicit the expertise of a coastal engineer. It provides 

transportation professionals with the information needed to determine scopes of work, prepare 

requests for professional services, communicate with consultants, and evaluate modeling 

approaches and results. Topics include modeling basics, preparing model input, interpreting input 
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and output, calibration and validation, model coupling, and the use of models in coastal 

vulnerability assessments. Written for the layperson, it is an introduction to the broader field of

coastal modeling, i.e. not just storm surge. 

Webb (2017) includes detailed information on the selection and use of riverine hydraulic and 

coastal hydrodynamic models. These two types of models are related and generally solve the 

same types of equations that describe fluid motion, but they are not all applicable to the same 

conditions and locations. The decision to employ coastal hydrodynamic modeling, riverine 

hydraulic modeling, or some combination of the two is complex, constrained in part by the 

proximity of the subject site to the coast and the criticality of the asset. Figure 4.17 graphically 

describes the function of a site's coastal proximity and criticality. The closer a site is to the coast 

(and by extension, coastal hazards such as storm surge and wave attack) and the more important 

the infrastructure or roadway is, the more likely coastal hydrodynamic modeling will be needed in 

analysis and design. 

The numerical modeling of coastal hydrodynamics is based on solving the fundamental fluid 

mechanics of motion, the continuity equation and the momentum equation, in a manner that is 

most efficient and appropriate for the problem. A rich history of this modeling has developed over 

the past forty years in both the nearshore physical oceanography and coastal engineering 

research communities. Different formulations of the equations and different solution algorithms 

have been applied. Much of the research and development of these models was done with funding 

from federal agencies with coastal interests including NOAA, FEMA, and USACE. 

Table 4.3 lists models discussed in this manual, along with a brief comment as to the type of 

possible application/use in the design and planning of coastal infrastructure. Engineers and 

scientists with specialized training in these coastal models should be included in teams applying 

them to assess transportation infrastructure. The specific use of some of these models is 

discussed throughout this manual. 

One of the more commonly used models which can estimate storm surge well is the 

ADvanced CIRCulation model (ADCIRC). ADCIRC was originally developed for the USACE as 

a hydrodynamic circulation model for coastal waters (Luettich et al. 1992; Westerink et al. 1994). 

It now has an active research and application community in academia, agencies, and consulting. 

The ADCIRC program is actually a collection of numerical models that can be used to simulate 

tidal flows, water levels, and constituent transport (e.g. salt, larvae, contaminants, etc.) in two or 

three dimensions throughout the water column. However, its two-dimensional version is used 

most commonly for storm surge. These programs utilize the finite element method in space 

allowing the use of highly flexible, unstructured, high-resolution grids. Because of its skill and 

ability to provide excellent results at a high resolution, ADCIRC is sometimes referred to as a 

“high fidelity” model (see Webb 2017 for more explanation of modeling terminology). 

Storm surge can be simulated well with modern coastal hydrodynamic models.
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Figure 4.17. When and where to apply riverine hydraulic and coastal hydrodynamic models as a 

function of distance from the coast and importance of infrastructure (Webb 2017)

Table 4.3. Numerical coastal models cited in this manual

Model/Program Comments

ADCIRC Hydrodynamic model (often used to model storm surge)

SLOSH Storm surge model 

ET-SURGE Storm surge forecasting model

DELFT-3D Hydrodynamics, waves, and morphology model 

MIKE-21 Hydrodynamics, waves, and morphology model

FVCOM Hydrodynamic model

WAM Wave model

STWAVE Wave model

CMS Hydrodynamics, waves, and morphology model

SWAN Wave model

CH3D Hydrodynamic model

EDUNE Dune erosion model 

SBEACH Cross-shore morphology model

XBEACH Hydrodynamics, waves, and morphology model

CSHORE Cross-shore wave and morphology model

CHAMPS Cross-shore wave and morphology model
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Although ADCIRC does not explicitly model wind waves, it can incorporate their effects on storm 

surge. Gradients in wave momentum can be specified as additional model inputs which are 

obtained from a separate wave model. Appropriate coupling between ADCIRC and a numerical 

wave model yields accurate storm surge elevations which include wave setup effects (Dietrich et 

al. 2011). Central to its use in modeling storm surge, the ADCIRC model includes Holland-type 

(Holland 1980) parameterization of tropical storms based on general storm characteristics like 

central pressure, maximum wind speed, radius to maximum winds, etc. These models, including 

some significant enhancements inherent to ADCIRC, make it a particularly useful tool for 

simulating peak storm water levels in tropical storms. At the time of producing this manual, typical 

practice of the USACE and other entities is to apply ADCIRC in many coastal surge modeling 

efforts (USACE 2011b, 2015, FEMA 2011, Webb 2017, CERA 2018). Aligning with such efforts 

(and being able to readily use available model inputs and outputs) makes the ADCIRC model a 

strong candidate for the higher levels of effort in vulnerability assessments (as discussed in 

Chapter 14: Analysis Methods for Assessing Vulnerability to Coastal Storms). 

Numerous hydrodynamic models can be used to estimate storm surge water levels. While their 

details vary, they all typically solve some similar set of governing conservation equations for mass 

and momentum, accept similar types of boundary conditions and forcing, and do not explicitly 

model the waves. One well-known example, the Sea, Lake, and Overland Surges from Hurricanes 

(SLOSH) model, was developed by the National Weather Service (Jelesnianski et al. 1992; Glahn 

et al. 2009). The SLOSH model has been used for years to develop coastal Storm Surge 

Inundation Maps (SSIMs) primarily for input to emergency evacuation decisions. Inherent 

limitations in SLOSH have led many investigators toward other models like ADCIRC for detailed 

mapping of coastal flooding. The SLOSH model accounts for a local balance between the sea 

surface slope induced by the wind stress and the restorative forces of gravity and Coriolis and 

bottom drag, but it does not include some of the governing physics explicitly such as momentum 

acceleration effects and wave setup effects. These can be important components of storm surge.

Other issues relate to the ability of SLOSH grids to highly resolve large areas adequately. 

Other storm surge models include the extratropical storm surge model ET-SURGE (or ETSS) 

developed by NOAA as an operational forecast tool providing surge guidance for extratropical 

storms like nor’easters. The ET-SURGE model is a variation of SLOSH forced by real-time output 

of winds and pressures from NOAAs National Centers for Environmental Prediction’s Global 

Forecast System (Kim et al. 1996; Glahn et al. 2009). Other hydrodynamic models which can be 

used for storm surge modeling include the Curvilinear Hydrodynamics Three-Dimensional (CH3D) 

model (Sheng 1990; Johnson et al. 1991), the DELFT3D model (Vatvani et al. 2012), the MIKE21 Model (Savioli et al. 2003) and the Finite Volume Coastal Ocean Model (FVCOM; Chen et al.

2006). These models offer fully three-dimensional results of extended coastal ocean processes. 

Others, like the USACE Coastal Modeling System (CMS) strictly provide two dimensional (depthintegrated) solutions (Buttolph et al. 2006). 

An example of a numerical mesh used for estimating storm surge in the Atlantic basin is shown 

in Figure 4.18. It has over 400,000 nodes and covers the entire Gulf of Mexico, Caribbean Sea, 

and much of the western North Atlantic Ocean. The spacing of the nodes is very large in the open 

ocean and so small near the coast that they can’t been seen in Figure 4.18 except as solid blue. 

The extensive coverage is considered necessary to adequately capture the large-scale aspects 

of hurricane storm surge. 
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Figure 4.18. Example of numerical mesh used for a storm surge model

Along the coast, the numerical storm surge model mesh should include upland areas to account 

for flooding. An example topographic-bathymetric mesh surface for a storm surge model of Mobile 

Bay, Alabama is shown in Figure 4.19. It includes both the water and the low-lying lands adjacent 

to the water. The topography shown includes all the areas with elevations below 50 ft. The 

extensive area in Figure 4.19 to the north of Mobile is the Mobile Delta area which is the historical 

river deltas of the Tombigbee and Alabama Rivers. 
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Figure 4.19. Detail of a seamless topographic-bathymetric mesh surface used for modeling 

coastal storm surge flooding

An example of output from a storm surge model is shown in Figure 4.20. This output is from a 

“hindcast” of Hurricane Katrina (2005) for the Alabama coast. Hindcasting is the use of a model 

to simulate a past time period and is often used to validate that a model is working. The 

colors represent the maximum water surface elevation during the passage of the storm. Modelled 

surge elevations exceeded 12 ft in the northern end of Mobile Bay but much less at the southern 

end of the bay. These values are consistent with the actual surge values in the bay during passage 

of the storm. The strong south winds were causing significant wind setup of the water surface in 

the bay, and the numerical model captures that well. The highest surge values shown are at the 

western boundary of this hindcast in coastal Mississippi. Hurricane Katrina was making landfall 

near the Mississippi-Louisiana border, roughly 100 miles to the west of the mouth of Mobile Bay 

at this time. 
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Figure 4.20. Example of storm surge model output (with input winds) along the Alabama coast 

during Hurricane Katrina (2005) 

ADCIRC also produces good storm surge forecasts. The Coastal Emergency Risks 

Assessment system provides on-line storm surge forecasts at a high resolution whenever a 

hurricane is approaching the US.3 Forecast exposure of individual coastal transportation facility 

locations, roads and bridges, can be evaluated. The storm forecast (track, windspeed, forward 

speed, size, etc.) from the National Hurricane Center (NHC) is the primary input to the model. The 

surge forecasts change as the NHC storm track and strength forecasts change. The surge 

forecasts are limited by the uncertainty in the input storm parameter forecasts with additional 

scientific uncertainty inherent in the model. However, the surge forecasts provide good results 

due to their high resolution (CERA 2018). North Carolina DOT has integrated the on-line forecasts 

from CERA with their internal GIS-based asset management database and can visualize and 

quantify which facilities are at risk as hurricanes approach (NCDOT 2018).

One promising approach for both developing risk-based design storm data and for forecasting 

storms is the general approach called a “surrogate model.” Essentially, a high-fidelity model for 

storm surge, with coupled models for waves and runup, are run for the full range of possible storm 

characteristics with the results placed in a matrix. Queries of that matrix are a surrogate to actually 

running the models. The big advantage is run time. Thousands of hours of computational time for 

 

3 https://cera.coastalrisk.live/ website for the Coastal Emergency Risk Assessment program 

which is affiliated with LSU and UNC (accessed September 12, 2018)
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the real models are reduced to seconds in the surrogate model. Smith et al. (2012) and Taflanidis 

et al. (2013) describe the approach with a surrogate model developed for coastal storms in Hawaii. 

Most of the available storm surge models compute the velocity of flow in the incoming and 

outgoing surge. Thus, the velocity of flow, usually depth averaged, is a part of the output. These 

flow values can be important in some damage mechanisms including hydrodynamic forces on 

structures and bridge scour. However, the resolution and detail in the velocity estimates from 

these models depends on the resolution and detail in the numerical grid/mesh of the model. 

Individual building edges and road embankments can be appropriately accounted for in these

models. 

4.3.3 Wave Setup

A phenomenon called wave setup is also a component of storm surge at some locations. Wave 

setup occurs primarily in the surf zone and landward of the surf zone such as on a flooding barrier 

island or behind a reef. Wave setup is an increase in mean (time-averaged) water level at a 

location due to the presence of the waves. For example, average water levels on a beach can 

be higher than those at the end of a beach pier due to wave setup. Wave setup is due to the 

momentum flux of the water by wave action and is greatest when waves are breaking (Dean and 

Walton 2018). Wave setup increases with wave height and the magnitude of the contribution of 

wave setup to storm surge can be up to several feet at some locations during major coastal 

storms.

Some models, including ADCIRC, DELFT3D and MIKE 21, are capable of dynamically 

incorporating wave contributions to storm surge through momentum transfer and wave setup. 

Such models are said to be dynamically coupled passing the circulation and wave model results 

back and forth in order to model the effects of waves on storm surge and currents, as well as the 

effects of storm surge and currents on wave generation, transformation and breaking. 

4.3.4 Impacts of SLR on Storm Surge

This section discusses the likely effects of SLR on storm surges and other factors that could 

influence the strength, frequency and damage of coastal storms in future environmental 

conditions. 

Sea level rise can clearly increase flood depths along the coast due to storm surge. However, this 

is not a “one-to-one” or linear relationship. A one-foot rise in sea level will not necessarily produce 

a one-foot rise in storm surge elevation at most coastal locations. This is because of the complex 

physics of coastal storm surge inundation and wave propagation. Storm surge at any location for 

a specific storm is a complex function of the storm’s interaction with the ground elevations around 

that location. Those interactions will likely change somewhat as different areas will be flooded at 

different depths. A significant portion of onshore storm surge is due to wave setup which is very 

sensitive to local depth and ground slope. There will also, likely, be geomorphological changes 

along the coast, such as wetland changes. These will also affect the relationship between sea 

level rise and increased storm surge elevations.

Detailed methods for incorporating future sea levels into quantitative storm surge modeling are 

outlined in Chapter 14. Several recent, numerical, hydrodynamic model studies of storm surge 

under different sea level rise scenarios have demonstrated this “nonlinear” response of storm 

One foot of future sea level rise can increase storm flood levels more than one 

foot! There is a nonlinear relationship between sea level and flood level.
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surge (and also waves) to increased water levels (Smith et al. 2010; Choate et al. 2012; Hagen 

and Bacopoulos 2012; Atkinson et al. 2013). In some cases, and locations, particularly in shallow 

water near the coast, the incremental change in storm surge can be greater than the 

corresponding sea level rise. In other words, the storm surge increase will be amplified compared 

with the sea level rise. In other cases, and locations, the storm surge increase will be less than 

the sea level rise increment but still greater than a similar storm surge on present-day sea levels. 

There has been a substantial increase in hurricane activity in the Atlantic since the 1970s and 

these increases have been linked, in part, to higher sea surface temperatures (Melillo et al. 2014). 

There have long been concerns about increases in the strength of individual coastal storms as 

well as the frequency of, and damage produced by, coastal storms. However, in spite of these 

logical linkages, research on coastal storms provides varied conclusions. There are significant 

issues with the historical data record and other factors (Church et al. 2001; IPCC 2007; Knutson 

et al. 2010). 

Knutson et al. (2010) summarizes the research into each of these aspects (strength, frequency, 

damage) related to tropical storms in the future. Bender et al. (2010) have suggested, based on 

atmosphere-ocean models, that there may be an increase in the number of large hurricanes but 

a slight decrease in the overall frequency of all tropical storms in the north Atlantic. 

4.4 Coastal-Riverine Flooding

Some extreme flooding is related to the interaction of coastal storm surge and runoff from rainfall 

on watersheds near the coast. Combined hydraulic and hydrodynamic simulations of flow and 

water levels reveal that rainfall-runoff can significantly affect coastal water levels and that the 

presence of storm surge generally results in higher flood levels upstream. Important to these 

results are the topography of the watershed and an associated time lag between the rainfall event 

and the storm surge (Blumberg et al. 2015, Klerk et al. 2015, McGuigan et al. 2015, Torres et al.

2015). 

Coastal flooding can exacerbate upstream flooding, subsequently impacting bridges some 

distance from the coast. The elevated coastal water levels act as a downstream control for storm 

related rainfall runoff within coastal watersheds. Until the surge recedes, this runoff does not have 

anywhere else to go, increasing the backwater and flooding effects. This flooding may occur over 

some time, possibly more time than the storm surge duration. Additionally, the probability of 

exceedance of the resulting flood level may be much greater than the frequency of either the 

storm surge event or the rainfall event. In other words, a storm with a 10-year return period storm 

surge and a 15-year return period rainfall might combine to produce a 100-year return period flood 

event at a specific location. 

Consider the following example from the Texas coast during Hurricane Harvey in 2017. Figure 

4.21 shows measured water level heights from a NOAA tide gage located at Sabine Pass on the 

Texas Coast and from a USGS stream gage located on the Sabine River near Orange, TX. The 

USGS gage is approximately 30 miles from the coast. In order to aid in their comparison, the 

predicted tidal signal was removed from the measured water levels at Sabine Pass, resulting in a 

time-series of the non-tidal residual (NTR) water levels (NTR = Measured Water Level – Predicted 

Tide). In this manner, the NTR (gray line) represents all non-tidal contributions to measured water 

levels at the coast. A 24-hour moving average applied to the NTR, shown by the black dashed

line, shows the slowly-varying behavior of water levels at the coast. Note how the coastal surge 

peaks just before August 30, 2017 as Hurricane Harvey makes its second and final landfall along 

the Texas coast. Flooding at the USGS gage closely mimics the behavior of the downstream 

coastal stage. 
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Figure 4.21. Water level measurements near Sabine, Texas (upper panel) and the path of 

Hurricane Harvey (lower panel)
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As the storm passes to the east of the NOAA tide gage at Sabine Pass, north winds on the west

side of the eye push the surge offshore resulting in a lowering of the coastal water levels at the 

tide gage. However, water levels at the USGS gage on Sabine River continue to increase until 

September 3, 2017. This fluvial flooding drives an increase in the coastal stage that persists over 

a nearly two-week period from 9/1/17 to 9/13/17. This example demonstrates well how coastal 

flooding contributes to backwater effects and how fluvial discharge can impact downstream 

coastal flooding. 

When considered individually or as components, the peak flow resulting from storm surge tends 

be much higher than that from rainfall-runoff, owing to the intense flood and ebb of storm surge 

during a short duration storm event. However, the total flood volume draining toward the coast 

tends to be dominated by rainfall-runoff (Torres et al. 2015). 

Hydraulic design associated with peak flow (e.g. bridge foundation, culvert, drain, etc.) would 

likely be sensitive to the increased flows associated with the rushing flood or ebb of storm surge. 

Design that is most concerned with flow volumes (e.g. detention basins, routing reservoirs, 

stormwater wetlands, etc.) would be more sensitive to long-lasting or delayed drainage due to 

combined rainfall-runoff and storm surge.

Because this compound flooding is so dependent on location-specific characteristics, the joint 

probability of rainfall-runoff and storm surge bears consideration. Sea level rise is one major 

cause of increased coastal flooding, but sea level effects will likely increase the probability of 

compound events (surge and heavy precipitation), augmenting flooding potential in some areas 

(Wahl et al. 2015).

4.5 Lake Water Level Fluctuations

The Great Lakes, the Great Salt Lake, and other very large inland lakes are tideless. They are 

completely separated from the oceans and are too small for any astronomical tides of their own. 

Water levels in these large inland lakes have significant fluctuations however in response to 

rainfall/snowmelt in their drainage basins and winds associated with major weather events. 

Many of these very large lakes have their own local lake level datums that are used for science 

and engineering related to the water level. A bulletin describing lake levels for the Great Lakes is 

available from the Detroit District of the USACE on-line (USACE – Detroit District 2018). 

Figure 4.22 shows an example of seasonal fluctuations of Lakes Michigan and Huron. The lake 

levels generally rise in the spring and fall in the fall. Also shown is the 6-month forecast lake levels 

at the time this graphic was downloaded (September 2018). The lake levels for 2017 and 2018 

shown in Figure 4.22 are higher than the long-term average. However, some of the historical 

water level lows occurred as recently as 2012 and 2013. 

Figure 4.23 shows a much longer historical record of average annual lake levels. The dashed, 

horizontal line is the average for the entire period of record. There have been 5 to 6 ft of maximum 

lake level change in the past century. A 15-year period of low lake levels prior to 2013 was 

followed by an increase in lake levels that was unprecedented (Gronewold et al. 2015). 

High lake levels cause increased bluff erosion as waves strike higher elevations. Interestingly 

though, there are erosion issues related to the historically low water levels, too. The lower levels 

expose the base of seawalls to different levels of air, ice and waves. 
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Figure 4.22. Example of seasonal lake level fluctuations (USACE-Detroit District 2018) 

Lake levels depend on the hydrologic balance, including precipitation, runoff, evaporation and 

outflow. Most studies conclude that lake levels will fall (Hayhoe et al. 2010, Angel and Krunkel 

2010) but some question the level of reduction (Lofgren et al. 2011). Mackey (2012) presents a 

range of possible scenarios with reductions of 0.66 ft (0.2 m) to about 3.3 ft (1 m) by the end of 

the century. An interactive, on-line “Great Lakes Water Level Dashboard,” Smith et al. (2016), 

allows for graphical presentations/comparisons of the long-term lake level forecasts. The 

magnitude of the range in fluctuations is similar to long-term water-level changes in the lakes (e.g. 

33-yr, 60-yr, 160-yr fluctuations), making it difficult to differentiate between change and variability 

(Baedke and Thompson 2000; Baedke et al. 2004).
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Figure 4.23. Annual lake level fluctuations – Lakes Michigan and Huron (dashed line is the longterm average)

One interesting phenomenon is the “seiching” of the lakes. A seich is a long-period (several hours 

duration in the lakes) oscillation of the water levels after passage of a weather front. Lake Erie 

and Lake Ontario are particularly susceptible to these extreme storm surge levels and subsequent 

seiching motions. A storm in 1985 caused so much wind stress across Lake Erie that the water 

level at Toledo dropped 4 ft while the water level at Buffalo rose 4 ft. After passage of the front to 

the east, the lake levels oscillated back and forth for several days. The water levels rose and fell 

at opposite ends of the lake while the water level in the middle of the lake, near Cleveland, 

remained nearly constant. Investigations of seiching in Lakes Michigan and Huron reveal that the 

two systems are coupled and cannot be considered separately.

One phenomenon that occurs in the Great Lakes, as well as in all water bodies, is the 

meteotsunami. Barometric pressure fluctuations can combine with wind fields to cause wave 

oscillations that can propagate alongshore in response to the distant forcing. For tsunamis, the 

distant forcing is a submarine landslide and for meteotsunamis the forcing is a meteorological 

disturbance on the water surface such as very quick increases in winds or a pressure disturbanceanomaly related to a weather front (Monserrat et al. 2006). These can explain freak wave 

occurrences that have occurred in the Great Lakes (Anderson et al. 2012) and along the Atlantic 

Coast where they are also called squall line surges (Dean and Dalrymple 2002). 

4.6 Design Water Levels

The selection of a design water level can be one of the most important coastal engineering 

decisions for the designs and structures discussed in Part 3 of this document. For example, the 

design still water level (SWL) often controls the design wave height, stone size and extent of 

armoring on coastal revetments. Also, wave loads on elevated bridge decks are extremely 

sensitive to water level. Essentially, the water level dictates the reach and the power of waves.

Typical practice in hydraulic engineering addresses design water level decisions using the 

traditional, risk-based approach of a "design return period." To illustrate, the FHWA regulation 23 

CFR § 650.105(b) defines the “base flood” as “mean the flood or tide having a 1-percent chance 

of being exceeded in any given year.” As “tide” indicates the storm tide (or storm surge), for HICE, 
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the "100-year storm surge level" is the surge elevation with a 1%-annual probability of 

exceedance. Each year, there is a 1% chance that a storm surge of this magnitude (or greater) 

will occur. Some coastal designs may justify a lower return period (e.g. 25- year or 50-year) in 

certain areas – balancing the greater risks affiliated with such design with engineering and 

economic considerations. Chapter 13 Engineering Risk at the Coast discusses these fundamental 

risk-based design concepts. 

The many potential sources of coastal water level data include federal agencies, state agencies, 

universities, and private consultants. This section describes only a limited subset of potential data 

sources. 

Obtaining water level data for the purpose of engineering design has long been a challenge 

especially in coastal regions. These existing sources of measured and analyzed data tend to be 

far from where they are needed for transportation design. Some form of modeling, ranging from 

simple to complex, is typically needed in order to overcome the geographic gap between where 

data exist and where they are needed. In recent years, the availability of coastal hazard data, 

including storm surge water levels, has improved greatly. 

The USACE's Coastal Hazards System is an online database for water levels and other “hazards” 

including waves. As a component of the North Atlantic Coast Comprehensive Study funded in the 

aftermath of Hurricane Sandy, the USACE initiated development of this system which contains 

high resolution hazard data in coastal regions around the US including the Great Lakes (USACE 

2019). This Coastal Hazards System database contains densely spaced points of hydrodynamic 

“data” along the coast in open water and on the coastal floodplain. These data are the output from 

hydrodynamic models. The density, location, and data available at each point make this dataset

useful for coastal transportation projects. Most data points provide water level, depth-averaged 

water velocity, wave height, wave period, wave direction, wind velocity, and atmospheric pressure 

time-series and/or representative statistics for each of the synthetic storms (hundreds or 

thousands) simulated with storm surge and wave models. Annual exceedance probabilities 

(AEPs) for water levels, wave height, and wave period ranging from 0.1 percent to 10 percent 

(10-, 20-, 50-, 100-, 200-, 500-, and 1,000-yr return periods) are available. In addition to the 

expected value AEP for each parameter, the 68 percent, 85 percent, 95 percent, and 98 percent 

confidence limits are also given. 

Consider an example from the Texas coast shown in Figure 4.24. That figure shows the expected 

AEP and associated 98 percent confidence interval for water level, significant wave height, and 

peak wave period for a point near the CR 257 bridge that crosses San Luis Pass between 

Galveston and Follets Islands. For example, the 50-year return period water level is slightly higher 

than 3 m (10 ft). 

FEMA, as part of their flood insurance mapping mission, has estimated 100-year flood levels and 

areas of subsequent inundation along much of the US coast. However, the techniques used to 

develop these coastal FEMA products are constantly evolving. The accuracy of the FEMA results 

can be limited and should be evaluated carefully before use in design and planning of coastal 

transportation facilities. 

Some of the more recent FEMA coastal modeling efforts related to mapping have used state-ofthe-art surge models including high resolution ADCIRC modeling. FEMA documentation specifies

these coastal products are for flood mapping for insurance purposes only. While similar FEMA 

The USACE Coastal Hazards System is a useful online source for design water 

level information.
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products are commonly used for upland riverine flood engineering analysis and flood plain 

regulation by local government, those riverine products are based on the same basic technologies 

(regression analysis for runoff and the standard back-step model for backwater computations) 

that have been used successfully for riverine flooding for decades. 

Many emergency management agencies have coastal inundation maps that are based on results 

from hydrodynamic models. The SLOSH model is usually used to estimate the worst possible 

flood level for each of the Saffir-Simpson scale storm categories. These may provide an estimate 

of extremely rare storms but do not provide risk-based information for design. Some USACE 

Districts have developed their own water level-return period relationship for design at many 

coastal locations. Some State resource management agencies, e.g. Florida's Department of 

Environmental Protection, have developed estimates of surge-return period relationships along 

the coast. 

Figure 4.24. An example of water level, wave height and wave period values near San Luis 

Pass, Texas obtained from the USACE Coastal Hazards System database

All available estimates of the surge-return period relationship should be collocated and evaluated 

carefully before use in design. Available estimates are often not adequate for design of sitespecific coastal works and the judgment/review of an experienced, qualified coastal engineer can 

be valuable. The Florida DOT has researched application of such analyses and developed a 

protocol that may be useful for others to review and adopt (Sheppard and Miller 2003). 

The literature provides examples of a probabilistic, numerical approach; using a hydrodynamic 

model for storm surge simulations and historical storm information; for application at major 
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infrastructure studies and projects (e.g. USACE 2015, Bosma et al. 2015). The model should be 

calibrated appropriately. Input storm conditions for historical hurricanes for the past 150 years are 

available from the NOAA HURDAT database. There are two general approaches to assigning the 

proper probability to historical storms and other "hypothetical storms;" 1) the Joint Probability 

Method (JPM) which is typically used by FEMA in their coastal flood studies (now usually with an 

optimal sampling scheme), and 2) the Empirical Simulation Technique (EST) which was 

developed by the USACE to develop site-specific water level-return period relationships (USACE 

2002). This type of analysis likely necessitates the integration of a qualified, coastal engineer or 

scientist into the design team. 

None of the available data for determining design water levels mentioned above includes 

the effects of future sea level rise (SLR). Chapter 14 – “Analysis Methods for Assessing 

Vulnerability to Extreme Coastal Events” presents techniques for including future SLR in design 

water levels. 
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Chapter 5 - Waves

Waves, one of the primary hydraulic forces affecting our coasts, are caused by a disturbance of 

the water surface. The original disturbance may be caused by winds, boats or ships (wakes), or 

other disturbances such as underwater landslides due to earthquakes (tsunamis). But wind 

generates most of the waves that strike our coasts. After waves are formed, they can propagate 

across the surface of the sea for thousands of miles. When waves break on a shoreline or coastal 

structure, they have fluid velocities and accelerations that can impart tremendous forces. When 

riding on top of storm surge, waves can strike roads and bridges that are typically not designed 

for such forces.

Practical wave mechanics is a blend of theories and empirical evidence. Several wave theories, 

including the small-amplitude wave theory, and Stokes 2nd order wave theory developed in the 

late 1800s, are still used today. Much of the practical scientific study of coastal waves changed 

during World War II. Plans for amphibious landings such as at Normandy on D-Day and on the 

Pacific Islands later in the war needed the highest possible quality predictions of the surf 

conditions that the landing craft could expect. Research led to equations for forecasting wave 

heights based on wind speeds as well as equations for estimating how waves break in shallow 

water. That research revolutionized nearshore oceanography and led to predecessors of the 

coastal engineering tools still used today and that this manual briefly summarizes below. 

5.1 Definitions, Theories, and Properties of Waves

This section introduces the basic definitions used in wave mechanics, some of the more useful 

engineering properties of waves, and in brief, several of the most important wave theories. Many 

engineering applications of wave theories rely on the small-amplitude wave theory. However, 

several important engineering properties can only be explained by more complex theories or by 

empirical methods.

Figure 5.1 depicts the basic parameter definitions in the simplest model of water waves. The wave 

in Figure 5.1 is assumed to be progressing toward the right and the individual waves are longcrested (such that the 2-dimensional plane shown in Figure 5.1 is sufficient) and part of an infinite 

train of repeating waves. The basic length scales used to define the wave are the wavelength (L) 

defined as the distance between wave crests, and the wave height (H) defined as the difference 

between the elevation of the crest and the trough of an individual wave. Waves are called 

monochromatic waves in this simplest model since the waves are all the same wavelength. The 

water depth (d) is defined as measured to the still water level (SWL), the level of the water if the 

waves were not present. Wave period (T) is the time a wave takes to travel one wavelength. 

Small amplitude wave theory provides estimates of many of the basic engineering properties of 

the monochromatic wave train on a fixed water depth. The result is a progressive, monochromatic 

wave solution to the boundary value problem consisting of the governing equations of motion for 

irrotational motion of an inviscid fluid (Laplace’s equation) and the appropriate boundary 

conditions. 

A fundamental assumption in the theoretical development of the theory is that the wave amplitude 

is small. Coastal engineers define amplitude (a) as 1⁄2 of wave height (H) or a = H/2. The smallamplitude wave theory is often called “linear wave theory” because the small-amplitude 

assumption allows for the boundary conditions to be mathematically “linearized” and thus solved. 
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Figure 5.1. Wave parameter definitions 

In spite of the seemingly limiting assumption of small waves, small-amplitude wave theory 

estimates many of the basic properties of waves very well. For more information on the 

theoretical basis and results of small-amplitude wave theory, see Dean and Dalrymple (1991) or 

Sorensen (1993). 

The primary small-amplitude theory solution for the water surface elevation (η) is a cosine wave 

(as shown in Figure 5.1) described by Equation 5.1. 

 

 (5.1) 

where:

η = water surface elevation (as measured from the SWL)

H = wave height

x = horizontal position

t = time

L = wavelength

T = wave period

Small-amplitude wave theory indicates that three of the four basic parameters describing the basic 

wave model are not independent. Specifically, the wavelength (L) is a function of water depth and 

wave period (T) (Equation 5.2):

 

 

 (5.2) 

where:

g = acceleration due to gravity
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d = water depth

tanh = hyperbolic tangent function

In deepwater, where the depth is greater than one-half the wavelength (d > L/2), Equation 5.2 

reduces to: 

 

 (5.3) 

where:

L0 = wavelength in deepwater

Wave speed, or celerity (C), is the speed at which the wave form moves across the ocean surface. 

Based on the parameters above, this is:

 (5.4) 

where: 

C = wave celerity

In deepwater, Equation 5.4 becomes:

 

 (5.5) 

where:

C0 = wave celerity in deep water

Note that Equation 5.5 suggests that waves of different periods move at different speeds in deep 

water. 

In shallow water, where depth is less than one-twentieth of the wavelength (i.e. d < L/20), Equation 

5.4 becomes: 

 (5.6) 

Equation 5.6 indicates that all waves move at the same speed in shallow water regardless of 

wave period and that waves slow down as they move into shallower water. 

Equation 5.2 is an implicit equation for L. One explicit approximation to Equation 5.2 is Eckart’s 

approximation:

 (5.7) 

Equation 5.7 gives results within 5% of those from Equation 5.2. Given the lack of precision of 

input conditions in many coastal design situations as well as the uncertainty inherent in the 

analytical methods this accuracy is often acceptable for engineering purposes.

Instantaneous water particle velocities in waves are given by the small-amplitude wave theory, 

as:

 (5.8) 

and 
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 (5.9) 

where:

u = horizontal component of water particle velocity

w = vertical component of water particle velocity

k = wave number = 2π/L

σ = wave frequency = 2π/T

z = vertical direction (measured from the SWL, see Figure 5.1) 

cosh = hyperbolic cosine function

sinh = hyperbolic sine function

Note that the velocity field in waves is oscillatory or orbital, with respect to the wave phase or the 

position of the wave crest. The velocity equations have essentially three parts: (1) an oscillatory 

term with the sine or cosine function, (2) a hyperbolic function of z which is an exponential 

decrease in velocity with distance below the free surface, and (3) a magnitude term, πH/T. 

Maximum velocities occur when the phase is such that the sine or cosine term equals 1.0, e.g. 

when cos ( kx – σt ) = 1 for Equation 5.8. Considering the vertical variation in velocity, maximum 

velocity occurs at the free surface (z = 0). 

Note that, based on the assumptions inherent in the small amplitude theory, the free-surface is 

taken as z = 0 instead of at some higher elevation such as z = η. The maximum forward water 

particle velocity occurs on the free surface of the crest of the wave and is:

 (5.10) 

The wave-induced horizontal velocity on the bottom (z = -d), which can control sediment 

movement on the bottom, becomes:

 (5.11) 

with a maximum value, where cos ( kx-σt ) = 1, is

 (5.12) 

The instantaneous water particle accelerations in a wave field are given by:

 

 (5.13) 

and

 

 (5.14) 

where:

ax = horizontal component of water particle acceleration.

 az = vertical component of water particle acceleration.

Water particle displacements or the paths of individual water particles in water waves can be 

estimated by small-amplitude wave theory. In deepwater the paths are circular with the magnitude 

of the circular motion decreasing with distance below the free surface (Figure 5.2). At a depth of 

about one-half the wavelength the wave-induced orbital movements die out (see Dean and 
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Dalrymple 1991, Sorensen 1993, or USACE 2002 for the particle equations). Below that depth, 

d=L/2, no surface wave motion is felt. 

Figure 5.2. Water particle paths under waves in deep water 

Figure 5.3. Water particle paths under waves in shallow and intermediate water depths 
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Water particle paths are elliptical in intermediate and shallow depths (Figure 5.3). The vertical 

amplitude of the elliptical motion decreases with increasing depth. At the bottom, the water 

particles move back and forth along the bottom. Scuba divers in shallow water are familiar with 

this back and forth motion and often refer to it as “wave surge.” The magnitude of the motion can 

cause difficult working conditions for divers as the corresponding accelerations can make for 

nauseous conditions.

The small-amplitude wave theory provides adequate approximations of the kinematics of wave 

motion for many engineering applications. However, when waves are very large or in very shallow 

water, small-amplitude theory results may not be adequate. Higher-order wave theories, such as 

higher order Stokes wave theories, cnoidal wave theory, and solitary wave theory address these 

important situations more appropriately. Numerical wave theories, however, have the broadest 

range of applicability.

Small-amplitude wave theory may not adequately predict the distortion of the water surface profile 

for large waves or for shallow water waves. The sinusoidal shape of the free surface of a water 

wave (shown in Figure 5.1) is a reasonable engineering model of the free surface of smaller 

waves in deepwater. 

However, larger waves are known to have water surface profiles that are more like those shown 

in Figure 5.4. Stokes 2nd order theory predicts water surface profiles that are the sum of two 

phase-locked sinusoidal waves with the smaller having half the wavelength of the first. The 

resulting water surface profile has more sharply peaked crests and flatter troughs than the 

sinusoidal profile from small-amplitude theory. The elevation of the crest of the wave is higher, for 

the same H, which can be important for bridge clearance.

Figure 5.4. Stokes 2nd order wave theory water surface profile 

Numerical wave theories can predict water surface shape and kinematics for large waves in deep 

or shallow water to any level of accuracy. The iterative power of the computer is used to more 

precisely solve the governing equations and appropriate boundary conditions. The most 

commonly available numerical wave theories are Dean’s stream function wave theory (Dean 

1965, Dean 1973) and Rienecker and Fenton’s potential theory (Rienecker and Fenton 1981). 

Two shallow water wave theories are the solitary wave theory and the cnoidal wave theory. These 

are both analytical theories for waves in very shallow water. The solitary wave considers a single 

wave. The cnoidal wave is part of a train of monochromatic waves. 
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The phenomenon of a non-sinusoidal shape of the water surface profile can become obvious for 

swell in shallow water. Cnoidal waves or the numerical theories can model this phenomenon well. 

The wave kinematics, including orbital velocities and accelerations, predicted by higher-order 

wave theories vary from those predicted by the small-amplitude theory. The velocities and 

accelerations under the crests of the waves will be larger but of shorter duration than those 

predicted by linear theory. However, the variation from the small-amplitude theory is often less 

than 20-30%. 

The total wave energy of a wave train is the sum of its kinetic energy and its potential energy. The 

kinetic energy is that part of the total energy due to water particle velocities associated with the 

orbital wave-induced motion discussed above. Potential energy is that part of the energy resulting 

from part of the fluid mass in the wave crest being above the wave trough. The total energy density 

(energy per unit surface area) in a wave train is given by small-amplitude wave theory as:

 (5.15) 

where:

 = total energy in a wave train per unit area of sea averaged over one wavelength

H = wave height

γ = specific weight of water

The implication of Equation 5.15 is that energy in a sea state is directly related to the square of 

the wave height. Wave height can be used as a measure of energy in a sea-state. Energy is very 

sensitive to wave height and doubling the wave height increases the energy in the sea-state fourfold. 

Waves propagate energy across the sea by moving in wave groups. Interestingly, the groups of 

waves, and thus the energy in the waves, can move at different speeds than the individual waves. 

The wave group velocity (Cg), or the velocity at which energy is propagated, is related to the 

individual wave celerity as:

 (5.16) 

where:

n = ratio of wave group velocity to wave celerity (given by Equation 5.17 below)

C = wave celerity (defined by Equation 5.4)

 

 (5.17) 

The value n varies from 1⁄2 to 1. In deepwater, it approaches n = 1⁄2. In shallow water, it approaches 

n = 1. Thus, in deepwater, the wave energy is propagated at about one-half (1⁄2) of the individual 

wave celerity. 

However, in shallow water the energy moves at the individual wave celerity:

 (5.18) 

The wave power, or wave energy flux in a wave train, is given by:

Wave height is a surrogate for energy in the sea state. Larger waves have much 

more energy.
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 (5.19) 

where:

 = wave power 

 = total wave energy density (defined in Equation 5.15)

Cg = wave group velocity (defined in Equation 5.16)

Wave energy flux entering the surf zone has been related to the longshore sediment transport 

rate, wave setup in the surf zone, and other surf zone dynamics as discussed in Chapter 6. 

5.2 Wave Transformation and Breaking

As waves move toward the coast into shallower water depths, they undergo transformations and 

ultimately break. The wave period of individual waves remains constant through the 

transformations until breaking but the direction of propagation and the wave height can change 

significantly. Transformations include shoaling, refraction, diffraction, attenuation and reflection. 

Waves break in different ways when they hit a shoreline or structure. The concept of a depthlimited wave height in shallow water can be very valuable in some coastal engineering 

applications. 

As a wave moves into shallower water the wavelength decreases (recall Equation 5.2) and the 

wave height increases. For two-dimensional propagation, i.e. straight toward shore, the increase 

in wave height can be theoretically shown, by conservation of wave energy considerations:

 

 (5.20) 

where:

 Ks = shoaling coefficient

 H = wave height

 = deepwater wave height

The shoaling coefficient increases from Ks = 1.0 up to perhaps as much as Ks = 1.5 as the 

individual wave moves into shallower water until the wave breaks via the depth-limited mechanism 

discussed below. 

Wave crests bend as they move into shallower water via refraction. As waves approach the beach 

at an angle, a portion of the wave is in shallower water and moving more slowly than the rest of 

the wave. Viewed from above (Figure 5.5) the wave crest begins to bend and the direction of 

wave propagation changes. Refraction changes the height of waves as well as the direction of 

propagation. 
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Figure 5.5. Bending of wave crests as they approach the shore due to refraction (adapted from 

USACE 2002)

There are two general types of models for monochromatic wave refraction: wave-ray models and 

grid-based refraction models. Wave-ray models, the older type, estimate the path of wave rays, 

lines perpendicular to the wave crests. These wave ray models are based on Snell’s Law. They 

can provide reasonable estimates of refraction but have problems with crossing wave rays or 

“caustics.” These caustics are physically impossible since they imply an infinite wave height. Gridbased refraction models solve some form of governing differential equation for the wave height 

field across arbitrary bottom contours and avoid this “caustic” problem.

Diffraction is the bending of wave crests as they spread out into quieter waters. An example of 

wave diffraction is the spreading of wave energy around the tip of a breakwater into the lee of the 

breakwater. The wave crest, as viewed from above, can wrap itself around the tip of the 

breakwater and appears to be propagating from that tip location into the quieter water. Diffraction 

also occurs in open water as waves propagate across varying depths. Thus, wave diffraction and 

refraction often occur together and any separation of the two mechanisms can be problematic in 

engineering modeling. 

The combination of wave refraction and diffraction can cause wave energy to be focused on 

headlands or reefs and de-focused in embayments as shown in Figure 5.6. Thus, wave heights 

can be increased on headlands and decreased in embayments. Figure 5.7 shows combined wave 

refraction, diffraction, and breaking occurring across the shallow reefs offshore of the Diamond 

Head Lighthouse.

Numerical wave refraction models are often combined with diffraction models. One such 

combined model is the REF/DIF model originally developed by Kirby and Dalrymple (1983). 

Modern numerical wave generation models (see Section 5.4) typically include some form of 

refraction and diffraction in the wave propagation algorithm.
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Figure 5.6. Wave energy focused on headland by a wave refraction and diffraction (adapted 

from USACE 2002) 

Wave energy can propagate very long distances across the ocean with very little loss of energy. 

However, wave height can decrease as a wave propagates across flat bottoms in shallow water. 

Energy can be lost due to bottom friction and other processes. These energy losses, or 

attenuation, can significantly reduce wave heights. Wave breaking across a shallow bar or reef is

also sometimes referred to as wave attenuation. 

 (5.21) 

where:

Cr = reflection coefficient

Hr = reflected wave height

Hi = incident wave height

The reflection coefficient can vary from 0 < Cr < 1 depending on the shoreline or structure type. 

Smooth, vertical walls have reflection coefficients of 0.9 < Cr < 1.0. Reflection from sloping walls, 

revetments and beaches is very sensitive to slope and can vary from 0.05 to 0.9 for different 

smooth slopes. The lower values are for very flat slopes. Typical values of reflection coefficient 

for sandy beaches and rubble-mound structures are 0 < Cr < 0.45 and 0 < Cr < 0.55 respectively 

(USACE 1984).

Waves break at two general limits:

• In deepwater, waves can become too steep and break when the wave steepness defined 

as, H/L, approaches 1/7. 

• In shallow water, waves break when they reach a limiting depth (see Figure 5.8 and Figure 

5.9.). 
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Figure 5.7. Waves refracting, diffracting and breaking on the shallow-water reefs south of the 

Diamond Head Lighthouse, Honolulu, Hawaii

Wave energy is usually partially reflected when it hits a shoreline or structure. The reflection 

coefficient is defined as the ratio of the reflected wave height to the incident wave height: 
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The photograph in Figure 5.9. shows depth-limited wave breaking across an area that is normally 

dry land near the shore of Mobile Bay. The surge and winds are from Hurricane Katrina which 

had made landfall about 100 mi west of this location several hours earlier. These waves were 

generated by those winds in Mobile Bay, which is on the right side of the photograph. The storm 

surge was inundating a low coastal bluff along the bay allowing waves to propagate into these 

trees.

Figure 5.8. Depth-limited wave breaking in shallow water

This depth-limited breaking can be very important in the design of coastal revetments protecting 

highways. For an individual wave, the limiting depth is roughly equal to the wave height and lies 

in the practical range:

 

 (5.22) 

where:

 

 = maximum ratio of wave height to water depth.

The variation expressed in Equation 5.22 is due to nearshore slope and incident wave steepness, 

H/L. 

Steeper nearshore slopes result in higher values of �H

d� �

max. 
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Figure 5.9. Depth-limited wave breaking during storm surge (permission to use photograph 

provided by Happy Partridge)

A practical value when there is a mild slope offshore of the structure is:

 

 (5.23) 

Which corresponds with a theoretical limit from solitary wave theory of:

 

 (5.24) 

The depth-limited wave height can be expressed as: 

 (5.25) 

where: 

 Hmax = maximum wave height

 d = depth of water (as shown in Figure 5.8) 

Equation 5.25 is often useful in selecting an upper limit for a design wave height for coastal 

structures in shallow water. Given an estimate of the water depth at the structure location, the 

maximum wave height Hmax that can exist in that depth of water is known. Any larger waves would 

have broken farther offshore and been reduced to this Hmax. Equation 5.25 is a nominal limit and 
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is not conservative on sloped bottoms. Note that depth, d, is the total water depth, including tides 

and design surge levels, and allowances for scour if applicable.

Waves striking built coastal infrastructure are often depth-limited, i.e. their height at a specific 

location is controlled by the water depth at and around that location. The depth-limited wave 

concept is part of numerous FEMA regulations, methods and guidance (e.g. FEMA 2011); USACE 

guidance in many situations (USACE 2002); and guidance in the ASCE-7 building code (ASCE 

2016). Numerical wave generation and propagation models use a similar concept when 

estimating wave breaking in shallow water. This rather simple concept plays a significant role in 

the final results of many analyses, vulnerability assessments, risk-based coastal flood elevations, 

insurance rate maps, and engineering designs. 

There are four different types of breaking waves: spilling, plunging, surging and collapsing 

breakers. Typical water surface profiles for these breaker types are shown in Figure 5.10. Breaker 

type is controlled by wave steepness (H/L), beach or structure slope, and local wind direction. 

Spilling and plunging are the most common breaker types on sandy beaches. The spilling breaker 

begins gradually at the top of wave crest with some tumbling white-water. The tumbling water 

increases until the entire wave face is tumbling white-water. An example of a plunging wave is 

shown in Figure 5.11. When waves plunge, the wave form stands up in vertical face that then 

plunges over often forming a “tube” of the sort that good surfers like. Local wind direction at the 

time of breaking can change the breaker type between spilling and plunging (Douglass and 

Weggel 1988). 

If a breaking wave plunges onto a rigid structure such as a seawall or highway bridge deck, a 

pocket of air can be trapped between the water and the structure. The air pocket can be 

compressed and produce extremely large, short duration loads on the vertical structure (see 

Figure 11.12). 

Figure 5.10. Wave breaker types 

The depth-limited wave height is often used as the design wave height in coastal 

engineering design.
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Figure 5.11. Example of a plunging breaker (from Douglass 2002) 

Figure 5.12. Example of a spilling breaker (Huntington Beach, California, 2017)
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Figure 5.13. Example of a collapsing breaker (Morro Bay, California) 

Surging breakers occur on very steeply sloped beaches and on coastal structures. The surging 

breaker type is really just a form of wave reflection. The collapsing breaker is intermediate 

between the plunging and surging types. Collapsing breakers are often the most damaging to 

coastal structures, particularly rubble-mound structures, because the entire wave front collapses 

on the structure generating extremely high wave particle velocities and accelerations. Figure 5.13

shows a rock breakwater being struck by a collapsing wave. 

5.3 Irregular Waves

The smooth water surfaces of monochromatic wave theories are not often realistic 

representations of the sea state on real ocean or bay surfaces. Figure 5.14 shows a long period 

swell approaching the Pacific Coast. For long-period swell situations such as this one, the 

monochromatic theories are appropriate. Note in the background a continuous train of waves that 

are of almost the same height with long, straight wave crests (except where they begin to refract 

and break). The wave profiles show some of the behavior of sharp crests and flat troughs 

discussed above for Stokes 2nd order wave and cnoidal wave theories. 

Another, more typical, water surface is shown in Figure 5.15. Individual smooth wave trains are 

not obvious and the sea state in the bay looks much more chaotic with short-crested waves. 

Figure 5.16 shows waves breaking in the nearshore and on a highway rock, rubble-mound 

seawall. This photograph was taken when there was a strong onshore wind generating waves in 

the Gulf of Mexico. The more typical storm sea-states, like those shown in Figure 5.15 and Figure 

5.16, can be referred to as “irregular waves,” or random waves, since they do not have the 

smooth, repeating shapes of monochromatic theories. 
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Figure 5.14. A train of long-period swell approaching the Oregon coast 

Figure 5.15. Irregular waves on San Francisco Bay, California 
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Figure 5.16. A storm-driven, irregular, sea state in the Gulf of Mexico (Florida SH 30E) 

“Significant wave height,” Hs, is a term with a long history of use in oceanography and coastal 

engineering. Oceanographers have developed two different sets of tools to describe realistic sea 

states and significant wave height. One is a statistical representation of the individual wave 

heights in a sea state. This leads to a primary wave height definition called a “significant wave 

height”, the average height of the one-third highest waves. The other, an energy-frequency 

spectrum representation of the water surface elevation, leads to a primary wave height definition 

that is also called the “significant wave height.” See Dean and Dalrymple (1991) or other text for 

more on wave spectral analysis. The notation for the statistically-based Hs is often:

 (5.26) 

and the notation used for the spectral significant wave height is:

 (5.27) 

The two definitions lead to values of significant wave height (Hs) that are approximately equal in 

deepwater seas. However, in shallow water, and especially in the surf zone, the two parameters 

diverge. The term "significant wave height" probably arose as a way for ship-board observers to 

estimate the wave height. Some argue that there is nothing truly “significant” about either 

definition since very few individual waves in an irregular sea will be of the same height as the 

significant wave height. The significant wave height (Hs) for a sea state is a statistical artifact. 

However, Hs (with either definition) provides a consistent, meaningful measurement of the energy 

in a given sea state. Most modern engineering methods use the spectral significant wave height 

definition.
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Individual wave heights vary in an irregular sea state. The distribution of individual wave heights 

follows a Rayleigh probability distribution (see USACE 2002). This one-parameter distribution 

allows for estimation of other wave heights that are sometimes used in design. Table 5.1 provides 

the relation of some of these other wave heights to Hs. Table 5.1 shows two types of statistics: 

• the average of waves with heights above a certain level ( ) 

• the wave height exceeded by a given percentage of waves in the irregular sea state (H10%, 

H1%). 

Table 5.1. Wave height statistics in irregular seas.

Wave statistic Description Multiple of = 

 10 average of the highest 10% of waves 1.27

 average of the highest 5% of waves 1.38

 1 average of the highest 1% of waves 1.67

H10% height exceeded by 10% of waves 1.07

H1% height exceeded by 1% of waves 1.52

Individual wave periods also vary in an irregular sea. The peak wave period, Tp, is the wave period 

corresponding to the peak of the energy density spectrum. That is the period of most of the energy 

in the irregular sea state. Peak wave period, Tp, is the output of almost all wave generation models 

discussed in the next section.

Each of the wave transformations discussed above for the simpler monochromatic wave train 

model occur in irregular seas. This includes refraction, diffraction, shoaling, attenuation, and 

depth-limited breaking. Several numerical approaches have been developed to model these wave 

transformations. 

5.4 Wave Generation and Modeling

Almost all water waves in the ocean and on bays are caused by winds. Wind first ripples the water 

surface and then begins to increase the heights of the ripples until they become small waves that 

propagate on their own. Wave heights continue to increase as the wind blows farther or harder 

across the water surface. This includes energy transfer from the wind and complex 

hydrodynamics of nonlinear transfers of energy between waves. If the water body is large enough, 

eventually the wave heights will stop growing unless the wind speed increases more. Once they 

are generated, waves often propagate for hundreds or thousands of miles across the ocean. They 

travel beyond the storm that generated them. Most waves that hit the shoreline were generated 

far out at sea. Waves that have traveled out of the winds that generated them are called “swell.” 

See Figure 5.14 for a photograph of swell waves. Waves that are still being acted upon by the 

winds that created them are called “sea.” See Figure 5.15 and Figure 5.16 for photographs of sea 

waves. 

Fetch (F) is the distance across the water that a wind blows to generate waves. For enclosed 

bays, this is the maximum distance across the water body in the direction of the wind. Duration is 

the time that a wind blows. Waves are called “fetch-limited” if their height is limited by the available 

fetch distance. Waves are called “duration-limited” if their height is limited by the duration that the 

wind has blown. If winds blow long enough, until the sea state is no longer duration-limited, the 

sea is considered “fully-arisen.” 
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One of the products that came out of the World War II efforts to forecast surf and wave conditions 

for amphibious landings was an empirical method for estimating wave generation (Sverdrup and 

Munk 1947). Bretschnieder improved it to form the method now known as the SMB method after 

those investigators. The USACE Shore Protection Manual (USACE 1984) replaced the SMB 

method with a similar method based on results from JONSWAP experiments (Hasselmann et al.

1980) and adjusted for depth-limited conditions. These equations are listed in Section 6.2.2.4 of 

AASHTO (2008) and also in Appendix C of Douglass and Krolak (2008). The equations can 

estimate design wave height and period for bays and lakes. These situations are “fetch-limited” 

in that the fetch is limited by the size of the bay or lake. 

On the open ocean, waves are almost never fetch-limited and they continue to move after the 

wind ceases or changes. Swell wave energy can propagate very long distances and into other 

storms. Waves striking the shore at any moment in time may include swells from several different 

locations plus a local wind sea. Modern wave modeling can numerically solve wave generation 

and propagation equations using a grid across the entire ocean. These models can include wave 

generation as well as the transformations of refraction and depth-limited breaking. The 

appropriate selection of a model will depend on the scale of the process being simulated, as well 

as the nature of the problem being evaluated. 

A number of available wave models include the Steady-state Spectral Wave (STWAVE) model 

(Smith et al. 2001), the Wave Analysis Model (WAM) (Komen et al. 1994) and the Simulating 

Waves Nearshore (SWAN) model (Booij et al. 1999). See Table 4.3 for a list of modern coastal 

engineering models. Wave generation and transformation modeling is an active area of research 

and the technology is still being developed and debated in the oceanography community. 

However, throughout the world, these and other models are used daily to forecast waves and aid

in engineering analysis for design. 

The prediction of storm surge and waves at the local scale (i.e. barrier island) often relies on a 

definition of wave characteristics at the basin scale (i.e. ocean). The Global Ocean Wave 

Prediction Model (WAM) is commonly used to provide wave boundary conditions for nearshore 

wave models (Group 1988). The model predicts the growth of wind waves from sea surface 

roughness and wind characteristics, and simulates nonlinear wave-wave interactions as well as 

the dissipation of short waves. WAM provides wave conditions at each model grid point at chosen 

times. While WAM is effective at simulating basin-scale wave propagation it should not be used 

to simulate the propagation and transformation of waves close to shore. 

Wave models account for the wave transformations that occur in intermediate to shallow water 

depths (when the local depth is less than one-half of the wavelength). Such processes include 

wave refraction and shoaling, current-induced refraction and shoaling, wave diffraction, and wave 

breaking (see Section 5.2). Appropriate treatment of these wave transformations becomes 

increasingly important during storm surge events as coastal topography is inundated. The SteadyState Spectral Wave (STWAVE) model provides steady-state solutions of wave transformationpropagation over variable bathymetry in two dimensions. The STWAVE model is also a windwave generation model and accepts depth-averaged currents from a circulation model to simulate 

current-induced wave shoaling and refraction (Smith et al. 2001). STWAVE is capable of 

simulating most nearshore wave transformations of interest on a rectangular, Cartesian grid. The

model simplifies the treatment of wave diffraction and does not consider wave reflections. 

Wave modeling is an excellent tool in coastal engineering. The techniques 

simulate wave heights and wave periods across the oceans and into shallow 

coastal locations.
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STWAVE has been used on a number of engineering project studies (Smith and Smith, 2001). 

STWAVE is a finite-difference model designed to simulate the nearshore transformation of a 

directional spectrum of wave energy. A typical application is to take known offshore wave 

conditions, such as those measured by a NOAA buoy, and transform them over complex 

nearshore bathymetry, often to the point of nearshore breaking. Typical coverage areas are 10-

20 km in the offshore direction and 20-40 km along the shore, with grid cell sizes ranging from 25 

to 100 m. 

The USACE CMS model has a stand-alone wave model, CMS-Wave, whose capabilities are 

similar to those of STWAVE with some enhancements (Lin et al. 2008). CMS-Wave is a twodimensional wave transformation model. Similar to STWAVE, the CMS-Wave model is capable 

of wind-wave generation and propagation over variable bathymetry, and includes nearshore wave 

transformations including shoaling, refraction, diffraction, reflection, and breaking. 

The Simulating Waves Nearshore (SWAN) model has been adapted for coupling with many 

different types of general circulation models. The SWAN model is capable of simulating twodimensional (horizontal) wave propagation in time and space in Cartesian or spherical coordinate 

systems. The SWAN model simulates random, short-crested wind-wave generation and 

propagation in coastal and inland waters (Booij et al. 1999; Ris et al. 1999). The wave model is 

capable of simulating nearshore wave transformations as well as transmission and reflection. It 

can be successfully coupled with ADCIRC (Dietrich et al. 2011) and CH3D (Sheng et al. 2006) as 

part of tightly coupled hindcast and forecast simulations of storm surge and waves. 

The same models used for wave forecasting can be used for wave “hindcasting.” Hindcasting is 

the application of the model to estimate wave conditions that occurred in the past. This can 

be done for historical storms or for long-term simulations. 

Figure 5.17 shows results from a hindcast of Hurricane Ivan (2004) using the SWAN model. It 

shows the maximum wave heights in the vicinity of the I-10 bridge across Escambia Bay in 

Pensacola, Florida. The model results are overlain on the Google Earth image and the diagonal 

line across the bay in the middle of the graphic shows the bridge location. The colors refer to 

estimated significant wave heights for each location and the arrows indicate wave direction at that 

maximum. The storm surge, about 11 ft at its peak, flooded the low-elevation land to the east of 

the bay. The wave heights on top of that flooded land are the blues to the right of the plot. The 

land to the west of the bay is much higher and did not flood. Wave heights at the bridge in the 

middle of the bay exceeded 8 ft but near the shoreline of the bay, they were lower. This reduction 

in wave heights near the shoreline of the bay effected the pattern of damage experienced by the 

bridge (see Figure 11.7). The modelled wave height distribution shown in Figure 5.17 explains 

the damage pattern well. Where the waves were lower, the damage was less severe. 

The USACE Wave Information Study (WIS) has developed a hindcast database of wave 

conditions around the US coastline. Ocean wave generation models were used with decades of 

wind estimates generated from historical barometric pressure fields. The results are numerical 

modeling estimates of wave conditions; Hs, Tp and wave direction at thousands of locations 

around the US coast. The resulting data and summaries are available on-line at the USACE 

Coastal Hydraulics Laboratory web-site.4 These hindcast WIS data are used to develop estimates 

of long-term wave statistics for engineering design and estimating longshore sand transport rates. 

Care should be taken in using these hindcast wave statistics for design since these results are 

not based on actual measurements but rather computer simulations. 

4 http://www.frf.usace.army.mil/ (accessed September 25, 2018) 
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Figure 5.17. ADCIRC+SWAN wave model hindcast of maximum significant wave heights 

generated during Hurricane Ivan (2004) at the I-10 bridge across Escambia Bay, Pensacola, 

Florida

The WIS data have been incorporated into USACE Coastal Hazards System discussed in Section 

4.6 Design Water Levels (see Figure 4.24 for an example). The Coastal Hazards System is an 

excellent source for design wave height information which includes exceedance probabilities 

corresponding to 10-, 20-, 50-, 100-, 200-, 500-, and 1,000-yr return periods. 

5.5 Wave Runup 

Wave runup is the rush of water up a beach, or other slope, as the wave breaks. The limit of runup 

defines what gets wet on a beach above the still water level due to waves. Wave runup on a 

natural beach during storms is a fairly complex hydrodynamic process in that there are several 

components beyond the swash (runup and rundown) of any single wave. Wave setup raises the 

water level in the surf zone. Sometimes oscillations of water level in the surf zone, called “surf 

beat” or infragravity energy, cause fluctuations, with periods of 30 sec to 3 min, in the water level 

in the surf zone. 

Runup is typically defined as the elevation difference between the upper limit of runup and the 

still water level (see Figure 7.22 for a definition sketch). One commonly used empirical equation 

to estimate runup on sandy beaches is Stockdon et al. (2006). Poate et al. (2016) propose a 

similar empirical equation for runup on cobble beaches. 

5.6 Ship Wakes

Ship wakes are sometimes the largest waves that occur at sheltered locations and thus can be 

the design waves for some revetments or other structures. Large ships can generate wakes with 
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wave heights exceeding H=10 ft and smaller vessels (including tugboats) can generate wakes of 

H=5 ft. 

The wake depends on the size, hull shape, speed of the vessel and distance from the sailing line. 

Engineering judgment based on observations can be used to establish a reasonable upper limit 

on wake size if the maximum speeds from all possible vessels are considered. Several 

methodologies for estimating ship wakes are available including Weggel and Sorensen (1986) 

and Kriebel et al. (2003) for large vessels and Bottin et al. (1993) for some smaller recreational 

watercraft. 

5.7 Tsunamis

Tsunamis are a series of waves resulting from a rapid, large-scale disturbance in a body of water 

caused by earthquakes, landslides, volcanic eruptions, and meteorite impacts. Many of the 

tsunamis have generated broad area destruction and fatalities. Tsunamis are some of the world’s 

most powerful water waves because they have extremely long wavelengths that transform 

significantly as they propagate into shallow water. They are often called “tidal waves” even though 

they have nothing to do with the tides. Tsunami is a Japanese word which loosely translates to

“harbor wave” (“wave” directly translates as “nami”). Tsunami has been adopted in the worldwide 

oceanography community to avoid the term “tidal wave.” 

The generation and propagation of tsunamis is an active area of oceanography research and the 

flow dynamics of a tsunami runup, including how it interacts with infrastructure such as buildings 

and roads, can be very complex and is an active area of civil engineering research. The 

destructive flows due to tsunami wave breaking and runup can vary greatly from location to 

location for the same tsunami based on local bathymetry and topography.

The deadly 2004 “Boxing Day” tsunami in the Indian Ocean struck an area that had previously 

been thought to be unlikely for major tsunamis. It was caused by a magnitude-9.3 underwater 

earthquake and devastated coastal areas around the northern Indian Ocean. It killed over 220,000 

and displaced over 1.5 million people. The Tohoku Japan Tsunami of 2011 was generated by a 

magnitude-9.0 earthquake and ran up to inundation heights in that region that exceeded all 

historical records. It killed over 19,000 people and caused extensive damage to ports, buildings, 

bridges and other coastal infrastructure (FEMA 2012). 

Tsunamis in 1946 and 1957 damaged bridges on the north coast of the Hawaiian island of Kauai. 

The 1964 Alaskan earthquake tsunami caused coastal infrastructure damage along the coasts of 

Washington, Oregon, and California. 

Tsunamis can hit any US coast but 95% of reported tsunamis in the US hit the Pacific states and 

territories (NTHMP 2018). Large portions of the US Pacific Coast and Hawaii have tsunami 

warning systems in place in recognition of the threat. Many US coastal communities have tsunami 

evacuation routes with signs that have been coordinated with the SDOT. Tsunamis can occur 

along the Atlantic and Gulf Coasts but the probability is extremely low. 

Analysis tools for assessing the vulnerability of coastal highways to tsunamis are presented in 

Section 14.7 and tsunami-induced scour is discussed in Section 12.3.7. 

Tsunamis are a rare, but potentially deadly and damaging, natural hazard that 

influences civil engineering designs along the US Pacific Coasts and Islands.
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Chapter 6 - Coastal Sediment Processes

Sediments along the coast are constantly being reshaped by waves and currents. These 

processes, primarily sand movement, can have significant implications for engineers tasked with 

working in this environment. The study of coastal sediment processes includes several specialty 

areas of coastal geology including “coastal geomorphology,” the study of coastal landforms and 

features, and “coastal sedimentology,” the study of the properties of beach sands. 

A good understanding of the terminology and concepts of coastal geology is valuable for 

coastal engineering because it is an underpinning science. The design function of many coastal 

engineering projects is to positively affect coastal sediment processes. Many coastal engineering 

projects which have improved navigation, such as inlet or harbor jetties and dredging, have 

caused nearby beach erosion. Inlet jetties can trap sand that was otherwise moving across the 

inlet naturally to the downdrift beaches. This trapping can cause beach erosion for miles downdrift. 

Many ship channels are dredged through a sand bar that is part of the beach’s littoral, sandsharing system. If the dredged sand is not placed back into the littoral system appropriately, 

downdrift beaches can be starved and recede. Thus, the engineering works themselves can 

cause erosion problems. A clear understanding of the coastal processes and the geological 

framework at work at each project location is needed for comprehensive coastal engineering 

projects. 

This chapter provides a brief introduction to some coastal sediment processes including an 

overview of coastal geology, beach terminology, coastal sediment characteristics and transport, 

and tidal inlets. Just a few of the other textbooks and references with much more detail on these 

topics include Komar (1998) and Dean & Dalrymple (2002) for coastal sediment processes; Davis 

(1994), Davis & FitzGerald (2009), and Davis (2014) for coastal geology; and the SPM (USACE 

1984), the CEM (USACE 2002), and Kamphuis (2010) for some of the coastal engineering and 

management applications. 

6.1 Overview of Coastal Geomorphology

This section is a very brief introduction to some of the coastal geomorphology concepts that are 

valuable in understanding the underlying geologic framework for some coastal road situations. 

Coastlines in the US include the extensive barrier islands systems of the south Atlantic and Gulf 

Coast as well as the coastal bluffs of New England, the Pacific, and the Great Lakes. A few ocean 

coasts are muddy shorelines (the “big bend” of Florida) or vegetated shorelines (mangroves of 

southwestern Florida) but these are the exceptions. Some coasts are rock cliffs that extend into 

the sea and are pounded by relentless ocean waves. Marsh shorelines are common on small 

bays with small waves. Most of America’s coasts have some form of sandy shoreline. 

Beaches, the accumulations of loose sediments along the shoreline can either be barrier islands 

or just short pocket beaches between two rock headlands. The type of coast and beach at each 

location is partially controlled by the “geologic framework” that created it. This framework includes 

the local geologic formations and the interplay between plate tectonics, sea-level changes and 

waves that have created each beach.

Coastal geomorphology is the study of coastal landforms. Many of the most obvious coastal 

landforms are products of either erosional processes or depositional processes. Sea cliffs, stacks, 

arches, caves and wave cut terraces are some erosional features found on retreating rocky coasts 

(see Komar 1998). 
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Figure 6.1. Sea cliff and pocket beach on the Oregon coast (Oswald West State Park)

Figure 6.1 shows a sea cliff on the Pacific Coast. Note a small beach at the base of the cliff. This 

is a pocket beach that forms from sand eroded out of the cliff and from a nearby creek. The beach 

is held in place by the headlands where the cliffs extend to the sea at the ends of the beach. 

Barrier islands, sand spits, bays and lagoons are some large-scale depositional features found 

along much of the US sandy coast. Figure 6.2 shows a barrier island on the Gulf of Mexico. The 

US barrier island system, that generally extends along much of the coast between New York and 

Texas, is the longest such system in the world.

One of the fundamental geologic controls on shoreline position and characteristics is sea level. 

Sea level has fluctuated tremendously throughout the past two million years. Section 4.1

discusses the sea level change experienced along the US shorelines during the past century. 

However, the history of sea level changes over the past 2 million years, and particularly the past 

20,000 years has had an impact on the coastlines we have today. During the ice ages, sea level 

fell as glaciations increased and rose as the glaciers receded. 

According to geologists’ estimates, 20,000 years ago, the global, eustatic (with land elevation 

changes removed), sea level was probably 300 ft lower than it is today. One estimate of the rate 

of sea level rise in the past 20,000 years is shown in Figure 6.3. This time period, particularly the 

past 12,000 to 20,000 years, is the Holocene Epoch at the end of the Quaternary Period. It is 

characterized by the rise of global sea level in response to the melting of the last of the Wisconsin 

ice-age glaciers (Davis and FitzGerald 2009).
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Figure 6.2. An example of a barrier island. Dauphin Island, Alabama (circa 2001, looking east 

with Gulf of Mexico to the right and Mississippi Sound to the left) 

The Holocene rise in sea level (Figure 6.3) has two distinct portions. Prior to about 5,000 to 7,000 

years ago, sea level rose at a much faster rate: about 3 ft/century. This fast rate potentially caused 

the coastline to move so rapidly that mature barrier islands did not have time to form. The rate of 

rise slowed significantly about 6,000 years ago. The slower rate allowed the shorelines to become 

more stable and wave-driven longshore sand movement to create the barrier island systems 

along many of our shores today. 

The question marks shown on Figure 6.3 represent the uncertainty about the way that the 

Holocene rise in sea level occurred. Although some investigators postulate that there was 

significant fluctuation and others do not, most agree with the general shape of the curve shown

(Davis 1994). 
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Figure 6.3. Sea level rise curve for the past 20,000 years

The position and characteristics of shorelines are partially controlled by global plate tectonics 

(Inman and Nordstrom 1971). The Pacific Coast of the US is on the “leading” edge (the edge of 

the plate that is in the front of the plate's movement) of the North American plate and the Atlantic 

Coast is on the “trailing” edge. The difference explains some of the general differences in 

shoreline characteristics including the presence of mountain ranges and a narrow continental 

shelf near the Pacific Coast but not the Atlantic Coast (Davis 1994). These are contributing factors 

to the lack of barrier island systems on the Pacific Coast and their extensive presence on the 

Atlantic Coast. 

6.2 Beach Terminology

The beach can be defined as the accumulation of unconsolidated sediment (sand, gravel, and/or 

cobbles) extending from some upland location, such as a sea cliff or sand dune or vegetation line, 

to the water line and extending out below the water to a depth where the sediment is not moved 

by wave action. The beach is commonly synonymous with the term “littoral” referring to this same 

area where waves can move sand (Komar 1998). The offshore limit of the littoral zone can be 

very deep during large storms but is often just assumed to be a depth of 20 to 60 ft depending on 

the wave climate. 

Terminology used to describe the processes of waves and currents in the nearshore is shown in 

Figure 6.4. The nearshore zone extends from the upper limit of wave runup on the beach to just 

beyond where the waves are breaking. The breaker zone or line is the portion of the nearshore 

region in which waves arriving from offshore become unstable and break (see Section 5.2 Wave 

Transformation and Breaking). The swash zone is the portion where the beach face is alternately 

covered by the run-up of the wave swash and then exposed by the backwash. The surf zone is 

the portion of the nearshore between the breaker line and swash zone. The surf zone can have 

bore-like, breaking or broken waves propagating across it. The field of “surf zone dynamics” is an 
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active area of research that focuses on the hydrodynamic motions of waves and currents as well 

as the sediment response to those motions in the surf zone.

Figure 6.4. Terminology used to describe processes of waves and currents in the surf zone

The shape of a beach profile, or transect or cross-section, has some typical features. The 

terminology used to describe the beach profile is shown in Figure 6.5. A longshore bar, or sand 

bar, is an underwater ridge of sand running roughly parallel to the shore. Sand bars can be 

exposed at low tide in areas with large tide ranges. Figure 6.6 shows a sand bar exposed at low 

tide at a location on a South Carolina coast where the tide range is about 7 ft. Because of the 

beach slope, the intertidal area here is several hundred feet wide. A longshore trough is a 

depression inside of a sand bar. The beach face is the area of the swash zone. The beach berm 

is the nearly horizontal portion of the beach formed by the deposition of sediments by waves. 

Some beaches have more than one berm at slightly different levels separated by a scarp. A scarp 

is a nearly vertical cut into the berm portion of the beach profile by wave erosion. Scarps are often 

at the top of the beach face when erosion is occurring. A scarp along a southern California beach 

is shown in Figure 6.7. Waves were actively eroding the berm at the time the photograph was 

taken. 
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Figure 6.5. Terminology used to describe the beach profile

Figure 6.6. Sand bar and trough exposed at low tide (Garden City Beach, South Carolina) 
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Figure 6.7. A beach scarp (Los Angeles County, California) 

Figure 6.8. A sand dune (St. George Island, Florida)
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Sand dunes are natural features behind many sandy beaches in the US (see Figure 6.8). Dunes 

are a unique type of habitat and host some endangered and threatened species of mice. Sand 

dunes also provide protective benefits during storm events by blocking or reducing storm surge 

flooding and wave action. Dunes function as sacrificial volumes of sand that minimize storm 

impacts until the dunes are eroded by waves or overtopped by storm surge. Dunes with sand 

fencing and vegetation trap and stabilize sand, leading to increases in dune volume and dune 

height over time. 

Repetitive measurements of beach profiles are a common tool in quantifying erosion and other 

coastal processes. Elevation of sand surface, both above and below the waterline, is measured. 

A variety of techniques have evolved over the years for obtaining these measurements. The 

problem is that neither traditional land surveying techniques nor traditional marine surveying 

techniques can easily span the offshore, the surf zone, and the upland portions of the beach 

profile. So, overlapping coverage with different techniques is necessary. 

Figure 6.9 shows a beach surveying crew using a traditional land surveyor’s level to measure the 

profile. The person holding the rod has to wade and swim in the surf zone and this can become 

problematic in large surf. One highly specialized modification of this approach is shown in the left 

side of the photograph in Figure 6.10 where a staff gage or total reflector station is attached to a 

CRAB (Coastal Research Amphibious Buggy) that drives out through the surf zone while 

measurements are made. The CRAB shown in Figure 6.10 is privately owned and used 

exclusively for measuring beach profiles in beach nourishment projects. 

Figure 6.9. Beach profile surveying crew using a traditional level and rod 
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Figure 6.10. A CRAB (Coastal Research Amphibious Buggy) used to measure beach profiles 

during beach nourishment

Marine surveying techniques have been adapted for the surf zone by placing fathometers and 

GPS or total stations on jet-skis (personal watercraft). This can improve the ability of the vessel 

to obtain data in very shallow water. 

A common method for measuring land elevations is airborne LIDAR, laser-based elevation 

measurements, from an airplane. LIDAR technology has the capability of measuring the dry beach 

elevation and the underwater portion of the profile at the same time with the same equipment. 

The water depth measuring LIDAR has some operational limitations related to water clarity and 

surf zone breaking. The laser can only penetrate water if it is clear enough: the air bubbles in 

white-capping in the surf zone can cause problems. However, the ability of LIDAR to collect large 

amounts of precise measurements over large distances in short periods of time is a significant 

advance for beach profile surveying. 

Many beach profiles have similar shapes. If the sand bar is ignored, many beach profiles are 

concave upward with slopes that are much milder than the angle of repose of the same sands on 

dry land. This shape is a response to the wave energy present in the surf zone. A useful concept 

is that of an “equilibrium beach profile” where the shape of the profile is in equilibrium with the 

wave energy. The shape of the offshore portion of the profile has been modeled with a variety of 

different expressions. One is shown in Figure 6.11. This simple relationship between depth and 

distance offshore fits many sandy beach profiles at different locations and has some physical 

LIDAR can measure water depths in some locations (clear water). 
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meaning related to the dissipation of energy in the surf zone (Dean 1974, Dean and Dalrymple 

2002). The addition of more parameters, including the use of a variable exponent in place of 2/3, 

can improve the fit of the relationship to any particular profile or set of profiles. The value of the 

“A” parameter has been shown to be a function of the sand grain size (typically between 

0.1<A<0.2). 

Figure 6.11. Dean’s equilibrium beach profile shape definition sketch

6.3 Coastal Sediment Characteristics 

The sediments on most American beaches are whatever hard, loose sediments are available, 

based on the local geology. The majority of coastal sediments are sands. Exceptions include the 

many cobble beaches of the Pacific, New England, and the Great Lakes. Cobbles and pebbles

are round stones, and shingles are flatter stones. 

Most sand-size sediments on American beaches are quartz or some other hard mineral. 

Exceptions to this general rule are the many beaches consisting of shell hash, ground up coral 

reefs, or other carbonate materials that exist in Florida, Hawaii, and to a lesser extent, along many 

other beaches. The mineral composition of the sand grains depends on the local geologic 

framework. Figure 6.12 shows the variation in color of beach sands throughout the nation. 

The size of the sand grains influences the way a beach behaves and is very important in beach 

nourishment engineering. Beach sand grain size can vary significantly. Beach sediment grain size 

can be evaluated with a sieve analysis much like grain size in other civil geotechnical engineering 
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analyses. The median diameter (D50) is the most common measure of sand grain size. Typical 

median grain sizes for American beaches are 0.15 to 0.60 mm.

Figure 6.12. Examples of colors of US beach sands (from Douglass 2002, photograph used with 

permission of the author) 

The results of a grain size analysis on beach sand are shown in Figure 6.13. The median diameter 

is about D50 = 0.25 mm. Figure 6.13 shows an important characteristic of beach sand grain size 

distribution - they can be extremely well-sorted. Essentially, waves can winnow all the other grain 

sizes away. Since almost all the grains are of the same size, care should be taken to include the 

full complement of available sieve sizes in order to adequately differentiate beach sand grain 

sizes with sieve analysis. 
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Figure 6.13. Example grain size distribution based on a sieve analysis for beach sand 

6.4 Cross-Shore Sand Transport and Dune Erosion Modeling

Waves have the capacity to move tremendous amounts of sand in the surf zone. This sand 

movement on beaches can be conveniently considered as either longshore or cross-shore sand 

transport. This distinction, cross-shore vs. longshore transport, is somewhat artificial, in that the 

individual grains of sand may be moved both in the cross-shore and longshore directions at the 

same time. The movement of individual sand grains in response to wave motion and currents in 

the surf zone is extremely complex. Movement is related to instantaneous near-bottom water 

velocities under breaking irregular waves, the resulting shear stress on the bottom sand grains, 

and the subsequent transport of sand including the rich variations in transport mechanisms 

(bedload, suspended load, ripple and other bedform effects, bed ventilation effects). The 

complexities of surf zone dynamics and sediment transport processes preclude any meaningful 

analytic approaches. Thus, coastal engineers and scientists typically look for simplifications of the 

dynamics of the processes that can be modeled and compared with empirical results. One of the 

simplifications adopted is the separation of transport into the cross-shore and longshore 

directions. 

Coastal practitioners have long understood that sand moves back and forth across a beach profile 

in response to changes in incident wave energy. This is shown schematically in Figure 6.14. 

Wave steepness, the ratio of wave height to wave length, H/L, has a significant impact on whether 

sand bars are moving onshore or offshore. When the wave steepness is low, such as with swell, 

sand bars typically migrate to the shore. The sand bars sometimes can move all the way into the 

dry beach and build up the berm making the dry portion of the beach wider. These low steepness 

wave conditions typically occur in the summer on the US Atlantic and Pacific Coasts and thus this 
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profile shape, with a wide beach berm, is called a “summer profile.” When waves are steep, such 

as with a locally generated short period wind sea, sand is eroded out of the berm and the sand 

bars form or are pulled farther offshore. These sea wave conditions typically occur in the winter 

and thus, this profile shape is called a “winter profile.” The beach is narrower than for the summer 

profile. Essentially, the beach profile shape is just moving toward a new equilibrium with the 

incoming waves. Since incoming waves are always changing steepness through time, the beach 

may never really reach an equilibrium shape but is always approaching one. 

These seasonal shifts of sand on the beach profile, which cause a narrowing of the dry, visible 

beach are not typically the cause of real beach erosion and long-term shoreline change. However, 

shoreline recession along a coast which is eroding because of a longshore deficit of sand will 

appear most obviously after storms. Furthermore, in very large storms, sand can be moved out 

into sand bar formations and take several years to return to the nearshore system. 

Figure 6.14. Typical beach profile changes in response to cross-shore transport of sand

Sand in the berm and dune can be moved out to sea into sand bars when storm surge temporarily 

raises the still water level. This storm-induced dune erosion can destroy large dune fields in a 

single major storm. Several models simulate storm-induced dune erosion. 

Two examples of simple cross-shore morphology (i.e. dune erosion) models are EDUNE (Kriebel 

and Dean, 1985; Kriebel, 1986) and SBEACH (Larson and Kraus, 1989, Larson et al. 1990). 

These models require a definition of the initial beach profile, or bathymetric profile, as well as 

storm wave and water level characteristics. The models seek to describe a modified beach profile 

that is in equilibrium with the storm conditions. While the models typically capture the dune erosion 

and overwashing events well, the simple cross-shore morphology models are not as capable of 

accurately predicting the redistribution of sediments to deeper portions of the profile. In most 

cases this would not present a limitation when considering impacts to infrastructure along the 

coast. Figure 6.15 shows some an example of modeled dune face erosion. 
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Figure 6.15. Dune erosion model results example

FEMA has adopted a simpler, parametric model for storm-induced dune erosion for the purposes 

of mapping coastal flood plains. FEMA’s model empirically relates the volume of sand eroded 

from the dune directly to the storm return period (Hallermeier and Rhodes 1988): 

 (6.1) 

where: 

(Vol) = volume of erosion from the sand dune above the storm surge elevation per unit 

length of shoreline

T = return period of storm in years

C = empirical coefficient: c = 86 when (Vol) is in ft2; c = 8 when (Vol) is in m2

Equation 6.1 estimates the volume of erosion for the 100-year and 5-year storm levels as 20 yd3

and 6 yd3 of sand per foot of shoreline, respectively. These values are for the volume of sand 

above the storm surge elevation (which of course can be much higher for the 100-year storm). 

This dune erosion model has been incorporated into FEMA’s Coastal Hazard Analysis Model 

(CHAMP) model that is available on-line. 

Equation 6.1 can be used to design sand dunes as nature-based solutions. Since there is no 

factor-of-safety in the parametric model, engineering judgment is needed in nature-based solution 

design applications. Dunes can be constructed as integral parts of larger beach nourishment 

projects or as stand-alone barriers to overwash. Small, FEMA funded dunes (erroneously termed

“berms”) are often designed using Equation 6.1 and built as emergency projects after major 

hurricanes to “jump-start” the dune recovery process and reduce overwashing. 

Numerical models of coupled waves, flow, and morphology are being used in the coastal 

engineering research community. These models simulate both cross-shore sand transport and 

longshore sand transport (see Section 6.5). The XBEACH (Roelvink et al. 2009) and CMS 

(Buttolph et al. 2006) models are both two-dimensional models capable of simulating the coupling 
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between waves, currents, sediment transport and morphology in a time-dependent fashion. 

Typically, sediment transport modeling, by its very nature, is not as well understood as 

hydrodynamic modeling. Even more complex, the modeling suite DELFT3D can estimate 

morphologic change (Roelvink et al. 1998). Although the EDUNE and SBEACH models are 

relatively easy to apply, they do not provide a time history of morphologic response during storm 

events, nor do they seek to describe morphologic change in a direction parallel to the coast. The 

XBEACH, CMS, DELFT3D, and MIKE 21 models, by comparison, provide estimates of the 

temporal and spatial morphologic change including dune erosion, overwashing, and breaching. 

Less complex, coupled, one-dimensional (transect) models are CSHORE (Johnson et al. 2012) 

and CHAMPS (FEMA 2007). 

6.5 Longshore Sand Transport and Shoreline Change Modeling

Waves have the ability to move tremendous amounts of sand down the coast in longshore sand 

transport. As wave energy enters the surf zone, some of the energy is transformed into nearshore 

currents and expended in sand movement. The nearshore current field is driven by the incident 

wave energy and the local winds. The largest currents, in terms of absolute magnitude, are the 

oscillatory currents associated with the waves. However, several forms of mean currents are 

important to sediment transport: the longshore current, rip currents associated with nearshore 

circulation cells, and downwelling or upwelling associated with winds. 

Longshore current is the mean current along the shore between the breaker line and the beach 

that is driven by an oblique angle of wave approach (see Figure 6.16). The waves provide the 

power for the mean current and also provide the wave-by-wave agitation to suspend sand in the 

longshore current. The resulting movement of sand down the beach is littoral drift or longshore 

sand transport. This process has been likened to a “river of sand” that flows along all our sandy 

shorelines. The amount or rate of longshore sand transport can be tremendous during large 

storms. When averaged over a year, it can exceed a million cubic yards per year moving down 

the beach along some parts of the US coast. Longshore sand transport, unlike rivers, reverses 

direction frequently in response to changes in the direction of wave approach. Thus, the net 

longshore transport rate is significantly less than the gross rate. If longshore sand transport is 

interrupted by a ship channel or other engineering works like a jetty system to stabilize an inlet 

for shipping, erosion can occur for many miles downdrift. 

The most common equation for estimating longshore sand transport rate is the so-called “CERC 

Equation” or energy-flux method (USACE 2002). It estimates the sand transport rate based on 

the longshore component of energy flux or wave power entering the surf zone. Using the 

expressions for wave power (Equation 5.19), the wave-energy flux factor evaluated at breaking 

can be derived as: 

 

 

 

 (6.2) 

where: 

Pls = wave energy flux factor

Hb = wave height at breaking

Cgb = wave celerity at breaking

α = angle of the breaking wave crest with the shoreline

If longshore sand transport is interrupted by engineering works, downdrift 

erosion can be extensive. 
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γ = specific weight of water

Figure 6.16. Definition sketch of wave angle at breaking

The CERC equation relating this to longshore sand transport can be written as: 

 (6.3) 

where: 

Q = longshore sand transport

K = Empirical coefficient (K=7,500 when Q is expressed in yd3/year and Pls in lb/s)

The relationship between transport rate and energy flux factor is not a precise relationship as 

shown in Figure 6.17 with prototype, field data. Also, there is often uncertainty in estimating the 

input wave parameters, such as Hb in the CERC equation. In many situations, the CERC equation 

can be considered as a good order of magnitude estimate of transport. 

Shoreline change models simulate the temporal change in shoreline position, i.e. the movement 

of the shoreline. The CERC equation, or some derivative of it, often is used to estimate the 

longshore sand transport rate at all locations along the shoreline. Then conservation of sand down 

the coast is modeled. The equations are solved repetitively in time for a discretized shoreline. 

Wave refraction and diffraction have been incorporated into most shoreline change models. The 

results of shoreline change models are estimates of the changes in shoreline position due to the 

construction of engineering works such as groins or beach nourishments. Several shoreline 

change models are available: Perlin and Dean (1983), GENESIS (Hanson and Kraus 1989), and 

ONELINE (Dabees and Kamphuis 1998) for example. Since shoreline change models are 

essentially multiple applications of a form of the CERC equation or some other longshore sand 
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transport model, their results include all the uncertainties inherent in such modeling. Thus, 

shoreline change models need to be adequately validated and judgment is needed in applications. 

The inherent uncertainties in coastal sediment modeling are much greater than those in coastal 

hydrodynamic and wave modeling. 

Figure 6.17. The CERC equation model for longshore sand transport rate plotted with field data 

(adapted from USACE 1984)

The more complex models mentioned in Section 6.5, XBEACH, CMS, DELFT3D, and MIKE 21, 

are numerical models of coupled waves, flow, and morphology being used in the coastal 

engineering research community. These models simulate both cross-shore sand transport and 

longshore sand transport. 

6.6 Tidal Inlets

Tidal inlets allow water to flow from the ocean to the bay and back. Two tidal inlets are shown in 

Figure 6.18. There are about 600 tidal inlets of various sizes in the US. Oregon Inlet, North 

Carolina, is an example of a large, unstabilized inlet. Tidal inlets are dynamic parts of the barrier 

island system that have important influences on the bays and the nearby beaches. 

While every inlet is unique, Figure 6.19 shows some common geomorphological features. The 

flood tide is the movement of water into the inlet and the ebb-tide is the flow of water out of the 

bay back to the ocean. Typical patterns of the strongest ebb-tide and flood-tide flows are shown 

by the flow arrows in Figure 6.19. The shoal, or bulge of sand, formed just seaward of an inlet is 

called the ebb-tidal delta or ebb-tidal shoal. Likewise, a shoal just inside of an inlet is called the 

flood-tidal delta or shoal. The outer bar of the ebb-tidal delta permits longshore sand transport to 
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naturally bypass an inlet to the downdrift beaches. There are often other shoals inside the outer 

bar of the ebb-tidal shoal. 

The gorge or throat section of the inlet is the main flow channel. It is typically the deepest part of 

the inlet with the highest, most concentrated ebb- and flood-tidal flows. Tidal inlets can exhibit 

significant migratory tendencies. These can be problematic for highway infrastructure. 

Tidal inlets are essentially in some dynamic equilibrium between the longshore sand transport 

system of the adjacent barrier island system and the tidal currents (Bruun 1966). The wave-driven 

longshore sand transport would seal off the inlet if not for the tidal currents scouring the sand out 

of the throat and depositing it on the inlet shoals. Most inlets are not symmetrical about their throat 

like that shown schematically in Figure 6.19 but rather skewed in the direction of net longshore 

sand transport. For example, the tidal channels and shoals in Figure 6.18 are skewed to the south 

by the dominant southerly direction of longshore sand transport along that portion of the coast. 

The hydraulics of tidal flows through inlets can be extremely complex due to the shoals, waves 

and currents. The primary tidal flows can be idealized as shown in Figure 6.20. Water flows into 

the inlet when the tide in the ocean has risen to a level that exceeds the elevation of the water 

surface in the bay. This vertical difference in elevation, the head difference, between the ocean 

and the bay drives the flow much as the downslope gradient in river elevation drives the flow in 

rivers. The flow in the inlet will continue to “flood” until the tide level in the ocean falls to an 

elevation below that in the bay. Thus, the bay tide always “lags” the ocean tide. The tidal lag can 

vary significantly depending on the shape of the bay and inlet. 

Tidal hydraulics can be simulated well with numerical hydrodynamic models like ADCIRC. These 

models usually provide depth-averaged water velocities but some can estimate 3-dimensional 

flows. See Section 4.3.2 and Webb (2017) for discussions of coastal hydrodynamic modeling. 

The amplitude, or range, of the tide in the bay can also be much smaller than in the ocean. This 

results in an attenuation of the tide range. This is common when the inlet is constricted to a 

level that does not allow enough time for the bay to completely fill up during the rising ocean tide 

before the ocean tide begins to fall. In many cases, the tide range can actually increase farther 

up an estuary due to inertial effects. There are a number of quasi-analytical parametric models of 

the solutions to the idealized ocean-inlet-bay system including solutions for maximum velocity in 

the inlet and bay tidal range amplitude (USACE 1984). Other parametric models relate to the 

stability of inlet systems. 

Beaches adjacent to and near tidal inlets are part of the dynamic littoral system of the inlet and 

exhibit much more shoreline change than beaches farther from inlets. Sometimes the shoreline 

movement is recession and sometimes it is accretion. The beaches near inlets can increase 

dramatically in width as some of the shoals migrate onshore. Inlet geometry can change 

dramatically in both the short-term and the long-term. A single storm can move hundreds of 

thousands of cubic yards of sand shoals into or out of an inlet. Some inlets have a tendency to 

migrate along the coast. Some migrate in the direction of net longshore sand transport and some 

migrate in the other direction. 

Many tidal inlets naturally migrate and this can cause transportation 

infrastructure problems. 
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Figure 6.18. Two tidal inlets on the southwest Florida coast (New Pass and Big Sarasota Pass, 

Lido Key is the barrier island between the two inlets) 
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Figure 6.19. Typical inlet morphology

Figure 6.20. Idealized ocean-inlet-bay system (adapted from USACE 1984)
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Empirical relationships have been recognized between the components of tidal inlet systems. 

Figure 6.21 shows one empirical relationship between tidal prism and inlet throat area. Tidal prism 

is defined as the amount of water that moves into and out of a tidal inlet during a tidal cycle. It is 

essentially the area of the bay multiplied by the bay tide range. 

All tidal inlets are evolving and changing over the long-term. This evolution is in response to many 

changing factors including sea level rise, changes in longshore sand transport rate and changes 

in tidal prisms. These factors change naturally but also can be changed by engineering. 

Engineered changes to the ocean-inlet-bay system include the stabilization of the inlet with jetties 

or the dredging of the inlet or bay for navigation. 

Figure 6.21. Tidal prism versus minimum inlet throat area for all major inlets on the Atlantic, 

Gulf, and Pacific Coasts (USACE 1984) 
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Less obvious changes include the impact of engineering works in the bay that affect the tidal 

prism. This impact can be caused by the filling of wetlands or the construction of causeways in 

the bay. The implication of the relationship shown in Figure 6.21 is two-fold: any change in the 

tidal prism of a bay can affect the inlet and, any change in the inlet including shoaling, scour, 

dredging and engineered structures can affect the tidal flow. 

The two inlets shown in Figure 6.18 are evolving in response to a number of factors including the 

original creation of Lido Key by filling many decades ago and the construction of causeways not 

shown inside the bay. Another factor in the evolution of those two inlets is the complexities added 

to the tidal hydraulics by the interconnectedness of the multiple inlets to the bay. Multiple inlet 

systems can evolve as one inlet captures more of the tidal prism and expands while allowing 

others to close. 

6.7 Future Sea Level Rise Effects on Coastal Processes

Sea level is an important component of coastal geology. The type of coast and beach at each 

location is partially controlled by the “geologic framework” that created it. This framework includes 

the local geologic formations and the interplay between vertical land movements, sediments, sea 

level changes and waves which have shaped our coasts over the past millennia. 

As sea levels rise, wave-driven erosional forces will act farther up on coastal bluffs and cause 

increased erosion (Hampton et al. 1999). Furthermore, as the timing, duration, and volumes of 

precipitation within a watershed, and as temperature modifies the state of precipitation 

(snow/rain/runoff), the quantities of water and sediment delivered to the coast in the form of fluvial 

discharge will undoubtedly affect some of these coastal processes. 

The long-term fate of some of the presently low-lying barrier islands as sea levels rise has been 

questioned. Is it reasonable to assume that a specific barrier island will still be in place in 2100 if 

sea levels have risen by 2 ft? by 4 ft? In response, some literature suggests this is probably a 

reasonable assumption at many barrier island locations in the US (FitzGerald et al. 2008, Moore 

et al. 2010, Miselis and Lorenzo-Truebas 2017, USGS 2018). 

Barrier islands have a number of natural migration mechanisms that have allowed them to move 

and survive the sea level rise of the past several millennia (Davis 1994). These mechanisms 

include barrier island rollover processes related to storm overwash processes (see Chapter 10), 

inlet migration and the movement of sand from the ocean to the bay in flood tidal shoals, longshore 

sand transport onto spits at the ends of barrier islands, and wind-blown sand transport. Some 

former barrier islands off the Louisiana coast have now become underwater sand shoals but those 

are areas which have suffered both from substantial natural subsidence and loss of all new sands 

due to the Mississippi River mouth migrations of the past several thousand years. US barrier 

islands will likely be able to survive the projected rates of future sea level rise except in areas 

where the natural sand sharing system is interrupted by natural or man-made reasons (e.g. see 

Douglass 2002). 

One complicating factor is the human response or adaptation. Within the past several centuries, 

human modifications have had significant impacts on our coasts. Human efforts, including 

dredging of navigation channels and trapping of sand by coastal structures, have removed over 

one billion cubic yards of beach sands from US beaches and caused many of the most severe 

beach erosion and shoreline recession problems in the past century (Douglass et al. 2003). 

Barrier islands and beaches have some natural mechanisms to survive relative 

sea level rise... they move. 
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However, most of those issues related to erosion caused by blocking of the natural littoral drift 

have been addressed with improved artificial sand bypassing during the past few decades. 

Artificial sand bypassing is the use of mechanical dredging equipment to place sand dredged from 

navigation channels or trapped by coastal structures on the downdrift beaches. Where artificial 

bypassing is not happening adequately, barrier islands might disappear. 

One of the primary responses to beach erosion and shoreline recession today is beach 

nourishment, the direct placement of large amounts of good quality sand on the beach to widen 

the beach (see Section 8.7). Beach nourishment has ceased and reversed recessional shoreline 

change trends along many of our highly-developed US coastal communities (Douglass 2002, 

Houston and Dean 2013). It appears that beach nourishment will be able to keep pace with the 

projections for future sea level rise (Houston 2019). Coastal wetlands will respond to future sea 

levels in different ways. The stability of a coastal wetland depends on sea and land levels, the 

biological production of the salt marsh, and the deposition of sediments that maintain the marsh 

surface in a dynamic equilibrium with the local mean sea level (Morris et al. 2002). Many presentday coastal wetlands have adjusted to the historical rate of sea level rise already. Protected 

coastal wetlands have the ability to vertically grow and adjust to some increments of rise. 

However, large areas of coastal wetlands in some locations, including coastal Louisiana and 

southern Dorchester County Maryland, are being converted from marsh to shallow open water 

habitat due to land subsidence (essentially an increased rate of relative sea level rise, RSLR), 

reduced sediment influx due to river engineering, and/or dredged canals. This process can be 

important in some of the modeling discussed in Chapter 14. For example, Smith et al. (2010) have 

adjusted Manning’s “n” values (the empirical coefficient quantifying surface roughness in most 

storm surge models) to account for changes, including conversion from marsh to open water, in 

coastal wetlands due to higher sea levels. Chapter 14 describes how to quantify the impacts of 

sea level rise on coastal highway infrastructure. The FHWA “Nature-based Solutions for Coastal 

Highway Resilience: An Implementation Guide” (Webb et al. 2019) addresses the issue of coastal 

marsh response to future sea level rise in more detail. 

6.8 Physical Models in Coastal Engineering

Coastal engineering, like the broader field of hydraulic engineering, relies on three complementary 

techniques to deal with the complex fluid flows typical of many projects: 

• field measurements and observations, 

• laboratory measurements and observations, and 

• mathematical and numerical calculations. 

Laboratory studies are generally termed physical models because they are miniature 

reproductions of a physical system. In parallel to the physical model is the numerical model, which 

is a mathematical representation of a physical system based on assumed governing equations 

and solved using a computer (Hughes 1993). 

The use of physical models in coastal engineering has evolved in response to the development 

of numerical models. For example, in the mid-1900s, large physical models of tidal estuary 

systems were used to understand the complex flows and analyze the influence of major 

engineering works. However, “large physical models of tidal estuary systems have now been 

almost totally replaced with numerical models that can predict flows with a good degree of 

success” (Hughes 1993). 

Physical models are still important for coastal engineering to address other types of problems 

(beyond the estuary tidal circulation problem). They are particularly important for understanding 
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complex flows around structures where wave and current-induced turbulence issues reduce the 

usefulness of mathematical-numerical approaches. They are important for newer fluid-structuresediment-interaction problems that have not been tested extensively. 

This evolution from physical models to numerical models is similar to that in river hydraulics 

engineering and research. For example, the J. Sterling Jones Hydraulics Laboratory at the FHWA 

Turner Fairbank Highway Research Center often applies numerical models to complex river flow 

problems but also maintains physical models to validate the numerical methods (FHWA 2019a). 

Physical model tests are often performed to calibrate empirical coefficients in the numerical model 

or to validate the results of the numerical models. “Hybrid modeling” is the use of both physical 

and numerical models to complement each technique (Hughes 1993). 

Physical models have a role in coastal engineering applications to highways for both reasons: the 

complex flows and the newer problems. For example, the problem of wave loads on bridge decks 

(Section 11.4) has been investigated with physical models in several different laboratories. 

Figure 6.22 shows a 1:20 scale physical model of a bridge deck subjected to waves in a 

laboratory. The tests are being conducted in a glass-sided wave flume which is 5 ft wide (and 75 

ft long) and the model bridge deck is the horizontal white object in the middle of the photograph. 

The model bridge deck is partially obscured by an aluminum hanger which is instrumented to 

measure loads and deflections. A single wave is visible at the left side of the photograph. It is 

propagating to the right and is about to impact the model bridge deck. This problem of wave loads 

on bridge decks involves extremely complex flows, including extreme turbulence levels and air 

entrainment, and thus the physical model tests are justified to validate the engineering analysis 

models used in design. 

The use of physical models in coastal engineering is very much of an art as well as a science. 

Model to prototype similarity issues are extremely complex. Hughes (1993) summarizes the 

issues and capabilities of physical models and laboratory techniques in coastal engineering. 

Physical laboratory models of waves can be very useful for education and training. Figure 6.23

shows a small-scale, portable, wave flume being used to wave model runup on a smooth slope. 

This flume is used in the FHWA’s National Highway Institute (NHI) Course No. 135082 Highways 

in the Coastal Environment. The participants making the measurements in the photograph are 

primarily SDOT professionals enrolled in the course (and their counterparts from some US 

Territories). Other coastal phenomena which course participants model in this small flume include 

wave properties, rock revetment stability in waves, wave loads on bridge decks, and road 

overwashing. 

The physical models shown in Figure 6.22 and Figure 6.23 only have fluids (including the air/water 

interface) and the structure. They do not include any sediment. So-called “movable-bed” models 

include sediments. The inclusion of sediments in physical modeling adds significant complexities 

because of the addition of new scaling issues. 

Figure 6.24. shows a moveable-bed physical model of an offshore breakwater system. The wave 

basin is 20 ft wide and the wave generator is behind the camera location. The progression of 

times shows the development of a salient, or bulge in the shoreline, in the lee of the middle 

breakwater. 

NHI Course No. 135082: Highways in the Coastal Environment is an 

opportunity for transportation professionals to learn coastal engineering 

principles while using a small, portable coastal wave flume model. 
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Figure 6.22. Physical model experiment on wave loads on a bridge deck in a wave flume

(University of South Alabama wave flume)

Figure 6.23. A small-scale wave flume being used to model wave runup for professional 

development training in NHI Course #135082 Highways in the Coastal Environment
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Figure 6.24. A movable-bed, physical model experiment on beach salient formation in a wave 

basin (University of South Alabama wave basin)

The initial condition, t1, is a straight beach with parallel bottom contours of sand. The subsequent 

times, t2 and t3, show laboratory waves diffracting through the breakwater gaps. The final time, t4, 

shows a salient (bulge in the shoreline) has formed on the beach in the lee of the breakwater. 

Nearshore breakwaters with beach nourishment are discussed as a nature-based engineering 

solution in Section 8.8. 
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Chapter 7 - Coastal Revetments

Rubble-mound revetments are often used as slope protection from waves for coastal highways. 

Revetments can be used for protection from four broad hydraulic situations: direct rainfall impacts, 

overland flow, stream or river currents, and waves. Only waves are addressed here. HEC-23 

(Lagasse et al. 2009) provides procedures for the design of riprap revetments for channel bank 

protection on streams and rivers. 

Consistent with USACE, AASHTO, and other practices, this chapter describes an approach for 

coastal revetments designed for waves based on determining a design wave and using Hudson’s 

equation to size the stones in the outermost, armor, layer (USACE 1984, USACE 2002, Lagasse 

et al. 2009, AASHTO 2014). This approach considers the nature of the higher forces caused by 

waves and can lead to designs with larger stones and narrower stone gradations than designs for 

non-wave situations. The chapter also discusses practical issues and common failure 

mechanisms of coastal revetments as well as wave runup and overtopping of structures. The 

chapter concludes with a detailed design example of a coastal revetment. 

7.1 Types of Revetments and Seawalls

This section describes some applications of coastal structures for embankment protection along 

the coast. Figure 7.1 shows a revetment along a Gulf of Mexico bay shoreline designed to protect 

a local road from erosion by waves during storms. This design has a stone revetment extending 

from below the water surface up to a sheet pile wall and pile cap near the roadway shoulder. 

Storm surges can exceed the pavement elevation at this location. 

Revetments, seawalls, and bulkheads are distinguished by their function (USACE 1984). 

Revetments are layers of protection on the top of a sloped surface to protect the underlying soil. 

Seawalls are walls designed to protect against large wave forces. Bulkheads are designed 

primarily to retain the soil behind a vertical wall in locations with less wave action. Design issues 

such as tie-backs and depth of sheets are primarily controlled by geotechnical issues. Figure 7.2 

provides a conceptual distinction between the three types of coastal protection based on the 

relationship between wave height and fetch (distance across the water body). Bulkheads are most 

common where fetches and wave heights are very small. Seawalls are most common where 

fetches and wave heights are very large. Revetments are often common in intermediate situations 

such as on bay or lake shorelines. 

Seawalls can be rigid structures or rubble-mound structures specifically designed to withstand 

large waves. Two very large, rigid, concrete seawalls with recurved tops to minimize overtopping 

are the Galveston Seawall (Figure 7.3) and San Francisco’s Great Highway Seawall (Figure 7.4). 

Sand has buried much of both seawalls in these photographs. Such massive structures are not 

commonly constructed in the US. Vertical sheet pile seawalls with concrete caps are common but 

require extensive marine structural design. A more common seawall design type in the US is a 

rubble-mound that looks very much like a revetment with larger stones to withstand the design 

wave height. Thus, the two terms, seawalls and revetments, can be used interchangeably with 

the former typically used for the larger wave environments. Figure 7.5, Figure 7.6, Figure 7.7, 

Figure 7.8 and Figure 7.9 are examples of rubble-mound seawalls protecting coastal roads 

exposed to open-coast storm waves. 
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Figure 7.1. A revetment protecting a coastal highway (Bayfront Road, Mobile, Alabama, 2019) 

Figure 7.2. Types of shore protection walls
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Figure 7.3. Galveston Seawall (Seawall Boulevard, 2018) 

Figure 7.4. San Francisco’s Great Highway Seawall (California SH 35, 1991)
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Figure 7.5. Seawall protecting a coastal highway (Venice, Florida, 2001)

Figure 7.6. Seawall protecting a coastal highway (Pacific Coast Highway, Pacific Palisades, 

California, 2003)
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Figure 7.7. Seawall protecting a coastal highway (Florida SH A1A, Flagler Beach, 2002) 

Figure 7.8. Seawall protecting a coastal highway (US 101, Curry County, Oregon, 2001)
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Figure 7.9. Wave striking a seawall protecting Kaumuali’i Highway on Kauai (Hawaii SH 50, 

2018)

Figure 7.10 and Figure 7.11 (as well as Figure 7.1) are examples of rubble-mound revetments 

protecting highways along coastal bays. Revetments are common on bay or lake shorelines 

where design waves are short-period, fetch-limited, locally-generated storm waves (see 0Waves). 

Revetments are criticized for a variety of reasons, including their aesthetics. Figure 7.12 and 

Figure 7.13 show two different types of protection designed for local roads that were threatened 

by bluff erosion. Figure 7.12 shows a rock revetment and Figure 7.13 shows a concrete wall that 

has been designed to look much like the natural bluff. The engineered seawall is in the middle of 

the Figure 7.13 image. The aesthetically pleasing seawall (Figure 7.13) was designed more 

recently than the rock revetment. This is an example of the evolving nature of seawall design in 

the US. 

7.2 Hudson’s Equation for Armor Stone Size

A well-designed and well-constructed rubble-mound revetment can protect embankments from 

waves. The underlying philosophy of the rubble-mound is that a pile of stones is efficient at 

absorbing wave energy and that it is robust enough to avoid catastrophic damage. It also can be 

relatively inexpensive. Some of the oldest coastal structures in the world are rubble-mounds. They 

have the inherent ability to survive storms in excess of their design storm. In the words of an old 

advertisement for a brand of watches, rubble-mound revetments “can take a licking and keep on 

ticking.” This ability to continue to provide some function even after experiencing storms that are 

more severe than their design storm is valuable in a coastal environment where costs often 

preclude selection of extremely rare design storms. 
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Figure 7.10. Revetment protecting a highway along a bay shoreline (Florida SH 60 crossing

Tampa Bay, 2003)
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Figure 7.11. Revetment protecting a highway along a bay shoreline (Washington SH 105, 

Willapa Bay, 2003)

Figure 7.12. Seawall protecting a local road (West Cliff Drive, Santa Cruz, California, 2001)
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Figure 7.13. Concrete seawall designed to look like the natural rock formation built on an 

eroding sea cliff to protect a local road (East Cliff Drive, Santa Cruz, California, 2001)

Figure 7.14 illustrates the important elements of a typical coastal revetment. An outer, armor, 

layer of larger stones is placed on a slope to absorb breaking wave energy. Hudson’s equation 

can be used to determine the appropriate size of the stones in the armor layer to withstand the 

wave-induced loads. There may be an underlayer of stones and usually a geotextile of some kind 

to prevent the underlying soils from being removed by wave action or rainfall. Both a toe at the 

bottom and a splash apron at the top are often included. 

Figure 7.14. Typical coastal revetment design cross-section 
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Hudson’s equation (USACE 1984) provides a basis for estimating the appropriate stone size in a 

sloped revetment that is struck by waves (e.g. Figure 7.15). It accounts for the most important 

variables, including design wave height, structure slope, and stone density and angularity. The 

appropriate median weight for the outer, or armor layer, stones is: 

 

 

 (7.1)

where:

 W50 = median weight of armor stone

 wr = unit weight of stone (~165 lb/ft3 for granite) 

 H = design wave height

 KD = empirical stability coefficient (= 2.2 for rip-rap gradations)

 Sr = specific gravity of stone (~2.65 for granite)

θ = slope

Note that Hudson’s equation relates the armor stone weight to the cube of wave height. Therefore, 

careful consideration of the design wave height is needed (see Section 7.3). 

Also, revetment armor stones are generally more stable if placed on a milder slope and this is 

reflected in Hudson’s equation. Steeper slopes call for larger, heavier stones. The range of 

recommended slopes here is up to 2:1 (horizontal: vertical). Note that, by definition, the cot θ = 2 

for a 2:1 slope and cot θ = 3 for a 3:1 slope, etc. Engineering practice does not recommend 

revetment structure slopes steeper than 11⁄2:1 (horizontal: vertical) (USACE 1984). Hudson’s 

equation also includes the influence of density of the rock and therefore the armor layer can use 

other rocks besides granite. 

The empirical coefficient in Hudson’s equation, KD, is based on laboratory tests and varies to 

include the effect of stone angularity/roundness, number of layers of armor stone, distribution of 

individual stone sizes about the median size, and interlocking characteristics. The value 

suggested here, KD = 2.2, for rip-rap is for a layer of rough-angular quarrystone at least two stones 

thick with a gradation of weights that varies between 

 (7.2)

where:

 W50 = median weight of armor stone

 W = weight of all the armor stones in the revetment

The gradation of the individual stones should be relatively uniform between the upper and lower 

limit of Equation 7.2. USACE (1984) and USACE (2002) discuss values of KD for a variety of other 

situations. 

Coastal revetment rock size to survive waves is based on Hudson’s equation.
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Figure 7.15. Storm waves breaking on a rock, rubble-mound breakwater in a bay (Mobile Bay, 

Alabama)

The equivalent diameter of the median stone weight can be found using a number of different 

empirical formulations. The CEM (USACE 2002) provides an approximate empirical equation for 

the typical diameter of an angular armor layer stone (using sieves at the quarry) as:

 

 

 (7.3)

where:

 D = typical size, diameter, of a stone

 W = weight of the stone

 wr = unit weight of stone (~165 lb/ft3 for granite)

An armor layer thickness of at least two stones is typical as a rule of thumb in all coastal design 

situations and the thickness, t, of the armor layer (and any underlayer) can be calculated using

 (7.4)

where:

 t = thickness of armor layer (or underlayer)

 n = number of stones per layer (at least 2)

 W50 = median weight of armor stone

 wr = unit weight of stone (~165 lb/ft3 for granite)
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Caution should be exercised when using standard specification language in coastal designs. The 

stone gradations recommended for coastal revetment rip-rap are narrower than those 

typically called for in highway specifications for other purposes. For example, the FHWA’s 

Standard Specifications for Roads and Bridges on Federal Highway Projects calls for Class 5 riprap to have a median weight of W50 = 525 lb, with 15% of the stones weighing between 0 to 190

lb and including spalls and rock fragments to provide a stable, dense mass (FHWA 2014).

However, the gradation recommended above for Hudson’s equation for a coastal revetment for 

the same median weight calls for all stones to weigh between 65 and 2100 lb. Thus, the 

recommended coastal revetment gradation precludes the smaller stones in that FHWA 

specification. These smaller stones are typically not included in coastal revetments for two 

reasons. One, smaller stones work against one of the fundamental purposes of a rock revetment 

under wave attack which is to have large open voids in the armor layer to cause turbulence that 

removes the incident wave energy most effectively. Two, they have more tendency to move in 

response to wave action. Thus, while there may be reasons to “chock” the larger stones with the 

smaller stones in a revetment designed for the riverine environment, there are reasons not to do 

so in a revetment designed for the coastal environment. 

A properly designed coastal revetment usually needs specially ordered armor stone. This is not 

unusual and most quarries understand these USACE coastal rock gradations. Most SDOTs have

rip-rap gradation specifications for revetments which were primarily developed for the riverine 

environment yet may state that they are for “wave protection for causeways and shoreline 

roadway embankments” and for different levels of “waves.” But, some of these standard 

specification statements are not consistent with the coastal revetment design concept outlined in 

this chapter. A few SDOTs have special coastal revetment design material which is consistent 

with sound coastal engineering practice and with this chapter. 

The use of an underlayer of rocks under the armor layer is common except when the stone size 

is less than 200 lb. The underlayer should have a median weight no smaller than one-tenth that 

of the armor layer stones (USACE 1984): 

 (7.5)

Undersized underlayer stones might be pulled out between the gaps of the armor stones. The 

underlayer should have a thickness of at least two underlayer stones.

Engineering practice recognizes that one component of a coastal revetment design cross-section 

shown in Figure 7.14 is use of a filter layer of geotextile or composite geotextile/geogrid between 

the rocks and the underlying soil (Lagasse et al. 2009, AASHTO 2014). The geotextile should 

provide rapid transfer of water through the material while holding soil particles, and it should be 

strong enough to survive the construction process without the overlying rocks puncturing it. The 

modern use of a plastic grid integrally welded to the geotextile can provide some additional 

strength to soft underlying soils. The geotextile should not allow the rocks to slide down the 

surface. The geotextile should be wrapped up and back under some of the armor stones at the 

seaward end so it does not “wave” in the waves when some scour occurs. This will protect the 

soil under the toe and protect the entire slope better.

7.3 Design Wave Heights for Revetment Design

Hudson’s equation is sensitive to wave height. The proper wave height for Hudson’s equation 

above for coastal revetment design is either the depth-limited maximum wave height or the 

average of the highest 10% of all wave heights in the design sea-state (H�10) whichever is lesser 

(USACE 1984). 
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The basis of using this wave height value within Hudson’s equation is engineering judgment and 

interpretation considering the origin of the equation (USACE 1984). Hudson’s equation was 

originally derived based on monochromatic laboratory tests. Thus, the proper selection of a 

corresponding wave height statistic from an irregular sea-state is not obvious. Experience has 

found that the use of the significant wave height, Hs, in Hudson’s equation is not conservative and 

can lead to undesired levels of damage to the revetment. 

Some researchers have suggested that the proper irregular wave height statistic for use in 

Hudson’s equation is H�10. To be more conservative, some engineers use the average of the 

highest 5% of all wave heights in the design sea-state (H�5). The relationships between significant 

wave height and these other statistics are H�10 = 1.27 Hs and H�5 = 1.38 Hs (see Table 5.1). 

Coastal revetments are often located where the design sea-state is depth-limited (see Section 

5.2), i.e. the depths are so shallow immediately offshore of the location of the revetment that the 

largest storm waves have broken and the largest waves remaining are, 

 (7.6) 

where:

 Hb = maximum breaking wave height

 ds = design depth at the toe of the structure

To account for the distance over which waves travel as they break, a depth some distance 

offshore of the toe (say one wavelength) sometimes is used in Equation 7.6. For steep nearshore 

slopes see USACE (1984) and USACE (2002) for guidance on depth-limited breaking. 

A depth-limited design wave height used in Hudson’s equation should account for any long-term 

erosion that may change the depths immediately offshore. The construction of a revetment, while 

it protects the upland, does not address the underlying cause of erosion. The depths at the toe of 

the revetment will likely increase if the erosion process continues. Also, the revetment or seawall 

does not allow the material in the bluff to naturally nourish the beach at the toe of the bluff. And, 

the presence of a revetment or seawall can increase the vertical erosion at its base by increasing 

wave scour (see Chapter 12). 

Hudson’s equation has no factor of safety. The KD values suppose that armor layer will sustain 

some small level of damage, represented as a percentage of the armor layer stones that moved 

under the design wave condition. The KD values represent 0-5% damage. Thus, it is entirely 

appropriate to add some conservatism or factor of safety to the design process based on 

engineering judgment. Selection of a conservative design wave height used (such as H�5) in 

Hudson’s equation or increasing the specified design median rock weight could provide this factor 

of safety. 

Fetch and depth limitations result in a design wave height for many revetments being less than H 

= 5 ft. Designs for larger wave heights use a narrower gradation of armor stone than specified in 

Section 7.2 above (see USACE 2002). When design wave heights get very large and the design 

water depths get very large, problems with the performance of rubble-mound structures can 

occur. These problems relate in part to wave groupiness (back to back large waves), design seastate specification, constructability and other issues. Seawalls with design wave heights much 

greater than H = 5 ft need more judgment, more experience, and input from a trained, experienced 

Hudson’s equation has no inherent factor of safety, so appropriate 

engineering judgment should be used for design applicatons.
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coastal engineer. Other details about the design of rubble-mound revetments are discussed in 

the CEM (USACE 2002). 

7.4 Other Issues for Coastal Revetment Design

This section discusses other related issues for coastal revetment design: common failure

mechanisms and alternative design approaches.

7.4.1 Common Failure Mechanisms of Coastal Revetments

Understanding common failure mechanisms is valuable when designing coastal revetments. This 

section briefly discusses those mechanisms and approaches to avoid them. Each can be 

prevented by an experienced, knowledgeable engineer. Common failure mechanisms are: 

1. inadequate armor layer,

2. inadequate underlayer,

3. flanking, 

4. toe scour, and

5. overtopping splash. 

The first failure mechanism, inadequate armor layer, can be avoided by using Hudson’s equation

to size the armor layer. Sections 7.2 and 7.3 outline the best proven way to design revetments for 

wave attack. Revetment failures have occurred because the rocks were just too small for the sitespecific wave conditions. 

Also, careful engineering judgment based on experience should be used when the design crosssection varies from the typical cross-section shown in Figure 7.14. Figure 7.16 shows a revetment 

protecting a highway that has a small, vertical bulkhead with stones on the seaward side and an 

almost flat stone section landward. This cross-section design essentially “trips” breaking waves 

when storm surge raises the water level and begins to inundate the highway. Thus, breaking 

waves plunge directly on the small stones behind the wall and move them onto and across the 

roadway during major storms. For very mild slopes approaching horizontal, Hudson’s equation 

estimates very small armor stones. However, sound coastal engineering judgment would suggest 

a larger stone weight should be used to prevent this type of failure. 

The second failure mechanism, inadequate underlayer, can be avoided with a design as outlined 

in Section 7.2. The use of a proper underlayer helps to ensure the stability of the soil underlying 

a revetment. Figure 7.17 shows a failed attempt to protect an embankment. The slope protection, 

used concrete slab panels, were available from some other project and were set on the surface 

of the eroding bluff. Although the panels were heavy enough to withstand the wave action itself, 

wave action during storms likely pulled, or pumped, the underlying soil out from between the gaps 

in the slabs. Consequently, the panels collapsed. The second photograph shows the panels after 

collapse. A rock revetment was subsequently placed farther back on the bluff. The original panel 

design did not adequately protect the underlying soil and did not have the flexibility of a rubblemound revetment. 
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Figure 7.16. A revetment with rocks too small to withstand the wave attack at this location

Figure 7.17. Example of a failed attempt at embankment protection (USACE archives 

photographs) 

The third failure mechanism, flanking, can be avoided with careful consideration of the ends of 

the revetment. Flanking occurs when adjacent, unprotected shorelines continue to recede. 

Erosion at the end of the wall allows wave action to remove the soil from behind the wall starting 

at the ends, then progressing along the walls as they fail. Flanking can be avoided by extending 

the revetment or wall to meet an existing revetment, a wall, or a natural rock outcropping. It can 

also be avoided by using a return wall. A return wall is aligned perpendicular to the shoreline. The 

length of the return wall should exceed the expected long-term and storm-induced recession of 

the adjacent shorelines. 

The fourth failure mechanism, toe scour, can be avoided with the use of an adequate toe scour 

apron. Vertical scour at the toe of a revetment or seawall can cause the underlying soil to be 

exposed to waves. One solution to toe scour problems is shown in the recommended revetment 

cross-section in Figure 7.14. A significant volume of stones is placed at the toe. This toe is 

Typical coastal revetment failure mechanisms can be avoided with 

engineering judgment.
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designed to collapse into any toe scour hole that develops without loss of the stones on the slope. 

For very erosive areas, more stones can be used in the toe (see USACE 2002). 

The fifth failure mechanism, overtopping splash, can be avoided with careful consideration in 

design. Overtopping splash at the top of a revetment or seawall can lead to failure by exposing

the underlying soil to waves, resulting in its removal. If the wall does not extend to a high enough 

elevation, waves will overtop the wall. Figure 7.18 shows indications of overtopping splash 

damage at the top of rock seawall. A solution to overtopping splash problems is to provide a 

splash apron as shown in the revetment cross-section in Figure 7.14. The rocks extend some 

distance back from the break in slope. The width of the splash apron varies depending on the 

severity of the expected overtopping. A minimal splash apron width is 5 to 10 ft. An example of a 

splash apron is shown at the top of the rock revetment in Figure 7.19. Splash aprons can raise 

issues with overall space limitations and with highway clear zones. 

Figure 7.18. An example of splash damage behind seawall (circa 2004) 
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Figure 7.19. Example of a splash apron (center of photograph) at the top of a coastal rock 

revetment protecting a local road along a bluff on Chesapeake Bay (Shore Road, Anne Arundel 

County, Maryland, 2017)

7.4.2 Alternative Revetment Concepts and Materials

Revetments come in all shapes and sizes, widely varying in form and material. Although the 

layered armor stone revetment is a proven, robust design in coastal situations, other approaches

are often employed. The USACE CEM discusses and summarizes methods to quantify the 

stability of revetments in wave attack beyond the Hudson’s equation approach outlined above 

(2002). Most of that USACE discussion is in terms of a “stability number;” likely necessitating 

services of a trained coastal engineer for design using these other methods. 

A “dynamic revetment” (or “berm revetment”) which contains a significantly larger volume 

of smaller stones with a wider gradation is one alternative to the traditional design of Figure 

7.14. The general design philosophy of a dynamic revetment is to allow the rocks to move in 

response to storm waves into an equilibrium shape much like a cobble or sand beach. This 

approach has the advantage of allowing a higher percentage of the stones from the quarry to be 

used in the breakwater than the traditional design approach (Baird and Hall 1984). The equilibrium 

cross-section shape is similar to the equilibrium shape for sandy beaches (see Figure 6.11) where 

the rocks act, essentially, as much larger grain sediments than sands and develop a concave 

upward shape. Stones at the location and elevation where the largest waves are striking the 

structure during design conditions get pulled offshore a short way and change the shape of the 

face of the structure. 

There has to be enough rock material so that the underlying soils are not exposed and the upland 

feature (roadway) is not attacked by the recession of the structure face during the design storm. 

This recession can be significant (see Andersen et al. 2014). Berm breakwater revetments thus

typically have a much wider footprint than traditional rubble-mound revetments. Many practical 

design examples of this approach around the world (see van der Meer and Sigurdarson 2017) 
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are for harbor breakwaters exposed to very energetic seas, i.e. not typical of shallow water 

revetments along most US coastal highways. However, the general concept extends to cobble 

beaches and several of those have been included in recent designs to protect US coastal 

highways as nature-based solutions (Surfers Point Managed Retreat Project in Ventura, CA and 

Cardiff Beach, CA). 

Concrete armor units provide an alternative to using extremely large stones in the armor layer of 

a traditional revetment design. These typically are lighter since they interlock better than 

quarrystone and thus have higher KD values. They are made of reinforced concrete and can be 

cast on site. A common reason for their use is that the wave energy is too energetic and thus the 

extremely large armor stones that could be used are extremely expensive. A number of shapes 

of artificial concrete armor units are available, including several patented shapes that may involve

licensing to use (thus augmenting cost). Concrete armor units are typically used for harbor 

breakwaters, not coastal highway revetments. 

Concrete block revetments, however, are problematic alternatives to rubble mound revetments. 

Some of these have some physical interlocking between individual blocks and others do not. The 

performance of interlocking blocks in severe coastal environments has not been good. One 

problem is that minor damage can lead to failure of a large portion of the revetment. Two examples 

are shown in Figure 7.20 and Figure 7.21. 

The failed revetment in Figure 7.20 has subsequently been covered by a sand beach through 

beach nourishment (see Figure 8.10). The failed revetment in Figure 7.21 has been covered by 

a sand beach through beach nourishment stabilized with offshore segmented breakwaters (see

Figure 8.12). 

Problems with concrete block revetments in coastal situations often develop at the ends of the 

revetment where the blocks abut a more rigid structure. Even a small amount of settlement can 

affect the performance and aesthetics of block revetments.

 

Figure 7.20. Example of rigid concrete-block revetment failure (Florida SH A1A, Delray Beach, 

circa 1972; University of Florida and USACE archive photos)
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Figure 7.21. Example of failed block revetment (Louisiana SH 82, circa 1980, USACE archives 

photos)

Other revetment systems include articulated concrete mats, flexible rock-filled marine mattresses, 

gabions, and sand-filled geotextile tubes or bags. Articulated concrete mats have concrete blocks 

interconnected by cables. The size and weight of the blocks should be a function of the wave 

height, slope, currents, etc. Proper installation includes adequate filtration material and secure 

anchoring at the top of the slope. The toe is sometimes unsecured to allow it to settle (scour). 

Flexible rock-filled marine mattresses are used as foundations and for scour control underneath 

marine structures like nearshore breakwaters; but they are not generally recommended for slope 

protection by themselves. Gabions are rock-filled "baskets" composed of steel wire or 

polypropylene grid which are stacked for embankment protection but their performance in coastal 

waves has been “poor” (Jackson et al. 2006).

Sand-filled geotextile containers (tubes or bags) are typically only used for temporary revetment 

protection in the coastal zone. They can be buried within the existing grade and designed to 

become exposed only during storm erosion (an example is illustrated in Figure 8.2). The 

structures are prone to damage or failure by vandalism, rolling, and natural deterioration when 

exposed. 

7.5 Wave Runup and Overtopping – Coastal Structures

Waves breaking on a sloping structure like a revetment or seawall cause water to run up the 

structure, potentially overtopping it. For situations where the embankment elevation is much 

higher than the expected level of wave runup during design conditions, a decision regarding the 

height of the revetment is needed. The height of wave runup (Ru) is shown in Figure 7.22. It can 

be estimated using: 

 with a maximum of 3.2 r (7.7) 

where:

 = runup level exceeded by 2% of the runups in an irregular sea

 Hs = significant wave height near the toe of slope

 r = a roughness coefficient (r = 0.55 for the stone revetments)

ξop = the surf similarity parameter as defined below
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 (7.8) 

where:

θ = angle of slope of structure (see Figure 7.22) 

 Hs = significant wave height

 Tp = wave period, peak period

 g = acceleration of gravity

Figure 7.22. Wave runup on a structure definition sketch

Note that R is defined as the vertical difference in elevation between the SWL and the peak of the 

wave runup. The level given by Equation 7.7 is for the 2% runup level - the runup level exceeded 

by 2% of the incoming waves. Thus, 2% of the waves will run up higher than this level. 

The roughness coefficient (r) accounts for the roughness of the surface of the revetment with r = 

1 for smooth slopes. For rock revetments, such as shown in Figure 7.14, the recommended value 

for r is 0.55, and Equation 7.7 has an upper limit of 3.2 r = 1.76. Thus, the 2% level of runup is < 

1.76Hs. Equation 7.7 is adapted from a methodology developed by van der Meer and summarized 

by Pilarczyk (1999). More detail including other structure geometries can be found in that 

reference. 

Wave overtopping of revetments and seawalls occurs when runup exceeds the top or crest of the 

structure. Wave overtopping onto coastal roads is fairly common in some parts of the United 

States. Figure 7.23 shows a wave overtopping a seawall protecting a local road in Lihue, Hawaii. 

The wave is moving from left to right striking the vertical wall in the foreground and some of the 

splash from the wave is landing landward of the wall. Overtopping splash from larger waves on 

high tide often splash so far that it lands on the other side of the second vertical wall by the road 

on the far right-hand side of the photograph. Local drainage systems channel the overtopped 

water back to the sea to the left from both the area between the two walls and the road. Local 

drainage is often designed, like this, to return overtopped water to the sea. Building seawalls high 

enough to completely prevent overtopping is often unacceptable because of aesthetics and costs. 
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Two aspects of overtopping which can be of interest to the highway engineer are the timeaveraged volumetric rate of overtopping and the tolerable limits of intensity or force of a single 

wave overtopping event. Accurately estimating volumetric overtopping rates can be vital to design 

of seawall crest elevations if inland flooding is caused. Unfortunately, accurately estimating 

overtopping rates can be very difficult for many situations. Many of the tools available for 

estimating overtopping rates are empirical results from physical model testing and are only valid 

for very specific circumstances (including structure type, material, and sea state). Guidance on 

estimating overtopping can be found in Goda (1985), USACE (2002), EurOtop (2007) and 

EurOtop (2016). 

Figure 7.23. Wave overtopping a seawall (Lihue, Hawaii, 2018)

The EurOtop model has been used to estimate the volumetric rate of flooding due to waves 

overtopping the entrance walls at the western portal to the I-10 tunnel near Mobile Bay. That 

analysis found that significant levels of tunnel flooding will occur when the storm surge elevation 

is 2 to 3 ft below the crest of the tunnel entrance walls due to wave overtopping (Douglass 2010). 

Interestingly, the entrance walls at that tunnel portal are 3 ft higher than the other portal of that 

tunnel which is in downtown Mobile and will not have any wave attack or wave overtopping (clearly 

the original tunnel designers understood the overtopping risk at the western portal well enough to 

raise those elevations an appropriate amount!). 

USACE (2002) condenses a number of field studies regarding tolerable limits and critical values 

of overtopping discharge as shown in Figure 7.24. Some rough guidelines of the critical 

overtopping values for various asset types are presented. The overtopping discharge rates shown 
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are in terms of q, the time averaged flowrate over the wall per unit length of the wall since this is 

a typical result from overtopping equations like the EurOtop model. It should be recognized that 

the overtopping discharge is very unevenly distributed in space and in time because it varies 

considerably from wave to wave. Most of the overtopping is due to a small fraction of the waves, 

the largest waves in the sea state. The intensity of overtopping from one wave can be more than 

100 times the average overtopping rate. The true critical values for any given site are extremely 

site dependent. What constitutes a "critical" value is itself very subjective, depending on individual 

opinion, local custom, and other factors. 

7.6 Design Example: Coastal Revetment 

This section presents an example procedure for the design of a rubble-mound revetment exposed 

to waves. The example applies and follows the engineering principles within this chapter. 

7.6.1 Example Situation

Consider the scenario of a coastal highway running along the top of an eroding bluff on a bay 

shoreline. Shoreline and bluff erosion presently threaten to undermine the roadway. Figure 7.25

shows the eroding bluff and roadway shoulder in cross-section. The design SWL (including tide, 

storm surge) has been estimated as +8 ft NAVD. The design wave height (Hs) has been estimated 

as 5 ft with a wave period (Tp) equal to 5 s (using a wave generation model for this fetch-limited 

bay location) (see Section 5.4). 

7.6.2 Design Steps

Hudson’s equation (Equation 7.1) is used to determine the armor layer stone weights needed to 

withstand the wave loads. The content and discussions outlined above reduces to the following 

steps:

Step 1 – Determine proper H to use in Hudson’s Equation, H10: 

Following Section 7.3 and using Table 5.1, the proper H to use in Hudson’s equation is H10, which 

is the average of the highest 10% of the waves in the sea state:

 

Step 2 – Specify slope, specific gravity and weight of stone

The slope of the revetment will be set at 2:1 (horizontal: vertical) to follow the recommendation of 

Section 7.2. This slope also generally matches the assumed existing slope shown in Figure 7.25. 

There may be issues related to width of right-of-way restrictions and encroachment into adjacent 

water bottoms that are site-specific. Milder slopes need more space and more, but smaller/lighter, 

stones. Cut and fill to match the desired revetment slope is often a consideration also. 

Assume that the stone used for construction of the revetment is granite having a specific gravity 

of Sr = 2.65 and a specific weight of wr = 165 lb/ft3 (typical values for granite). 
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Figure 7.24. Critical values of average overtopping discharges (adapted from USACE 2002)
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Figure 7.25. Eroding profile input to coastal revetment design example

Step 3 – Apply Hudson's Equation to determine design armor stone median weight, W50

Next, determine the median weight of armor stone needed for stability given the design wave 

height and other specified parameters by applying Hudson's equation (Equation 7.1): 

 

 

 

 

 

 

This value can be rounded up to 2,500 lb. A Hudson’s stability coefficient of KD = 2.2 is used 

following Section 7.2 for rough, angular quarrystone. Since no factor of safety is inherent in the 

Hudson's equation calculation, one way to make the design more conservative is to increase the 

median weight by some rounding up of the median weight, say to W50= 2,500 lb, with a 

corresponding increase in the full gradation. Another way to make the design more conservative 

would have been to round up the input H. 

Step 4 – Specify armor stone weight gradation

The standard USACE coastal riprap gradation used with Hudson’s equation (see Equation 7.2) is 

0.125 W50 < W < 4 W50. Thus, the lower and upper bounds of the riprap gradation for use in the 

revetment are: 

 

 

Thus, all the stones in the revetment armor layer should weigh between 300 lb < W < 5 tons. 

Alternatives which would reduce the armor stone median weights include using milder slopes or 

using a dynamic revetment (see Section 7.4). 

Step 5 – Determine corresponding design armor stone median size, d50

The equivalent diameter of the median stone weight can be estimated with Equation 7.3: 

 

and the armor sizes will range from
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to

 

Thus, the size of the individual stones in the armor layer will range from 1.4 to 4.5 ft with a median, 

or typical, of 2.8 ft.

Step 6 – Specify armor layer thickness

The armor layer of the cross-section should be at least two stone widths thick. Using Equation 

7.4 this thickness is:

 

Step 7 – Determine the level of runup

Given the low elevation of the top of the bluff, relative to the design SWL, as shown in Figure 

7.25, runup will likely exceed the elevation of the road pavement. This can be checked with the 

runup equation given above, Equation 7.7: 

 

 

 

 � 

 

Thus, wave runup from the largest (2%) waves in the sea state could be expected to reach the 

design storm surge elevation plus this value, 8 + 11 = 19 ft NAVD if the revetment extended up 

that high. Since it does not, severe overtopping and overwashing of the road is expected during 

design conditions. Thus, a splash apron as part of the design cross-section is needed and care 

should be taken during construction at the location where the rocks meet the pavement shoulder. 

Step 8 – Specify splash apron

The splash apron serves to armor the top of the coastal revetment embankment to prevent failure 

due to wave overtopping. Given the proximity of the highway to the eroding bluff, the splash apron 

can reach all the way to the edge of the pavement and it should include the armor layer with the 

large stones to survive design conditions. 

Step 9 – Specify toe protection

Toe protection is necessary at the foot of the revetment. As a rule of thumb, the maximum wave 

height is a conservative upper limit of scour depth at the toe of the structure (see Chapter 12). 

Thus, a toe depth of 6.5 ft should be adequate. This is between 2 and 3 of the median stone 

widths. Construction of a toe often involves excavation. 

Step 10 – Specify underlayer and geogrid/textile

The design cross-section should include an underlayer that consists of large enough stones that 

they do not get pulled out of the holes in the armor layer. The median rock weight of the underlayer 

should be, using Equation 7.5,
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This is the median weight of the underlayer. The underlayer can also follow a riprap gradation

with minimum and maximum weights of: 

0.125 W50 / 10 = 30 lb

4 W50 / 10 = 1,000 lb

Thus, all the stones in the revetment’s underlayer should weigh between 30 lb < W < 1,000 lb. A 

geogrid is included under that and a layer of gravel is often included above the geogrid to avoid 

damage to the geotextile during construction. 

The diameter of the underlayer stones will range from (using Equation 7.3)

 

to,

 

with a median, or typical, size of, 

 

The underlayer thickness (also set two stones thick) using Equation 7.4 will be: 

 

So, the armor layer will be 5 ft thick and the underlayer will be about 2.3 ft thick. 

Step 11– Sketch design cross-section

The design cross-section (Figure 7.26) of the revetment will have armor stones weighing between 

300 lb and 10,000 lb with a median stone weight of 2,500 lb. The slope of the armor layer will be 

2:1 (H: V), and the armor layer will be 5 ft thick. The armor layer will include a flat splash apron 

reaching to the edge of the pavement and toe protection reaching down about 6.5 ft below the 

original grade. The cross-section will include an underlayer of rocks weighing between 30 lb and 

1,000 lb with a median weight of 250 lb, 2.3 ft thick. Below the underlayer is filter layer of geotextile 

or geogrid. Given that this design extends to the edge of pavement, a guardrail is likely needed 

(not shown) for driver safety concerns. 
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Figure 7.26. Coastal revetment design example cross-section
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Chapter 8 - Roads in Areas of Receding Shorelines

Many roads in the US near the coast are exposed to waves on a daily basis, and many more are 

occasionally exposed to waves during storms. Also, much of the US coastline is experiencing 

long-term recession (erosion) due to sea level rise and other factors. Consequently, roads near 

these receding shorelines will eventually be subjected to waves. While this chapter does not 

present any FHWA requirement, it does attempt to provide an understanding of receding 

shorelines and associated issues faced in HICE by transportation officials. Therefore, this chapter

outlines a potential process for how to assess and design for roads in areas of receding 

shorelines. It covers how long-term shoreline changes can be quantified and used to estimate 

future shoreline positions. It describes the general engineering options available for highways 

exposed to coastal waves and erosion including relocation/abandonment, and the alternative 

shoreline stabilization techniques available for protecting a highway in place. These alternatives 

include a spectrum of solutions from seawalls, revetments, and bulkheads to nature-based 

solutions like beaches and marshes. 

8.1 Examples of Issues

The “Stump Hole” area of Cape San Blas, Florida has a roadway with a rubble mound revetment 

seawall protecting it from waves (see Figure 5.16). In the 1970s the road was located over 300 ft

landward of the shoreline. The beach here is eroding at a high rate so that the shoreline has been 

moving toward the road at an average rate of 15 ft per year for the past 35 years. Shoreline 

recession progressively narrowed the beach until an emergency rock revetment/seawall was 

constructed. The revetment has not slowed the recession of the adjacent beaches. Tree stumps

are exposed in the surf and on the beach face as a result of the recession. Shoreline recession 

has continued on both sides of the revetment and the road is extending farther out into the sea. 

The seawall is now protecting the road and functioning like an artificial headland. 

Figure 8.1 shows the Cape Shoalwater area of Washington SH 105 built along another rapidly 

receding shoreline. At the time of this photograph (April 2003) there was a rock revetment at the 

base of the bluff and a groin in the background. There had been some limited beach nourishment. 

The shoreline recession has continued in this general location and now there is a rock revetment 

along more of this shoreline (2018). Figure 8.2 shows a road on a narrow, low-lying barrier island 

in New Jersey. The road parallels the beach on one side and a back-bay wetland on the other. 

The shoreline here has been receding for decades and the road is threatened. Several shoreline 

stabilization and roadway protection projects have been attempted. A sand-filled geotextile tube 

was built and covered with a sand dune to protect the highway but was being repaired after a 

small storm in 2003 and is visible here. This beach and dune system were restored with a major 

beach nourishment project in 2016 which was a joint effort of the USACE and the New Jersey 

Department of Environmental Protection. 
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Figure 8.1. A highway initially built inland now threatened by long-term shoreline erosion. Cape 

Shoalwater area of Washington SH 105 (2003)

This location is typical of coastal road problems in two ways. One, the problem areas often remain 

problem areas for the transportation entity for a long time. Two, many former road problem areas 

are no longer problem areas because of beach nourishment by non-transportation organizations.

Figure 8.3 shows a local road undermined by bluff erosion on Lake Erie. This road used to 

continue straight ahead until bluff erosion undermined the pavement. The bluff erosion has been 

exacerbated by sand starvation of the beaches at the base of the bluff by an updrift jetty system.

Figure 8.4 shows Texas SH 87 along the east Texas coast of the Gulf of Mexico destroyed by 

shoreline recession. A twenty-mile stretch of this highway along the coast has been closed since 

1989 when a storm caused significant pavement damage. 
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Figure 8.2. A local road threatened by long-term shoreline recession (Ocean Drive, Whale 

Beach area of Cape May County Road 619, Ludlam Island, New Jersey 2003)

Figure 8.3. A local road being undermined by bluff erosion and long-term shoreline recession on 

the Great Lakes (Painesville, Ohio, 2001)
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Figure 8.4. Road destroyed by shoreline recession: a) broken pavement on the beach at the old 

location; b) south end of the closed section; c) location map. (Texas SH 87, Jefferson County, 

circa 2002)

8.2 Quantifying Shoreline Change Rates

This section describes a potential process (1) to quantify shoreline change rates and (2) to 

estimate future shoreline positions. It also describes the shortcomings of such estimates, and the 

concept and role of sediment budgets. 

Coastal erosion rates are often given in terms of the change in average annual shoreline position 

with time, e.g. 2 ft per year. These are actually shoreline change rates rather than erosion rates. 

The terms “recession” and “accretion” are typically used to describe the direction of 

shoreline movements. A beach that is widening in response to sand deposition has an accreting 

shoreline. A beach that is narrowing in response to erosion has a receding shoreline. 

Shoreline change rates typically vary with location and time. Shoreline change rates should be 

evaluated over as long a time period as possible with as many observations as possible. “Longterm” shoreline change usually refers to multi-decadal time scales. Many observations in a single 

year can give some estimate of the seasonal variability in shoreline position as sand moves crossshore on the profile. However, these data are typically not useful for developing “long-term”

shoreline change trends. 

Historical shoreline data are available from a variety of sources. The USGS has significant 

shoreline change databases covering all US coasts. Often a state resource agency, such as 

the State of Florida’s Department of Environmental Protection, has the most 

comprehensive shoreline change information. 

No accepted national standard exists for shoreline change analyses. The quantity and quality of 

shoreline change data vary significantly. Each location has different types of historical data and 
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analyses. New analysis of existing data may be needed to develop a clear understanding of 

historical shoreline changes for a project. 

Historical shoreline positions can be measured by repetitive surveys or by remote sensing such 

as air photograph interpretation. Historical and current vertical air photographs can provide the 

basis for shoreline location data with proper interpretation and positioning analysis. One source 

for estimates of older historical shoreline locations is NOAA’s National Ocean Survey surveys and 

the surveys of their predecessor organization, the US Coast & Geodetic Survey (USC&GS). Highquality estimates of shoreline position can extend as far back as the 1850s. The USC&GS 

significantly improved the accuracy of coastal surveys at about that time. Pre-1840 estimates of 

shoreline position are not as accurate as those done after 1850. USC&GS “t-sheets” and “hsheets” are the summary plots of specific surveys and correspond with the dates of the actual 

survey. Navigation charts, however, are updated continuously and the date of the chart does not 

correspond with the date of all of the information shown on it. Accuracy of these historical 

shoreline estimates often can be adequate for the purpose of shoreline change analysis (Crowell 

et al. 1991). 

An example of a shoreline change analysis is shown in Figure 8.5. The plot is for five locations, 

spaced 1,000 ft apart, centered on the location where the road in Figure 5.16 extends into the 

sea. The plot shows the measured shoreline locations through time and the lines are splines fit to 

the data for visual convenience. A recessional (negative) trend is obvious at all five locations and 

is very consistent at four of the five locations. Some variability in the overall trend at station R-106 

may be explained by effect of the revetment protecting the road. The nomenclature and 

designations of the stations are those of the Florida Department of Environmental Protection 

FDEP). Given the natural temporal variability of shoreline location, the strong trends shown in 

Figure 8.5 are not typical. Similar plots often show much more variability through time and the 

trend is not always clear. The site analyzed in Figure 8.5 has a very clearly recessional shoreline. 

Figure 8.5 shows a nonlinear trend in shoreline position through time. The recession rate appears 

to be greatest in the most recent years. A relatively large number of major storms have impacted 

this coast since 1997. 

More results from the same shoreline change analysis are shown in Figure 8.6. The average 

annual recession rate along 30,000 ft of shoreline on the west-facing shoreline of St. Joseph 

Peninsula is shown. The average annual rate depends on location and the time period over which 

the average is taken. Clearly the recession rate is much greater to the south (higher R-monument 

numbers). 

Recession rates shown in Figure 8.6 have been calculated by the “end-point method” which 

averages the change in shoreline position from the beginning to the end of the time period. An 

alternative to the end-point method is linear regression (Crowell et al. 1997). Linear regression is 

typically preferred to the end-point method because it uses all the available data and is less 

sensitive to one spurious or aberrant value. 

US Coast & Geodetic Survey maps of historical shorelines became very 

accurate due to changes in land surveying techniques in the 1850s. 
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Figure 8.5. An example of shoreline position changes through time. Stump Hole area of St. 

Joseph’s Peninsula, Florida

8.3 Estimating Future Shoreline Positions

An estimate of future shoreline locations can be valuable in planning highways near areas of 

receding shorelines. The most common method for estimating future shoreline positions is direct 

extrapolation of historical shoreline change rates to the present shoreline (Crowell et al. 1997). 

Figure 8.7 shows some historical shoreline positions as well as projected future positions at one 

location. The historical shoreline data were obtained from the FDEP on-line database. Florida 

originally obtained the older (1868 and 1934) data from the USC&GS, made appropriate datum 

corrections and added their own data from beach profile surveys. The projected shorelines are 

extrapolations from the 2005 shoreline location, at 1,000-foot intervals along the coast based on 

the average annual rate of shoreline recession. The average annual rate of shoreline recession 

was based on the most recent 32 years (1973 to 2005). The result shows that more and more of 

the highway will be threatened by recession in the coming decades. Such projections with 

graphical presentations can be valuable in planning alternative responses. 

8.3.1 Shortcomings of Shoreline Change Assumptions

From a coastal science perspective, shortcomings exist in the underlying assumptions inherent 

in using historical shoreline change rates to estimate future shoreline position: 

• Natural shoreline change processes are often not linear in time,

• Engineering may have influenced historical shoreline changes, and

• Engineering may influence future shoreline changes.

 
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Figure 8.6. An example of shoreline change rates along 30,000 ft of coast showing temporal 

and spatial variations but a significant recessional trend. Western-facing shoreline of St. 

Joseph’s Peninsula, Florida

It has long been recognized that shoreline change can be episodic. An individual storm may cause 

significant erosion or even trigger the beginning of an erosional period. The natural dynamic 

equilibrium on some beaches involves years of recovery after major storms. Large storms on lowlying barrier islands can cause island rollover and migration. Large storms on some coasts may 

remove large amounts of sand from the beach, via longshore and cross-shore sand transport 

resulting in bluff erosion. Subsequent times of lesser storm activity can result in the replacement 

of much of that sand by similar processes. 

Shoreline position in many US locations has been influenced either positively or negatively by 

engineering works. Engineering works can include seawalls, groins, breakwaters, inlet jetties, 

dams (on the US West Coast), dredging of ship channels, and beach nourishment. For example, 

a groin that traps sand will often widen an updrift beach while narrowing a downdrift beach. Over 

1 billion cubic yards of sand have been trapped or removed from US beaches by such projects 

(Douglass et al. 2003). Beach nourishment projects can widen beaches significantly. 

 
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Figure 8.7. Example of projected future shoreline positions at Stump Hole
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8.3.2 Sediment Budgets

Sediment budgets are one tool to estimate future shoreline positions. Sediment budgets are 

estimates of the rate at which sand is entering and leaving a specific reach along the coast. The 

difference between the volume entering and the volume leaving an area yields the volume gained 

or lost by that area. Sediment budgets typically make use of much more data and analysis than 

simple shoreline change extrapolation. 

8.4 Relocation/Abandonment Considerations

One obvious solution to the problem of a roadway threatened by shoreline recession is to relocate 

the road. Roads have been moved or abandoned at different locations along the US coast for 

decades. One example is Washington SH 105 in the Cape Shoalwater area (see Figure 8.1). This 

road was moved several times because this area has experienced some of the highest, long-term 

shoreline recession rates in the nation. By 1998 however, a series of coastal structures has been 

employed to protect the existing highway. Relocation of the road had again been considered but 

not selected as the preferred alternative. 

One example of an abandoned road is Texas SH 87 between High Island and Sabine (see Figure 

8.4). The road was closed indefinitely due to damage by Hurricane Jerry in 1989. Prior to that, 

the road had been damaged repeatedly by coastal storms. A road in that location had been there 

for over a century. 

A primary issue when considering road relocation is the new route. The logical location is farther 

inland from a receding shoreline. However, those areas are often already occupied by private 

property or wetlands. Developing private property is extremely expensive due to its location near 

the coast. Wetlands are protected for their habitat value. 

The stretch of Texas SH 87 that is closed today is in front of wetlands that are part of the 

McFadden National Wildlife Refuge. Relocating the road landward would involve filling the 

wetlands. Likewise, relocation of CR 30E in the Stump Hole area of Cape San Blas (see Figure 

8.7) would involve the filling of wetlands currently managed by the state as an outstanding aquatic 

preserve. Alternative relocation options considered for Washington SH 105 in the Cape 

Shoalwater area include private cranberry bog farms. 

Abandoning even small section of local roads may cause significant legal issues. More discussion 

of the issues with coastal road abandonment is included in Section 15.2.5. 

8.5 Shoreline Stabilization Options 

Some form of shore “stabilization” is a popular option to protect a road along a receding shoreline.

Stabilization is essentially holding the line and resisting the recession. The shore protection 

generally is in one of two forms. One could be some form of traditional “hard” structural shoreline 

protection such as a seawall or revetment (e.g. Chapter 7). Two could be some form of “soft” sand 

shoreline protection such as beach nourishment, marsh, or other nature-based solution. 

Nature-based solutions are options which mimic characteristics of natural features, but 

are created by human design, engineering, and construction (Webb et al. 2019). They 

include a range of features that serve as alternatives to, or ecological enhancements of, traditional 

shoreline stabilization and infrastructure protection techniques. A wide variety of terminology 

describes nature-based approaches, some of which are listed in the text box. The common thread 

connecting these approaches is the desire to protect or improve the built environment while 

maximizing the ecological function of the overall system. Because they address a specific 

ecological or ecosystem function, nature-based solutions are often site-specific and their design 
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requires a cross-section of expertise drawing on the fields of coastal ecology, coastal geology, 

coastal oceanography, and coastal engineering. 

In 2019, the FHWA developed the “Nature-Based Solutions for Coastal Highway Resilience: An 

Implementation Guide” (Webb et al. 2019) in parallel with the development of this HEC-25 3rd

edition revision. Some of the information in that implementation guide has been incorporated here. 

The FHWA developed this implementation guide to help transportation practitioners understand 

how and where nature-based solutions can be used to improve the resilience of coastal roads 

and bridges. Upfront, it summarizes the potential flood reduction benefits and co-benefits of these 

strategies. From there, the implementation guide follows the steps in the design process: 

• how to consider nature-based solutions in the planning process, 

• how to conduct a site assessment to determine if nature-based solutions are appropriate, 

• what engineering and ecological design considerations are crucial, 

• what permitting approaches to use, 

• what to consider for construction, and

• what monitoring and maintenance strategies to incorporate. 

The implementation guide also includes examples of nature-based solutions, a site 

characterization questionnaire, an evaluation tool, performance metrics, additional tools and 

resources, and technical fact sheets. Another product from that FHWA study is a detailed “white 

paper” that summarizes the literature on nature-based solutions (Webb et al. 2018). 

8.6 The Functional Design of Coastal Structures 

Similar to other concepts within science and engineering, coastal “functional design” refers to how 

a structure behaves, or “functions,” within the coastal environment (USACE 2002). Coastal 

structures can be categorized in terms of their primary function as follows:

• seawalls, revetments, bulkheads – shore-parallel structures on the shoreline designed to 

protect upland property from waves (Chapter 7),

• groins – shore perpendicular structures designed to control longshore sand transport, 

• breakwaters – shore-parallel structures located seaward of the shoreline to reduce the 

wave energy in their lee and to control longshore sand transport,

• hybrid structures – some functional combination of groins and breakwaters including “thead groins” or “headland breakwaters” 

Names for Nature-Based Solutions:

• Coastal green infrastructure

• Nature-based infrastructure

• Living shoreline

• Natural and nature-based features (NNBF)

• Engineering With Nature (EWN)

• Building with Nature (BwN)

• Working with Nature (WwN)
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Groins were probably the most common shoreline stabilization technique in the first half of the 

20th century. Figure 8.8 shows a groin field. Groins are typically placed as shown in groups or 

“fields.” They are often called “jetties” by laypersons but that term is typically reserved by the US 

coastal engineering community for structures that stabilize inlets. 

Figure 8.8. Groin field in Long Branch, New Jersey (2006)

Groins can stabilize a shoreline via two mechanisms if there is adequate sand in the littoral 

system: 

• Groins can locally realign the shoreline to reduce the longshore sand transport rate. 

• Groins can shelter the area adjacent to them from the wave energy especially when waves 

approach the shore at an angle. 

Groins can trap sand on one side while causing erosion on the other. The shoreline on the updrift 

side of a groin accretes while the shoreline on the downdrift side recedes. Thus, groins are often 

built in groin fields so the one just downdrift stabilizes the next portion of the shore. Shoreline 

recession downdrift of the last groin at the end of a groin field can be severe. 

Groins are much less acceptable today as a shoreline stabilization technique than they were prior 

to the 1960s. New groins were discouraged or prohibited in many states for years because of 

their potential downdrift negative impacts. However, in the past decade their use is becoming

acceptable when modern coastal engineering principles adequately address the downdrift 

impacts. 

“Functional design” refers to how the coastal structure functions in 

influencing coastal hydrodynamics, sediments, and habitats.
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8.7 Beach Nourishment

Beach nourishment is the placement of large volumes of good quality sand on a beach to 

widen the beach. Sand dunes can be constructed at the back of a nourished beach. Beach 

nourishment is a viable engineering alternative for shore protection and has become the principal 

technique for beach restoration (NRC 1995). Beach nourishment is a “nature-based solution.” 

Figure 8.9 shows a beach nourishment project under construction. Sand is being pumped from 

an offshore dredge (not shown) to the beach and then down the beach to where the sand-water 

slurry discharges from the pipe. The beach is then shaped by bulldozers. As the new beach 

extends farther down the beach, the dredge pipe is extended. 

Beach nourishment projects usually need to be maintained through subsequent renourishment 

as the sand moves out of the project limits. Many of the policy, management, and engineering 

issues related to beach nourishment projects are qualitatively described in Douglass (2002). Many 

of the quantitative engineering tools used in beach nourishment planning and design are 

presented in Dean (2002). The quantitative tools for beach nourishment for shoreline stabilization 

include methods for evaluating the performance of potential nourishment sands, estimating the 

short-term performance and the long-term renourishment intervals, and evaluating the ability of 

structures (if desired) to extend the renourishment interval. Each of these can be important 

aspects of beach nourishment planning and design. 

Beach nourishment projects protect a number of roads in the US. Two examples are shown in 

Figure 8.10 and Figure 8.11. The beach and dune in Figure 8.10 were constructed by the City of 

Delray Beach on top of the failed seawall shown in Figure 7.20. The sidewalk and parapet wall 

on the crest of the seawall in Figure 7.20 is the same as the sidewalk and bench shown in Figure 

8.10. Since originally constructed in 1973 this beach nourishment project has protected the road 

while providing a beach. The site has been renourished five times since 1973. The protective and 

aesthetic benefits of the beach outweigh the costs of renourishment. 

The nourishment project at Sea Bright, New Jersey, shown in Figure 8.11 is a federal shore 

protection project funded through the USACE's shore protection authority. The beach was initially 

constructed in 1994 directly seaward of the seawall. Nourishment was the preferred alternative to 

further seawall repairs. It has been renourished after passage of Hurricane Sandy. 

Proponents for beach nourishment projects have typically not been DOTs, even when the project 

protects a state highway. Rather, local government, a state resource management or economic 

development agency, the USACE, or a private entity typically sponsors beach nourishment. 

However, SDOTs have sponsored or co-sponsored several beach nourishment projects. 

Beach nourishment is a nature-based engineering solution that protects many 

coastal highways. 
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Figure 8.9. A beach nourishment project under construction in Gulf Shores, Alabama (2001)
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Figure 8.10. Beach nourishment project with constructed dune on top of old, failed revetment 

protecting Florida SH A1A (Delray Beach, 2001)

Figure 8.11. Beach nourishment seaward of a seawall protecting a road. (New Jersey SH 35, 

Sea Bright, 2001)

The USACE shore protection program has the authority to consider and build either beach 

nourishment or seawalls to protect upland property. Almost all the USACE’s federally authorized 

beach nourishment projects require a matching cost contribution from a non-federal sponsor. The 

USACE shore protection program typically has an annual budget of around $100 million, and the 

program has not grown significantly during the past several decades. 

Sand dunes are often part of major beach nourishment projects because they provide protective 

benefits during storm events by blocking or reducing storm surge flooding and wave action. Dunes 

function as sacrificial volumes of sand that minimize storm impacts until the dunes are eroded by 
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waves or overtopped by storm surge. Post-Sandy assessments demonstrate that the presence 

of dunes contributed substantially to reductions in storm damage (Tomiczek et al. 2017) and 

flooding (Walling et al. 2014). Dunes with sand fencing and vegetation trap and stabilize sand, 

leading to increases in dune volume and dune height over time. 

Beach nourishment has proven effectiveness and broader societal benefits of aesthetics, 

recreation and environmental enhancement. It should be considered by transportation engineers 

where a road is threatened by a receding shoreline. 

8.8 Combining Beach Nourishment with Structures

Modern nature-based coastal engineering shoreline stabilization solutions often combine beach 

nourishment with coastal structures. The purpose of the structures is to extend the interval 

between periodic renourishment. Some of these “hybrid” soft-hard solutions attempt to emulate 

natural geomorphological features such as pocket beaches and tombolos. The names of these 

“hybrid” solutions are evolving. 

Figure 8.12 shows some nearshore segmented breakwaters with beach nourishment protecting 

a highway in Louisiana. This system extends over 7 mi along the coast with 85 nearshore 

breakwaters. This is the same general area of the highway once protected by the failed 

interlocking concrete block revetment in Figure 7.21. 

Figure 8.12. Offshore segmented breakwaters with salients in beach nourishment protecting a 

highway (Louisiana SH 82, Holly Beach) 

Figure 8.13 and Figure 8.14 show a portion of another extensive system of offshore segmented 

breakwaters and nourishment sand to protect a road and other infrastructure. In the system at 

Presque Isle, Pennsylvania (on Lake Erie), there are 58 breakwaters with nourishment along over 

5 mi of shoreline. Tombolos do not form (that is by design), i.e. the beach does not extend out to 

the breakwaters. The bulges in the shoreline in the lee of the breakwaters are called “salients.” 

This system reduces longshore sand transport in the lee of the breakwaters. Figure 8.13 is an 
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historical photograph showing the initial three breakwaters and beach nourishment at a popular 

beach in the state park. This project was built as a “proof-of-concept” demonstration project which 

was so successful that the other 55 breakwaters were added in the 1980s. 

Figure 8.13. Offshore segmented breakwaters with salients in beach nourishment (Presque Isle 

State Park, Pennsylvania, circa 1980, USACE archive photograph) 

The formation of salients or tombolos is controlled by the geometry of the breakwater system as 

shown in Figure 8.15. The Coastal Engineering Manual (USACE 2002) provides more guidance 

on the functional design of nearshore segmented breakwaters. 

Figure 8.16 shows a nearshore segmented breakwater system with terminal groins used to build 

a small recreation beach. The beach was created with nourishment on the bay side of a long 

seawall that protected a road but did not have any sandy beach. The breakwater and groin 

structure system were designed to retain the nourishment sand. The beach was built to provide 

access to the bay for wind surfers and others. 

Coastal structures are often designed to extend the life of the beach 

nourishment.
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Figure 8.14. Offshore segmented breakwater system at Presque Isle, Pennsylvania
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Figure 8.15. Empirical relationships for shoreline effect of offshore segmented breakwaters. 

(after Pope and Dean 1986, and USACE 2002)

Figure 8.16. Offshore segmented breakwaters with groins and beach nourishment on Corpus 

Christi Bay (Ocean Drive, Corpus Christi, Texas)
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Figure 8.17 and Figure 8.18 show headland breakwater-pocket beach systems designed to retain 

beach nourishment sands on bay shorelines. Both were constructed in front of seawalls that had 

previously been damaged by erosion. These headland breakwater-pocket beach systems use 

structures to retain sand by providing artificial headlands. Figure 8.17 shows a headland 

breakwater that incorporates a “T-head groin” in the middle. The structures in Figure 8.18 do not 

include the stem of the “T” because tombolos were expected to form. The Virginia DOT was a 

partial sponsor of the project in Figure 8.18 since the system protected a short stretch of road. 

Functional design parameters for the design of headland breakwater-pocket beach systems 

include the distance offshore as well as the gap spacing. The goal is the creation of a pocket

beach with the sand fill. The shorelines as shown in Figure 8.17 and Figure 8.18 are curved 

because they have responded to the wave energy coming through the gaps in the artificial 

headland structures. Wave heights and directions are modified as waves diffract through the 

gaps. The structure layout can essentially be “tuned” to the local, site-specific wave 

climate to produce a beach with a desired curved shape and width (Bodge 1998). More 

guidance for the design of these systems including methods for estimating the final equilibrium 

shoreline shape and location are given in Silvester and Hsu (1993) and Hardaway and Gunn 

(2000). The potential complexity associated with design of solutions which combine nourishment 

and structures may necessitate involvement by an experienced, qualified coastal engineer during 

project delivery process. 

Figure 8.17. Constructed pocket beach stabilized with a T-head groin breakwater system (Point 

Clear, Alabama, circa 2006) 
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Figure 8.18. Beach nourishment project stabilized as pocket beaches with a headland 

breakwater system protecting a road (Water Street, Yorktown, Virginia, circa 2006) 

8.9 Marsh Restoration/Creation

Coastal marshes protect many roads along bay shorelines in the US. Restoration of natural 

marshes, or construction of new marshes, can be a nature-based solution to protect these roads. 

Coastal marshes, mangroves, and even coral reefs in some locations can reduce wave heights,

erosion of roadway embankments, and vulnerability to shoreline retreat (Webb, et al. 2018).

Keeping a healthy fringe marsh between a coastal roadway and bay waters provides habitat value 

as well as protective services. When thinking about any nature-based solution, perhaps consider 

the adage “it is best to let nature be your guide.” Most successful engineered nature-based 

solutions mimic some nearby natural feature (beach, dune, marsh, mangroves, etc.) with some 

additional coastal engineering. 

Hybrid approaches are common in bay settings. The combination of marsh with breakwaters

enhances the resilience of both the infrastructure and the ecosystem. The structural features 

serve at least three purposes: first, they improve the reliability of the system; second, they improve 

the performance of the system; and, third, they provide the specific hazard reduction benefits that 

a nature-based solution, on its own, may not. Examples of some common, proven hybrid 

approaches along bay shorelines are:

• Constructed marsh with a stone or timber sill structure, 

• Marsh/mangrove with breakwaters or reefs, and

• Beach nourishment with breakwaters and/or groins (Section 8.8). 

Figure 8.19. and Figure 8.20 show constructed marshes built with nearshore breakwaters. The 

primary value of this type of solution is its habitat. Note the horseshoe crab on the sandy pocket 

beach in the foreground of Figure 8.20. The constructed interface between the land and bay 

functions much more naturally, in terms of connectivity, than a bulkhead or revetment. The 

horseshoe crab is able to climb directly up on a sandy beach as its ancestors have done in this 

bay for millennia.

“Nothing is invented, for it’s written in nature first.” 

- Antonio Gaudi (1852-1926)
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Some form of structure to attenuate waves and/or stabilize shorelines in addition to marsh grass 

plantings are common in medium-to-high energy bay wave environments (e.g. fetch greater than 

1 mile). Meeting project objectives dictates the design of the size, characteristics, and location of 

the structure (e.g. rock breakwaters or sills, timber structures, or complex habitat-type devices). 

Common mistakes include the following: 

• Under- or over-designing structures for their intended application, 

• Using non-traditional structures (e.g. alternatives to rock or timber breakwaters) where 

their performance is not well understood, and

• Placing structures in sites where they that may exacerbate shoreline erosion. 

The project shown in Figure 8.19. uses a breakwater composed of multiple small concrete units

arranged as breakwaters with gaps. The project shown in Figure 8.20 uses more traditional rock 

breakwaters with gaps (see Section 8.8). The layout of the breakwaters and gaps for both projects 

were designed to reduce the wave energy to a tolerable level for the marsh grass to thrive. A rule 

of thumb is that marsh grasses cannot tolerate storm wave heights of H > 1 ft. See Webb, et al.

(2018) for more information on the design of bay marshes. 

As allowed under FHWA regulation 23 CFR § 777 (Mitigation of Impacts to Wetlands and Natural 

Habitat), transportation officials may consider constructed marshes as an alternative

where receding bay shoreline threatens a road. In addition to their effectiveness in mitigation, 

they may also serve the broader societal benefits of environmental habitat enhancement. This 

type of solution will typically need engineering beyond the width of the normal highway right of 

way. FHWA regulation 23 CFR § 777 also allows a project to consider partnering with other 

entities, such as the state resource agency. 

Figure 3.1 depicted a typical situation where a constructed marsh system would be an effective 

nature-based solution. Shoreline recession threatens a local road on a small bay. However, other 

parts of the road nearby are well-protected by natural marshes. Construction of a new marsh with 

some small breakwaters is a solution that will protect the threatened road while mimicking the 

habitat value of a marsh (FHWA 2017c). 

See the “Nature-Based Solutions for Coastal Highway Resilience: An Implementation Guide” 

(Webb et al. 2019) for more implementation guidance about nature-based solutions for coastal 

highways. 
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Figure 8.19. Example of a constructed marsh stabilized with a nearshore breakwater (Little Bay, 

Bayou LaBatre, Alabama)
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Figure 8.20. Example of a constructed marsh protected by nearshore rock breakwaters (Holts 

Landing State Park, Indian River Bay, Delaware, 2009)
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8.10 Unproven “Experimental” Shoreline Stabilization Approaches

This section briefly discusses the many so-called “experimental, or “innovative,” shoreline 

stabilization techniques which have been suggested for decades. SDOTs may not even have 

these techniques within their design and construction standards and specifications (FHWA, 

2012a). When such techniques do not conform to criteria or specifications, on a project basis, 

FHWA may approve such experimental features using its authority under 23 CFR § 625.3(f)(i)

(Exceptions). Proposed solutions have included artificial seaweed, used tire breakwaters, 

different types and shapes of rigid submerged and emergent devices, and beach dewatering. 

Many rely on expensive, patented devices and systems. The marketing of these systems often 

argues that they function differently than more traditional approaches. But all follow the same 

general principles of physics including mass (sand) conservation. Some of these systems have 

survivability problems in large surf conditions. The USACE (2002) describes new or innovative 

coastal project design concepts as being more susceptible to failure due to lack of experience 

with similar designs. 

If the wave attenuation characteristics of rigid breakwater units are adequately quantifiable, and 

the design level of wave attenuation will clearly be beneficial at that location, some coastal 

structures can be designed to function very similarly to the more traditional rock structures. If the 

placement of a device or apparatus in the surf causes beach sands to deposit, it functions much 

like the more traditional structures described above (and has the capability of causing downdrift 

erosion). 

Sometimes, so-called “innovative” approaches are marketed based on questionable coastal 

science such as a linkage between calmer waves and the presence of submerged aquatic 

vegetation (SAV). However, SAV health is often more closely tied to broader water clarity issues 

than those related to wave action level. 

Most innovative solutions are serious attempts to address a challenging problem. But unproven

shore protection solutions for highway applications should be pursued judiciously if not recognized 

as typical practice. The broader research and development community should continue to 

evaluate all potential solutions to beach erosion problems. Meanwhile, prudent highway 

engineering planning and design should focus on proven solutions: relocation, 

nourishment or other proven nature-based approaches, structures, or some combination 

of these approaches. 
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Chapter 9 - Increased Flooding due to Relative Sea

Level Rise

Communities in almost every coastal state are experiencing problems with more frequent, and 

more severe, road flooding at high tide and during small storms. This problem has only recently 

become apparent and recognized. Previous editions of this document did not address it. 

Figure 9.1 shows a poignant example of the problem: the sidewalk around the Tidal Basin is under 

a foot or two of water because of an abnormally high tide exacerbated by moderate flooding in 

the Potomac River. The construction of waterfront infrastructure in most US cities involved filling 

wetlands decades or centuries ago. The perceived local flood stage at the time of construction 

was likely the controlling factor for the height of fill. Sea levels have risen at almost all US 

waterfront locations, and the old local flood stage is now exceeded more frequently. 

This chapter briefly describes the problem, how the transportation community has begun to 

assess the problem, and one of the mitigation countermeasure options being used to address the 

problem (i.e., raising the road). Typically, the choice is to raise local flood thresholds or experience 

more flooding. 

9.1 Description of the Problem

This section describes the issues of increased flooding due to relative sea level rise (RSLR) with 

a focus on road examples. Since sea levels are rising along most US coasts and the rate of this 

rise is projected to increase significantly this century (see Figure 4.11), this flooding will become 

progressively more problematic. As a result, SDOT’s and local transportation agencies will face 

a need to address these issues more in the coming decades. 

Figure 9.2 is a typical example of this flooding on a local street. This intersection floods with salt 

water from a tidal lagoon about 2,000 ft away flowing up through the storm sewer system at high 

tide. Local citizens often detour around this area to avoid driving in salt water. Similar situations 

occur in hundreds of coastal communities in Florida (see Figure 9.3 and Figure 9.4), Maryland 

(see Figure 9.5), South Carolina, North Carolina, Washington, Texas, Alabama, Massachusetts, 

Hawaii, New Jersey, Georgia, Virginia, California, and New Hampshire. 

Figure 9.3 shows similar flooding of a primary road in the Miami area. The salt water on the road 

to the right in the photograph has backed up through the storm sewer system into the traffic lane. 

The Atlantic Intracoastal Waterway is visible on the right side of the photograph. The flooding is 

not restricted to the road immediately adjacent to the waterway: Figure 9.4 shows a local street 

to the east on the barrier island flooding the same day (the Atlantic Ocean is at the far end of that 

local cross-street). The flooding is also due to salt water from the waterway backing up through 

the local storm drainage system.

Figure 9.5 shows similar flooding of a local road which is affecting traffic flow and parking in 

Annapolis, Maryland. Flooding in this block has extended into the businesses on the left of the 

photograph (and at least one business, a coffee shop, has relocated from this area to a nearby, 

higher location to avoid the repeated flooding).

“Increased flooding due to relative sea level rise” is the term used in this 

document for this issue.
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Figure 9.1. Flooding of the Tidal Basin in Washington, District of Columbia (June 2018; 

permission for use of photograph provided by Loic Pritchett)
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Figure 9.2. Example of coastal flooding on an urban road (Honolulu, March 30, 2017) 

Figure 9.3. Example of coastal flooding on a primary road (Florida SH A1A, Hollywood Beach, 

October 21, 2017)
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Figure 9.4. Example of coastal flooding on a local road that crosses a developed barrier island 

(Hollywood Beach, Florida, October 21, 2017)

Figure 9.5. Example of flooding of a coastal road which is affecting traffic and parking (Dock 

Street, Annapolis, Maryland, 2017 photograph provided by Joe Krolak)
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This type of flooding has been called “nuisance flooding,” “sunny-day flooding,” “high-tide 

flooding,” “more flooding,” “storm-tide flooding,” “chronic flooding,” “recurrent flooding,” and “kingtide flooding.” It is associated with either weather or tidal conditions, or both, including the 

following: 

• the peak of the high tide for the day, 

• periods of the year when the astronomical high tides are above average, 

• wind blowing across coastal waters and bays that causes a local increase in mean water 

level (wind setup), 

• local rainfall runoff,

• storms (although it can occur on sunny days), or

• some combination of the above. 

In this document, this type of flooding is called “increased flooding due to relative sea level 

rise (RSLR).” Both depths and the frequency of flooding are higher as a result of RSLR. While 

flooding happened before, the gradual RSLR over the past decades has brought the sea levels 

closer to the infrastructure levels. Areas that once flooded only every few years during major 

storms may be flooding several times per year. Areas that once flooded only during the most 

extreme storm tides may be flooding during any high tide when the position of moon makes the 

tide higher than the usual daily high tide. And areas that once flooded an inch deep now flood 

several inches deep and for longer durations. 

RSLR has broader societal impacts than coastal highways. Increased frequency of coastal 

flooding is a reason for people to move out of low-elevation neighborhoods (e.g. Norfolk, Miami, 

and Tampa). Keenan et al. (2018) finds a correlation between housing prices in the Miami area, 

since 2000, and increased flooding due to RSLR. Lower elevation areas have a lower rate of 

property value appreciation. Increased flooding due to RSLR may have already contributed to 

real estate value losses in the Miami area and other coastal areas (McAlpine and Porter 2018, 

First Street Foundation 2019). These concerns have been discussed in the common press 

extensively (e.g. Smith 2018, Kusisto and Campo-Flores 2018). 

As mentioned, coastal highway surface flooding is often caused by bay waters essentially backing 

up through the culverts and storm drains in the local surface water drainage system. The coastal 

bay water surface is higher than the road surface. Flood water on the road surface is problematic 

for drivers until the water levels recede off the pavement. The driver can either drive more slowly 

though the standing water or take alternative routes around the lower portions of road. Drivers 

often avoid these areas because of the known effects of saltwater corrosion on vehicle 

undercarriages. This type of flooding often recedes a few hours after the peak of the high tide or 

when the wind changes speed or direction. The effect of increased flooding to RSLR on vehicle 

travel times has been quantified by Jacobs et al. (2018a, see Section 9.2.3). 

Knott et al. (2017) discusses how increased flooding due to RSLR can affect the service life of 

pavements in coastal roads. Coastal groundwater tables are expected to rise with RSLR and will 

intersect the unbound layers of road base. This can lead to pavement service life reductions 

related to fatigue and rutting. 

The FHWA held a small workshop on the coastal flooding issue in 2017 and a series of regional 

peer workshops around the US in early 2018. Those workshops informed this chapter. The 

Appendix of this manual lists workshop participants. 
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9.2 Assessing the Problem

This section discusses some potential approaches for quantitatively accessing increased flooding 

due to RSLR. Three examples from the literature are presented: 

• Sweet et al. (2014)/Sweet and Park (2014) – quantitative examples of the historical 

problem and the future problem (considering sea level rise projections), 

• Kriebel and Henderson (2018) - an expanded example that presents a simplified method 

for evaluating the increase in flooding due to future RSLR at a specific 

transportation asset, and 

• Jacobs et al. (2018a) - an analysis of vehicle delay increases over a multi-state area due 

to the problem. 

All three examples estimate how exposure to this problem will increase in the coming decades. 

Much of the work on increased flooding due to RSLR follows landmark papers by Sweet et al.

(2014) and Sweet and Park (2014). Others who have described and quantified this type of flooding

well include Kriebel and Geiman (2014), Kriebel et al. (2015), Moftakhari et al. (2015), Sweet et 

al. (2018), and Kriebel and Henderson (2018). 

9.2.1 Sweet et al. (2014)/Sweet and Park (2014)

The Sweet et al. (2014) report cover has a photograph of a flooded public park and statue at the 

City Dock in Annapolis that illustrates the issues facing communities. The report presents results 

by region, explores the seasonality of the phenomenon, and concludes that flood frequencies will 

increase dramatically at many US locations. 

Sweet et al. (2014) explain the increased flooding due to RSLR in a straight-forward way - by

considering the local NOAA tide gage data. Because sea levels have risen, the hourly water levels

have shifted upward as shown in Figure 9.6. The figure shows hourly water level observation data

at the NOAA gage at Sewells Point (Norfolk), Virginia for (a) 1950 and (b) 2012 and those (c) 

yearly water level distributions. All the elevations in Figure 9.6 are relative to the 1980-2001 MSL 

datum. Panels (a) and (b) show a monthly spring-tide influence and a few discrete storm events. 

Readily apparent in panel (c) is a shift of the distribution. 

The shift of the distribution to the right in panel (c) of Figure 9.6 is due to the RSLR between 1950 

and 2012 at that location. Also shown is the MHHW elevation of the 1983-2001 epoch. The plot 

shows that MHHW elevation was exceeded in several hundred hourly observations in 1950. But 

it was exceeded in several thousand hourly observations in 2012. The dashed, pink-yellow lines 

in all three panels represent the elevation (about +1 m) of a local nuisance flood elevation 

threshold in that area. Although that elevation was not exceeded in 1950 it was exceeded several 

times in 2012. The threshold is exceeded more often now due to RSLR. 
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Figure 9.6. The increased frequency of flooding occurs because the full distribution of tides rises 

with RSLR (from Sweet et al. 2014)

Figure 9.7 shows the projected future increase in frequency of flooding with RSLR at two US 

coastal cities. The number of “minor tidal floods” in a year will increase rapidly this century to near 

or over 100 days per year by mid-century and almost every day by 2100 regardless of the 

assumption of future RSLR (see Section 4.1.6 for an explanation of RCPs and sea level rise 

projections). Minor tidal floods are those above the NOAA “minor” threshold. NOAA has 

established coastal flood severity thresholds for use when that agency issues coastal flood 

advisories and warnings. The thresholds are based upon water level heights empirically calibrated 

to tide gage measurements from years of impact monitoring by NOAA local Weather Forecast 

Offices and emergency managers. The threshold elevations differ around the US due to the extent 

of infrastructure vulnerabilities, which vary by topography and relief, land-cover types or existing 

flood defenses. Sweet et al. (2018) describes these thresholds and the patterns and projections 

of increased flooding due to RSLR in terms of them. 

9.2.2 Kriebel and Henderson (2018)

Kriebel and Henderson (2018) present a straight-forward, effective, way to quantify the increase 

in flooding due to RSLR at any roadway location. This type of analysis can be applied at any 

location experiencing problems with increased flooding due to RSLR. It can inform the evaluation 

of engineering countermeasures (e.g. see Figure 9.15). 

Kriebel and Henderson (2018) consider McNair Road on the US Naval Academy, Annapolis, 

Maryland. A portion of the road floods occasionally and these occasions are becoming more 

frequent. Figure 9.8 shows that portion of McNair Road flooded. The water in the foreground is a 

tidal creek connected to the Severn River and Chesapeake Bay. 
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Figure 9.7. Increased flooding due to RSLR including historical exceedances (orange), future 

projections through 2100 based upon the continuation of the historical trend (blue), and future 

projections under median RCP2.6, 4.5 and 8.5 conditions, for Charleston, South Carolina and 

San Francisco, California (from Sweet et al. 2017a adapted from Sweet and Park 2014) 

Figure 9.8. Flooding of McNair Road, US Naval Academy, Annapolis, Maryland (permission for 

use of photograph provided by David Kriebel)
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Kriebel and Henderson (2018) find that the distribution of high tides fairly closely follows a normal, 

or Gaussian, distribution at many eastern US locations. Figure 9.9 shows the cumulative 

exceedance probability of the high tides at the nearby Annapolis NOAA tide station. The blue line 

represents the data points and the red line (which is obscured because it overlays the wider blue 

line) represents the normal distribution. The median value of high tide is about 0.5 ft above MSL. 

Several major floods in the high tide data are the blue circles towards the right side of the plot. A 

plot like Figure 9.9 shows the relationship between the tides and any specific elevation. For, 

example, roughly 20% of high tides exceed +1 ft (MSL). 

To apply to coastal infrastructure, the tidal elevation data are usually converted from the tidal 

datum (MSL) to the survey datum (NAVD88). See Section 4.2.2 Tidal and Survey Datums for a 

discussion of this conversion. The surface of McNair Road begins to flood when the tide elevation 

is +1.98 ft (NAVD88). 

Figure 9.9. Probability distribution for high tides at Annapolis approximately follows a normal 

distribution (from Kriebel and Henderson 2018)

The lower, right side of the exceedance probability distribution curve in Figure 9.9 is where most 

of the problem with increased flooding due to RSLR is being experienced in the US with today’s 

sea levels. That portion of the distribution can be evaluated with a more focused graphic that 

changes the cumulative probability scale to an “average number of floods per year” scale as 

shown in Figure 9.10 for the Annapolis data. The black circle on the graphic highlights the 

Transportation officials could choose to apply a Kriebel and Henderson (2018) 

type analysis to any road location to inform planning and design decisions 

related to increased flooding due to RSLR.
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elevation at which McNair Road floods (+1.98 ft NAVD88). With today’s sea levels, McNair Road 

floods 21 times per year on average. The other circles on the graphic highlight the elevations at 

which other infrastructure in downtown Annapolis begin to flood: 

• Dock Street storm drain = 1.90 ft (see Figure 9.5) 

• City Dock storm drain = 1.71 ft

• Compromise Street storm drain = 1.67 ft

• Newman Street storm drain = 1.44 ft

The Newman Street storm drain floods almost 90 times per year. Figure 9.10 shows that the 

number of annual flooding events changes dramatically with just a few inches of elevation. 

Figure 9.10. Average annual flood events above threshold water levels at Annapolis, 2014-2017

(from Kriebel and Henderson 2018).

Future RSLR projections can be added to the analysis by shifting the distribution as shown in

Figure 9.11. The annual number of flood events is shown for today’s sea level and 0.5 ft, 1.0 ft, 

1.5 ft, and 2.0 ft of future RSLR. Note that the upper end of vertical scale is near 730 high tide 

events because Annapolis has a semi-diurnal tide, and thus, two high tides per day. This analysis 

assumes that the shape of the probability curve does not change with future SLR (which seems 

like a reasonable initial assumption for engineering). 
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Figure 9.12 shows how this information can be used to quantify the increase in flood frequency 

of a specific asset with RSLR. The right panel is specific to the +1.98 ft (NAVD88) elevation. The 

number of times that the McNair Road is flooded will increase from an average of 21 times per 

year, with today’s sea level, to about 80 times per year with 0.5 ft of RSLR, to about 280 times 

per year with 1 ft of RSLR. 

Figure 9.11. The effect of RSLR on annual flood events at Annapolis (from Kriebel and 

Henderson 2018)

This extreme sensitivity to less than 1 ft of RSLR in this example may be indicative of the 

seriousness of the problem the US will face with RSLR in the coming decades. With sitespecific information like this, the transportation planning entity can consider the effects of a range 

of RSLR scenarios and the expected timing of that much RSLR (see Section 4.1 Sea Levels). 

Further analysis of this McNair Road example is presented in Section 9.3 Raising Roadway 

Elevation (see Figure 9.15). 

The left panel in Figure 9.12 shows two other elevations along with the McNair Road elevation. 

The “NOAA Minor” and “NOAA Moderate” elevations refer to NOAA’s coastal flood severity 

thresholds (see discussion of these thresholds in Section 9.2.1 above or see Sweet et al. 2018). 
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Figure 9.12. Applying the annual flood events curves to McNair Road, Annapolis (from Kriebel 

and Henderson 2018)

9.2.3 Jacobs et al. (2018a) 

Jacobs et al. (2018a) analyze vehicle delay increases over a multi-state area caused by increased 

flooding due to RSLR. The study identifies vulnerable roads and quantifies the risk from increased 

flooding due to RSLR in the eastern US by combining public road information from the FHWA’s 

Highway Performance Monitoring System with flood frequency maps, tide gage data, and future 

projections of annual minor tidal flood frequencies and durations. This is a form of an inundation

mapping approach (see Section 14.1). Jacobs et al. (2018a) find that increased flooding due to 

RSLR threatens 7,500 miles (12,000 km) of roadways including over 400 miles (644 km) of 

interstate roadways in the eastern US. Vehicle-hour delays induced by this flooding are estimated 

to exceed 100 million vehicle-hours annually and that value is estimated to increase to 160 million 

vehicle-hours of delay across the eastern US by 2020 (85% increase from 2010). Figure 9.13

summarizes state-by-state vehicle-hour delay findings. 

9.3 Raising Roadway Elevation

This section discusses a constructed solution to the problem of increased flooding due to RSLR 

– raising the elevation of the road. A broader discussion of mitigation approaches is in Chapter 

15. 

Figure 9.14 shows a Miami Beach road raised to address the problem of increased flooding due 

to RSLR. The road to the right of the photograph was raised several feet, and the retail businesses 

at the left of the photograph (see the café tables) are at the original road elevation. Biscayne Bay 

is a block away in the far background of this photograph (not visible). Prior to construction, the 

road and these businesses suffered from increased flooding due to RSLR. The local stormwater 

drainage system was re-engineered as part of the road elevation project. Pumps move rainfall 

stormwater runoff up and out of the area behind the raised road. 
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Figure 9.13. Annual vehicle-hours of delay projected for major roads (principal arterials, minor 

arterials, and major collectors) due to increased flooding due to RSLR by state; year (2020, 

2060, 2100), and RSLR scenario (Intermediate-Low, Intermediate, and Extreme) (from Jacobs 

et al. 2018a) 
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A significant portion, sometimes more than half, of the roads in southern Dorchester County, 

Maryland are flooded on some high tides. Many yards are flooded more frequently also. The 

residents of parts of the county have adjusted to this reality though lifestyle changes (e.g. realizing 

that on some days they cannot drive to the city, buying vehicles with higher clearance, etc.). The 

southern portion of Dorchester County is an area with a high regional subsidence rate (VLM). The 

high rates of RSLR combined with the low elevations raise other land use planning issues that 

are related to the transportation issues (e.g. see discussion near the end of Section 4.1.7). 

The analysis methods outlined in this chapter may provide transportation officials with potential 

approaches for planning and design of countermeasures and adaptations for the problem of 

increased flooding due to RSLR (e.g., raising the road). For example, Kriebel and Henderson 

(2018) relate how an engineer could extend the McNair Road analysis to consider the reductions 

in future flood frequency afforded by raising the road. 

Figure 9.14. Example of a road which has been raised to reduce flooding due to RSLR (Sunset 

Harbour Drive, Miami Beach, 2017)

The left panel of Figure 9.15 shows the increase in the number of annual flood events with four 

future RSLR assumptions. The bottom (blue) curve is the existing, historic, linear rate of RSLR 

(at Annapolis this is 3.61 mm/yr, which is about one ft above today’s level by 2100). The other 

three curves are for an additional +1, +2, and +3 ft of RSLR by the end of the century above that 

existing linear rate. All the values on the left-hand panel are likely to be problematic in reality. For 

example, if RSLR increases at the existing, historic, linear rate; the roadway will be flooded about 

80 times per year by 2060. Note that sea level is projected to rise faster than the historical rate 
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(e.g. see Figure 4.11). The number of annual flood events on McNair Road is estimated to 

increase to 170, 300 or 430 times per year by 2060 if RSLR follows the other curves shown on 

Figure 9.15. 

The right panel of Figure 9.15 shows the reductions in flooding of McNair Road if it were to be

raised 1 ft. Such an adaptation-countermeasure would essentially “turn back the flooding clock.” 

The annual number of floods would reduce to just a few initially. However, with time, the number 

of floods across the road would begin to increase as shown in the panel to the right. The elevated 

road would flood as frequently as it is now by about 2065 to 2070 about 2 ft of RSLR this century

(that is the orange line of +1 ft above the Existing Trend). While each transportation entity and 

project has their own potential approaches and constraints, this type of information can still 

inform planning and design decisions related to engineering adaptions.

Figure 9.15. Flooding of McNair Road due to RSLR without (left panel) and with (right panel) a 1 

ft increase in road elevation (from Kriebel and Henderson 2018)
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Chapter 10 - Highway Overwashing

Coastal highways are vulnerable to storm damage during overwashing during storms. In some 

cases, overwashing leads to complete damage of the roadway surface through a process termed 

the “coastal weir-flow damage mechanism.” This chapter provides an overview of the issues 

associated with this type of damage. 

10.1 Description of Overwashing

Coastal highway overwashing occurs when coastal storm surge elevations with wave runup 

exceed those of the roadway and water flows across the pavement with velocities high enough to 

cause embankment erosion. 

Figure 10.1 shows an example of post-storm damage from overwashing. Hurricane Ivan (2004) 

caused this damage along the Gulf of Mexico Coast near the Florida/Alabama border. The road 

pavement elevation was about +8 ft (NAVD) and the storm surge peak from Hurricane Ivan here 

was roughly +11 ft (NAVD). Figure 3.3 shows overwashing damage caused by Hurricane Michael 

in 2018. 

Figure 10.1. Example of pavement damage due to storm surge (Florida SH 292 on Perdido Key 

after Hurricane Ivan, September 2004). 
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10.2 The Coastal Weir-Flow Damage Mechanism

This section describes the coastal weir-flow damage mechanism in detail, including the use of 

diagrams and figures that draw on common hydraulic engineering concepts of weir flow 

mechanics. 

Pavements subject to overwash can be damaged primarily by three mechanisms: direct wave 

attack on the seaward shoulder of the road, flow across the road and down the landward shoulder

(“weir-flow” damage mechanism), and flow parallel to the road as water moves to “breaches” or 

lower spots in the road as the storm surge recedes.

Paradoxically, much of the damage to road pavements observed after Hurricane Ivan (2004) was 

on the landward side of the road. The Gulf of Mexico is to the right side of Figure 10.1 (behind the 

buildings). Figure 10.2 shows another example of similar damage, partial pavement undermining 

on the landward side of the road. Hurricane Ivan damaged over 50 miles of roads with partial 

damage as shown or complete damage. Weir-flow was likely the primary cause of the failure 

mode with contributions from parallel flow. This is based on post-storm damage assessments. 

The barrier islands were evacuated during the hurricane.

Figure 10.2. Example of pavement damaged by hurricane (photograph looking west on Florida 

SH 399, Gulf Islands National Seashore, 2005)

The specifics of the damage mechanism are as follows: the road embankment acts like a 

broad-crested weir to the incoming storm surge and the pavement is essentially the crest of 

Road embankments can act like broad-crested weirs during incoming and 

outgoing storm surge flooding.
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the broad-crested weir. As the surge elevation exceeds the elevation of the crown of the road, 

water flows across the road. Flow across a broad-crested weir passes through the critical flow

condition (Chow 1959). In particular, flow down the landward shoulder is super-critical (high 

speed) and erodes the shoulder material. If the scour reaches the edge of the pavement, water 

continuing to flow over the edge of the pavement forms a hydraulic jump and undermines the 

pavement. The same mechanism can scour the seaward shoulder later in the storm as the surge 

returns to the sea. 

A similar mechanism is responsible for damage to road embankments in a riverine environment 

(Chen and Anderson 1987, Clopper and Chen 1988). Figure 10.3 shows the general flow regimes 

that are established when a roadway embankment is overtopped. Damage can occur with or 

without tailwater (see Figure 10.4). 

Figure 10.3. Flow regimes leading to failure of embankments in riverine flooding situations (after 

Clopper and Chen 1988)

Figure 10.5 shows a road destroyed during Tropical Storm Arlene (June 2005). This road was 

under construction after having been destroyed the previous year by Hurricane Ivan (September 

2004). Hurricane Ivan removed all the sand dunes and allowed this portion of the barrier island to 

overwash during smaller storms. 

During several small storms in 2005, coastal weir-flow damage was observed. Prior to those 

storms, the barrier islands were typically evacuated during major storms and the islands had sand 

dunes that prevented overwash during minor storms. Figure 10.6 and Figure 10.7 show the 

mechanism, as it was occurring, at two different locations during Tropical Storm Cindy (July 2005). 

The storm surge flow direction is from the ocean to a bay in both pictures. Flow is from right to 

left across the pavement in Figure 10.6 and from left to right in Figure 10.7. A small hydraulic 

jump is visible on the downstream side in each picture due to the elevation drop across the edge 

of the pavement. 
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Figure 10.4. Embankment failure mechanisms (after Clopper and Chen 1988) 

Figure 10.5. Pavement destroyed by the weir-flow mechanism (Fort Pickens Road, Gulf Islands 

National Seashore, near Pensacola, Florida) 
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Figure 10.6. Weir-flow damage beginning (Fort Pickens Road, Gulf Islands National Seashore, 

Florida, July 2005; FHWA photograph) 

Figure 10.7. Weir-flow damage occurring. (Fort Pickens Road, Gulf Islands National Seashore, 

Florida, July 2005; FHWA photograph) 
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10.2.1 Coastal Weir-Flow Damage Mechanism Investigations

An FHWA-funded study conducted jointly by the University of South Alabama (USA) and Texas 

A&M University (TAMU) investigated the coastal weir-flow damage mechanism prototype-scale 

in a laboratory in early 2005. Figure 10.8 shows a schematic of the laboratory set-up and Figure 

10.9 and Figure 10.10 show schematics of the results. The experiment was conducted in a 12-

foot wide and 10-foot deep flume. The flume contained a constructed, sandy road embankment 

with a roadway on its crest consisting of 12, 2-foot wide concrete slabs. The sand shoulders were 

unconsolidated as is typical of many coastal highways. Water was pumped across the road 

section until failure as shown in Figure 10.9 and Figure 10.10. Figure 10.11 and Figure 10.12

show the failure. The USA/TAMU tests showed that the weir-flow is the likely cause of pavement 

damage observed in post-storm damage assessments. The damage can occur with only a little 

depth of water flowing across the road. 

Figure 10.8. USA/TAMU laboratory experiment model setup schematic 

Waves likely exacerbate the weir-flow damage mechanism. Waves moving across the pavement 

on the storm surge will increase the instantaneous flow velocities on the downstream shoulder 

which lead to more scour. Some levee failures in the greater New Orleans area during Hurricane 

Katrina were also due to downstream side erosion due to overtopping waves. 

Clopper and Chen (1988) discuss uplift on overtopped pavements on a riverine embankment. 

Uplifting may be an even greater problem in the coastal environment because of the easier 

transmittal of pore-pressure under the pavement due to the sandy nature of the coastal road 

bases and the accelerations in water waves. There was some evidence of pavement lifting during 

Hurricane Ivan as shown in Figure 10.13. 
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Figure 10.9. Schematic of results from USA/TAMU laboratory experiments test run one 

Figure 10.10. Schematic of results from USA/TAMU laboratory experiments test run two 
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Figure 10.11. Laboratory tests of the weir-flow damage mechanism showing scour destroying 

the downstream shoulder and beginning to undermine the edge of pavement. (USA/TAMU 

flume tests, June 2005) 

Figure 10.12. Laboratory tests of the weir-flow damage mechanism showing scour has 

continued to the point of undermining failure of 3 sections (6 ft) of roadway surface. (USA/TAMU 

flume tests, June 2005) 
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Figure 10.13. Pavement moved landward by overwash processes. (Florida SH 399, Gulf Islands 

National Seashore, Florida after Hurricane Ivan, 2004)

The same weir-flow mechanism that can damage the landward shoulder of a coastal road can 

damage the seaward shoulder too. Later in the storm, as the storm surge recedes, the water 

elevation on the landward side of the road embankment may be higher than the elevation on the 

seaward side. Flow is back to the sea and the downstream shoulder is now the seaward shoulder. 

Figure 10.14 shows pavement damage likely due to this return flow. The Gulf of Mexico is to the 

right in Figure 10.14. The damage looks more like it is due to the weir-flow damage than direct 

wave attack. Notice the background showing the pavement edge buried in sand (the buried 

pavement was not damaged); in the foreground, the pavement is exposed and damaged. This 

damaged section was at a slightly lower elevation than the undamaged section in the background. 

It appears that the water flowed back to sea over this lower pavement late in the receding surge 

hydrograph. 

Another related damage mechanism is parallel flow (parallel to the road direction) along the 

landward side of the coastal highway embankment as the storm surge recedes. Late in the storm, 

the embankment can begin to act like a dam holding the flood waters on the barrier island. If a 

portion of the embankment is lower due to failure or breaching, then water will flow laterally toward 

the low spot in the embankment. This flow scours the foundation material along the shoulder and 

contributes to its damage or failure. Lateral flow along the shoulders during coastal storms has 

been observed by Florida DOT personnel at US 98 near Destin, Florida. There is post-storm 

evidence of this flow in many locations (including the location shown in Figure 10.1). 
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Figure 10.14. Evidence of weir-flow damage to the seaward edge of pavement due to return 

flow late in the storm (Alabama SH 182, after Hurricane Ivan, Gulf Shores, 2004) 

10.3 Strategies for Roads that Overwash 

This section presents some potential strategies for reducing or preventing the roadway damage 

commonly encountered during coastal overwashing events. Four strategies for minimizing 

pavement damage due to overwash have been successful for coast-parallel roads on barrier 

islands. They can be used in combination with each other: 

• re-locating the road to a portion of the barrier island farther from the ocean, 

• lowering the elevation of the road to be at or below much of the existing grade to 

encourage burial by sand during overwash,

• constructing a sand dune seaward of the road to reduce the likelihood of overwashing and 

to provide a reservoir of sand to bury the pavement when overwashing occurs, and

• armoring of the shoulders of the road to resist erosion during overwashing. 

10.3.1 Road Location Considerations

Storm overwash on barrier islands often naturally erodes elevation from the front portion of the 

island and deposits sand on the landward portion of the island, shown schematically in Figure 

10.15. This geological process is one of the primary mechanisms for barrier island migration (see 

Chapter 6). This process, called barrier island “rollover,” has been likened to the tread of a tank 

in that the sand from the front is thrown to the back and then gets rolled over by more sand from 

the front in later storms. 
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Frontal dunes are often the highest elevations on a barrier island. These dunes and the beach 

berm seaward of them often erode in major storms through dune erosion and overwash 

processes. Sand is pulled offshore until the dune crest is breached or overtopped by the storm 

surge. Then sand moves landward and is deposited in lower elevations on the back of the island. 

These deposits, called overwash fans, can extend back into the bay.

Figure 10.15. Schematic of sand erosion and deposition on a barrier island resulting from 

overwash 

If the roadway is located where a cross-section erodes, it will be subject to severe wave attack 

and scour. If, however, it is located in the deposition zone, it can be buried by sand early in the 

overwashing event. Some roads, found under this layer of sand after a coastal storm, have been 

undamaged. A highway scraper or plow can scrape the sand off the road and the road can be

opened to traffic shortly after the storm. Roadway relocation away from the ocean has been done 

on low barrier islands susceptible to overwash in Florida, North Carolina and elsewhere to work 

with the natural process depicted in Figure 10.15 (see Section 15.2.5). 

10.3.2 Road Elevation Considerations

Another approach to reducing damage due to the coastal weir-flow damage mechanism is to 

lower the elevation of the road to near adjacent ground elevations. This lower elevation can 

prevent the weir flow from occurring since the crest of the pavement is not the highest portion of 

the grade. Figure 10.16 shows a road buried by overwash sand that survived a major hurricane. 

The piles of sand along the road were scraped off the road as part of the post-storm maintenance. 

Some practical limits to lowering the road depend on drainage and safety. Lower roads may also 

need more maintenance such as sand sweeping. Installation of sand fencing and vegetation can 

reduce drifting sand and the frequency of sweeping. Experience in west Florida suggests that 

constructing a typical road embankment elevated above the adjacent ground elevations can result 

in significant damage even if the road is relocated away from the ocean. 

10.3.3 Construction of Sand Dunes

Sand dunes seaward of the road, either facilitated through sand fencing and vegetation plantings 

or through construction, reduce the likelihood of overwashing and provide a reservoir of sand to 

bury the pavement when overwashing occurs. Many states and local government have attempted 

to construct sand dunes seaward of roads to protect against storm surge and waves. North 

Carolina has used this approach to protect portions of North Carolina SH 12 along the Outer 

Banks. Figure 10.17 shows a portion of that highway north of Buxton, North Carolina, where a 

large, artificial sand dune has been constructed on the seaward side of the highway. 
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Figure 10.16. Example of road buried by overwash sand after it was opened by plows 

Figure 10.17. Artificial sand dune constructed seaward of a highway to protect the highway 

(North Carolina SH 12; looking north, the Atlantic Ocean is to the right and Pamlico Sound is to 

the left, 2002) 
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Dune erosion analysis and modeling tools can be used to design the size and shape of the dune

(see Section 6.4 Cross-Shore Sand Transport and Dune Erosion Modeling). Construction of a 

healthy sand dune usually involves vegetative plantings to stabilize the dune and to establish a 

dune that functions like a natural dune. 

All three of the above approaches to reducing damage to pavements during overwashing can be 

implemented together. The schematic of Figure 10.18 shows a new road located as far from the 

ocean as practical, built at a low elevation, with small dunes constructed near it. The dune 

vegetation also acts to reduce wind-blown sand from covering the road during normal (non-storm) 

conditions. 

Figure 10.18. Schematic summarizing three approaches (bayward location, low elevation, 

constructed sand dunes near road) to minimize damage to roads that overwash

10.3.4 Armoring of Shoulders

The downstream shoulder of roads that experience overwashing damage can be armored to 

withstand high velocity flows. This approach has been adopted to protect a section of US 98 on 

Okaloosa Island, Florida. The armoring includes sheet piling (Figure 10.19) and gabions (Figure 

10.20). The sheet piling is located on the shoulder of the pavement farthest from the sea. This is 

the edge of pavement that has suffered the most damage due to the weir-flow damage 

mechanism in past hurricanes. The top of the concrete sheet pile wall cap section is visible in the 

photograph of Figure 10.21. Most of the engineering is not visible since it is buried under sand to 

the right of the roadway in that photograph. The bay is shown at the right-hand side of the 

photograph and the incoming surge flow is from left to right. Buried gabions are used elsewhere 

where the overwashing flow may be lower but flow parallel to the road during the storm is expected 

to be strong enough to cause damage. This design was constructed in 2005, after Hurricane Ivan, 

and had not been tested by a major overwashing event as of 2018. 

This US 98 design was evaluated recently as a case study of an engineering adaptation to 

increase coastal highway resilience to rising sea levels (FHWA 2017b). The evaluation included 

a range of future relative sea level rise scenarios and the effect of that sea level rise on storm 

surge return period elevations. Engineering economic analysis conducted under this 

evaluation concluded that the cost of this design was justified with today’s sea level and 

that the economic justification becomes much stronger as sea levels rise.
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Figure 10.19. Sheet pile, with buried gabions for scour protection, at edge of pavement to resist 

pavement damage due to coastal storm surge overwash. (Florida DOT figure) 

Figure 10.20. Gabions at edge of pavement to resist pavement damage due to coastal storm 

surge overwash. (Florida DOT figure)
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Figure 10.21. Highway shoulder which has been designed to withstand the weir-flow damage 

during overwashing events (US 98, Okaloosa Island, Florida, 2016) 

Clopper and Chen (1988) provide results of research into armoring shoulders that might be 

applicable to the coastal problem. They conducted laboratory experiments on different possible 

countermeasures to resist the flow of water across a highway embankment. Their tests were 

based on riverine overflow situations and focused on soil types not as sandy as those typically 

found at the coast. They only considered current flow forces and not wave forces. However, 

Clopper and Chen (1988) found that a concrete block revetment system with relatively heavy 

blocks, horizontal and vertical interlocking cables, and anchors was able to resist the hydraulic 

forces due to overtopping better than a number of other alternatives. They tested flow rates 

generated by up to 4 ft of differential head over the embankment which is probably more than is 

typical in coastal storms. However, the addition of waves in coastal storms likely increases 

turbulence, shear stresses, and erosion significantly. Figure 10.22 is a sketch of how that concept 

could be implemented as a retrofit to protect a coastal highway. The capabilities of interlocking 

A buried wall was built under the shoulder of the highway to reduce damage in 

future storms.
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blocks to withstand the overtopping condition was confirmed by laboratory tests by Clopper 

(1989).

Figure 10.22. Conceptual design to resist pavement damage due to coastal storm surge 

overwash 
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Chapter 11 - Coastal Bridges 

Engineering of coastal bridges involves consideration of forces and processes unique to the 

coastal environment including loads from waves. Over 36,000 bridges are within 15 miles of coast 

in the US. But as this is only 6% of the approximately 600,000 bridges contained within the 

National Bridge Inventory, riverine-based hydraulic design approaches are often applied to these 

coastal bridges. This chapter provides an overview of some of the unique aspects of bridges in 

the coastal environment including: 

• Bridge settings within the coastal floodplain, 

• wave loads on bridge decks (with examples of damage, a description of the problem, a 

review of the technical literature, a revised method for estimating wave loads on bridge 

decks, and estimating bridge deck elevation needed to avoid wave loads),

• possible countermeasures for wave loads (and problems with them), and

• selection of design storm surge and wave heights.

The method of estimating wave loads on bridge decks presented below (Section 11.4) is revised 

from the previous edition of this manual. The limited research on countermeasures or retrofits that 

have been proposed to mitigate coastal bridge damage is summarized in Section 11.5. The 

important conclusion is that attempts to resist wave loads can lead to damage of some other 

bridge component later in the storm. Chapter 12 addresses the related issues of coastal scour. 

11.1 Coastal Bridge Settings 

Coastal bridges are found at a variety of general types of location settings within the coastal 

environment including inlets, open-water bays or sounds, tidal embayments or arms, and river 

crossings (Figure 11.1). Each presents different issues and challenges for the coastal practitioner. 

These issues and challenges are described here to better orient the reader to subsequent 

descriptions of wave and hydrodynamic loads on bridges and coastal bridge scour (Chapter 12). 

11.1.1 Bridges at Inlets

Tides move between the ocean and a bay through inlets (see Section 6.6 Tidal Inlets). They are 

the entrance to many estuaries and other water bodies of ecological importance. These interior 

bays and estuaries can store significant volumes of water. Inlets experience complex 

hydrodynamics, some with extremely intricate interactions between currents and sands. Most 

shoreline change occurs near inlets and many inlets are “evolving” geologically in response to 

engineering and natural changes. There can be multiple inlets to the interior water body (bay, 

sound, etc., e.g. see Figure 6.18) 

The US has over 600 tidal inlets, many with bridges across their throat. These can range from 

very large structures (e.g. Golden Gate Bridge) to relatively small spans (Figure 11.2).

A revised method for estimating wave loads on bridge decks is presented in this 

Chapter (see Section 11.4).
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Figure 11.1. Conceptual schematic of four typical bridge settings within the coastal environment

11.1.2 Bridges over Bays and Sounds

Bridges link coastal barrier islands and peninsulas to the mainland. They are bridges or a 

combination of bridges and elevated causeway embankments. The floodplain crossing can be 

some combination of open water and wetlands. For example, Figure 11.3 shows the F.J. Torras 

Causeway between Brunswick and Saint Simons Island, Georgia. The causeway bridge, like 

many such structures, serves as an evacuation route during storm events. The total causeway 

length is over 21,000 ft and contains five bridges crossing tidal creeks and rivers. The total length 

of bridge structures, however, is less than 6,000 ft. The remaining 15,000 ft of the causeway is a 

roadway on a constructed embankment. 

11.1.3 Bridges Spanning Tidal Embayments

Another common location for coastal bridge crossings is a tidal embayment. As opposed to inlet 

bridges, these bridges are located in interior water bodies or a distance “upstream” on an open 

bay or estuary. Bridges at these locations can vary in width and span length: from two-lane, 20-

foot spans over tidal creeks to multi-mile interstate spans.

Bridges spanning tidal embayments are distinct from causeway bridges in that they are in open 

waters. Examples range from a small tidally influenced creek to large tidally influenced 

waterbodies such as Mobile Bay (Figure 11.4); Knik Arm, Alaska; or Chesapeake Bay. 
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Figure 11.2. Bridge spanning a small inlet (Alabama SH 182 in Gulf Shores)

Figure 11.3. The F.J. Torras Causeway bridges between Brunswick and Saint Simons Island, 

Georgia
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Figure 11.4. Interstate 10 bridge crossing Mobile Bay, Alabama

11.1.4 Bridges Crossing Rivers

Bridges also cross rivers in the coastal setting. The locations of these crossings and the influence 

of tides, sea level rise, and coastal storm impacts are unique to their location. Such bridges may 

be located a short distance upstream from a tidal embayment on a bay, sound, or estuary, or 

perhaps even a considerable distance upstream, and still exhibit some tidal influence on hydraulic 

processes. 

A distinction can be made between “tidally-dominated” and “tidally-influenced” areas of coastal

rivers. Tidally-dominated areas have reversals in flow direction due to the tide. “Tidally-influenced” 

areas have unidirectional river flow with fluctuations in discharge controlled by the tide (by 

tailwater control). Examples of major rivers with miles of both types of flow are the Columbia River, 

Hudson River, and Cooper River. 

Bridges also cross river mouths near the open ocean. Numerous such bridge crossings are along 

the Pacific Coast and on some Pacific Islands. Many of these crossings are over smaller rivers 

and creeks. Figure 11.5 shows four of these crossings. 
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Figure 11.5. Pacific Coast river mouth crossings (clockwise from top left: Big Creek, Oregon; 

Pistol River, Oregon; Yachats River, Oregon, and Redwood Creek, California)

These river bridge crossings differ from other locations described above in several ways. First, 

the local geographical features, such as hills and mountains extending to the shoreline, often 

result in a narrower floodplain. Second, the long-term geomorphological changes and shorterterm interactions between fluvial discharge and coastal processes, namely wave-driven 

longshore sand transport, result in a complex setting that may impact bridge hydraulics over time. 

Third, some of these rivers carry a notable sediment load to the littoral zone. Severe lateral 

channel migration may occur especially within the backshore beach zone. Thus, the shoals shown 

in Figure 11.5 change significantly in time.

11.2 Wave Loads on Bridge Decks: Examples and Description

Highway bridges along the north-central Gulf coast were damaged during landfall of Hurricanes 

Ivan (2004) and Katrina (2005). These include the Interstate-10 (I-10) bridge across Escambia 

Bay in Florida, the I-10 bridge across Lake Pontchartrain in Louisiana, the US 90 bridges across 

Biloxi Bay and Bay St. Louis in Mississippi, and an on-ramp to the I-10 bridge across Mobile Bay 

in Alabama (see Figure 11.6). The bridge to Dauphin Island, Alabama, that now carries Alabama 

SH 193, was destroyed by Hurricane Frederic in 1979. The two US 90 bridges in Mississippi 

bridges damaged in Hurricane Katrina had previously been damaged by Hurricane Camille in 

1969. The replacement bridges built after Hurricane Camille (1969) were built at the same 

elevation. After they were damaged in Hurricane Katrina (2005), they were rebuilt at much higher 

elevations to avoid future storm surge and wave loads. 
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Figure 11.6. Location map of some of the highway bridges damaged by wave loads in 

hurricanes along the north-central Gulf Coast (from Douglass et al. 2006a) 

Other bridges in the region suffered damage during Hurricane Katrina when vessels that had 

broken their moorings collided with them. A comprehensive listing of bridges damaged by 

Hurricane Katrina can be found in the ASCE Technical Council on Lifeline Earthquake 

Engineering (TCLEE) report (2006) and discussion of the overall civil engineering damage issues 

can be found in Robertson et al. (2007). 

Reviewing information related to several of these damaged bridges reveals potential failure 

modes and commonalities. This section describes some of the forensic observations of two of the 

case studies of bridge damage in 2004-2005 and concludes with a description of the damage 

mechanism – waves on storm surge striking the bridge decks. The investigations and lessons 

taken from these two bridges could similarly describe many of other wave load impacted bridges. 

11.2.1 I-10 Escambia Bay (Hurricane Ivan)

Interstate 10 (I-10) crosses Escambia Bay just east of Pensacola, Florida on elevated twin span 

bridges. The bridge was severely damaged both west and east of the high-span portions during 

passage of Hurricane Ivan September 15-16, 2004 (see Figure 11.7). The hurricane made landfall 

on the Gulf Coast west of this bay and generated extreme storm surge and large waves 

propagating up the bay toward the bridge as shown. Like many other bridges, the decks were

simply supported spans atop pile caps (bent beams) with very minimal structural connections 

between the deck girders and pile caps. Waves riding on top of the elevated storm water levels
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displaced the bridge decks. Higher elevation twin spans replaced this bridge following Hurricane 

Ivan.

Figure 11.7. Location map and damage overview for the I-10 bridge across Escambia Bay, 

Florida in Hurricane Katrina

Figure 11.8 shows a photograph of damage on the west side of the high span as a result of 

Hurricane Ivan (the area identified as “Damage W” on Figure 11.7). At the time of this photograph, 

the evening after landfall the previous night, the storm surge elevation had already receded below 

its maximum and the wave heights had also diminished significantly. 

Note that the spans in the right of the photograph have moved to the left (in the direction of wave 

propagation) and some have fallen off the pile caps (bent beams). The spans in the foreground

of the photograph have not moved even though they are at the same elevation as the spans that 

have moved. This implies that wave heights were slightly lower due to the partial sheltering of the 

shore and slightly shallower water near the shore. Numerical model hindcasts of the storm surge 

and wave fields which occurred during passage of Hurricane Ivan confirm this (see Figure 5.17). 

The spans in the background of the photograph have not moved because they are elevated higher 

above the bay for the high-span for navigational clearance. 

The connections between the decks and the pile caps were small steel clips (angles) with bolts 

embedded only a few inches into the concrete. The repeated extreme loads from individual waves

broke the bolts out of the concrete. 

The spans on the westbound bridge (left side of photograph) were less damaged than the ones 

on the eastbound bridge because the eastbound bridge provided shelter during the peak of the 

storm and reduced wave heights, and thus loads, on the westbound bridge. The large storm 
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waves were propagating from the right to the left during the peak of the storm (see wave travel 

direction indicated in Figure 11.7). The photographic evidence indicates that the wave-induced 

loads were just large enough to begin to move the decks at the peak of the storm surge. Some 

shifted a short distance but some moved far enough to topple off the pile caps. As the storm surge 

receded from its peak that night, wave loads on the bridge decks diminished.

Figure 11.8. Interstate-10 bridge across Escambia Bay, Florida, after Hurricane Ivan. 

Photograph looking northeast from Pensacola September 16, 2004 (photograph used with a 

license from AP Images)

Wave loads on bridge decks are extremely sensitive to water levels when the water level

is near the deck elevation. This sensitivity is clear in forensic investigations and in the methods 

for estimating these loads (see Section 11.4). 

Recent forensic modeling of the storm surge and waves in Escambia Bay during Hurricane 

Katrina shows that long-term relative sea level rise (RSLR) likely contributed to the damage which 

occurred in Hurricane Ivan (Kilgore et al. 2019a). The magnitude of wave loads in 2004 when the 

bridge was actually damaged and those which would have occurred if a storm with the exact same 

characteristics had occurred with sea levels reflective of those 30 to 40 years prior, when the 

bridge was designed, are compared. ADCIRC and SWAN for both sea levels are used in the 

hindcasts. 
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The Pensacola NOAA tide gage records indicate that the MSL for the year 1970 was about 0.33 

ft lower than in 2004 (annual average sea level). After performing the 2004 hindcast of Hurricane 

Ivan, a second simulation applied a water surface that was offset 0.33 ft lower in the models (1970 

condition). The models capture all of the potential nonlinear effects that lead to changes in storm 

surge between the two different time periods (see Section 4.3.4 Impacts of SLR on Storm Surge). 

The Douglass et al. (2006a) method is used to estimate the maximum vertical wave loads for both 

the 1970 and 2004 condition simulations. The only variable in the method that changes between 

the two simulations is the elevation of the maximum wave crest (since the bridge elevation was 

fixed). The resultant wave loads on the bridge decks would have been 70,000 lb less in 1970 due 

to the lower MSL at the start of the storm. 

In other words, RSLR, which occurred in the three to four decades between the design of the 

bridge and Hurricane Ivan, caused an increase in wave-induced loads on the bridge of about 

70,000 lb. This is within the range of the resistance of the minimal connections between the deck 

girders and pile caps (bent beams). 

Also, the higher elevation MSL and maximum wave crest elevations, due to the RSLR, caused a 

prolonged duration of wave attack during the storm event. The increased exposure time of the 

bridge to very large waves in the storm would have been about 2 more hours. Using the peak 

wave period values extracted from the model simulations at the bridge (~6 s), that extended 2-

hour duration would have been equivalent to the bridge being hit by an additional 1,200 waves. 

In summary, this bridge may not have been damaged, or would not have experienced as much 

damage, if MSL had not risen because of RSLR in between the time of design and the landfall of 

Hurricane Ivan.

11.2.2 US 90 Biloxi Bay (Hurricane Katrina)

Figure 11.9 (and Figure 3.4) shows the US 90 bridge across Biloxi Bay, Mississippi after Hurricane 

Katrina. The extreme storm surge during the hurricane raised the water level to an elevation where 

waves could impact and inundate the bridge decks (superstructure). The simply supported-span 

bridge decks moved landward off the pile caps (the Gulf of Mexico is to the left in Figure 11.9). 

However, no bridge deck movement occurred at higher deck elevations (i.e. the approach to a 

ship channel - shown between the deckless pile caps and an open drawbridge across that 

channel). The forensic evidence is entirely consistent with the fact that these bridges were 

damaged by storm waves on the storm surge striking the decks.

Some of the details of the damage are visible in Figure 11.10. Inspection of the damage revealed 

three consistent observations. One, there was damage to the bearing pad assemblies with the 

exception of the seaward side of the seaward-most bearing pads with only seaward bolts surviving 

(see upper left inset). Two, there was more damage to the concrete surface of the pile caps on 

the landward side than on the seaward side. Three, some of the bridge decks flipped over during 

the storm and are visible to the right of the bridge in the shallow water.

RSLR has already contributed to damage of one major US bridge during a 

hurricane.
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Figure 11.9. US 90 bridge over Biloxi Bay, Mississippi showing the spans below a critical 

elevation were removed by Hurricane Katrina (photograph looking southwest from Ocean 

Springs, February 19, 2016) 

Figure 11.10. Details of damage to US 90 bridge over Biloxi Bay caused by Hurricane Katrina
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11.2.3 Description of the Wave Load Damage Mechanism

Wave loads were the primary force causing much of the damage to coastal bridges in the north, 

central Gulf coast due to Hurricanes Ivan (2004) and Katrina (2005). The damage occurs as the 

storm surge raises the water level to an elevation where larger waves can strike the bridge 

superstructure. The individual waves produce a force on the deck with a vertical (uplift) 

component and a horizontal component (see Figure 11.11). The magnitude of wave uplift force 

from individual waves can exceed the weight of the simple span bridge decks (Douglass et al.

2007). The total resultant wave force is able to overcome any resistance provided by the (typically 

small) connections. The decks begin to progressively slide, “bump,” or “hop” across the pile caps 

in the direction of wave propagation in response to individual waves. This condition can occur 

before the storm surge elevation exceeds the bridge deck elevation. 

The buoyancy of the bridge decks is a secondary influence. This includes any additional buoyancy 

produced by air pockets trapped under the bridge decks. The buoyancy contributes to the total 

force on the individual bridge decks when the deck is submerged, i.e. when the storm surge 

elevation exceeds the bridge decks. However, bridge decks that were elevated above the storm 

surge still water elevation were damaged in both Hurricanes Ivan and Katrina (Douglass et al.

2006a). 

Due to the nature of hurricane hazards and evacuations, there are no videos of the prototype 

damage occurring. But post-storm forensic inspections indicate a similar pattern in every case. 

Physical model laboratory experiments subsequently confirmed this damage mechanism (e.g. 

see video of Douglass et al. 2006b). The laboratory results have remarkable qualitative and 

quantitative similarities with the prototype forensic damage. For example, some of the bridge 

decks flipped over in the laboratory experiments as seen in the prototype (Figure 11.10). 

Figure 11.12 shows a schematic of the typical time-history of one component (either vertical or 

horizontal) of wave-induced loads on a rigid structure like a bridge deck. Such loading is 

consistent with measured laboratory loads reported in the literature by numerous investigators. 

One part of the wave-induced force is a longer-duration slowly “varying” force. This “varying” force 

changes magnitude and direction with the phase (crest or trough) of the wave as the wave passes 

under or across the structure. This part of the wave-induced load has been called “quasi-static,” 

or simply “wave” force by others in the coastal engineering literature (Section 11.3). The duration 

of the “varying” load corresponds with the period of the incident waves that is typically on the 

order of 3 to 15 seconds. The horizontal slowly varying loads are in the landward direction (based 

on direction of wave propagation) for the wave crest but can reverse to the seaward direction in 

the wave trough. Likewise, the vertical slowly varying loads are directed up (i.e. lift) for part of the 

wave but can be downward for part of the wave. The downward-directed wave load can be due 

to both the mass and downward momentum of the portion of the wave crest above the bridge 

deck. The uplift loads appear to be typically greater than the downward-directed loads. 

Wave loads were the primary force causing much of the damage to coastal 

bridges in 2004 and 2005.
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Figure 11.11. Schematic of resultant wave-induced load, with both uplift and lateral 

components, on a bridge deck

Figure 11.12. Schematic of typical time-history of wave loads on rigid structures
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The other part of the wave-induced load shown in Figure 11.12 is a very short-duration (maybe 

less than 0.1 to 0.001 seconds long) “impact” force as the wave crest first begins to hit the deck. 

This force is directed in the horizontal direction of wave propagation and in the upward vertical 

direction. This impact force does not typically reverse direction. The impact force is often 

associated with the trapping of a small pocket of air between the structure and the wave face, and 

is sometimes referred to as the 'slamming' force. Wave impact loads have been studied most for 

horizontal loads on vertical walls. The duration of the “impact” and the magnitude of the peak 

“impact” force are inversely proportional for this type of wave load (Weggel 1997). Whether or not 

this extremely short impact load should be considered in the design of coastal bridges subjected 

to wave impacts is not clear and likely best left to the structural engineer. The large mass of the 

bridge decks may make these extremely short-duration loads less important than the “varying” 

loads due to structural dynamic response times as discussed in the next section. 

11.3 Wave Loads on Bridge Decks: Literature

The engineering literature on wave loads on bridge decks has expanded significantly since 2004-

2005. Douglass et al. (2006a) provided a comprehensive literature review at that time. 

Hayatdavoodi and Ertekin (2016) is an excellent update to the literature review of the field which 

includes tsunami wave loads. Most of the literature prior to 2004-2005 was on the more general 

problem of piers and other horizontal decks subjected to wave loads and not specific to typical 

US bridge deck geometries. That is still a shortcoming in the available literature as discussed 

below. 

A number of investigators measured wave uplift loads on flat, horizontal decks (not bridge deck 

geometry) subjected to monochromatic waves in physical models (e.g. El Ghamry 1963; Wang 

1970; French 1970; Isaacson and Bhat 1996). A number of investigators presented methods for 

estimating wave loads on decks (not highway bridge decks) with significant clearance above the 

SWL which is typical of the offshore oil and gas industry (Kaplan et al. 1995; Bea et al. 1999; and

McConnell et al. 2004). McConnell et al. (2004) also measured wave-induced loads on decks in 

the laboratory with irregular waves. 

The US Naval Facilities Engineering Command (NAVFAC) has design memorandum with a 

simple equation for vertical uplift loads due to waves on bulkhead whalers, horizontal timbers as

support structures on the seaward side of vertical walls (NAVFAC 1982). The NAVFAC equation

is similar to ones considered by French (1970), Wang (1970), and a “practical design formula” 

used by Overbeek and Klabbers (2001) to explain wave uplift damage to a pier in the Caribbean. 

Physical laboratory model tests of wave loads with models of more realistic highway bridge 

geometries have been reported by Denson (1978), Denson (1980), Cruz-Castro et al. (2006), 

Douglass et al. (2007), and Sheppard and Marin (2009). 

McPherson (2008) conducted 1:20 scale physical model tests with simplistic girders and 

diaphragms to trap air under the deck like highway bridges. He used a range of monochromatic 

waves and, based on his observations and results, suggested modifications to improve the 

Douglass et al. (2006a) method for estimating wave loads on bridge decks. 

Bradner et al. (2011) conducted very large-scale (1:5) physical model tests, with simplistic girders 

and diaphragms, and allowed the model decks to move to evaluate the structural dynamic 

response to the wave loads. Seiffert et al. (2014) conducted small-scale laboratory tests (1:35) 

and found that diaphragms, or “end caps,” between girders can cause a significant increase in 

wave-induced uplift loads. Douglass et al. (2007) reported a similar effect but Seiffert et al. (2016)

suggest the intentional design of “air-relief-openings’ to reduce wave loads. Hayatdavoodi et al.

(2015) conducted extremely small-scale tests using nonlinear, cnoidal waves which may be 

representative of very-long period waves and tsunami wave loads on bridge decks. 
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Some of the literature addresses the magnitude and importance of the “impact” loads. It has long 

been suggested that structural engineers can ignore this component due to the extremely short 

duration relative to the dynamic response of the structure. Bradner et al. (2011) found these very 

high amplitude, but very short-duration, pressure spikes in some of their large-scale laboratory 

experiments. However, the “impact” spikes did not transfer to similar loads in the pile cap (bent 

beam) connections because of the structural dynamics of the overall deck system. 

Guo et al. (2015) conducted large-scale (1:10) physical laboratory tests with decks with simplistic 

girders and diaphragms. They found that the Douglass et al. (2006a) method for estimating wave 

loads on bridge decks generally agreed well with their experimental results. Specifically, they 

found that their vertical force data were bracketed (below and above) by the varying load estimate 

(C=1) and the sum of the varying load estimate and the impact load estimate (C=1+3=4). Guo et 

al. (2015) recommended combining the two portions (varying and impact) of the Douglass et al. 

(2006a) method for vertical loads and setting the combined coefficient to C=2 to better match their 

data for all situations except total submergence. As an example of such approaches, Section 

11.4.1 (below) adopts and presents this revision of the method. 

Probabilistic bridge fragility, or damage, models for coastal bridges exposed to storm surge and 

waves are described in Ataei and Padgett (2013a), Ataei and Padgett (2013b), Ataei and Padgett 

(2013c), Ataei and Padgett (2015), Ataei et al. (2010), and Padgett et al. (2012). Bridge fragility 

curves conditioned on wind speed and water depth are provided in Chorzepa et al. (2016) as part 

of a study on the vulnerability of coastal bridges in the state of Georgia. 

There is also a growing body of published literature addressing tsunami impacts on coastal 

bridges. Recent published literature addresses reliability assessments and fragility models, 

describes experiments and methods for estimating tsunami-induced forces, and describes

damage from tsunamis. For example, studies by Akiyama et al. (2012) and Akiyama and 

Frangopol (2014) describe probabilistic estimation of reliability based on the development and 

application of probabilistic seismic hazard data combined with tsunami fragility curves, like those 

described in Gidaris et al. (2017). Numerous publications describe numerical or physical 

simulations of tsunami loads on bridges. A comprehensive overview of existing studies is given 

in Hayatdavoodi and Ertekin (2016). More specific information related to the numerical simulation 

of tsunamis and their resulting loads on bridges is found in Azadbakht and Yim (2014), Bricker 

and Nakayama (2014), Wei et al. (2015), Wei and Dalrymple (2016), Salem et al. (2016), and 

Winter et al. (2018). There are comparatively fewer physical studies, but descriptions of tsunamiinduced loads on bridge superstructures are given in Shoji et al. (2011), Shijo et al. (2015), and 

Maruyama et al. (2017), the latter also included detailed observations of the Great East Japan 

earthquake of 2011. Detailed post-tsunami assessments of bridge damage are provided in Chock 

et al. (2013) and Kawashima and Buckle (2013). 

At the time of the preparation of this document, a multi-state and federal study is developing 

bridge design guidelines for the estimation of tsunami loads on highway bridges (Transportation 

Pooled Fund Program 2019).

Cuomo et al. (2007) developed empirical equations for wave loads that are fairly similar to those 

of Douglass and Krolak (2008) but include wave height directly in the dimensional parameters. 

AASHTO (2008) presents a method of estimating wave loads on bridge decks which is 

fundamentally different than the method presented in this chapter. Practitioners use both methods 

Guo et al. (2015) recommends modifications, which are adopted here, to the 

Douglass et al. (2006a) equations for wave loads on bridge decks.
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with one sometimes being a check on the other. Independent laboratory test investigations have 

found that the method of estimating wave loads on bridge decks in AASHTO (2008) overestimates 

loads for some conditions and underestimates loads for other conditions (Hayatdavoodi et al.

2015, Guo et al. 2015). The laboratory data used to develop and calibrate the wave loads equation

in AASHTO (2008) are not available for independent verification or use because Sheppard and 

Marin (2009) has a flawed printout of the data table (Appendix B: Table B-2 Significant Force 

Value for All Model Tests). 

11.4 Wave Loads on Bridge Decks: Method for Estimating 

This method for estimating wave loads on typical US bridge decks is a revision to the method 

originally suggested in Douglass et al. (2006a) and included as Appendix E of the previous edition 

of this manual (Douglass and Krolak 2008). The revision is based on some of the subsequentlypublished, independent laboratory experiments discussed above. 

The following sections present the revised wave load equations, a brief example calculation, and 

an explanation of the basis of the method and it how ties into the expanded body of knowledge 

and literature in more detail.

11.4.1 Equations for Estimating Wave Loads on Bridge Decks

The wave-induced loads imparted on elevated highway bridge decks by waves are estimated in 

terms of their vertical and horizontal components (see Figure 11.13) as: 

 (11.1) 

and

 (11.2) 

where: 

 = maximum of the vertical wave-induced load 

 = maximum of the horizontal wave-induced load

 Cv = an empirical coefficient for the vertical load (recommended value is Cv=2)

 Ch = an empirical coefficient for the horizontal load (recommended value is Ch = 2)

 cr = a reduction coefficient for reduced horizontal load on the internal (i.e. not the 

waveward-most) girders (recommended value is cr = 0.4)

 N = the number of girders supporting the bridge span deck

 Av = the area of the bridge contributing to vertical uplift, i.e. the projection of the bridge 

deck onto the horizontal plane (width of the deck times the length of the deck)

Δzv = difference between the elevation of the crest of the maximum wave and the 

elevation of the underside of the bridge deck or the low-chord of the diaphragms 

under the deck if diaphragms are present (see Figure 11.14 for definition sketch)

γ = unit weight of water (64 lb/ft3 for salt water)

 Ah = the area of the seaward side of the bridge contributing to horizontal loads, i.e. 

the projection of the bridge deck onto the vertical plane (height of the deck, 

including girders and solid rails, times the length of the deck)

Δzh = difference between the elevation of the crest of the maximum wave and the 

elevation of the centroid of Ah (see Figure 11.14 for definition sketch).

Note that Av and Ah are shown in the definition sketch of Figure 11.13 as the deck width and 

height. Thus, the lengths shown on the sketch are multiplied by the span length to get the areas 

applied in Equations 11.1 and 11.2. 
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When the wave crest elevation does not exceed the top of the bridge (solid rail elevation), a 

reduced area and lowered centroid corresponding to the area below the wave crest elevation can 

be used to define Ah and Δzh. 

The wave crest elevation used in Δzv and Δzh should be that corresponding to a very large wave 

height estimated in the design sea state, ηmax. Given a design sea state with a significant wave 

height (Hs), this elevation can be estimated as (see Section 11.6): 

 (11.3) 

The design sea state is typically computed using the design storm surge elevation (see Figure 

11.14). 

The recommended value of both empirical coefficients, in Equation 11.1 and 11.2, is 2 (i.e. Cv = 

2 and Ch = 2). The selection of these recommended values is discussed below. They are not 

intended to be conservative. Thus, they should be increased for conservative design values. 

Given the uncertainties involved in the application of the available methods for estimating wave 

loads on US highway bridges, the load factor of 1.75 required by AASHTO (2008) may be 

appropriate (see Section 11.4.3 Discussion of the Method).

. 

Figure 11.13. Horizontal and vertical wave-induced loads on bridge decks
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Figure 11.14. Definition sketch for Δzh, Δzv, Ah, Av, and ηmax in Equations 11.1 and 11.2 for 

estimating wave loads on elevated bridge decks) 

The method assumes that the two components (horizontal and vertical) of the wave-induced loads 

act in phase, i.e. their maximums at the same instant, to resolve the maximum resultant load. The 

two components will likely not be completely in phase but this is a reasonable initial approximation

for engineering. 

Equations 11.1 and 11.2 are for the maximum of the total loads. They include the magnitude of 

varying load and the impact load. Separate equations for the two types of loads were dropped in 

this revision following the recommendation in Guo et al. (2015). 

11.4.2 Example Application of Wave Load Equations

Application of the method is demonstrated using values which correspond to the US 90 bridge 

across Biloxi Bay, Mississippi during Hurricane Katrina. For the purposes of this example, a 

specific span has been selected as representative. This span is roughly in the middle of the 

western side of the bridge (see Figure 3.4). The low-chord elevation of the span (bottom of girders) 

was about +13 ft NGVD with the bottom of the diaphragms at +14 ft, top of the bridge deck at 

+16.5 ft, and the bottom of the deck at +16 ft. In this portion of the bay, the depth is fairly shallow, 

and the bottom, mud-line elevation is assumed to be about -4 ft NGVD. The decks were 33.4 ft 

wide and 52 ft long. 

Storm surge and wave hindcast modeling results indicate that at 8:00 a.m. CDT on August 29, 

2005, the mean water level had risen to an elevation of = 11.9 ft and there was a significant 

wave height at the bridge location of Hs = 6.2 ft (Douglass et al. 2006a). Thus, the waves were 

striking the span by that time in the storm. 

The wave loads on the deck at that time are estimated as follows:

 

 
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Δzh = (elevation maximum crest) - (elevation centroid of Ah) = 20 – 15.7 = 4.3 ft

 

In this example, Ah is 286 ft2 with a centroid elevation of +15.7 ft. These values are obtained by 

accounting for the design of the rail based on engineering plans obtained from Mississippi DOT 

for the old Biloxi bridge. There were 6 girders. 

So, in summary, at 8:00 a.m. the wave-induced loads on this span are estimated as being cyclical 

with maximum “varying” loads of 1,300 kips of vertical uplift and 470 kips of horizontal landward 

force. 

These bridge decks weighed about 340 kips, and their connections provided essentially no 

resistance to uplift. Thus, these example calculations imply that the uplift from some of the largest 

waves in the sea state at this time was enough to exceed the weight of the bridge span at the 

same time it was experiencing large lateral loads. Thus, these spans had likely already been 

bumped, by individual large waves, up and over on the pile caps by this time in the storm. Such 

behavior is consistent with the evidence. Katrina’s storm surge (and wave heights) continued to 

increase to a peak SWL of about +21.5 ft at around 10:30 a.m. CDT. 

11.4.3 Discussion of the Method

As mentioned above, Equations 11.1 and 11.2 are relatively simple to apply, can be applied in a 

conservative manner, are consistent with the available literature, and can be used to provide 

estimates of wave loads on bridge decks. These equations are slightly revised from the way this 

method was originally suggested in Douglass et al. (2006a) and included as Appendix E of the 

previous edition of this manual (Douglass and Krolak 2008). 

The primary revisions are the combining of the two aspects of the typical load signature, “varying” 

and “impact;” and the new coefficient values. These changes are based primarily on the 

recommendation of Guo et al. (2015). That recommendation was for the vertical load component 

only. However, there is still tremendous uncertainty surrounding the horizontal component of 

these wave loads (see discussions below about trapped air potential below). 

The older method did a good job of explaining the prototype 2004-2005 damage to bridges (see 

Douglass et al. 2006a) and these revisions do not change that. The older method was intended,

at that time, to be tested and improved upon relatively easily in the future as independent 

laboratory and prototype experimental data become available. 

The fundamental concept behind Equations 11.1 and 11.2 (that these loads are proportional to 

the relative relationship between wave crest elevation and deck elevation) is consistent with most 

of the literature including McConnell et al. (2004), Wang (1970), French (1970), Overbeek and 

Klabbers (2001), and NAVFAC (1982). When the coefficient is set to one, this method is identical 

to the method for estimating wave uplift loads on horizontal whalers on the front of vertical walls

presented in NAVFAC (1982). 

Inputs for the approach outlined above include the basic bridge deck cross-section and elevation 

information and estimates of storm surge elevation and wave height. 

This approach is not necessarily conservative. These revisions likely reduce the overall 

conservativeness for the vertical loads. But this reduction seems appropriate given the much 

greater availability of more appropriate laboratory data than existed in 2006. 
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The method can be conservatively applied with an appropriate factor of safety. A factor of safety 

can be applied for design by increasing the two coefficients. This is justified based on the 

complexities of the process, the uncertainties in estimating design wave conditions, the limited 

available lab-scale load data, the lack of bridge-specific lab results, and the relatively small scales 

of the available lab data. A similar load factor of two (2) has been adopted in ASCE Standard No. 

7: Minimum Design Loads for Buildings and Other Structures for wave loads on buildings for 

similar reasons (ASCE 2005). AASHTO (2008) requires a load factor of 1.75 for coastal 

hydrodynamic loads. Either of those values is appropriate. 

The method only provides an estimate of the total overall load without information concerning 

where that load is applied on the structure. Essentially, it thus implicitly assumes the load is 

applied through centroid of the cross-sectional area of the bridge. This is not particularly realistic. 

Also not considered are the details of wave phase and the fact that the down-wave width of bridge 

decks will likely cause spatially-varying loads, particularly uplift, that will impart a moment. These 

moments may be important aspects of bridge deck response. 

The approach outlined above should be used primarily for the case where storm surge elevation 

is roughly near the bridge deck elevation. Analyses indicate that this was the case during 

Hurricanes Ivan and Katrina. However, there are two other situational cases for bridge decks: 

• The deck is much higher than the storm surge such that only the crests of a very few 

waves in the storm sea state hit the girders (and loads are low), 

• The surge is so high that the bridge deck is completely inundated. The method here likely

overestimates that condition significantly. 

More research on wave loads on bridge decks is justified by the magnitudes of the estimated 

wave loads, the seriousness of the implications of them for design, the significant uncertainty in 

the available methods for estimating the loads, and the likelihood that the uncertainty can be 

reduced with more research. This research need includes quantitative laboratory force 

measurements for the cross-sectional geometry typical of simple-span bridge decks used in US 

highways across coastal waters for different levels of relative inundation (particularly the 

submerged bridges). 

These new equations for estimating wave loads can be conservatively applied 

with an appropriate factor of safety.
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Figure 11.15. Example of concave seaward face of a coastal bridge (I-10 bridge over Mobile 

Bay, Alabama)

A number of investigators have suggested that, while the vertical loads estimated by the method 

of Douglass et al. (2006a) are conservative but reasonable, the horizontal loads estimated are 

too high (i.e. too conservative). The revision outlined above effectively reduces the level of 

conservatism for the vertical loads. Also, none of the other investigators have used model 

geometries which are fully representative of one of the most important aspects of the geometry 

of US coastal highway bridges. Specifically, this is the likelihood of waves striking concave 

portions of the seaward side of the prototype structures. Figure 11.15 shows a typical seaward 

face of a prototype bridge deck. The combination of the seaward-most girder (which has a 

concave surface) and the overhanging deck has the potential to “catch” significant momentum in 

waves as well as allowing air pockets to form for an instant between the wave and the structure 

when storm surge is high enough. Whenever air is trapped against a rigid structure by waves, 

very high “impact” forces can be generated. Also, the coefficient for the individual girders, cr = 0.4, 

is based solely on judgment and not on any analysis or experiments. Research with more realistic 

geometries is needed to better understand the coefficients in Equation 11.2.

McPherson (2008) suggested a logical, simple modification of the Douglass et al. (2006a) method 

which adds the weight of water on top of the bridge decks to the vertical load equation. The 

addition of this term improved the agreement of the method with the McPherson data. This is a 

promising approach that has not been adopted in this revision but should be considered in future 

revisions. 
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The needed research is both physical laboratory model tests and numerical “laboratory” model 

tests. Recent computational fluid dynamics (CFD) investigations show promise (e.g. 

Hayatdavoodi et al. 2019). 

11.5 Wave Loads on Bridge Decks: Countermeasures

A wide variety of engineering adaptations, or countermeasures, to withstand the extreme wave 

loads occasionally placed on highway bridge decks during storms and tsunamis have been 

suggested. However, there is no evidence that any of them will work (except elevation to avoid 

the loads). 

Lehrman et al. (2011) found that none of the connections, between bridge deck girders and the 

pile caps (i.e. between the bridge superstructure and substructure), which are commonly used 

along the northern Gulf coast of the US are capable of resisting the extreme loads generated by 

hurricane waves on storm surge. 

FHWA (2017a) and Cleary et al. (2018) considered four potential engineering adaptionscountermeasures to the I-10 bridge across Mobile Bay: 

• strengthening the connections (two options were evaluated), 

• increasing span continuity, 

• modifying the shape of the bridge deck to reduce lateral wave-induced loads, and 

• increasing the deck elevation in combination with other retrofit options. 

This work followed the load path through the structure, traditional structural engineering practice, 

and found that none of the adaptations by themselves, with the exception of increasing the deck 

elevation, will ensure the survival of the structure in a reasonable, but severe, storm scenario. 

If the existing connections between the bottom of the girders and the pile caps (bent beam) were 

reinforced adequately to transfer those extreme loads to other structural components, the 

structure would fail under the wave-induced loads due to negative bending in the girders and deck

(FHWA 2017a). Failure of the substructure could eventually occur if the other bridge components 

could be reinforced to resist the loads. 

It should be possible to design a highway bridge structure that can survive the most extreme 

wave-induced loads. One oft-mentioned approach is to build it at a lower elevation with the idea 

that this will avoid more severe wave loads in depth-limited situations. Unfortunately, our 

understanding of wave loads on submerged structures is limited. More research is needed for this 

submerged situation. 

Design-specific laboratory tests of wave loads may be justified in the design of any high-cost asset

that is expected to be subjected to extreme wave loads. 

11.6 Bridge Deck Elevation to Avoid Wave Loads

The most common design approach is to avoid superstructure wave forces by elevating the bridge 

so that the storm waves crests pass under the low-chord of the bridge. This elevation is shown 

schematically in Figure 11.16. 

Strengthening the connections between bridge decks and the substructure will 

lead to another failure mechanism – “negative bending” of the deck and girders.
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Figure 11.16. Definition sketch of wave parameters and water levels for determining elevation 

of bridge deck for clearance from wave crests

The elevation can be set by adding some additional clearance or freeboard above the crest of the 

largest wave in the design sea state: 

 (11.4) 

The low chord elevation is taken as the elevation of the bottom of the girders (see Figure 11.16). 

The maximum wave crest elevation can be calculated as: 

 (11.5) 

where:

 SWL = design still water level

 Ymax = difference between the SWL elevation and wave crest elevation for the 

maximum wave in the design sea-state (defined below)

In general, the value for Y is the portion of the wave height, H, above the SWL. A useful 

engineering estimate of Y for this purpose is 75% of H. Thus, Ymax above can be estimated as: 

 (11.6) 

where:

 Hmax = design maximum wave height (defined below)

11.6.1 Nominal Maximum Wave Height Approach

The design maximum wave height (Hmax) depends on the site-specific conditions. The design seastate can be estimated using a wave generation model applied to that site for specific wind and 

water level conditions. Given a design significant wave height (Hs), the design maximum wave 

height can reasonably be set as: 
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 (11.7) 

The value of 1.7 given in Equation 11.7 corresponds with a wave height statistic on the Rayleigh 

Distribution (see Table 5.1) that is slightly higher than the average of the highest 1% of wave 

heights ( � ). This 1.7 value corresponds with the probable maximum wave height for 200 waves. 

This is a reasonable number of waves for the typical durations of the peak of a storm surge and 

average wave periods in storm surges. 

Combining Equations 11.6 and 11.7 yields: 

 (11.8) 

11.6.2 Depth-Limited Maximum Wave Height Approach

In some cases, however, the maximum wave height might be depth-limited, i.e. very large waves 

in very shallow water. Larger waves in the design sea state may break farther offshore of the 

bridge and the largest waves will not reach the bridge. In this case, check the depth-induced 

breaking criterion (or similar criteria): 

 

 (11.9) 

This can be written as:

 (11.10) 

where:

 ds = depth at bridge structure during design conditions (i.e. including the storm surge)

For the depth-limited case, combining Equations 11.6 and 11.10 yields: 

 (11.11) 

11.6.3 Estimating the Maximum Wave Crest Elevation

The difference between the SWL elevation and wave crest elevation for the maximum wave in 

the design sea-state (Ymax) used in Equation 11.5 should be the lesser of the values yielded from 

Equation 11.8 and Equation 11.11. Therefore, considering the potential for non-depth-limited and 

depth-limited maximum wave heights, the primary equation estimating the elevation of the 

maximum wave crest (see Figure 11.16) becomes: 

 (11.12) 

This equation can be used to set the elevation of the low-chord of bridge decks that span coastal 

waters. The next section discusses the use of additional freeboard above this elevation. 

11.6.4 Freeboard Considerations

“Freeboard” can be added to the maximum wave crest elevation. The approach outlined above 

does not provide “freeboard” above the wave crests. In riverine systems, SDOTs may require one 

or two ft of “freeboard” to be added above the design water surface elevation to account for wave 

action or debris as well as for uncertainty in the analysis. This freeboard, if added in the coastal 

situation, will also account for higher waves in the sea-state. The uncertainties involved in coastal 

surge SWL analysis are likely at least as great as those in the riverine situation (if not significantly 

greater). Thus, some additional freeboard for the low-chord elevation of coastal bridges may be 

appropriate. 
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AASHTO (2008) sets a required vertical clearance to avoid wave loads for coastal bridges as 1 ft 

above the maximum wave crest elevation in the 100-year return period sea state at that location 

accounting for storm surge. Bridges with less clearance should be designed to withstand the wave 

loads. 

However, complete clearance from all wave forces may not be needed to ensure bridge integrity 

during major coastal storms. Post-storm inspections of damage to bridge decks along the northcentral Gulf coast in 2004 and 2005 indicate that some bridge decks survived that were exposed 

to some wave loads. Apparently, the loads were small enough that they did not cause damage. 

This is likely due to resistance to wave forces provided by the weight of the bridge spans and the 

limited connections. The damage pattern suggests that there was a critical elevation at each 

location for that specific bridge deck design and those site-specific and storm-specific surge and 

wave conditions. Spans below that elevation were displaced off the pile caps; spans above that 

elevation were not. The critical elevation was below the elevation for complete wave clearance 

given by Equation 11.12. 

For example, Figure 11.9 shows some simply-supported spans on the US 90 bridge across Biloxi 

Bay, Mississippi still in-place even after removal of lower spans by Hurricane Katrina. These 

remaining spans had a higher low chord elevation than those displaced. The maximum wave 

crest elevation was +34 ft at the peak of the storm (based on hindcast modeling and Equation 

11.12), yet a bridge span as low as +24.5 ft “survived.” Thus, a bridge span with a low-chord 

elevation almost 10 ft lower than the maximum wave crest elevation apparently did not move. 

Chen et al. (2009) found that the Biloxi Bay bridge damage level corresponded with the crest of 

the significant wave height (not the maximum wave height in the sea state). This would suggest 

the Ymax could be set to 0.75Hs without any additional freeboard. A preferable approach is to set 

the deck elevations of new bridges based on an improved understanding of the wave loads. 

Projections of future relative sea level rise (RSLR) can be considered in freeboard design (see 

Figure 4.11). 

11.7 Other Coastal Bridge Issues

This section very briefly discusses other design and maintenance issues related to coastal bridges 

including increased concrete spalling due to wave splash, and lateral loads on pilings. 

Some low-elevation coastal bridges have suffered increased concrete damage near their 

landward end just above vertical retaining walls. Wave splash during storms sprays salt water on 

the underside of the bridge deck concrete and, over time, these areas can become areas of 

concern for bridge inspectors. The use of reinforced concrete in the marine environment typically 

calls for additional considerations, including the use of air entrainment admixtures and increased 

minimum thickness of specified concrete cover over reinforcing bars. Newer bridges, with higher 

clearances and longer, higher approach sections, often avoid this problem by elevating all of the 

bridge deck well above the elevation of splash. Wave runup and splash on existing low bridges 

could be reduced by placing rip-rap on the vertical walls. Clearance issues for coastal bridges 

over navigation channels are primarily controlled by the US Coast Guard. 

Waves can produce lateral loads on bridge pilings and pile groups. These loads in riverine 

situations are well modeled by the traditional fluid mechanics approach of estimating drag as a 

function of the water velocity squared and an empirical drag coefficient. However, the nature of 

The proven way to handle wave loads on bridge decks is to avoid them with 

elevation.
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wave motion produces loads beyond those due only to drag. The oscillatory water particle motion 

below waves can impart significant forces on structures due to the fluid accelerations as well as 

the velocities. Thus, it is neither adequate nor appropriate to simply increase the velocity used in 

the drag equations to account for the maximum wave orbital velocity. The acceleration generated 

forces, also called inertia forces, bear consideration. 

Morison’s equation from ocean engineering estimates the horizontal force per unit length of a 

vertical pile in waves as: 

 

 

 

 

 

 (11.13) 

where: 

 fp = horizontal force per unit length of a vertical pile

 fi = inertial force per unit length of pile

 fD = drag force per unit length of pile

 D = diameter of pile

ρ = density of water (1,025 kg/m3 for seawater)

 u = horizontal water particle velocity at the axis of the pile (as if the pile were not 

there)

 ax = horizontal water particle acceleration at the axis of the pile (as if the pile were 

not there)

 CD = drag coefficient

 CM = inertia or mass coefficient

The first term in Morison’s equation, the inertia term, accounts for the dynamic force on the 

structure due to the acceleration in the waves. The second term is the drag term and it is 

analogous to the drag load on a piling in unidirectional flow. The absolute value is used in the 

drag term because the load reverses direction with wave phase. In a wave, the water particle 

velocity, direction and acceleration at different points are constantly changing with phase. They 

also vary with depth below the surface and the total force on the pile is the depth-integrated sum 

of these changing loads. The two terms are out of phase and thus not maximum at the same time. 

More information, including values for the coefficients and appropriate applications, on Morison’s 

equation can be found in other references (Sarpkaya and Isaacson 1981; USACE 1984). 

An inherent assumption in Morison’s equation is the “thin piling” assumption that velocity and 

acceleration do not vary over the structure in the direction of wave propagation and that the piling 

is thin enough to not cause much of an effect on the wave. Because of the complexities involved 

in applications of Morison’s equation, a coastal or ocean engineer should be included in the design 

or analysis team for estimating wave loads on pilings. Empirical consideration of these forces is 

described in Wiegel (1964) and NAVFAC (1982). In cases of shallow water and/or wave breaking, 

where water particle velocities and accelerations will be significantly under-predicted by simple 

linear wave theory, higher-order theories are used (see Section 5.1). Dean's stream-function 

approach is a nonlinear wave theory that was developed to predict wave kinematics and forces 

on structures in deep and shallow water settings (Dean 1973). 

11.8 Selection of Design Storm Surge & Design Wave Heights

This section very briefly describes the process by which an engineer determines design water 

level and wave characteristics for resilient coastal bridge design. 
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11.8.1 Design Storm Surge SWL 

The selection of the design storm surge SWL (still water level) can be based on an analysis of 

historical storm surge elevations at the specific site or on an analysis that incorporates 

site-specific modeling of historical (hindcast) storm surges (see Section 4.6 Design Water 

Levels). Projections of future relative sea level rise (RSLR) can be included (e.g. Figure 4.11). 

USACE Coastal Hazard System, FEMA FISs and FIRMs provide SWL for many coastal areas. 

These may be suitable sources for these data, as long as study and methodological caveats 

are well understood. A nearby tide gage may not provide a reasonable first approximation of 

surge at a site. Storm surge elevations can vary significantly from location to location. 

Site-specific, probabilistic modeling of storm surge (and waves) is appropriate for the design of 

major new bridges and decisions concerning modifications to existing bridges. The 

potential damage justifies a comprehensive hydrodynamic surge analysis. Developing a 

probabilistic basis for this design storm surge elevation is consistent with the process for 

riverine bridge design considerations, risk-based flood maps for coastal management done 

by FEMA (and other agencies), and our modern understanding of future sea level rise. The 

historical gage analysis can be used as a check on the reasonableness of the results of the 

modeling approach. 

11.8.2 Design Wave Heights 

The design wave height (Hs) used in Equation 11.12 is the significant wave height at the bridge 

location during design conditions. Chapter 5 outlines appropriate ways for determining 

significant wave height. For fetch-limited situations, the parametric wind-wave generation 

modeling method may be adequate (see AASHTO 2008). For some situations in shallow water 

without much storm-surge, depth-limited wave conditions may apply (see Equation 5.25). Many 

situations, including those exposed to open ocean storm waves, may call for probabilistic 

oceanic wave modeling (see Section 5.4). 

Some USACE Coastal Hazards System data are available. As a check, FEMA FISs contain 

wave height estimates. However, these may not report Hs, but some other wave height statistic. 

Apply such estimates with knowledge of these and other study caveats. 

11.8.3 Coastal Engineer Involvement 

Given the importance and complexity of these considerations to the integrity of the 

highway structure, the FHWA suggests transportation officials might consider involving a 

qualified coastal engineer in the bridge project's planning, design, and pre-construction review 

(see Section 3.6 Coastal Engineering as a Specialty Area). 
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Chapter 12 - Coastal Scour

This chapter addresses coastal scour issues relevant to highway engineers working in the coastal 

environment. Coastal scour is the erosion/removal of granular bed material by hydrodynamic 

forces or processes in the vicinity of a coastal structure. This description of coastal scour, 

borrowed from the Coastal Engineering Manual (USACE 2002), specifically distinguishes it from 

a much broader interpretation of scour as simply erosion. Structures in the coastal environment—

bridge foundations, abutment walls, revetments—often cause scour by their very presence. Their 

presence modifies the flow of water thereby resulting in a change in sediment transport behavior. 

However, coastal structures are also subjected to scour resulting from geomorphological 

processes that are much larger both spatially and temporally. Geomorphological processes such 

as tidal inlet migration, barrier island retreat, and others, are less impacted by the presence of 

transportation infrastructure in the coastal environment, but interactions between the two are often 

problematic (see example Section 8.1). 

The FHWA’s National Bridge Inspection Standards (23 CFR part 650 subpart C) requires an 

appraisal of scour criticality for all bridges over water. In coastal environments, these bridges 

range in complexity from small, local roads crossing tidal creeks to limited access highways (e.g., 

interstate and toll roads) along the open coast. The practitioner should account for the potential 

range of hydraulic conditions at a bridge crossing, then select the set of conditions that govern 

scour for the purpose of design. It is important to note that the combination of flow velocity and 

depth ultimately drive the formation of scour around bridge foundations. Similar combinations of 

flow velocity and depth, and subsequent scour estimates, may exist for many different bridge 

crossings and hydraulic settings. However, the practitioner should apply the appropriate 

evaluation methods and tools to obtain those values for their given bridge setting and hydraulic 

conditions. 

This chapter covers a range of typical coastal scour issues and provides some potential 

processes and examples for their evaluation and estimation of interest to transportation 

professionals. The material begins with an overview of hydraulic conditions for coastal bridge 

locations and crossings. That discussion, and much of the discussion in this chapter, makes 

reference to Section 11.1 and the four general types of coastal bridge settings summarized there: 

tidal inlets, bays and sounds, tidal embayments, and rivers. Since scour depths are determined 

by parameter values (e.g., water depth and velocity), this chapter further categorizes bridges by 

their hydraulic setting and their exposure to tidal and/or other coastal processes that make those 

parameter values unique. The text then describes the various types of coastal scour. The chapter 

also includes pertinent examples of coastal scour and describes relevant applications of methods 

and tools for its estimation. The text briefly describes some scour countermeasures with the 

understanding that many strategies used in the riverine environment are transferable to coastal 

settings with appropriate modifications that account for wave attack and other coastal processes. 

12.1 Introduction

Coastal hydraulics and hydrodynamics impacts many US bridges. Depending on the region of the 

US and the coastal setting, design conditions may be related to tides, tsunamis, hurricanes, 

Experience and sound engineering judgment should be relied upon when 

evaluating the scour predictions for coastal bridges.
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nor’easters, or strong frontal systems (i.e. Great Lakes region). The stability of bridges is 

dependent upon an appropriate foundation design, which uses knowledge of both the long-term 

and single event-driven scour based on the in-situ soil properties within the waterway crossing. 

Coastal bridge scour occurs when bridge foundations, decks, abutments, and/or causeway 

islands interact with currents and waves in a way that alters sediment mobility and transport. 

The basic mechanisms driving scour are similar in both the riverine and coastal environments: 

flow velocity and flow depth. However, the coastal scour response to design events is often unique 

for at least four reasons: 

1. No distinct flow from an “upstream” source of sediment exists,

2. Flows are unsteady and multi-directional,

3. The presence of waves complicates the scour processes, and

4. Large surge/stage events do not always produce large velocities.

No distinct “upstream” source of sediment exists in the coastal environment. Rather, net sediment 

transport initiates from either side of a bridge due to the presence of flood and ebb tidal currents, 

incoming and outgoing surge or tsunami flows, and wind-generated waves. Coastal bridge 

foundations are also subject to geomorphologic changes that occur in estuaries, bays, sounds, 

and at tidal inlets along the coast. These long-term changes in sediment transport and bed 

elevation impact foundation design as noted in Section 12.3.1. 

Flows in the coastal environment are unsteady and multi-directional. There is no single “upstream” 

condition for coastal bridges. Existing methods for estimating equilibrium scour depths are almost 

universally based on two assumptions of the flow field: that it is steady, and that it is unidirectional. 

Tides are a type of “quasi-steady” flow. Their velocities persist over multiple hours of the day 

during the flood and ebb phases of the tide. The persistent direction of tidal currents during the 

flood and ebb phases means that the unidirectional assumption is not overly limiting. During 

design storm events, the currents associated with the rising and falling limb of a storm hydrograph 

may overwhelm the quasi-steady and unidirectional characteristics of tides. Further complicating 

matters, the orientation between flow velocity and the bridge foundation is also potentially unique 

for a given design event. Section 12.2.3 provides additional information regarding the complexity 

of coastal hydraulics at tidal bridge crossings. 

The presence of waves further complicates the scour processes. Waves are much shorter in 

period than the timescales of other coastal flows yet mobilize sediment and produce scour 

depressions at bridge foundations. Existing engineering practice clearly supports the concept of 

a maximum scour depth based on foundation parameters for pier scour (see Section 12.3.6), and 

on wave parameters for abutment scour (see Section 12.3.5). This is due to the nature of wave 

interactions with each foundation element where wave reflection from abutment walls drives the 

scour process but plays little role in scour at the base of a pier or pile. 

Large discharge events do not always yield large velocities. During a coastal flood event, lateral 

expansion of the floodplain often leads to exceptionally large cross-sectional flow areas. This 

expanded flow area yields comparatively smaller velocities and, therefore, smaller estimates of 

scour. Accordingly, evaluating scour potential for lower and higher exceedance probability events 

(longer and shorter return periods, respectively) is good practice. 

12.1.1 Bridge Scour Policy, Guidance, and Reference Materials

An established body of knowledge, products, and tools exist for the evaluation of scour at bridges 

crossing rivers and streams. The FHWA uses these products to develop and provide national 
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scour policy and guidance. As described in Chapter 2, the National Bridge Inspection Standards5

(NBIS) serve as one of the current policies for the inspection of bridges and the appraisal of scour. 

Other relevant and/or related policies include 23 CFR § 625, 23 CFR § 650 subpart A, and 23 

CFR § 650 subpart C, and (via 23 CFR § 625.4) relevant chapters of the AASHTO LRFD Bridge 

Design Specifications (namely chapters 2, 3, 10, 11, and 12). 

The FHWA currently has four reference documents containing information relevant to the topic of 

bridge scour in general:

• HEC-18 “Evaluating Scour at Bridges” (5th edition, Arneson et al. 2012)

• HEC-20 “Stream Stability at Highway Bridges” (4th edition, Lagasse et al. 2012)

• HEC-23 “Bridge Scour and Stream Instability Countermeasures: Experience, Selection, 

and Design Guidance” (3rd edition, Lagasse et al. 2009)

• Hydraulic Considerations for Shallow Abutment Foundations Technical Brief (FHWA 

2019b) 

Taken together, these documents provide information pertinent to the design of new bridges to 

resist scour impacts, for evaluating the vulnerability of existing bridges to scour, and for the design 

of countermeasures to mitigate scour. These reference materials provide important contextual 

information related to scour processes but do not specifically address most coastal scour issues. 

With some appropriate modifications, some of the tools developed for riverine scour are 

transferable to the coastal environment. This chapter identifies those opportunities, as are other 

methods, resources, and tools needed to develop estimates of the key hydrodynamic parameters 

(depth and velocity) that drive coastal scour formation. 

The NBIS, and other regulatory requirements described above, make no distinctions between

riverine and coastal scour situations. However, as a result of the complex hydrodynamics, 

sediment processes, and other coastal features, the FHWA cautions that in the coastal 

environment, the “hydraulic” expertise brought to bear on interdisciplinary teams should consider 

the inclusion of coastal engineering expertise, where appropriate. 

12.2 Hydraulics for Coastal Bridge Locations & Crossings

Waves, tides, storm surges, sediment transport, inlet dynamics and stability, and other coastal 

processes determine the hydrology and hydraulics that are most applicable for coastal bridges. 

Some bridges near the coast also experience varying levels of tidal influence. For those bridges, 

the practitioner should consider a combination of fluvial and coastal processes in order to 

determine the processes that govern bridge hydraulics. The following subsections describe many 

of these processes based on the degree of tidal influence on a bridge. The following text 

categorizes these bridges, in order of increasing tidal/coastal impacts, as tidally-influenced, 

tidally-dominated, and simply tidal bridges. The text also includes in its description of coastal 

bridges those that cross non-tidal waters but experience significant coastal processes. For 

example, bridges found throughout the Great Lakes. Section 12.4. provides suggestions for scour 

evaluation for each type of bridge crossing. 

12.2.1 Tidally-Influenced River Crossings

A tidally-influenced bridge is one that crosses a river or stream where astronomical tides modulate 

the fluvial discharge and stage, but where the flow does not reverse under normal tidal conditions. 

 

5 23 CFR part 650 subpart C
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These bridges often cross rivers, streams, or tributaries within coastal watersheds (see Figure 

11.1 for a graphical example). Figure 12.1 shows a tidally-influenced discharge record taken from 

the Suwanee River near Wilcox, Florida. The influence of the mixed, semi-diurnal tidal signal 

characteristic of this part of the Gulf of Mexico is evident in the record of discharge shown (see 

Section 4.2 for more information on astronomical tides). 

Figure 12.1. Discharge record for a tidally-influenced location (image downloaded from USGS 

website May 2019)

The flow at tidally-influenced bridges moves downstream with a magnitude that may slightly 

diminish during the flood tide. On the other hand, an ebbing tide enhances the downstream flow. 

At these bridges, the flow does not reverse direction during the flood tide. If the tidally-influenced 

bridge is well outside of the coastal floodplain, a typical hydraulic analysis of the bridge, using 

tools and equations typical of the riverine environment, is likely adequate. 

Coastal processes may impact a tidally-influenced bridge during major storm events. The degree

to which a tidally-influenced bridge experiences coastal impacts depends on the bridge’s location, 

the characteristics of the coastal floodplain, and the magnitude of the coastal storm event. As 

Three categories of coastal bridges are (in order of increasing tidal/coastal 

impacts): 1) tidally-influenced, 2) tidally-dominated, and 3) (simply) tidal 

bridges.
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noted in Sections 12.4 and 12.5, one may need to evaluate the fluvial and coastal events in order 

to determine the hydraulics that govern the design of bridges in these settings.

12.2.2 Tidally-Dominated River Crossings

Tidally-dominated bridges cross clearly definable rivers or streams where flow reverses during 

the astronomical flood tide. Like tidally-influenced crossings, these bridge crossings exist in 

coastal watersheds but are closer to the tidal source (see Figure 11.1 for a graphical example). 

At these locations, the flooding tide overwhelms the fluvial discharge and the mean (i.e. timeaveraged) flow is in the upstream direction for some period of the incoming tide. Figure 12.2 

shows the discharge record for a gage closer to its tidal source than the one shown previously for 

the Suwanee River (Figure 12.1). The periods of negative discharge, appearing twice daily with 

unequal magnitudes, correspond to the mixed, semi-diurnal tides from the Gulf of Mexico. Note 

that periods of high river discharge and/or weak tides may temporarily suspend flow reversals. 

One such period corresponds to the weak, neap tidal forcing on September 5 – 6, 2015 in that 

gage record. 

Tidally-dominated bridges are often in the coastal floodplain and, therefore, vulnerable to coastal 

storm surge and waves. In these cases, either the riverine flood event or the coastal flood event 

may govern bridge hydraulics. The practitioner should evaluate both conditions to determine 

which event leads to more scour and/or less freeboard (see Section 12.4.1 for a suggested 

approach).

Figure 12.2. Discharge record for a tidally-dominated location (image downloaded from USGS 

website May 2019)
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12.2.3 Tidal Crossings

Tidal bridge crossings cross open bodies of water such as estuaries, sounds, bays, bayous, and 

tidal inlets (see Figure 11.1 for examples). In these situations, the crossings are continuously 

elevated bridges with approach embankments on either side of the water body, or a series of 

elevated bridges connected by causeway islands. The physical coastal processes impacting tidal 

bridge crossings include relative sea level rise (RSLR), daily astronomical tides, storm surge, 

waves and wave processes, and potentially tsunamis, with all but the first producing combinations 

of flow velocity and flow depth that lead to scour. The influence of fluvial discharge on tidal bridge 

crossings is not always obvious. Depending on the characteristics of the fluvial discharge and the 

tidal water body, the watershed contributions may contribute only minimally to flow velocity, 

discharge, and depth. 

Coastal processes over timescales ranging from short to long, often dominate scour at tidal bridge 

crossings. Tidal crossings are subject to two different flow events during coastal storms and 

tsunamis: the first associated with the initial flood-directed surge or tsunami propagation (i.e., the 

rising limb of a hydrograph), and the second associated with the ebb-directed return flow or 

tsunami drawdown (i.e., the receding limb of a hydrograph). Each of these events has the potential 

to produce unique, time-dependent depth and velocity combinations that impact the scour 

formation process. 

12.2.4 Non-Tidal Waters

Bridge crossings over non-tidal waters may experience many of the coastal processes mentioned 

previously except for sea level rise and astronomical tides. Such bridges are located within the 

Great Lakes region. Bridges near the primary lake shorelines are vulnerable to significant 

seasonal or decadal lake level fluctuations. During storm events, these bridges often experience 

large waves, increased lake levels, and long-period lake level fluctuations (i.e. seiching). Methods 

and tools for evaluating scour at tidally-dominated bridge crossings are applicable to this category 

of bridges as well. 

12.2.5 Other Considerations

The combination of fluvial and coastal flood events, or compound flooding, was addressed earlier 

in Section 4.4. These types of maximal flooding conditions, while certainly complex, may not 

necessarily be the same conditions as those that would produce the worst scour. This is because 

when comparing the effects of depth and velocity, the two primary hydraulic variables associated 

with scour, velocity plays a greater role. In addition to stage, the practitioner should consider 

relationships between stage and discharge or stage and velocity to determine the conditions that 

lead to larger scour estimates. 

In the coastal environment, the maximum velocity and maximum storm tide elevation typically 

occur at different times. Maximal coastal scour formation conditions likely occur when the velocity 

is the greatest value. Such conditions arise during two scenarios: first, when surge is entering a 

tidal inlet or embayment on the rising limb of a storm hydrograph. The second occurs during the 

recessional period, when combined surge and the storm-derived rainfall flows back to the ocean. 

Consider Figure 12.3 as an illustrative example of this concept from a USGS gaging station on 

the Neches River near Beaumont, Texas. For the same stage value of 2.7 feet occurring before 

and after Hurricane Harvey on August 25, 2017 and September 19, 2017, respectively, the 

discharge and velocity are almost twice as large during the recessional period as they are 

preceding the storm.
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Figure 12.3. Representation of recession-dominated flow following Hurricane Harvey

12.3 Types of Coastal Scour

The types of scour that occur at bridges in the coastal environment include the same general 

categories (long-term change, contraction scour, and local scour) as found at riverine bridges. 

Additionally, coastal bridges can experience scour as a result of wave action (wave scour) and 

localized areas of high velocity associated with wave-current interactions. Coastal bridges, 

including tidally-influenced bridges located some distance from the coast, are experiencing sea 

level rise. Engineers can consider, as part of their bridge hydraulics evaluation, the effects that 

sea level rise will have on coastal bridge scour and freeboard requirements. See Section 4.1.6

for more information and discussions on projections of sea level rise for coastal transportation 

design. 

Important caveats and differences are associated with scour in the coastal environment. One is 

geomorphologic changes that impact deposition and erosion at a coastal bridge. Others include 

the potential impact of bedforms on scour depressions; the influence of abutments and causeway 

islands on contraction scour; alternative methods for estimating local pier scour in coastal 

environments; and wave-induced scour at piers and abutments. 

12.3.1 Long-term Changes

Long-term changes influencing scour at tidal bridge crossings include deposition and erosion of 

the bed, lateral migration of the channel thalweg (i.e. the deepest part of the channel) and/or 

shoals, shoreline recession, and tidal inlet migration. Changes in river or stream bank position are 
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also important considerations for tidally-influenced and tidally-dominated bridge crossings. 

Analyzing the effects of these long-term changes is important for two reasons. First, long-term 

changes alter hydrodynamic conditions including flow speed and direction, as well as wave height 

and direction which affect scour estimates. Second, these long-term changes may lead to 

undermining of bridge foundations even in the absence of the design scour event. Understanding 

and considering the impacts of long-term coastal change on bridge foundations are within the 

coastal engineer’s area of expertise. 

Deposition (aggradation) and erosion (degradation) refer to the long-term changes in bed 

elevation along a reach of a stream, channel, or water body. In the riverine environment, changes 

in upstream and downstream sediment supply, flow characteristics, and/or changes in the 

downstream hydraulic conditions drive the deposition and erosion that lead to bed elevation 

changes over time. For coastal bridges, deposition and erosion are often associated with changes 

in net sediment transport volumes, rates, or patterns. In the coastal environment, such changes 

occur over timescales ranging from short (i.e. hours) to long (i.e. centuries). Coastal storms and 

sea level rise, respectively, are the driving factors at each end of this scale. In addition to these 

natural processes, construction of jetties, groins, seawalls, bulkheads, causeway islands, and 

other works of man, alter the natural sediment delivery processes within the coastal environment. 

Evaluations of deposition and erosion at coastal bridges rely on measurements of bed elevation 

over time. These data are typically available as part of routine bridge inspection surveys. The 

engineer uses the change in bed elevation between bridge inspection surveys to determine rates 

of deposition or erosion for their scour appraisal. 

Bridges at or along the coast may not have foundations in well-defined stream or river banks and 

channels, but they are nevertheless vulnerable to lateral migration of the channel thalweg, as well 

as migration of the channel shoals. Some of the general techniques for evaluating stream stability 

mentioned in HEC-20 (Lagasse et al. 2012) apply here. One major exception is the use of aerial 

imagery, which by itself is not enough to determine channel stability. This analysis also uses

historical hydrographic survey data. Hydrographic survey data are available in the form of 

historical nautical charts and, more recently, high-resolution side-scan sonar bathymetric 

mapping data.

Shoreline recession represents an important form of long-term change (see Section 8.2). Bridge 

approaches or foundations near receding shorelines are subject to undermining as the shoreline 

retreats and erodes in response to long-term changes in net sediment transport and sea level 

rise. While the bridge approaches, abutments, and foundations may withstand some amount of 

scour, complete undermining and/or flanking of protection is possible as shorelines retreat over 

long periods of time.

In the coastal environment, navigation channels often have a clearly defined cross-section where 

the deepest portion is usually at the channel thalweg. Typically submerged, the shallow areas 

adjacent to the channel are referred to as “shoals” instead of banks, since the latter often indicates 

emergence above the water. Federally managed navigation channels typically have well-defined 

and maintained alignments but, like natural channels, may shift in response to changes in 

hydrodynamic conditions and net sediment transport. This is particularly true in estuaries and 

natural tidal inlets. 

Tidal inlets are in a dynamic equilibrium. Wave-driven longshore sand transport leads to lateral 

migration of the inlet throat, whereas ebb/flood tidal flows try to scour these deposits to maintain 

channel depths (see Section 6.6). Tidal inlets, even those stabilized by jetties, groins, or seawalls, 

can exhibit substantial morphologic changes in response to the net longshore sand transport, sea 

level rise, and changes to the back bay, sound, or estuary. Some, like Oregon Inlet in North 

Carolina, are extremely dynamic and change dramatically even over relatively short time periods, 
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as shown in Figure 12.4. Substantial lateral migration of the channel thalweg at this bridge 

contributed to significant local pier scour and even temporary closures during inspection and 

response efforts (Krolak and Henderson 2016).

Figure 12.4. Aerial imagery of Oregon Inlet, North Carolina from 1993, March 2011, and August 

2011

A much smaller tidal inlet, Matanzas Inlet near St. Augustine, Florida illustrates well the potential 

for tidal inlets, shoals, and channels to change over time as shown in Figure 12.5. The substantial 

changes in morphology, channel position, and channel depth attributed to natural tidal inlets can 

undermine and/or flank bridge foundations. Engineering of tidal inlets and/or modification of the 

tidal prism are known to impact scour as well (see Section 12.3.1.1 for one such example).

Figure 12.5. Aerial imagery of Matanzas Inlet, Florida from 1995, 2012, and 2016

12.3.1.1 Example: Indian River Inlet (Delaware)

Indian River Inlet, Delaware (see Figure 12.6) has experienced progressive scour since it was 

originally dredged and stabilized with jetties in the 1930’s. Scour holes near the bridge piers 

exceeded depths of 100 ft in 2000. As the inlet has deepened and its minimum cross-sectional 

throat area increased, more tidal flow has moved through it. Thus, its tidal prism has increased. 
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And as the tidal prism has increased, it has continued to scour out the throat area. Essentially, 

the artificially constructed and stabilized inlet has not reached its evolutionary equilibrium since 

its original opening in the 1930’s. Placing large very large riprap around the bridge piers as scour 

countermeasures only exasperated the issue, as the resulting mounds (see “red” areas on the 

roadway alignment in Figure 12.6) reduced throat area until the tidal prism again worked to 

reestablish equilibrium; unfortunately, resulting in further scour. 

Ultimately, scour concerns led to the construction of an entirely new cable-stayed bridge that 

spanned the entire inlet. However, even this new bridge had approach sections and piers within 

the locations of storm surge and tides. 

Figure 12.6. Bathymetric elevations showing scour holes inside Indian River Inlet (circa 1999)

12.3.2 Bedforms

Waves and currents lead to the development of bedforms in the coastal environment. Bedform 

lengths and amplitudes, while often small, contribute to the total scour at bridge foundations and 

abutments. Sand wave shoaling in navigation channels can generate very large bedforms, called 

megaripples, having heights up to 6 ft and lengths over 15 ft (Levin et al. 1992). Wave-generated 

bedforms, often termed “vortex or orbital ripples” because of their genesis, form by the interaction 

of wave orbital velocities with bed sediments. Characterizing the bedform dimensions of length 

and amplitude under field conditions is difficult. The Coastal Engineering Manual (USACE 2002) 

and some textbooks (e.g., Nielsen 1992) provide equations for predicting bedform dimensions 

over a range of sediment and hydrodynamic conditions. These reference materials provide 

equations for predicting bedform characteristics under regular and irregular wave conditions. In 

many cases, general equations represent the bedform length (Equation 12.1) and amplitude 

(Equation 12.2) with some amount of conservatism (see Figure 12.7 for definition sketch): 

 (12.1) 

 (12.2) 

 (12.3) 
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where:

λ = ripple length;

 A = horizontal orbital semi-displacement;

ηmax = maximum ripple amplitude (one-half the ripple height); and 

φ = sediment angle of repose.

Figure 12.7. Definition sketch showing the representative variables used to estimate the height 

and length of orbital ripples

12.3.3 Contraction Scour

Contraction scour refers to the removal of bed, channel, bank, and shoreline sediment resulting 

from the contraction of flow due to physical obstructions. The localized increase in velocity within 

the contraction leads to enhancements in bed shear stress that mobilize and entrain bed 

sediments. Sediment transported out of the region of the bridge by currents and waves passing 

through the contraction result in an overall lowering of the bed elevation throughout the crosssection. In the coastal environment, natural channel narrowing, causeway islands, bridge 

abutments, bridge approach embankments, or any encroachment of the bridge structure lead to 

contraction of flow during normal and storm conditions (Figure 12.8). 

The severity of contraction scour in the coastal environment depends greatly on the setting, the 

orientation between the obstruction and the flow, and the relative elevations between the 

obstruction and the adjacent terrain. For example, causeway islands may completely inundate 

under storm conditions, diminishing the severity of the contraction. Bridges crossing tidal inlets 

may experience similar conditions. In such cases, the portion of the embankment above the 

coastal flood elevation constitutes the only remaining obstruction that constricts the flow. The next 

two subsections include methods for estimating horizontal and vertical contraction scour.
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Figure 12.8. Examples of possible horizontal contractions at coastal bridges (thin lines denote 

possible streamlines, double-ended arrows denote the potential for flow reversals)

12.3.3.1 General Approach for Contraction Scour

The methods presented in HEC-18 (Arneson et al. 2012) for estimating contraction scour are 

applicable to coastal bridges, with some special considerations. Contraction scour at coastal 

bridges is most often like the Case 1 examples shown on page 6.2 in HEC-18. Contraction scour 

may occur under clear-water or live-bed conditions at coastal bridges. In order to determine the 

appropriate set of equations to apply, compare the critical velocity for sediment mobilization to 

the design flow velocity. During clear-water conditions localized increases in velocity within the 

contraction mobilize sediment and carry it away from the bridge crossing. During live-bed 

conditions the design velocity upstream of the bridge crossing is greater than the critical velocity, 

and sediment is moving toward the bridge crossing. HEC-18 describes both scenarios and 

provides the necessary equations for estimating contraction scour under either condition.

Selection of an upstream section needs special consideration in coastal contraction scour 

analyses. During a coastal storm surge event, velocities may approach the contraction from one 

side early in the event storm event, and from the opposite side as the storm surge recedes. It is 

important to evaluate contraction scour resulting from both “upstream” conditions. Receding storm 

surge flows may combine with watershed contributions resulting in flow velocities larger than 

those estimated during the initial surge propagation. A similar situation occurs during daily tidal 

exchange at each bridge, where the flood- and ebb-directed velocities approach the contraction 

from opposite sides during either phase of the tidal cycle. 

Engineering judgment is needed in the selection of an upstream cross-section location for scour 

calculations. The most appropriate upstream cross-section locations are free from major 

constrictions or other changes in channel/flow alignment. In other words, the most appropriate 

section aligns perpendicular to the flow field, and the upstream flow streamlines are straight and 

parallel. 

The HEC-18 contraction scour equations, originally based on a form of Laursen’s (1960) 

equations, may be overly conservative for coastal storm surge conditions. Steady, uniform flows 

are the assumed conditions leading to the ultimate scour condition. Coastal storms can produce 

extremely large velocities. However, the high velocity condition may not persist for long enough 

to attain the ultimate scour condition. This is also true of tidal velocities that persist either in the 

flood or ebb phases for no more than six to twelve hours per day. Performing two- or threedimensional hydrodynamic and sediment transport numerical modeling is a viable alternative for 

overly conservative estimates. Physical modeling is another alternative, but geometric scaling 

considerations often make these experiments cumbersome to perform in most laboratories (see 

Section 6.8). 
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12.3.3.2 Vertical Contraction Scour

Partially or fully submerged bridge superstructures during flood events may contribute to another 

form of contraction scour. Vertical contraction scour, often termed “pressure scour,” develops due 

to the vertical contraction of flow below the bridge deck and girders/beams (see Figure 12.9). 

Note that vertical and horizontal contraction scour occur simultaneously when bridge 

superstructures are overtopped. HEC-18 presents a methodology for estimating the vertical 

contraction scour for partial or complete bridge submergence (see also FHWA 2019b for related 

information). The primary consideration when applying these equations to coastal bridges is that 

the vertical contraction may occur both early and late in the storm event on the rising and falling 

stages of the storm surge hydrograph. This may also apply during tsunami events as well. Low, 

at-grade, bridge crossings and bridge approach spans are most susceptible to vertical contraction 

scour. 

Figure 12.9. Vertical contraction or pressure scour schematic (Arneson et al. 2012)

12.3.4 Local Scour at Bridge Piers

Local scour describes the scour that forms when flows interact with piers, piles, caissons, or other 

similar vertical, surface piercing structures. Section 12.3.5 addresses abutment scour, which is 

another type of local scour. Local scour develops around the base of the structure. Unlike in the 

riverine environment where the scour hole primarily develops on one side of the pier or pile, the 

multi-directional characteristics of coastal flows yield scour holes that develop on either side and 

often coalesce into one complete scour depression at the base of the structure (see Figure 12.10

for example). 

The following processes affect local pier scour:

• Increased mean flow velocities and pressure gradients near the structure,

• Development of secondary flows in the form of vortices, and

• Increased turbulence near the structure.
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The development of secondary flows near the structure are known to influence the scour 

development process as a result of two specific flow mechanisms. First, separation of the flow 

around the structure produces wake vortices downstream of the structure. Second, horseshoe 

vortices, wrapping around the structure near the bed and free surface, develop due to the variation 

in stagnation pressure along the upstream face of the structure as well as flow separation at the 

limit of the scour depression (FDOT 2012). This discussion of local pier scour focuses on scour 

processes initiated by tidal or non-tidal currents. Waves also produce their own type of local pier 

scour (see Section 12.3.6). The mechanisms described above are generally responsible for scour 

development in both cases, the difference being the time scale of wave-induced effects on the 

mean flow field is much shorter in duration than that attributed to the tidal or non-tidal currents. 

Like contraction scour, local pier scour happens during clear-water and live-bed conditions. If 

sediment is in motion upstream of the bridge, live-bed scour occurs. If sediment is not in motion 

upstream of the bridge and sediment mobilization only occurs near, and due to, the structure, 

then clear-water scour occurs. Recall that “upstream” conditions occur on both sides of the bridge 

during tidal and storm conditions and that the ebb tidal flows and receding storm flows can 

combine with fluvial discharge resulting in larger velocities. 

12.3.4.1 General Approach

Existing local pier scour equations were developed specifically for use in riverine conditions under 

the assumption of steady and uniform flow. However, they are based upon fundamental hydraulic 

processes and sediment characteristics that are transferable to the coastal environment. For

example, a design velocity and mobilization velocity for sediment are definable in both the riverine 

and coastal settings. Still, their application in the coastal environment uses some special 

considerations and sound judgment. 

This document describes two general approaches for estimating local pier scour at coastal 

bridges. They include the methods presented in HEC-18 and an alternative set of scour equations, 

originally developed by Sheppard et al. (1995) and described in the FDOT Bridge Scour Manual 

(FDOT 2005). Both documents provide local scour equations for clear-water and live-bed 

conditions. When the total estimated scour (i.e. the sum of long-term, contraction, and local scour) 

is greater than 5 feet, the FDOT Bridge Scour Manual recommends using the Sheppard Pier 

Scour Equations over the HEC-18 equations for coastal bridges. 

12.3.4.2 Wide and Complex Pier Geometry

Coastal bridge foundations are composed of multiple elements between the soil and the 

superstructure: bent beams, columns or piers, pile caps or platforms, and multiple piles. The 

elements that interact with currents and waves modify the flow field in ways that lead to local pier 

scour. The combination of these elements often results in complex shapes and geometries. In the 

case of bascule piers, the geometry of the foundation is very large compared to the flow field. 

However, existing scour equations were developed based on the formation of a scour hole around 

an individual obstruction. A correction factor or an effective diameter account for the geometry of 

complex piers in existing scour equations.
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Figure 12.10. Local scour depression at the terminus of a coastal structure

HEC-18 includes methods to compute pier scour for standard and complex pier geometries. The 

HEC-18 equations include wide pier correction factors that may be applicable to bascule piers 

when the pier is wide in comparison to the flow depth. HEC-18 also outlines a procedure 

for evaluating scour at complex piers consisting of a combination of pile groups, pile caps, and 

piers. Hoffman and Verheij (1997), Melville and Coleman (2000), and Sheppard (2001) 

present alternative local pier scour equations that are infrequently used in practice. 

The Sheppard pier scour equations from FDOT (2005) use an alternative definition of pier 

diameter. The complex pier method consists of developing an effective diameter, taken as the 

sum of the effective diameters of each component as shown in Figure 12.11: 

 Dcol

* Dpc* Dpg* (12.4) 

where: 

 = effective diameter of the complex pier,
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 Dcol

∗ = effective diameter of the column, 

 Dpc

∗ = effective diameter of the pile cap, and

 Dpg

∗ = effective diameter of the pile group.

This effective diameter methodology, described fully in FDOT (2005), assumes that each 

individual complex pier element can be modeled as a simple surface piercing pile with its own 

unique effective diameter. The pier elements are shown graphically in Figure 12.12. The basic 

concept is that the scour produced by the complex pier element and the scour produced by its 

effective diameter are equal in magnitude. However, the scour depth for the complex pier 

geometry is not simply the combination of each scour depth associated with an element. The total 

scour depth for the complex pier geometry is estimated using the total effective diameter, because 

there is a nonlinear relationship between scour depth and effective diameter in the general local 

scour equations (FDOT 2005) (see Figure 12.13). 

Figure 12.11. Complex pier components for Sheppard pier scour equations (FDOT 2005)

Complex pier geometries may consist of elements that become exposed during local scour. In 

these cases, the shape of the structure and its effective diameter change as scour progresses. 

Estimation of the equilibrium scour depth is an iterative process that accounts for changes in the 

effective diameter, flow, and/or sediment characteristics. This is a much more complex process. 

Therefore, the procedure is presented in terms of three specific cases (Figure 12.14) where the 

only difference is the location of the pile cap relative to the bed elevation prior to initiation of the 

local scour process. Note that the bed elevation used in this procedure should already account 

for the types of scour mentioned previously: long-term scour, bedform height, and contraction 

scour. The three cases are as follows: 

• Case 1: for situations where the complex pier shape does not change as local scour 

progresses (the pile cap is above the eroded bed elevation);

• Case 2: for partially buried pile caps; and

• Case 3: for completely buried pile caps.

The FDOT Bridge Scour Manual (FDOT 2005) fully describes these methodologies for estimating 

effective pier diameters. 
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Figure 12.12. Individual elements for Sheppard pier scour equations (FDOT 2005)

Figure 12.13. Concept of effective diameter for complex piers in the Sheppard pier scour 

equations (FDOT 2005)
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Figure 12.14. Three cases for Sheppard pier scour equations (FDOT 2005)

12.3.4.3 Example: Johns Pass (Florida)

Large Bascule piers are a contributing factor to local scour for a bridge at Johns Pass, Florida 

(Figure 12.15). Johns Pass is still evolving in response to engineering that occurred decades ago. 

Most of Florida’s inlets have been artificially created and stabilized by engineering works. Their 

tidal prisms have been significantly affected by engineering of the bays and by other inlets 

connected to those bays. 

Johns Pass illustrates three important lessons regarding scour and coastal bridges. First, because 

of its relative size, the presence of a Bascule pier has a larger than normal effect on the resulting 

scour prediction. This usually is handled with application of HEC-18’s wide or complex pier scour 

approaches (see also FDOT 2005). Second, the multiple inlets into the bay illustrate an important 

concern about attempts to numerically model such bridges and locations (Figure 12.16). 

Each inlet could require a separate boundary condition to ensure overall hydrodynamic 

circulation. Additionally, the direction of the surge event could complicate the hydrodynamics, and 

thus adequacy of the modeling results. Third, the multiple inlets are evolving geomorphologically

in response to previous engineering and thus the tidal flows through the pass may be gradually 

changing (see Section 6.6). 
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Figure 12.15. Johns Pass, Florida (2002)

Figure 12.16. Location overview for Johns Pass, Florida
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12.3.5 Local Scour at Bridge Abutments

Scour at bridge abutments is another form of local scour. HEC-18 provides methods for estimating 

abutment scour due to currents. Application of these methods, originally developed for the riverine 

environment, should account for the many complexities mentioned previously regarding coastal 

processes. For example, estimation of abutment scour at a coastal bridge should account for the 

flood and ebb tidal velocities, storm velocities associated with the rising and falling limb of the 

storm hydrograph, and/or potential combinations of ebbing or receding flows and fluvial inputs. 

Waves also lead to scour at abutments, or vertical walls more generally, and erosion of approach 

embankments during coastal flood events. Wave reflection from abutments, or wave breaking at 

abutments, increases the bed shear stress at the base of the wall leading to toe scour. The 

Coastal Engineering Manual (USACE 2002) provides methods for estimating toe scour due to 

regular, irregular, nonbreaking, and breaking waves. Figure 12.17 graphically demonstrates 

typical toe scour due to nonbreaking regular and irregular waves. Note that pile bents near 

abutment walls may experience additional local scour due to the development of bars and troughs 

running somewhat parallel to the abutment wall. 

Figure 12.17. Typical scour profiles due to waves at the toe of a vertical wall (from USACE 

2002)
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Xie (1981, 1985) developed an empirically-based estimate of maximum toe scour by assuming 

normally incident, nonbreaking, regular waves impinging on an impermeable vertical wall: 

 (12.5) 

where:

Sm = maximum scour depth at a distance L/4 from the wall;

H = incident wave height at the wall;

d = water depth at the wall; and 

L = normally-incident wavelength.

Hughes and Fowler (1991) proposed an equation for toe scour due to normally incident, 

nonbreaking, irregular waves: 

 (12.6) 

where:

urms = the root-mean-square of the horizontal nearbed wave orbital velocity;

Tp = peak wave period; and

Lp = wavelength associated with the peak wave period.

Toe scour due to breaking waves is often greater in magnitude than that attributed to either regular 

or irregular nonbreaking waves. The CEM (USACE 2002) presents some equations for estimating 

toe scour due to irregular, breaking waves but use of a simple approximation is more common. 

That approximation is: 

 (12.7) 

In other words, the maximum expected toe scour depth due to breaking waves is approximately 

equal to the maximum wave height at the abutment or wall (see Figure 12.18). This wave height 

is often equivalent to the water depth at the abutment under breaking conditions. If the depth at 

the abutment is extremely large, then the maximum wave height is instead fetch- or durationlimited instead of depth-limited. In those cases, application of a suitable equation from the CEM 

is prudent. 

Figure 12.18. Scour due to breaking waves at a vertical wall (USACE 2002) 

Use of a spill through or sloping abutment, instead of a vertical wall, reduces the maximum 

expected toe scour for all types of wave conditions. This is because the amount of wave reflection 
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from a sloping abutment is less than that from a vertical wall. The use of appropriately sized riprap 

or armor stone in front of the abutment also reduces wave reflection, ultimately reducing the scour 

while providing toe protection. 

Another form of scour may occur when elevated water levels allow waves to impact approach 

embankments. Wave action can easily remove embankment soil and undersized riprap during 

storm events. This erosion can lead to undermining of the approach and flanking of the abutment 

wall. Figure 12.19 shows an example, where large waves occurring during Hurricane Ivan in 2004 

removed small riprap and eroded soil from under approach slabs. The result was undermining 

and collapse of the approach slab. One way to prevent this damage is to provide an appropriate 

revetment cross-section including armor stone sized and graded to withstand wave attack (see 

Section 7.2). 

Figure 12.19. Damage to bridge approach behind abutment due to wave action in a hurricane

Some states, like Florida, do not require abutment scour estimates when the design includes 

minimum abutment protection (FDOT 2012). The minimum abutment protection, including stone 

size and weight, is a function of either the design velocity (Isbash equation, HEC-23) or the design 

wave conditions (Hudson’s equation, Section 7.2), with the larger of the two governing the design. 

In the coastal environment, armor stone sized to withstand wave attack is typically adequate to 

withstand movement by design flow velocities. Also, in fetch-limited situations boat wakes may 

serve as the design wave event. Section 12.6 provides additional information about scour 

countermeasures. 

12.3.6 Wave-Induced Scour

Wave-induced pier scour occurs when the wave orbital velocities interact with bridge foundations 

and sediments. The literature suggests that the amount of wave-induced scour is typically much 

less than that estimated using the local pier scour equations mentioned previously (Sumer et al. 

1992; USACE 2002). Equilibrium scour depths associated with wave action may attain a 

maximum value of about 1.3 times the pile diameter, though slightly larger multipliers are possible. 

While these scour depths are often small compared to those associated with currents, the 
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combination of waves and currents together lead to additional scour. Also, wave-induced scour 

may be large for complex pile groups as described below. Note that the time periods associated 

with severe wave attack during storm events are generally short—on the order of hours—and that 

the equilibrium scour depth due to waves may not occur during the design event. 

Wave scour processes occur due to different mechanisms. A dimensionless parameter called the 

Keulegan-Carpenter (KC) number describes the relative magnitude of the flow and structure 

length scales as: 

 (12.8) 

which describes the ratio of wave orbital velocity (flow) length scale to the size of the pile or pier,

where: 

Um = maximum nearbed wave orbital velocity (see Section 5.1) 

T = wave period; and

D = pile diameter.

When KC<1, interactions between the wave field and pile or pier dominate the scour process. 

These conditions typically occur due to very large pile or pier diameters. Wave-induced scour at 

the base of a bascule pier may fall into this category. When 6<KC<1,000, vortex shedding of wave 

orbital velocities around the pile or pier govern scour formation. In this range, the flow length scale 

is approximately six times greater than the pile, so the structure is characterized as a slender 

element. Beyond KC>1,000 the scour process attains an equilibrium value due to the quasisteady nature of the flow relative to the size of the pile. 

Separate equations exist for the steady streaming (e.g., Sumer and Fredsoe 2001; Khalfin 2007) 

and vortex shedding (e.g., Sumer et al. 1992; Dey et al. 2006; Sumer et al. 2007) ranges of KC. 

However, a transition zone between them (1<KC<6) is mostly ignored in published literature. An 

evaluation of numerous published scour studies by Webb and Matthews (2014) focused on this 

transition zone. They proposed a set of equations that covered the continuous range of KC values 

from 0.1 to infinity. Figure 12.20 shows the empirical data used to derive the equations and the 

best fit of the suggested equations to those data. The dashed gray line represents the best fit to 

all scour data considered in the analysis (N = 256). The dotted black line is a more conservative 

upper bound equation that captures nearly 95 percent of all values considered in the analysis. 

These two equations are as follows: 

 (best fit) (12.9) 

 (upper estimate) (12.10) 

where: 

S = equilibrium scour depth; 

D = pile diameter; and

KC = Keulegan-Carpenter number.

For complex pier geometries, or where the spacing between piles is relatively small, individual 

scour holes will coalesce or the pile group will behave as one larger pile and a deeper scour hole 

Equilibrium scour depths associated with wave action may attain a maximum 

value of about 1.3 times the pile diameter (though slightly larger multipliers are 

possible). 
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will form. The literature for estimating wave-induced scour for complex pier geometries and pile 

groups is still quite immature, but Webb and Matthews (2014) proposed a temporary solution for 

pile groups. When the ratio of the gap between piles (G) to the individual pile diameter (D) is less 

than five (G/D < 5), an effective diameter (De) is substituted in place of D in the scour prediction 

equations above. However, the individual pile diameter (D) should still be used to estimate the 

value of KC in those equations. This methodology could be applied to Case 1 and Case 3 

conditions illustrated by Figure 12.14. 

Figure 12.20. Wave-induced local pier scour (Webb and Matthews 2014)

This effective diameter, which is different than the one described for the FDOT scour equations, 

yields a cross-sectional pile area that is equivalent to the area of the entire pile group. The 

effective diameter is estimated as demonstrated in Figure 12.21): 

 (12.11) 

where:

 De = effective diameter of the pile group;

 L = length of the pile group; and

 W = width of the pile group.

Breaking waves, as might occur during a storm event, will exacerbate the scour process. The 

FHWA and SDOTs have documented several situations where significant scour occurred during 

severe coastal events. For example, during Hurricane Katrina, upland portions of the US-90 Biloxi 

Bay bridge became subject to surge and waves. As seen in Figure 12.22, a large scour hole 

formed in the vicinity of a pile bent. While bridge superstructure failure occurred for other reasons 

(see Section 11.2), the size and extent of the scour hole was significant. This figure also shows 

the potential for large scour holes to develop around entire pile groups as previously noted. 
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Figure 12.21. Definition sketch for effective diameter of pile groups for wave-induced pier scour 

(Webb and Matthews 2014)

Figure 12.22. Scour hole formed by Hurricane Katrina (permission to use photograph provided 

by Joe Krolak) 
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12.3.6.1 Example: Jensen Beach Causeway (Florida)

In 2005, the Jensen Beach, Florida causeway (Figure 12.23) experienced wave scour episodes. 

The passage of two successive tropical events along similar storm tracks produced waves within 

the embayment. These waves struck the causeway abutments and bridge piers producing scour 

to depths of over 30 ft (Figure 12.24). FDOT engineers had concerns about structural integrity of 

the foundations should a third storm event occur before installation of scour countermeasures. 

In trying to determine what had occurred, FDOT expressed concerns that standard HEC-18 

approaches did not predict such scour depths even when using advanced two-dimensional 

hydrodynamic and wave modeling. Only when investigators also considered and modeled, using 

SED2D, sediment transport resulting from both currents and waves did the simulations agree with 

post-event measurements (Glasser and Gosselin 2006). 

Figure 12.23. Jensen Beach Causeway bridge 

12.3.7 Tsunami-Induced Scour

Tsunamis are a coastal hazard for many bridges along the US West Coast, Pacific Islands, and 

Alaska. Tsunami-induced scour around foundations is known to cause building collapse. 

However, tsunami-induced bridge scour is not a topic that has received much attention. While 

some information exists, the understanding has not matured to the point of describing all of the 

scour processes that can occur at a bridge crossing. Most published literature describes local 

scour. There are no specific methodologies or data for contraction scour in published literature. 
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Figure 12.24. Jensen Beach Causeway bridge post-event scour bathymetry (2005)

Our current understanding of scour during tsunamis acknowledges contributions from both 

velocity/shear and pore pressure relief. Regarding the latter, published literature describes the 

potential effects of liquefaction on scour enhancement during rapid tsunami drawdown. An 

evaluation of scour at roadways and bridges reported in Francis (2006) found that a set of 

predictive equations proposed by Tonkin et al. (2003) explained observed scour depths very well 

for the 2004 Sumatra tsunami. The Tonkin et al. (2003) equations include a scour enhancement 

factor that accounts for the pore water mechanics that lead to liquefaction. However, their complex 

equations use input data that are not always readily available. 

Extensive field observations following the 2004 Sumatra tsunami and the 2011 Tohoku tsunami 

found that scour depths were generally less than 10 ft (Francis 2006; Chock et al. 2013; Bricker 

et al. 2015). The vast number of scour depth observations contained in published reports may 

point to a practical limit on local scour due to the relatively short duration of tsunami inundation 

and drawdown. 

Some very simple qualitative estimates of tsunami induced local scour are perhaps appropriate 

for screening bridge foundations for tsunami scour vulnerability. One relationship is cast in terms 

of a simple ratio of maximum scour depth (ds) to tsunami height (ho). Triatmadja et al. (2011) and 

Kuswandi et al. (2017) suggest that ratio ranges from 0.25 to 0.60. In other words, maximum 

estimated scour depths range from 25 percent to 60 percent of the tsunami height. 
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Another simple method for estimating scour depth is based on a percentage of the flow depth (d). 

This concept, based on the work of Dames and Moore (1980), is presented in FEMA (2012) as 

guidance for estimating approximate scour depths resulting from tsunami inundation events. The 

scour depths are further refined based on soil type and distance from the shoreline (coast) in 

Table 12.1. For sand, the scour depths range from 35 to 80 percent of the tsunami flow depth 

depending on distance from the coast. 

Table 12.1. Tsunami-induced scour guidance from Dames and Moore (1980) and FEMA (2012).

Soil Type Scour Depth (% of d)

(< 300 ft from Shoreline)

Scour Depth (% of d)

(> 300 ft from Shoreline)

Loose sand 80 60

Dense sand 50 35

Soft silt 50 25

Stiff silt 25 15

Soft clay 25 15

Stiff clay 10 5 

Triatmadja et al. (2011), who performed an evaluation of tsunami scour around coastal structures, 

found good agreement between measured equilibrium local scour depths and estimates using 

the HEC-18 local pier scour equations. ASCE (2016) outlines an Energy Grade Line (EGL) 

method for estimating the maximum tsunami inundation depth and velocity. This depth and 

velocity could be used along with the HEC-18 equations to arrive at a very conservative estimate 

of local pier scour. 

Bricker et al. (2015) also found reasonable estimates using the HEC-18 equations. They 

compared eight separate scour equation predictions to field measurements following the 2011 

Tohoku tsunami. The various methods underestimated or overestimated the measured scour 

depths by approximately a factor of 0.5 to 2, respectively. Of the methods evaluated, the HEC-18 

equations for local pier scour and abutment scour were included and generally overestimated 

measured scour by a factor of 1.5. The tsunami bore propagates with a long period and the flow 

dynamics may have similarities with the riverine conditions for which the HEC-18 equations were 

derived, but not sufficiently long enough to produce equilibrium scour. 

12.3.8 Time-Dependent Scour

Time-dependent scour equations have been suggested as more appropriate in the coastal 

environment. In addition to the typical physical processes, the short duration of the typical design 

storm is considered. Also, piers that are impacted by waves are subjected to very short duration 

pressure gradient fluctuations and shear stress variations which are difficult to quantify. 

The University of Florida has conducted research and developed such a set of equations 

(Gosselin and Sheppard 1998; Miller 2003). The Florida equations use a time-marching solution 

for the depth of scour adjacent to bridge piers. In other words, the governing equation is 

discretized and solved at successive time intervals using the results of preceding time steps. Input 

time-varying estimates of depth-averaged storm surge velocities at the bridge are based on 

numerical modeling of the hydrodynamics. The Florida equations include calibration coefficients 

which are primarily based on laboratory investigations. Miller (2003) discusses how the equations 

can be used to estimate scour at prototype coastal bridges. Gosselin and Sheppard (1998) 

concluded that more research would be needed before meaningful relationships could be 
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developed for time-dependent local scour. More recent research on this topic exists but is limited 

to scour at the base of monopiles used in offshore structures. Those results may not be 

transferable to bridge foundations. Therefore, this remains an area of needed research. 

12.3.9 Other Types of Coastal Scour

Scour also occurs at other types of coastal structures, such as a revetment, breakwater, groin, or 

jetty. Fewer semi-empirical scour equations exist for these rubble mound structures. In place of 

scour equations, rules of thumb are often applied to evaluate scour potential at the base of rubble 

mound structures. 

At the toe of a sloping rubble mound structure, such as Figure 7.14, the maximum scour is often

less than toe scour at an impermeable vertical wall under the same conditions. The CEM (USACE 

2002) states that a conservative estimate of scour depth is then: 

 (12.12) 

where: 

Sm = maximum scour depth; and 

Hmax = maximum wave height at the structure toe. 

Decreasing a structure’s reflection coefficient leads to similar reductions in scour depth. Flattening 

the structure slope and/or increasing its porosity reduce wave reflections and, therefore, scour 

depth. Currents flowing parallel to the structure in the presence of waves generate more scour. 

Waves approaching at oblique angles to a structure will also lead to larger scour depths due to 

two separate effects. First, oblique waves generate currents that flow parallel to the structure. 

These currents are effective at transporting sediment away from the structure. Second, shortcrested waves will increase in height along the structure due to the mach-stem effect (USACE 

2002). Larger wave heights have the potential to cause more scour. 

12.4 Bridge Scour Evaluation Methods

12.4.1 Tidally-Influenced Bridge Scour Evaluation

Many of the methods and tools used in the evaluation of riverine scour today are likely transferable 

to tidally-influenced bridges. Figure 12.25 shows a proposed workflow for evaluating design event 

scour at tidally-influenced bridge crossings. A practitioner should consider the downstream 

tidal/coastal boundary condition in evaluations of scour and bridge freeboard for tidally-influenced 

bridge crossings. The scour analysis should consider the design riverine discharge event, yielding 

the worst-case scour conditions, with a downstream boundary condition at the MLLW tidal datum. 

This set of conditions should lead to enhanced velocities and larger estimates of scour. Evaluating 

bridge freeboard, on the other hand, calls for an elevated downstream boundary condition. A likely 

choice there is a return period coastal flood elevation. Regulatory flood maps show the 100-yr 

return period (1 percent annual chance) coastal flood elevations. The corresponding Flood 

Insurance Study reports may provide coastal flood elevations for higher and lower probability 

events as well. 
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Figure 12.25. Proposed design scour analysis workflow for a tidally-influenced bridge crossing

12.4.2 Tidally-Dominated Bridge Scour Evaluation

The evaluation of scour at tidally-dominated bridges may involve consideration of fluvial, tidal, 

and coastal flood events, as well as their combinations, in order to determine which governs the 

design. Figure 12.26 illustrates a possible analysis workflow for evaluating design event scour at 

a tidally-dominated bridge crossing. Note that riverine and tidal event return periods, while 

specified as a 100-yr event in the graphic, should be selected to produce the worst-case scour 

for an appropriate design event. 

Evaluation of the riverine and coastal flood events may call for different modeling approaches 

depending on the exposure of the bridge to wave action. A one-dimensional or two-dimensional 

HEC-RAS model, or a two-dimensional SRH-2D model, may be appropriate for estimating design 

event parameters for the riverine flood event. These are standard approaches for scour evaluation 

in the riverine environment that most practitioners use today. 

Modeling the design coastal flood condition suggests, at a minimum, a two-dimensional unsteady 

hydraulic or hydrodynamic model. If the distance between the bridge and the coastal boundary 

condition is large, the influence of local storm winds on water levels and currents may contribute 

to flow velocity and depth in a substantial manner. A modeler can incorporate those processes 

into an appropriate coastal hydrodynamic model. Standard hydraulic models, like those listed 

above, generally do not have those capabilities. Appropriate hydrodynamic models will provide 

the flood depths, velocities, discharges, and wave characteristics needed to estimate contraction 

and local scour. Some coastal models, like the Coastal Modeling System and Delft3D, also 

simulate sediment transport and changes in bed elevation. However, sediment transport models 

have much greater uncertainties than hydrodynamic models. 

When the effects of wind on waves, currents, and water levels are negligible, a two-dimensional 

unsteady hydraulic model, like SRH-2D, may prove useful. In this case, the coastal storm surge 

hydrograph serves as the downstream, time-varying boundary condition for the hydraulic model. 

The hydraulic model will propagate the coastal flood throughout the model domain in a manner 

like that of a hydrodynamic model. Obtaining or defining an appropriate coastal storm surge 

hydrograph is not a trivial matter. Representative hydrographs from past storms are often suitable 

candidates. 

Tidally-dominated bridges may experience increased flow velocities and enhanced scour during 

the recession of a coastal flood event. As coastal flood waters recede, their velocities combine 

with fluvial inputs to the coast, thereby resulting in higher velocities (see Figure 12.3). Although 

the joint probability of fluvial discharge and coastal storm events is low in many cases (Wahl et 

al. 2015), simulating some appropriate amount of fluvial discharge may be prudent in order to
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develop the worst-case scour conditions for the design and check event frequencies (e.g., 100-yr 

and 500-yr return period events). This is particularly true for crossings of tidal inlets (next section), 

where the inland bay or estuary can have a significant storage effect on ebb-directed velocities. 

Figure 12.26. Proposed design scour analyses workflow for a tidally-dominated bridge crossing

12.4.3 Tidal Bridge Scour Evaluation

Determining the design event parameters for tidal bridges will, in almost all cases, call for the use 

of two-dimensional hydrodynamic models. The widespread availability and use of these models 

make it difficult to justify application of simple, analytical approximations of coastal flows. For 

example, the tidal prism method uses a lot of assumptions: the bay-ocean tidal phase lag, friction, 

entrance/exit losses, etc. Its results are strictly only applicable to simple tidal inlets and do not 

work well when applied in other coastal settings. 

Figure 12.27 shows a proposed analysis workflow for evaluating design event scour at a tidal 

bridge crossing. Note that while the graphic indicates use of the 100-yr event return period, it is 

best practice to also simulate the 50-yr and 500-yr return period coastal events to evaluate the 

behavior of stage and velocity through the bridge opening. 

The evaluation of scour at tidal bridges should consider the relevant design conditions that govern 

bridge hydraulics (see 23 CFR § 625, § 650.115 and § 650.117). Depending on the region of the 

US and the coastal setting, design conditions may be related to tides, tsunamis, hurricanes, 

nor’easters, or strong frontal systems (i.e. Great Lakes region). The selection of an appropriate 

hydrodynamic model will depend on which of these design conditions governs the depths, 

discharges, and velocities that lead to bridge scour. The practitioner should also consider relevant 

combinations of forcing in these models (i.e. tides+surge+waves, tides+surge+waves+discharge, 

and others). 

It is necessary to simulate the design event parameters on both the rising and falling limbs of the 

coastal flood event, as one will likely dominate over the other. To that end, the simulation should

include the complete time history of the event, not just one phase or stage. The practitioner should 
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consider the time histories of stage, velocity, and discharge during the design event to determine 

which combinations lead to greatest scour. 

Finally, it should be stressed that defining the design event frequency for scour in the coastal 

environment is not analogous to the riverine environment. Coastal bridge hydraulic design is 

typically related to an event frequency based on extreme water levels. Riverine bridge hydraulic 

design is related to an event frequency based on discharge values. In the coastal environment, 

for example, the 100-yr return period water levels are often the basis of design for vertical 

clearance and loads. Scour estimates are often based on the velocities and depths corresponding 

to this design event threshold, though their values are also checked for the 500-yr event. But 

strictly speaking, the maximum velocity, and therefore the resulting scour depths, determined 

from such an event do not necessarily have the same return period or exceedance probability. 

The maximum scour may occur for a coastal event with a stage that is lower or higher than the 

100-yr event. See Figure 12.29 for an example illustration of stage and velocity data for a bridge

crossing in Texas. In Figure 12.29 the velocity corresponding to the 100-yr coastal flood event is

less than that of the 50-yr event. The practitioner should consider combinations of velocity and

flow depth for enough design event return periods, or stages, to discern their potential trends

before determining the appropriate conditions for estimating bridge scour.

Figure 12.27. Proposed design scour analyses workflow for a tidal bridge crossing

12.4.4 Example of Tidal Bridge Scour Calculations

The following text provides a general example of design event scour calculations for a tidal bridge 

crossing exposed to hurricane storm surge and waves. Note that some of the values used in the 

preparation of this example may not reflect actual design conditions or parameter values at this 

location. The bridge is located in Pensacola Bay, Florida along the northern Gulf of Mexico (see 

Figure 12.28). There is no appreciable fluvial input to this system and the bridge hydraulics are 

governed by water levels, waves, and velocities associated with extreme coastal storm events. 

The following example text summarizes the appraisal of long-term scour; the effects of bedforms 

on scour depths; horizontal contraction scour; local pier scour; and total resulting scour for the 

100-yr design event and the 500-yr check event.
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Figure 12.28. Location overview for the Pensacola Bay Bridge scour example

Figure 12.29. Return period still water levels and corresponding event velocity magnitudes on 

Neches River, TX
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Step 1 – Determine the long-term bed elevation changes:

Historical bed elevation cross-sections parallel to the bridge alignment were reviewed using data 

provided as part of bridge inspection reports over a 15-year period. An average rate of erosion 

along the entire bridge length was calculated as -0.02 ft/yr. Over the specified 75-year service life 

of the bridge, this average erosion rate leads to an estimated 1.5-ft lowering of the channel bed, 

which is rounded to 2.0 ft for conservativism. This value for long-term scour will be added to the 

local scour values at the end of this design example. The lateral stability of the channel was 

similarly evaluated using the historical channel cross sections. The channel is stable and has not 

migrated during the period covered by those reports. 

Step 2 – Estimate bedform height and length:

The potential for large wave-generated bedforms is estimated using the equations described in 

Section 12.3.2. A simulation of the design coastal storm event provides the water depths and 

wave characteristics (height and period) needed in this step. At a specific pile bent, the water 

depth is 38 ft, the wave height is 14 ft, and the wave period is 6 s. A sediment analysis determined 

the sediment angle of repose as 30 degrees. Using these parameters, the estimated bedform 

length and height are: 

 

 

  

 

 

 

 

 

 

 

 

 

The calculated ripple height is rounded to 1.0 ft and incorporated into the total scour calculation 

at the end of this example. 

Step 3 – Estimate contraction scour:

Horizontal contraction scour values are found using the Laursen equations in HEC-18. Conditions 

at the bridge were evaluated to determine whether scour occurs as live-bed or clear-water scour. 

For this example, the contraction scour occurs as live-bed scour and the Modified Laursen live 

bed contraction scour equation in HEC-18 provides the scour estimate. An appropriate “upstream” 

section was identified on either side of the bridge alignment, chosen such that flow streamlines 

were as straight and parallel as possible. The Laursen equations were applied for each 

“upstream” section to determine the maximum contraction scour during the rising and falling limbs 

of the storm surge hydrograph. The width of, and discharge across, each “upstream” section was

extracted from the storm surge model output. The resulting live-bed contraction scour calculations 

produced a 100-yr design event value of 2.0 ft and a 500-yr check event value of 8.1 ft. 

Step 4 – Estimate local pier scour:

Estimates of local scour around the complex bridge piers (Case 1, Figure 12.14) were found using 

the FDOT (2005) method, pier and pile foundation design sections, sediment characteristics, and 

storm surge model output. These calculations account for additional local scour induced by the 

foundation of the twin span alignment, as well as scour associated with the remains of a nearby 

abandoned bridge foundation. The influence of adjacent and abandoned foundation elements on 
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local scour should always be evaluated, particularly when the ratio of effective pile diameter to 

the distance between foundations is small. Taking these issues into account, the estimated local 

pier scour at a representative bent location is 15.3 ft for the 100-yr design event, and 16.6 ft 

for the 500-yr check event. Note that FDOT (2005) contains detailed calculation examples for 

Case 1, Case 2, and Case 3 complex bridge pier geometries. 

Step 5 – Estimate wave-induced scour:

Because of the greater depths along this bridge alignment, the contributions of wave-induced 

scour to local pier scour are expected to be small. Wave-induced scour could potentially be 

important for approach foundations that experience small velocities, due to friction from shallow 

flooding depths, during surge events. 

The foundation design for this bridge contains an array of 24-inch-square concrete piles below a 

rectangular cap. The piles are spaced 10 ft apart in the longitudinal direction and 15 ft apart in the 

transverse direction. Therefore, the gap to diameter ratio is greater than or equal to 5 in both 

cases; the effective diameter does not apply under these conditions. The equations described in 

Section 12.3.4 are applied to estimate the wave-induced scour at each foundation pile as follows:

 

 

 

 

 

 

 

The contribution of wave-induced scour to local pier scour is approximately 1 ft. 

Step 6 – Calculate total scour:

The total scour is the sum of values determined in Steps 1-5. Note that the scour values calculated 

in Steps 2, 4, and 5 are location-dependent and will be unique for each pile bent. The values 

determined in Steps 1 and 3 are applied to each bent location. Therefore, for the bent location 

selected in this example, the total estimated scour is as follows: 

100-yr Total Scour = 2.0 ft + 1.0 ft + 2.0 ft + 15.3 ft + 1.0 ft = 21.3 ft

500-yr Total Scour = 2.0 ft + 1.0 ft + 8.1 ft + 16.6 ft + 1.0 ft = 28.7 ft

Step 7 – Estimate abutment scour:

The abutments at this bridge are only exposed to wave action during the design flood event. The 

method of Hughes and Fowler (1991), described in Section 12.3.5, is used to estimate the local 

toe scour due to nonbreaking irregular waves. Using the same values described earlier for 

maximum wave orbital velocity, wave period, and water depth the calculations are as follows: 

 

 
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The local toe scour under breaking wave conditions could be substantially larger, growing to 

approximately 80 percent of the flood depth at the abutment wall under the design event 

conditions. 

12.5 Scour Appraisal Methodologies

12.5.1 Low-Risk Crossings

Small, low-risk bridges crossing tidal creeks and minor tributaries in the coastal environment are 

unlikely candidates for sophisticated coastal hydrodynamic modeling. For very small bridges or 

bridge openings, their scale may actually prohibit all but the most advanced forms of numerical 

modeling. However, for these bridges (as per 23 CFR § 650 subpart C), an owner may determine 

whether they may be unstable as a result of scour based on either the observed or evaluated 

scour condition (23 CFR § 650.305). For the evaluated condition, this requires analysis of scour, 

which typically requires hydraulic computations of flow velocity and depth (see 23 CFR § 

650.117). One simple way to estimate flow velocity for a given flood event is to consider the phase 

speed of a shallow-water wave described in Section 5.1 (Equation 5.6). This is a conservative 

estimate of velocity that increases with the total flow/flood depth. This estimate provides a 

magnitude but no direction. For very small bridge openings the flow direction may align with the 

tidal creek or tributary that conveys the flow. If this is not the case, assume an orientation or skew 

that yields a conservative estimate of bridge scour. 

12.5.2 Typical Crossings

The evaluation of scour at most typical bridge crossings will almost always call for the use of an 

appropriate numerical model. Exceptions may include cases where design event data are 

available within a database, or where the flood depths, discharges, and velocities can be 

estimated with some certainty. These exceptions notwithstanding, it is difficult to justify not using 

numerical models to estimate scour at all but the smallest of bridges in the coastal setting. For 

tidally-influenced bridges, this may be a 1D or 2D hydraulic model. The HEC-RAS (1D) and SRH2D (2D) models mentioned previously are appropriate for bridge crossings over clearly definable 

rivers or streams not directly exposed to open waters and/or substantial wave action. For tidallydominated bridges adjacent to open water, for tidal bridges crossing open water, and for non-tidal 

bridges exposed to coastal processes, the use of 2D coastal hydrodynamic and/or wave models 

is necessary. 

Numerical models are typically used in a complete bridge hydraulics study. The flood depths, 

discharges, velocities, and waves predicted by these models serve as input to the contraction and 

local scour equations mentioned previously. Because the scour prediction equations were derived 

for steady, uniform flow conditions, and because the coastal flood events are typically of short 

duration (on the order of a few hours), ultimate scour may not be reached at some coastal bridges. 

Due to the uncertainties inherent in defining the storm event, the model predictions, and those of 

the scour equations themselves, it is difficult to state that the resulting scour estimates are 

conservative. Experience and sound engineering judgment should be relied upon when 

evaluating the scour predictions for coastal bridges. 

12.5.3 Special Cases

More sophisticated numerical models or reduced scale physical models are useful in resolving 

uncertainty or complexity in the scour predictions. The practitioner may consider these more 

comprehensive evaluation methodologies when the existing scour equations yield questionable 

results, or when the bridge crossing and coastal setting are extremely complex. This level of 
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analysis is likely only justifiable for some bridges as the level of effort (i.e. time and cost) is 

considerable. 

The use of more sophisticated numerical models may include the direct simulation of sediment 

transport and/or morphologic updating. These models are typically two- or three-dimensional (3D) 

models. For very complex coastal settings, the use of a 3D model is preferable. Furthermore, the 

use of a morphologic model can explicitly simulate the positive feedback that occurs between 

hydrodynamics and morphology as the bed elevation changes. It may be impractical to resolve 

the disparity in spatial scales small enough to simulate development of local scour around a pier 

as well as the hydrodynamic conditions of the entire bridge crossing, simultaneously. In such 

cases, a larger scale numerical model could be used to first develop an estimate of the contraction 

scour, with those conditions then used as input to higher resolution numerical models that better 

refine hydrodynamics and sediment transport around a set of bridge piers or the bridge abutment 

and approaches. 

The development of reduced scale physical models is challenging but may be necessary in some 

special situations. Here the physical dimensions of the problems are scaled down from prototype 

(true) to model (reduced) scale using Froude scaling. The horizontal and vertical scaling may be 

the same or distorted depending on the geometry of the problem and the size of the laboratory 

facility. In order to be truly compatible between the prototype and model scales, Reynolds scaling 

would also have to be satisfied for the sediments. However, simultaneously satisfying Froude and 

Reynolds number scaling in a laboratory is often impossible. Scour experiments conducted at 

reduced scale often assume that the Reynolds scaling can be relaxed due to the fully turbulent 

conditions typical of design events. For those conditions, the variation in Reynolds number is said 

to be insignificant for scour development. 

12.6 Scour Countermeasures

Scour prevention, or scour mitigation, can be achieved through the use of countermeasures. A 

countermeasure is typically a layer or some arrangement of less erodible material on the seabed 

or wherever scour is expected to occur. The countermeasure is typically placed near the toe, or 

at the base, of the structure where scour is to be prevented. A revetment prevents erosion of 

earthen embankments exposed to flow or wave attack. However, the toe of the revetment is also 

vulnerable to scour so it is designed with scour countermeasures integrated into its cross-section 

(see Section 7.4). A vertical wall can need a separate countermeasure as part of its design. 

In the coastal environment, scour countermeasures often consist of scour blankets, appropriately 

sized riprap or armor stone, and possibly even precast reinforced concrete armor units. For all of 

these examples, an appropriate geotextile fabric or layer of adequate bedding stone should be 

used as an underlayment to prevent piping of the soil through the scour countermeasure. In the 

case of scour blankets, also called marine mattresses, these fabrics are often integrated into the 

blanket/mattress framework. 

The design of scour countermeasures depends on two primary criteria. First, the stability of the 

countermeasure itself should be analyzed under design conditions. If the countermeasure is 

exposed to strong currents and wave action, both stability criteria should be satisfied, resulting in 

stone sizes for each criterion. Unlike revetments that only consider waves, and thus can apply 

Hudson’s equation (Section 7.2), designers need to consider additional criteria to also capture 

current. Eckert (1983) suggests that the larger of the two stone sizes be increased by a factor of 

1.5 to resist the combination of waves and currents. The stability of countermeasures to resist 

strong currents can be determined using the Isbash equation in HEC-23. Armor unit stability to 

wave attack can be evaluated using Hudson’s equation (Section 7.2). 
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The stability of marine mattresses and special concrete armor units may use alternative methods. 

Equations for sizing scour apron stone and for evaluating scour blanket stability in current fields 

are outlined in the CEM (USACE 2002). 

Second, the appropriate extent or dimensions of the countermeasure needed to protect against 

scour and/or slip failure should be estimated. The extents of the countermeasures will be different 

for each type of structure. The following methods provide suggested scour apron widths for 

cantilevered or anchored retaining walls, gravity retaining walls, sloping structures, and piles. 

For cantilevered or anchored retaining walls, providing a scour apron can prevent scour at the toe 

and/or to provide the stability needed to prevent slip failure. Failure of the apron’s edge may occur 

due to bed scour and/or lowering of the channel bottom over time. Consult FHWA (2019) for 

additional information on shallow abutment foundation countermeasures for tidally-influenced and 

tidally-dominated bridge crossings. 

Eckert (1983) provides equations for the recommended scour apron width based on geotechnical 

conditions and hydrodynamic conditions, where the appropriate size will be the larger of the two 

estimates (see definition sketch Figure 12.30). To protect against slip failure, Eckert (1983) 

recommends that the scour apron width (W) be: 

 

where:

de = depth of wall penetration below the seabed; and

φ = sediment angle of internal friction.

The width of the scour apron to satisfy hydrodynamic conditions should be taken as the greater 

of the following two expressions: 

 (12.14) 

or, 

 (12.15) 

where:

Hi = incident wave height; and

ds = depth at the toe of the structure.

In the case of gravity retaining walls, the scour apron need not be as wide as for cantilevered 

walls due to the lack of slip failure vulnerability. Eckert (1983) recommends a scour apron width 

approximately equal to the height of the incident nonbreaking wave. 

Mitigation of local scour at the base of existing at-risk piles or piers may be accomplished through 

the use of scour aprons as well. For individual piles exposed to tidal flows, Hoffmans and Verheij 

(1997) recommend an apron width that is at least four times the pile diameter in each direction 

parallel to the dominant tidal velocities, and a width of at least 2.5 times the pile diameter in each 

transverse direction. Therefore, the total apron size would be 8D in the streamwise dimension, 

and 5D in the spanwise dimension. 
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An alternative formulation by Carstens (1976) relates the scour apron width to the maximum scour 

depth (Sm): 

 (12.16) 

where: 

Fs = a factor of safety; and 

φ = sediment angle of repose.

Figure 12.30. Definition sketch for scour protection at the base of vertical walls

The design of coastal scour countermeasures for complex pier geometries and pile groups, 

however, lacks general processes and applicability. This is partially a result of the unique 

conditions encountered at coastal bridges. For example, as described in Section 12.3.1, addition 

of very large dumped rip rap at complex piers in the Indian River Inlet exasperated the scour 

formation. As related in that earlier discussion, after years of continued scour and stability 

concerns, the “countermeasure” for this crossing required spanning the entire inlet with a 

replacement bridge. Likewise, various coastal conditions at Oregon Inlet resulted in situations 

where scour removed bed material almost entirely at specific bridge pile groups, necessitating 

emergency remediation (e.g., lane and bridge closures, adding concrete jacks, building pile 

crutches, forced sand replacement, etc.) (Henderson and Krolak, 2016). After such episodes, the 

bridge owner engaged a multi-disciplinary team that instituted a comprehensive and aggressive 

scour monitoring plan of action with a variety of countermeasures (Henderson and Krolak, 2016). 

The first example demonstrates that simply adding countermeasures does not resolve the issue. 

The second demonstrates that not acting could have potentially disastrous consequences. 

However, the lessons learned in both examples were that any scour countermeasure for such 

complex coastal bridges requires a proactive approach, working through a multi-disciplinary team

(including coastal engineers), and focused on solutions that encompass the service life of the 

structure. 
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Scour protection at the base of piles may also be provided to withstand wave-induced scour. 

While specific relationships are lacking, an estimate of the scour apron dimension is 

approximately twice the expected scour depth (USACE 2002). This general approximation of 

scour apron size has many additional uses for mitigating wave-induced erosion and is applied in 

some upland situations as well to protect against erosion initiated by wave overtopping of walls 

and revetments. 

Additional considerations for scour countermeasures at highway bridges are found in HEC-23 

(Lagasse et al. 2009). However, caution should be exercised when using these countermeasures 

in the coastal environment. The unsteady, multi-directional flow conditions and wave action make 

the design of scour countermeasures unique to the coastal environment. The Coastal Engineering 

Manual should be taken to heart: 

“It should be recognized that scour protection in coastal engineering differs from scour 

protection encountered in typical transportation projects both in the magnitude of the 

forces and in their reversing directions. Though scour protection design for highways is a 

well-developed art with extensive documentation, the direct transferal of highway riprap 

experience to coastal problems is usually unsatisfactory.” Page III-1-4, USACE (2002) 
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Chapter 13 - Engineering Risk at the Coast

The broad concept of “risk” typically captures both (a) the probability of an extreme event, and, 

(b) the consequences to human health, the environment and economic activity associated with 

that event. This chapter discusses the tools appropriate for the quantitative engineering evaluation 

of probability and risk applied to coastal highways. While the general concepts of return period 

and probability of occurrence are exactly that same as for all of engineering, the coastal analysis 

tools differ significantly from riverine flooding tools. 

13.1 Return Period and Probability of Occurrence

This section discusses the common engineering concept of design storm based on return period, 

e.g. the “100-year storm” or the “1%-Annual-Chance-Flood.” This section also discusses a

common misunderstanding of risk levels implied by the return period terminology. 

Extreme natural events, such as river floods or coastal floods, vary in ways that appear to be 

random and can often be considered to be random for the purposes of engineering analysis and 

design. Techniques of probability and statistics are used to analyze these random flood events in 

water resources engineering. Historical records are used to understand the frequency of 

occurrence of a range of flood event magnitudes. 

Probability of exceedance is often referred to in terms of the average “return period,” T, of a 

given flood magnitude. For example, a flood with a return period of 100 years has a 1 percent 

(1/100) probability of being equaled or exceeded in any year. Note that the return period is the 

average number of years between floods equal to or greater than this magnitude. Return period, 

T, is related to probability of exceedance, Pe, by the equation, 

 (13.1) 

It should be emphasized that the return period or recurrence interval T is a long-term average. It 

is an average over a very large number of floods over a very long time span. To fully determine 

this average, one would need to observe and analyze a very long record of floods – perhaps 

thousands of years of floods. These fundamental concepts of return period are explained in many 

engineering textbooks (e.g. Roberson et al. 1998) and other FHWA manuals including HDS 2 

(FHWA 2002). 

The use of the term “1%-annual-exceedance-probability-flood” is often preferred to the 

equivalent “100-year flood” which confuses the general public, including decision-makers.

For example, to avoid such potential confusion, the FHWA regulation 23 CFR § 650.105(b) 

defines the base flood as “... the flood or [storm] tide having a 1-percent chance of being 

exceeded in any given year.” The flood with a return period of 100 years (P = 0.01) has a 0.01 or 

1% chance of occurring or being exceeded this year or any year. Likewise, a 50-year flood has a 

2% chance of occurrence in any year. A layperson may assume a 100-year flood is not going to 

occur for another hundred years or that there is only a 1% chance it will occur in the next 100 

years. The terminology may not adequately communicate the risk (nor address statistical 

complications, such as a non-homogeneous historical record). 

From the perspective of an asset’s service life (or design life), the longer the service life, the 

greater the risk of such an extreme event level occurring. The probability that a design flood level 

will be equaled or exceeded at least once during the service life of the project (the probability of 

occurrence) 

 (13.2) 
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where:

 P = probability that the design flood level will be equaled or exceeded in n years

 n = design or service life, years

 T = the return period of the design storm, years

This equation is a reduced form of the binomial distribution common in quantitative risk analysis 

(see Equation 4.81 of Hydraulic Design Series No. 2, 2nd ed., FHWA 2002). 

For example, if the design storm for a transportation asset is a 50-year storm, T = 50 and the 

design or service life is 30 years, then the probability of occurrence in the next 30 years is 

P(occurrence in 30 years) = 1 - (1 - 0.02)30 = 0.45 = 45%. There is a 45% chance that the 50-year 

flood or greater will occur at least once in the next 30 years. 

Figure 13.1 shows a family of lines corresponding to different risk levels expressed as P = 

probability of occurrence, as a function of T = design return period, and n = years of service. The 

same relationship is shown in Table 13.1. For example, as shown in both Figure 13.1 or Table 

13.1, the probability of occurrence of a 100-year storm increases from 1% for a service life of 1 

year (by definition), to about 10% for a service life of 10 years, 18% for a service life of 20 years, 

39% for a service life of 50 years, and about 63% for a service life of 100 years. In other words, 

there is a 63% chance that the 100-year flood will be equaled or exceeded at least once in the 

next 100 years. 

Figure 13.1. Probability of occurrence as a function of return period, T, and years of service, n

There is a 39% chance that the 100-year storm will occur in 50 years... by 

definition!
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Table 13.1. Probability of extreme event occurrence for various periods of time

Length of 

Service 

(Years)

Frequency – Recurrence Interval

10-year 25-year 50-year 100-year 500-year 1,000-year

1 0.10 0.04 0.02 0.01 0.002 0.001

10 0.65 0.34 0.18 0.10 0.02 0.01

20 0.88 0.56 0.33 0.18 0.04 0.02 

25 0.93 0.64 0.40 0.22 0.05 0.02 

30 0.96 0.71 0.45 0.26 0.06 0.03 

50 0.99+ 0.87 0.64 0.39 0.10 0.05 

75 0.99+ 0.95 0.78 0.53 0.14 0.07

100 0.99+ 0.98 0.87 0.63 0.18 0.10

Another way to use Figure 13.1 or Table 13.1 is to consider what design storm level is needed to 

attain a given risk level. For example, it might be desired for some facilities to reduce the 

probability of failure to a smaller level, say a 5% risk of occurrence in the next 25 years. Figure 

13.1 or Table 13.1 show that the design storm level would have to be T = 500 years to attain that 

low level of probability. For extremely important facilities, even less frequent design storms can 

be adopted. For example, 2,000-year events are used for some spillway designs or some 

structural designs for earthquake forces and some designs related to nuclear power plants 

consider design return periods of up to 100,000 years. Design storm return periods used for the 

evaluation of most types of transportation infrastructure range from 10-year to 500-year 

depending on the type of infrastructure. 

A transportation asset may not experience any damage with a flood level that exceeds its design 

level. The relationship between flood level and damage depends on numerous other factors 

including the type of asset (bridge, tunnel, or roadway), the characteristics of the asset (on piles, 

at grade, on an embankment, etc.), the site-specific topography, and the wave heights at that site. 

Damage mechanisms common in coastal transportation systems are discussed briefly in Section 

15.1 – Typical Coastal Damage Mechanisms. 

13.2 Coastal Storm Flood Frequencies

This section discusses how the storm surge elevation - return period relationship is typically 

estimated in coastal areas. Storm flood elevation is often the parameter of primary interest in 

coastal hazards assessment because it is often responsible for damage, along with waves (which 

are controlled in part by flood level). The established, general approach for estimating the 

frequency, or return period, of coastal storm levels was initially developed in the 1970’s with the 

advent of modern computing. It has evolved with better computers and improved methodologies. 

The numerical modeling involves investments in both computing power and trained personnel 

skilled in coastal modeling. The approach is fairly complex and varies in different parts of the US 

because of the regional differences in key coastal processes. 

13.2.1 Atlantic and Gulf Coast Surge Frequencies

The general approach for estimating the surge-frequency relationship along the Atlantic and Gulf 

Coasts was originally developed in the 1970’s and 1980’s by NOAA, USACE, and FEMA. The 

general approach is to numerically simulate the full complexity of the physical processes 
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controlling coastal flooding and to derive statistics from the results (the local response) of that 

analysis (FEMA 2003). Multiple runs of numerical, hydrodynamic storm surge models with some 

frequency analysis controlling the input modeled storm parameters are used. The surgefrequency relationship is then determined at all locations based on the surge results of the multiple 

model runs. The numerical model provides the link between the known statistics of the generating 

forces and the desired statistics of coastal flood levels. The primary reason for the development 

of such an approach is that the historical coastal storm surge elevation at any location is very 

dependent on geography and on the path, or track, taken by any individual storm. Thus, if any 

given historical storm had taken a slightly different track the surge at many locations would have 

been different. 

Computer modeling allows for consideration of the impact of similar storms with different tracks 

and different strength parameters (wind speed, barometric pressure, forward speed, size etc.). 

Sometimes historical, actual storms are input to the model. “Synthetic” or hypothetical storms 

which have not actually occurred but could have, based on the frequency analysis of the storm 

parameters of the historical storms, can also be included so that a realistic coastal flood risk 

estimate at all locations can be developed. This general approach assumes that future storms will 

be similar to past storms and thus does not account for possible changes in storm characteristics 

and frequency. The biggest problem with this approach may be that it traditionally does not include 

the impacts of future relative sea level rise (RSLR). However, techniques to do just that, and 

examples of how it has been done, are presented in Chapter 14. 

The most common approach to the selection of the input parameters for synthetic hurricanes is 

the Joint Probability Method (JPM) originally used by NOAA (Ho and Myers, 1975). Historical

storm parameters (e.g. wind speed, storm size, landfall location, etc.) are gathered and used to 

develop a set of statistically representative input storms. The JPM assumes that the probabilities 

for each parameter are independent and fit some probability distribution. Recently, the JPM has 

been extended for “Optimal Sampling” to reduce the number of modeled storm events with a goal 

of not impacting the resulting statistics (Niedoroda et al. 2010; Toro et al. 2010a; Toro et al.

2010b). This JPM-OS technique is needed because the high-performance computational storm 

surge models have expanded to increase resolution as computing power has increased. An 

alternative technique to the JPM for developing the surge-frequency relationship at any location 

is an Empirical Simulation Technique (EST) (Scheffner et al. 1996). 

Any numerical storm surge model can be used in the general approach for developing coastal 

surge-frequency estimates (see Section 4.3.2 - Storm Surge Modeling). However, several 

agencies including FEMA and USACE have begun to use the Advanced CIRCulation Model 

(ADCIRC) for much of their coastal storm surge modeling for developing surge frequencies (e.g. 

Algeo and Mahone 2011). The ADCIRC model is used for the storm surge modeling in both the 

Level 2 and the Level 3 case studies of how to include the effects of sea level rise in vulnerability 

assessment described in Section 14.8. 

Details on risk-based vulnerability assessment for coastal highways along the Atlantic and Gulf 

Coasts are presented in Sections 14.3 and 14.4. 

Simulating many “synthetic” coastal storms, which have the same statistics as 

historical storms, results in a reasonable estimate of coastal flood frequency 

statistics.
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13.2.1.1 Pacific and Great Lakes Coast Surge Frequencies

The methods typically used for the Pacific and Great Lakes Coasts to estimate the surgefrequency relationship differ from those just described for the Atlantic and Gulf Coasts. This is 

primarily because the damaging storms are so different from the Atlantic/Gulf hurricanes. In 

Pacific and Great Lakes regions, historical storms are simulated, i.e. hindcast, with numerical 

surge and wave models to develop a long, say 50-year, database of water levels at all locations. 

These results are then evaluated directly to estimate the surge-frequency relationship. FEMA 

(2005) outlines that agency’s guidelines for the Pacific Coast. A “total water level” approach is 

suggested which directly includes wave runup as well as the storm surge. This may also be 

referred to as the “storm response method” in some literature. Where hindcast storm water levels 

and waves for a period of at least 30 years can be developed, the extreme values such as the 

1%-Annual-Flood are estimated with the Generalized Extreme Value (GEV) Distribution Method. 

Similarly, for the Great Lakes, the literature recommends a total water level approach for 

estimating 1 percent-annual-flood levels based on numerical hindcasts of 50 years of historical 

storms (Melby et al. 2012; FEMA 2014; Nadal-Caraballo et al. 2012). The historical approach 

typical of the Pacific and Great Lakes Coasts avoids the JPM for developing input storms but the 

numerical modeling effort is still very complex, often with ADCIRC or a similar storm surge model, 

and needs significant computing resources and experienced coastal modelers. 

Details on risk-based vulnerability assessment for coastal highways along the Pacific and Great 

Lakes Coasts are presented in Sections 14.5 and 14.6, respectively. 

13.3 Available Estimates of Storm Surge: SSIMs and FIRMs

Two commonly available coastal mapping products are storm surge inundation maps (SSIMs) 

and flood insurance rate maps (FIRMs). SSIMs are developed by NOAA primarily for input to 

emergency evacuation decisions. As such, they are created to be conservative so that local 

decision-makers can use them for evacuation decisions as hurricanes approach. SSIMs show 

areas which can possibly flood due to different Saffir-Simpson categories of hurricanes. They are 

essentially maps of modeled (typically with SLOSH) potential inundation areas from the worst 

possible storms (track and strength) of each Saffir-Simpson storm category. They provide an 

upper-bound, or worst-case, flood level for each category storm. They are generated by running 

the storm surge model for numerous possible tracks and strengths in each category storm and 

recording the highest flood levels from all the storms. Thus, they are not flood maps from any 

specific storm but the worst-case at each location. Their usefulness for engineering is limited. 

There is no risk or probability level associated with SSIMs. The “Category 5” SSIMs can provide 

an overall upper-bound, or worst-case, flood zone for all hurricanes. 

A problem with using SSIMs, even for evacuation decisions, is that each storm has its own 

location and path and may not be the worst-case possible surge generator. For example, the 

location of Hurricane Irma, a Category 4 storm as it approached the southwest coast of Florida in 

2017, meant its track could not physically generate the highest possible Category 4 storm surge 

in Naples, Florida. That maximum from a Category 4 storm could only have been generated by a 

storm located west of Naples and moving east. But Irma was south of Naples and moving north. 

Thus, the forecast surge using the SSIMs was needlessly far too high. This will lead to a major 

catastrophe if a major hurricane actually approaches Naples from the west in the next few years. 

People who needlessly evacuated in Irma may not believe that next forecast. 

SSIMs for Category 5 storms estimate the “worst-case” flood at every location 

from any hurricane with that maximum wind strength.
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Coastal FIRMs are generated by FEMA to be consistent with FIRMs in riverine systems. They are 

maps of the 100-year coastal flood plain developed with the general approach to surge-frequency 

estimation described above. The primary purpose of the FIRMs is to establish flood insurance 

rates in coastal flood plains on a rational, consistent basis. Coastal FIRMs also show the FEMA 

“Base Flood” depth at specific locations. The base flood includes additional elevation above the 

still water level (SWL) to account for the wave crest elevations which can exist in that local depth 

of water. One difference between FIRMs and SSIMs is that the FIRMs are consistent with typical 

risk-based analysis. The differences between these two commonly available coastal surge map 

products, the FIRMs and the SSIMs, are explained in FEMA (2011a). 

13.4 Other Approaches to Quantifying Coastal Risk

This section discusses other coastal flood risk quantification approaches. One potential 

alternative for developing the coastal surge-frequency relationship is to use frequency statistics 

developed directly from measured, historical water level data. Water level data is recorded at a 

series of tide gages around the United States (see Section 4.1). However, caution should be 

exercised using these data for vulnerability assessments or designs for extreme events as they 

often do not reflect the extreme water levels that have occurred. The gages often do not record 

during extreme events due to physical damage, lost power, or surge elevations beyond their 

design parameters. Also, because of the complexities of the interaction of storms with coastal 

geography (bathymetry and topography) surge varies significantly within relatively short 

distances. Further caution in using water level data directly is that such an approach could be 

storm-specific, i.e. emphasize planning/designing for the last big storm. Coastal peak flood 

elevation data can be supplemented with careful high-water mark surveys after storms. However, 

those data are often subject to interpretation error and have only been collected with significant 

spatial coverage after major coastal storms in the past several decades. In general, not enough 

storm surge observations are available to make robust estimates of storm surge-frequency for 

engineering design. 

The Saffir-Simpson Hurricane Scale cannot be used for assessing the vulnerability (or for design) 

of most coastal transportation infrastructure. This is because wind is not the primary 

environmental design issue and the Saffir-Simpson scale is a wind speed scale. The winds create 

the surge and waves, which do the damage, but there is usually only marginal correlation between 

a hurricane’s wind strength and either surge or waves at any specific location along the coast. 

For example, the same hurricane will have very different surge and wave effects along the coast 

to the left of landfall, where peak winds are offshore and reducing surge and waves, than to the 

right of landfall, where peak winds are onshore and increasing surge and waves. Thus, 

statements like, “this should be designed to survive a Category 3 hurricane,” as a basis for 

planning and design decisions are problematic and should be avoided (unless direct wind loads 

are the damaging phenomenon, e.g. for traffic signs or building roof design). 

Life cycle cost analysis approaches for quantifying risk and incorporating it into 

engineering design are gradually replacing frequency-based analyses in many areas of 

civil engineering and planning. The Coastal Engineering Manual (USACE 2002) describes 

methodologies for risk-based analysis in coastal engineering, and the reader is directed to 

Thompson et al. (1996) and Almodovar et al. (2008) for specific case study application of the life 

cycle approach to coastal infrastructure (not directly related to highways). The life cycle approach 

addresses a number of possible realizations of project evolution during its design life. It can

simulate different cumulative damage scenarios; can incorporate management decisions (i.e. 

The Saffir-Simpson Hurricane scale is not used for coastal engineering design.
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repair, replace, etc.) and accounts for changing climatic conditions over time. The life cycle 

approach develops some understanding of the probabilities of key variables and is often a datadriven exercise. Ultimately, probabilities and risk describing the project elements are derived 

through an analysis of project performance of numerous (i.e. thousands) estimates of the project 

life cycle. This is most commonly done with Monte Carlo-like simulations or other statistical 

techniques. Transportation infrastructure/assets typically have long life cycles. Also, the 

characteristics of the areas surrounding coastal transportation facilities are subject to change 

either through natural or human impacts. And, within the life cycle of some assets, the statistics 

and/or characteristics of key variables may change. Economics can be integrated into the life 

cycle approach and considered simultaneously with technical performance in order to more 

appropriately describe risk and vulnerability. 
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Chapter 14 - Analysis Methods for Assessing 

Vulnerability to Extreme Coastal Events

The US coastal transportation system is vulnerable to extreme storms today and this vulnerability 

will increase as sea levels rise (Jacobs, et al. 2018b). Assessments of this vulnerability are 

becoming common in planning and design. Vulnerability assessment, the process of identifying 

and quantifying the vulnerabilities of a system, is a part of risk management in many different 

aspects of society (business, national security, infrastructure, etc.). Vulnerability is function of an 

asset's (or system's) exposure, its sensitivity, and its adaptive capacity (Figure 14.1). 

“Exposure” refers to the degree to which the asset is impacted. In this case, this is the amount of, 

or level of, damaging forces/hazards, e.g. the wave height or loads in a storm. 

“Sensitivity” refers to how the asset responds. This can be how, and how much, physical damage 

is done to the asset (and the cost of that damage). “Sensitivity” mechanisms for coastal roads, 

i.e. how they are damaged in storms, are discussed briefly in Section 15.1. 

“Adaptive capacity” refers to the ability of the asset (e.g. road, bridge, etc.) or the larger system 

containing the asset, to accommodate and/or recover from the impact. This can include 

considerations of whether, and for how long, the asset is out of service. Adaptation strategies for 

coastal highway infrastructure are briefly discussed in Section 15.2. 

Figure 14.2 schematically summarizes the FHWA’s (non-binding) vulnerability assessment 

“framework.” 

.

Figure 14.1. Vulnerability of highways in the coastal environment
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Figure 14.2. FHWA’s Vulnerability Assessment and Resilience “Framework” (FHWA 2017d)
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This “framework” outlines five broad components for such studies: 

• Set objectives and define scope, 

• Compile data, 

• Assess vulnerability,

• Analyze adaptation options, and 

• Incorporate results into decision making. 

The first component, defining objectives and scope, includes articulating the objectives, 

selecting/characterizing the relevant transportation assets, and identifying variables for study. 

This includes consideration of the intended outcomes, the target audience, the geographical and 

temporal limits of the study, and the level of detail needed. 

This chapter presents detailed, technical methods for assessing the “exposure” of coastal 

transportation infrastructure to extreme events including future sea level rise. These 

methods were originally outlined in Douglass et al. (2014). 

This chapter describes three levels of effort/analysis and provides some specific regional 

approaches. The availability and location of existing products, data, tools, and methodologies that 

can be considered at each level of effort for each region, are discussed. Examples of specific 

information include where to find and how to use flood hazard maps, the selection of an 

appropriate hydrodynamic model(s) to simulate the coastal processes of interest in that region, 

and some references to existing methodologies for estimating water levels, waves, currents, and 

erosion. The coastal tsunami hazard is included in this chapter because of its increasingly more 

frequent inclusion in engineering planning and design considerations in the Pacific states and 

territories. 

Section 14.8 summarizes three case studies which are examples of excellent applications of the 

techniques outlined in this chapter. Vulnerability assessments should be done for the planning 

and design of all high-value US coastal transportation assets using analyses like these case 

studies (particularly the Level 2 and Level 3 case studies). 

14.1 Literature on Assessing the Impacts of Future Environmental 

Conditions

This section is a brief summary of the literature on assessing the impacts of future environmental 

conditions on coastal highways. Most investigations have focused on the impacts of sea level rise 

and some have included the impacts of changing storm characteristics. 

Hyman et al. (2008) summarized the limited research into the effects of future environmental 

conditions on highways available at that time. The literature in the field has expanded rapidly in 

the past decade. ICF (2009) summarized the literature on vulnerability assessments, risk 

assessment, and adaptation approaches, including some efforts outside of the US. 

Many investigators have used a mapping overlay technique with projected future sea levels and 

roadways for beginning to characterize the extent of road inundation as a surrogate to vulnerability

(e.g. Titus 2002). The technique has been dubbed the “bathtub” approach because it considers 

primarily inundation by sea level rise. Gesch et al. (2009) provides a partial list of the 

investigations which have used the basic technique and NOAA (2012) describes many of the 

details related to the mechanics of how to apply a basic inundation mapping technique. 

Earlier studies used only sea level but later studies combine that with storm surge estimates. The 

sea level rise and the storm surge after sea level as risen can be estimated by linear addition of 
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the two components (Gill et al. 2009). Douglass et al. (2005) used a form of the technique to 

assess the existing vulnerability of the US coastal highway system to storm surge alone (with no 

consideration of sea level rise) by overlaying the FEMA 100-year flood plains on road maps to 

estimate that there are roughly 60,000 road miles occasionally exposed to coastal surge and 

waves today (see Section 3.2). 

Kafalenos et al. (2008) used the basic inundation mapping technique in what is commonly referred 

to as the “Gulf Coast” study. That study was a multi-year, multi-phase study. Phase 1 of the study, 

“Gulf Coast 1,” quantified the highway miles in areas with ground elevations below levels selected 

to account for storm surge and sea level rise in the study area from Galveston, Texas to Mobile, 

Alabama. 

Phase 2 of the study, “Gulf Coast 2,” used a more complex method for mapping exposure that is 

highlighted as a case study in this chapter (see Section 14.8.2). Numerical models with different 

assumed input future sea levels are used to map future exposure to storm surge and waves. 

Thus, any nonlinear aspects of the relationship between future sea levels and surge levels are 

inherently accounted for. Some of those mapping results were directly included in the 3rd NCA

(Melillo et al. 2014). 

Long-term Relative Sea Level Rise (RSLR) has already contributed to damage of one major US 

bridge during a hurricane. Interstate 10 (I-10) crosses Escambia Bay just east of Pensacola, 

Florida on elevated twin span bridges. The bridge was severely during passage of Hurricane Ivan 

September 15-16, 2004. Section 11.2.1 explains that the RSLR which occurred in the three to 

four decades between the design of the bridge and Hurricane Ivan, caused a significant increase 

in wave-induced loads on the bridge during the storm which caused the worst damage. 

Coastal exposure and vulnerability mapping efforts not specifically focused on transportation 

infrastructure have been done by others using similar approaches. Smith et al. (2010) evaluated 

storm surge and waves across the marshes of southern Louisiana with assumed scenarios of sea 

level rise. They found that increases in maximum surge elevations are not linear with the sea 

level rise assumptions. In particular, for moderate hurricanes the surge levels could increase 

significantly more than the sea level rise (see Section 4.3.4). The reason is the interaction of the 

surge with the bathymetry and coastal topography. Li et al. (2013) assessed the impacts of future 

sea level rise on the naval facilities in the Norfolk area. Atkinson et al. (2013) modeled sea-level 

rise effects on storm surge and nearshore waves on the Texas coast. They conclude that because 

of the complexities, “there is no one-size-fits-all response to relative sea level rise descriptive of 

all locations” and that “site-specific computer modeling should be used to evaluate the risk facing 

coastal communities.” Hagen and Bacopoulos (2012) did probabilistic modeling of extreme event 

surge for some Florida counties with sea level rise. They found the nonlinear effect of sea level 

rise on surge, which they called the “dynamic” response of flooding as compared to simply 

estimating the static response (i.e. the “bathtub” approach), was significant in terms of both 

coastal flood inundation areas and depths. 

Transportation vulnerability to sea level rise is described in studies by Caltrans (2018), Han et al.

(2017), Lu and Peng (2011), Neumann et al. (2015), and Oswald and Treat (2013). Fakhruddin 

et al. (2015) find that transportation systems in American Samoa are the most vulnerable 

infrastructure sector with regards to impacts of sea level rise. Some recent studies provide, and 

demonstrate, assessment tools or methodologies for sea level rise vulnerability. These include 

Bloetscher and Romah (2015), Demirel et al. (2015), and Lambert et al. (2013). Transportation 

Relative Sea Level Rise (RSLR) has already contributed to damage of a major 

US bridge.
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adaptation for sea level rise is described in Cooper and Dolan (2012), Filosa et al. (2016), and 

Shilling et al. (2016). In addition to sea level rise vulnerability, many numerical modeling studies 

address coastal bridge and coastal highway vulnerability to combinations of sea level rise and 

coastal storms. Examples of such numerical studies include Chorzepa et al. (2016), Dompe et al.

(2015), Filosa et al. (2016), Kim et al. (2015), and Tang et al. (2013). A recent study by Anarde et 

al. (2018) provides an example of a numerical modeling study that incorporates expected 

landscape changes/responses to sea level rise into an assessment of coastal bridge vulnerability 

to storm surge and sea level rise. Examples of studies that focus on transportation vulnerability 

to tsunami hazards include Yu et al. (2014) and Capozzo et al. (2017). 

A few papers describing general aspects of coastal highways are also available. Knott et al. (2017) 

demonstrate, in a study of coastal New Hampshire roads, that rising groundwater attributed to 

sea level rise leads to reductions in pavement service life. The magnitude and timing of pavement 

degradation is sensitive to the current depth to groundwater, the pavement structure, and the 

nature of the subgrade (and the sea level rise scenario governs the timing). Other states likely 

have similar vulnerabilities. Other recent literature describes the impact to roads, or of roads, near 

receding shorelines (e.g. Kim et al. 2011; Zoulas and Orme 2007). 

The FHWA has co-sponsored “pilot” planning level studies on vulnerability and risk assessments 

of infrastructure to the projected impacts and extreme weather events (FHWA 2012b). Most of 

the coastal studies use some form of an inundation mapping approach for sea level rise, storm 

surge, or some combination (ART 2012; NJTPA 2011; Oahu Metropolitan Planning Organization 

2011; and Washington DOT 2011). Most found a high level of vulnerability to coastal storms today 

that will increase with increasing sea levels: 

• Metropolitan Transportation Commission, San Francisco Bay, in a multi-agency 

collaboration including NOAA, assessed the vulnerability of transportation systems to sea 

level rise and flooding, as well as impacts to shoreline protection systems. The Adapting 

to Rising Tides project is one of the case studies of coastal vulnerability assessments 

highlighted at the end of this chapter (see Section 14.8.1). 

• New Jersey Transportation Planning Authority assessed the vulnerability of roads, 

bridges, tunnels, transit, freight rail, maritime assets, and airports to sea level rise and 

coastal flooding. 

• Oahu Metropolitan Planning Organization, Hawaii evaluated five high-priority sites with 

existing vulnerability to extreme weather events including the impacts of sea level rise, 

storm surge, and strong winds. 

• Virginia DOT evaluated the vulnerability and sensitivity of transportation infrastructure in 

the Hampton Roads region. 

• Washington State DOT assessed the vulnerability of all state-owned and statedmanaged assets to 2-ft, 4-ft, and 6-ft sea level rise scenarios. 

• Alaska DOT and FHWA Western Federal Lands assessed the vulnerability of Kivalina 

Airport to sea level rise and coastal storm impacts. 

• Caltrans District 1 evaluated the impacts of sea level rise on California SH 101 as well as 

the potential performance of various adaptation strategies to reduce flooding. 

• Hillsborough Metropolitan Planning Organization, Florida assessed the vulnerability of 

surface transportation systems to flooding from sea level rise and storm surge. 
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• Maine DOT evaluated the impacts of sea level rise and storm surge on state-owned roads, 

bridges, and culverts, as well as the impacts of transportation systems on marsh migration 

with sea level rise. 

• Maryland State Highway Administration evaluated the vulnerability of roads and bridges 

to sea level rise, storm surge, and flooding in two coastal counties. 

• Massachusetts DOT evaluated the impacts of sea level rise and extreme event flooding 

on the Boston I-93 Central Artery/Tunnel system. This study is highlighted below at the 

end of this chapter as an excellent example of a coastal transportation vulnerability 

assessment (see Section 14.8.3). 

• Oregon DOT evaluated the vulnerability of roads and bridges in two coastal counties to 

coastal flooding and storm surge. 

• In South Florida, three MPOs and a County planning department collaborated to assess 

the vulnerability of major freeways, arterials, and rail systems to sea level rise and storm 

surge. 

14.2 Levels of Effort in Assessments

This section provides a broad overview of the proposed levels of effort for performing a 

vulnerability assessment of coastal transportation infrastructure to extreme coastal events and 

sea level rise. Vulnerability assessments may range from broad planning overviews involving 

systems of assets to highly detailed, project-specific investigations employing state-of-the-art 

modeling tools. In addition to considering the type of assets that may be affected by extreme 

events, investigators have varying levels of budget and available expertise that may also define 

the scope of an investigation. Therefore, this chapter presents techniques for different “levels of 

effort” for the assessments. 

The amount of detail and degree of complexity grow with each subsequent level of effort, as does 

the quality of the assessment. The degree of uncertainty in the results, however, decreases with 

each subsequent level of effort. The potential levels of effort for coastal planning are broadly: 

• Level of Effort 1: Use of Existing Data and Resources - Use of existing inundation 

(FEMA or USACE) or tsunami data to determine the exposure of infrastructure under 

selected sea (lake) level change scenarios, and sensitivity to depth-limited wave or wave 

runup processes. 

• Level of Effort 2: Original Modeling of Storm Surge and Waves - Modeling of surge 

and wave fields for specified storm and sea level rise scenarios, or modeling of tsunami 

inundation under sea level rise scenarios. 

• Level of Effort 3: Modeling in a Probabilistic Risk Framework - Modeling of surge, sea 

levels, currents, and waves, or tsunamis, including the impacts of sea level rise, in a 

probabilistic risk framework. 

14.2.1 Level of Effort 1: Use of Existing Data and Resources

The purpose of the “Level 1” approach is to provide some meaningful information about 

the level and coverage of exposure to damaging storm parameters without having to 

perform complex modeling or calculations. This lowest level of effort is simpler to perform and 

relies on the use of established maps and tools to determine the degree to which a particular 

asset or area is exposed to the effects of extreme events with future sea level rise. The analysis 

team for Level 1 should include a trained coastal engineer to insure proper interpretation and 
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integration of the input coastal flood map products (see Section 3.6 Coastal Engineering as a 

Specialty Area). 

Since the Level 1 approach is relatively simple, a number of sea level rise scenarios can be 

considered. The goal of a Level 1 study might be to capture the sensitivity of a particular asset or 

area to the effects of sea level rise instead of predicting an accurate value of flooding depth or 

wave height, etc. Thus, the Level 1 approach can be used as a screening tool to identify those 

areas or infrastructure assets that are exposed to the effects of sea level rise. These specific 

areas can be evaluated in more detail through additional refinements of a Level 1 approach or by 

including them in assessments with higher levels of effort. 

The degree of uncertainty in results obtained in a Level of Effort 1 assessment will be relatively 

high. The results will include all of the assumptions and uncertainties inherent in existing 

inundation or flood hazard maps (or other existing tools used). The most commonly available, 

existing data are typically the FEMA flood insurance rate maps (FIRMs) and the corresponding 

Flood Insurance Studies (FIS) or USACE Coastal Hazards System Annual Exceedance 

Probability data (USACE 2019). In some cases, other agency flood maps and studies may be 

available. 

An example of a comprehensive Level 1 study is highlighted in Section 14.8.1 Case Study: 

Adapting to Rising Tides - San Francisco Bay. This case study developed inundation maps 

for one county for planning purposes. Two sea level rise scenarios (16 inches and 55 inches) 

were evaluated. The primary process of interest was flood depths, but some additional evaluation 

of waves was included. 

14.2.2 Level of Effort 2: Modeling of Storm Surge and Waves

The purpose of the “Level 2” approach is to provide detailed information about exposure under 

extreme events with sea level rise. In such a study, one or more sea level rise scenarios can be 

explicitly incorporated into model simulations so their effects on the important coastal processes 

can be determined. Performing a Level 2 analysis uses sophisticated hydrodynamic models that 

simulate storm surge and waves, or tsunamis. The development and application of these models, 

as well as interpretation of their results, should generally be performed by a trained coastal 

engineer with expertise in hydrodynamic modeling. 

Part of a Level 2 study is thoughtful selection of extreme events of interest for the modeling. These 

could be events of record for a region (e.g. a specific hurricane), a storm that had a notable impact 

on a specific piece of infrastructure (e.g. bridge failure), or perhaps even an event that has not 

yet occurred (e.g. hurricane with a shifted track, tsunami, etc.). A goal of the Level 2 study could 

be to demonstrate the degree to which sea level rise will modify the exposure of an asset or area 

relative to a present-day (baseline) scenario. The results of a Level 2 study will generally not be 

probabilistic in nature. However, it may be possible to assign a return period to a scenario if an 

historical storm or event (i.e. one that has previously occurred) is selected for analysis or if 

assumptions about the appropriate storm characteristics are developed. It may be helpful to 

The USACE Coastal Hazards System is a powerful online resource for 

vulnerability assessments.

Level 2 and Level 3 coastal assessments use sophisticated hydrodynamic 

models.
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consider a range of possible event and sea level scenarios that span from more frequent, lowerintensity events to infrequent, higher-intensity events. Practically, the Level 2 study may be limited 

to modeling less than approximately two dozen, extreme event and sea level scenarios. 

While each Level 2 study will be unique, an underlying methodology will be common to all studies. 

Every Level 2 study should include: 

• selection of extreme event and sea level rise scenarios appropriate for the region, 

• development of suitable hydrodynamic modeling tools, 

• validation of the hydrodynamic model(s) using hindcast simulations and observations, 

• simulation of the extreme event and sea level rise scenarios, and

• mapping of the hazards (e.g. inundation, waves, wave runup) as assessments of exposure 

under each scenario. 

While a Level 1 study uses inundation or flood hazard maps developed by others, a Level 2 study 

should develop new maps. Such mapping constitutes a significant amount of work and serves 

purposes beyond the exposure assessment, like communication and public outreach. Therefore, 

it is crucial that great care is taken in the preparation of these maps and that they are readily 

understood by laypersons. 

The time and effort inherent in a Level 2 study is significantly greater than a Level 1 study. 

Accordingly, the cost of a Level 2 study is likely to be much higher than a Level 1 study. But with 

an increase in time, effort, and cost comes a significant reduction in uncertainty and a narrower

range of possible answers. The reduction in uncertainty is mostly attributed to use of the 

hydrodynamic models, which provide specific estimates of the important regional coastal 

processes of interest, like water levels, wave heights and periods, velocities, etc. Note that the 

quality and utility of these estimates is not that they are necessarily “larger” values than those 

obtained in the Level 1 study, but rather that they are more accurate. 

An example of a comprehensive Level 2 assessment is highlighted in Section 14.8.2 Case Study: 

The Gulf Coast 2 Study – Mobile, Alabama below. That study used original, high-resolution 

modeling to map storm surge and waves in extreme events for a variety of sea level rise 

scenarios.

14.2.3 Level of Effort 3: Modeling in a Probabilistic Risk Framework

A “Level 3” study characterizes exposure in terms of probability and risk. While the general 

methodology is similar to that of a Level 2 study, probability is known. The Level 3 study uses

many more simulations in order to determine the probability of events. Whereas the Level 2 study 

is based on perhaps tens of unique event scenarios, this type of study may use on the order of 

one hundred simulations. This probabilistic approach is essentially the same as that currently 

used by most federal agencies for developing modern flood hazard maps (e.g. FEMA 2003), 

except here relative sea level rise (RSLR) scenarios are included. 

The Level 3 study involves a significant investment. It takes longer to complete and has a higher 

cost than the Level 1 and Level 2 assessments. However, this highest level of effort also provides 

the greatest reduction in uncertainty. Overall project costs, including construction costs, can 

be reduced by the use of a higher level of analysis for design since the lower analysis 

levels can overestimate both the magnitude and the duration of coastal flooding (Bosma 

2016). The number of simulations allows for a range of return period events to be considered and 

the results may be applicable over a much larger area. For instance, a Level 2 study might 

consider storm events that lead to failure or damage of a specific asset, like a bridge. These 
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results are valuable but only near that bridge. The event probabilities derived from a Level 3 

assessment may be applicable to more than one area when multiple storm tracks or tsunami 

events are modeled. 

A Level 3 study incorporates expertise in coastal engineering, numerical modeling, hazard 

analysis, probability and risk. Accordingly, such studies should be performed by accomplished 

engineers with demonstrated expertise in modeling extreme events as well as an understanding 

of the appropriate regional RSLR scenarios. 

An example of a comprehensive Level 3 approach is highlighted in Section 14.8.3 Level 3 Case 

Study: Central Artery Project, Boston, Massachusetts. That case study used high-resolution storm 

surge modeling to develop probabilistic inundation maps, and other products, for all coastal 

storms with sea level rise. 

14.3 Gulf of Mexico and South Atlantic Coast

The multi-level approach to evaluating exposure is described on a regional basis, beginning in 

this section and continuing through Section 14.7. Each region of the US experiences hazards, or 

combinations of hazards, unique to its particular characteristics. 

This section outlines the levels of effort, existing tools and data, appropriate models, and 

methodologies for determining exposure in the Gulf of Mexico and along the South Atlantic Coast

(see Figure 14.3 for one South Atlantic shoreline). Some of the tools outlined here are also useful 

in the other regions. In this region of the US, tropical storms and hurricanes are the predominant 

extreme events, and the storm surge and waves they generate are the important coastal 

processes of interest. This section describes a multi-level approach for evaluating exposure to 

extreme events with future RSLR in the Gulf of Mexico and along the South Atlantic Coast. 

14.3.1 Level of Effort 1: Gulf of Mexico and South Atlantic Coast 

The major steps for developing the information needed to perform a Level of Effort 1 exposure 

assessment on the Gulf of Mexico and South Atlantic Coasts are summarized in Table 14.1. The 

steps include the processes of interest: 

• Flood depths (Step 6),

• Wave heights (Step 7),

• Wave crest elevations (Step 8), and

• Flow velocities (Step 9).

If only flood depths are needed, steps 7-9 are unnecessary. 

A Level 1 approach in this region can be based upon the use of existing FEMA inundation maps, 

or FIRMs, to determine the exposure of infrastructure to coastal flooding. An alternative may be 

the USACE Coastal Hazards System (USACE 2019). In this region of the US, the FEMA flood 

hazard maps delineate the 100-year (1 percent annual chance) flood plain and in some cases the 

500-year (0.2 percent annual chance) flood plain. For coastal areas, these flood plains are mostly 

determined by storm surge from tropical storms and hurricanes. Existing flood hazard maps, 

however, do not account for future changes in sea level, nor do they explicitly provide information 

about wave heights or flow velocities. 
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Figure 14.3. Deerfield Beach, Florida (2018)

FEMA FIRMs describe the 1 percent annual chance still water elevation at various locations 

throughout the flood plain, as well as the base flood elevations (BFE) at specific locations (see 

Section 13.3). The BFE accounts for the elevation of wave crests that could exist in the local 

flooded depth. Use of the FEMA products calls for an understanding of their terminology (BFE, 

FIRM, etc.) as well as basic coastal engineering terminology (e.g. HEC-25). 

Four of the important, extreme event, coastal exposure processes can be evaluated in a Level 1 

study: 

1. spatial coverage of flooding, 

2. depth of flooding, 

3. wave characteristics, and

4. flow velocity. 
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Table 14.1. Exposure assessment steps for level of effort 1: Gulf of Mexico/South Atlantic Coast

Step Activity

1 Obtain appropriate FEMA flood map elevations for study area

2 Choose desired RSLR scenarios

3 Modify RSLR increments to account for nonlinearity

4 Add the result of Step 3 to the elevations obtained in Step 1

5 Obtain appropriate ground surface elevation maps for study area

6 Subtract ground elevations (Step 5) from values found in Step 4 to obtain flood depth

7 Multiply flood depths (Step 6) by 0.8 to determine maximum wave height

8 Multiply wave heights (Step 7) by 0.75, add to Step 4 for wave crest elevations

9 Use Equations 4.1 and/or 4.2 to estimate flood flow velocity

10 Consider shoreline retreat and erosion based on historical trends in study area

11 Map the damaging coastal processes (results from Steps 6-9)

12 Evaluate exposure of transportation infrastructure 

Only the first of these steps is explicitly provided by the FEMA flood hazard maps. The most 

current FEMA flood hazard maps can be obtained from the FEMA Map Service 

Center (https://msc.fema.gov) for use in GIS software applications. These maps 

are now part of what FEMA refers to as the National Flood Hazard Layer (NFHL). The 

maps describe the spatial coverage of the flood plain(s), as well as different flood hazard 

zones, in a geographic reference system. The flood hazard layer can be combined 

with other spatially-explicit data organized into GIS layers, like transportation systems, 

to quickly determine whether a specific area or asset falls within the FEMA flood 

plain. But this methodology does not immediately reveal whether an asset like a road or bridge 

will be affected by flooding, flood flows, or waves. It also does not account for increased 

exposure due to future RSLR impacts. 

After the appropriate FEMA flood map has been obtained, RSLR should be considered. In 

this region, the relevant sea level change scenario is the RSLR that accounts for both land 

subsidence and global eustatic sea level rise. Appropriate regional RSLR rates, increments, 

or targeted values at specific planning horizons (e.g. 2020, 2050, 2100, etc.), should be 

determined for the study area. 

14.3.1.1 RSLR Data 

Section 4.1 provides discussions and material on sea level rise (including relative sea level rise 

or RSLR). A practical way to apply this information is with the USACE online sea level change 

calculator that accounts for local rates of subsidence and eustatic sea level rise by using 

NOAA tide gages near any selected study area. 

14.3.1.2 Increased Flood Levels: Nonlinear SLR-Surge Relationship 

The effect of RSLR on storm surge can be nonlinear (see Section 4.3.4). In other words, if the 

depth of flooding under present-day conditions is A and the sea level rise increment is B, 

the resulting future flood depth may be greater than A+B. This nonlinearity can be accounted 

for by multiplying each selected relative sea level change increment considered by a constant 

ranging from 1 to 5. A value of 1 represents a linear coupling while a value of 5 represents a 

strongly nonlinear coupling (i.e. the surge increases 5 times the sea level change increment). 

The degree of nonlinearity is likely both site and scenario specific, as described in Smith et al.

(2010) who found only isolated cases of surge increasing 5 times the sea level rise increment. 
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Instead Smith et al. (2010) found widespread areas where the surge increased 1 to 2 times 

the sea level rise increment. 

Therefore, an appropriate singular value to apply over an entire study area might be 1.5. Currently 

very little literature about this nonlinearity exists and all of the published documentation comes 

from studies in the Gulf of Mexico. This is an area of research that needs more attention. Selection 

of a larger value could be justified to provide a more conservative estimate. This uncertainty is 

one of the limitations of a Level 1 approach. 

After multiplying the selected sea level change increments by an appropriate nonlinear constant, 

the resulting values should be added to the still water elevations shown on the FEMA flood map 

for each sea level scenario considered. Recall that FEMA flood maps provide the elevation of the 

still water (i.e. waves removed) relative to a fixed vertical datum, like the North American Vertical 

Datum (NAVD). It is important to note that these values are elevations, not flood depths. 

14.3.1.3 Coastal Land Elevation Data

In order to determine flood or total depths within the study area under the selected sea level

scenarios, the local ground surface elevations should be known and subtracted from the 

modified still water elevations. Appropriate sources of information include USGS topographic 

maps for ground surface elevations (http://www.usgs.gov/) and NOAA nautical charts for 

bathymetric elevations (http://www.nauticalcharts.noaa.gov/). Such maps, however, may not 

always provide the needed resolution to determine elevations near a specific asset. Many 

coastal areas of the US have been mapped using high-resolution light detection and ranging 

(LiDAR) systems that provide closely spaced data. A good source for LiDAR and other coastal 

elevation data is the NOAA Coastal Services Center Digital Coast web data portal 

(https://coast.noaa.gov/digitalcoast/). Other excellent sources of combined topographic and 

bathymetric elevation data sets, commonly available as GIS layers, include the NOAA National 

Geophysical Data Center’s Coastal Relief Models and the USGS National Map Viewer:

(http://www.ngdc.noaa.gov/mgg/bathymetry/relief.html) and 

(http://viewer.nationalmap.gov/viewer/). State organizations, like the Texas Natural Resources 

Information System (TNRIS) at https://tnris.org, may also provide more detailed elevation data

than national sources. 

14.3.1.4 Wave Characteristics

Wave characteristics and flow velocities can be determined as part of the exposure assessment 

once the spatial coverage and depth of inundation are estimated. In a Level 1 approach, waves 

can be assumed to be depth-limited and will break when their height is approximately 80 percent 

of the water depth (see Section 5.2). 

Also, the elevations of the wave crests under depth-limited conditions can be approximated by 

adding 75% of the maximum wave height to the still water elevation flood level. See Section 11.6

for more information about estimating wave crest elevations. This is essentially the same 

computation that FEMA uses to determine the base flood elevation (BFE) shown on many coastal 

flood maps. 

One foot of future sea level rise can increase storm flood levels more than one 

foot! There is a nonlinear relationship between sea level and flood level.
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14.3.1.5 Storm Surge Flow Velocity Estimates

The estimation of flood flow velocity magnitude and direction during a storm event is difficult. 

Conservatism and professional judgment should be applied when estimating and applying these 

values as part of a hazard analysis. ASCE (2016) provides general guidance on the estimation of 

flood flow velocity as a function of flood depth. A recommended lower bound on the velocity is 

equivalent to the flood depth divided by one second: 

 (14.1) 

where:

ds = design flood depth

For example, if the design flood depth is 5 ft, a low estimate of the velocity magnitude is 5 ft/s. A 

recommended upper bound on the velocity is given as the square root of the product of the 

gravitational constant and the flood depth: 

 (14.2) 

where:

g = gravitational constant having a value of 32.2 ft/s2 (9.81 m/s2) 

This equation is the same as that used to estimate the shallow-water wave celerity as described 

in Section 5.1 Definitions, Theories and Properties of Waves. Using the same example as before, 

an upper estimate for the magnitude of flow velocity using a design flood depth of 5 ft is nearly 13 

ft/s, more than twice the lower estimate. 

14.3.1.6 Other Considerations in Level 1 Studies

The spatial coverage and depth of inundation, wave heights, wave crest elevations, and flow 

velocity for extreme events with selected sea level scenarios can be mapped and used for 

exposure assessments. This information can be used to determine if an area or asset falls within 

the modified 100-year flood plain, but it can also be used to determine the sensitivity of 

transportation infrastructure to waves and flow velocity. Some common types of damage are 

described in Section 15.1 Typical Coastal Roadway Damage Mechanisms. Additional guidance 

for mapping hazards in sheltered waters, like coastal embayments, is available in FEMA (2008). 

One specific limitation of all these approaches is the inability to account for short- or long-term 

shoreline change. This may be a particularly important process to consider for highways near 

receding shorelines. The reader is encouraged to consider future shoreline positions that result 

from retreat and erosion in their exposure assessment. Additional information about where to 

obtain and how to use shoreline change data is provided in Chapter 8 - Roads in Areas of 

Receding Shorelines. 

14.3.2 Level of Effort 2: Gulf of Mexico and South Atlantic Coast 

A Level 2 study uses some original modeling of storm surge and waves for selected 

extreme storm events under specific sea level scenarios. Extreme event scenarios may 

include storm intensification through increased central pressure deficit and maximum winds. The 

fundamental steps in a Level 2 analysis for the Gulf of Mexico and South Atlantic Coast are 

summarized in Table 14.2. 
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Table 14.2. Exposure assessment steps for level of effort 2: Gulf of Mexico/South Atlantic Coast

Step Activity

1 Identify damaging coastal processes of interest (e.g. storm surge, waves, etc.)

2 Select and characterize storm and RSLR scenarios of interest

3 Select, develop, and prepare appropriate numerical modeling tools

4 Validate and/or calibrate the models through hindcast simulations and analyses

5 Incorporate RSLR scenarios into numerical model simulations

6 Perform model simulations of selected storm and sea level scenarios

7 Map the damaging coastal processes for each scenario

8 Evaluate exposure and sensitivity of transportation infrastructure for each scenario

In the Gulf of Mexico and along the South Atlantic Coast, storm surge and waves generated by 

tropical storms and hurricanes are the primary concerns. Episodic, storm-induced shoreline 

change may also be of concern for highways near receding shorelines, causeway islands, bridge 

approaches, etc. While the tide range in the Gulf of Mexico is generally small (less than 2 ft), it 

does grow larger moving north along the South Atlantic Coast and should be considered in 

exposure assessments. This is especially true for low-lying areas that are particularly sensitive to 

the effects of sea level rise on tidal inundation (see Hagen and Bacopoulos 2012). Also, the 

contributions of coastal watersheds through increased stream flows may play an important role in 

the increase in local water levels, especially in areas distant from the coastline (see Section 4.4). 

Finally, the combination of the wind-driven storm surge, tidal stage, and river stage are of concern 

when they occur simultaneously. 

14.3.2.1 Storm Selection and Characterization 

A range of storm levels can be considered in a Level 2 study. The selection and characterization 

of storm events for a Level 2 study should not be based solely on the worst-case scenario. Since 

risk is not a product of this approach it is important to consider the exposure and sensitivity of 

infrastructure to a wide range of storm and future sea level rise scenarios. The scenarios may 

include the frequent but low-intensity tropical storms, as well as infrequent but high-intensity 

hurricanes, that have previously impacted the study area. Other storms having properties that fall 

somewhere between these limits may also be considered, especially if they produced notable 

impacts to an area or piece of infrastructure in the past. This is the approach that was applied to 

the selection of storm events for the Gulf Coast 2 study described in Section 14.8.2. 

Some numerical models, like ADCIRC, use only basic storm parameters to develop 

meteorological forcing for the model. The time and geographic position of the storm, its central 

pressure, maximum winds, and radius to maximum winds generally constitute the minimum 

information to model the storm. Some models also allow the user to characterize the asymmetry 

of a storm by defining winds in each of its four quadrants. Historical storm characteristics and 

information can be obtained from the NOAA National Hurricane Center (NHC) data archive 

(http://www.nhc.noaa.gov/data/). The NHC Best Track data archive provides general storm 

characteristics in six-hour increments for the duration of the storm event. Detailed storm 

characteristics are found in the archived NOAA NHC storm forecast and observation reports. 

Synthetic storms, storms based on historical storms but with some possible differences, can be 

evaluated directly in a Level 2 modeling study. In particular, shifting the track of a storm is a 

common exercise in model studies because of the sensitivity of coastal storm surge to storm 

track. Other parameters which can be modified are storm movement speed, wind strength, etc., 

but care should be taken to alter only the characteristics of interest without affecting those that 
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should remain constant. For example, shifting the track and landfall location of Hurricane Katrina 

for the Gulf Coast 2 study (Choate et al. 2012) was done while simultaneously preserving the 

storm’s forward speed and decay characteristics appropriately. 

Expected future changes in the damaging coastal processes include RSLR and storm 

intensification. Also, local rates of subsidence can be very different. Care should be taken when 

developing RSLR scenarios for your study area. Section 4.1 (Sea Levels) provides discussions 

and materials related to sea level rise. 

The selected sea level rise scenarios should be incorporated into the numerical model before the 

simulation is performed. Doing so provides the deterministic results to capture the nonlinear 

effects of sea level rise on storm surge and waves. Incorporating relative sea level rise into 

hydrodynamic models can be achieved in two possible ways: first, the sea level position can be 

increased relative to its present-day elevation or; second, the ground surface elevations can be 

reduced relative to their present-day positions. If the model provides an option for the user to 

prescribe a sea level offset, this will generally be the simpler and preferred method for 

incorporating sea level change as it does not call for one to modify the computational ground 

surface grid or mesh. 

Associated effects of increasing sea level in a model simulation can be considered. Many 

hydrodynamic models call for the user to prescribe frictional constants throughout the 

computational domain. Frictional constants for dry land are typically based on land use and land 

cover data and represent the “roughness” of coastal landscapes. As new, low-lying areas are 

inundated by each sea level rise scenario, their frictional resistance should be lowered to be 

consistent with values of the surrounding water body (Smith et al. 2010). It may also be 

appropriate to consider and incorporate other major changes to the newly-inundated landscape. 

Examples include the abandonment and/or removal of major infrastructure; the recession and 

erosion of barrier islands; and the incorporation of new coastal engineering defenses like sea 

walls, dikes, levees, and beach fill projects. 

While the future intensification of storms is still an area of ongoing research, limited guidance is 

available in Knutson and Tuleya (2004), Knutson et al. (2007), Knutson et al. (2010), and other 

documents. For example, Knutson et al. (2010) state that tropical storms and hurricanes will grow 

in intensity by +2 percent to +11 percent by the year 2100. This roughly corresponds to a +3 

percent to +21 percent central pressure fall. This information can be used to alter storm 

characteristics like wind speed and central pressure. 

14.3.2.2 Surge and Wave Models

One or more numerical models will be used in a Level 2 study. Few models are capable of 

simulating storm surge, waves, and shoreline change in a fully-integrated package. The selection 

of appropriate modeling tools is somewhat dependent upon the regional coastal processes of 

interest, the type of extreme event being considered, and the spatial coverage of the region to be 

modeled. The common storm surge models are briefly described in Section 4.3.2. In the Gulf and 

South Atlantic region of the US, USACE modelers and FEMA modeling contractors typically use 

the ADCIRC model to simulate storm surge from tropical storms and hurricanes because of its 

proven ability to model surge characteristics in a variety of complex situations. 

A number of numerical models for simulating wave characteristics that may be used in a Level 2 

study. See Section 5.4 for a discussion of the merits of some of the available wave models. Any 

appropriate shallow water wave model can be used for Level 2 studies. The case study in Section 

Future sea level rise will increase the vulnerability of our coastal highways.
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14.8.2 used the ADCIRC model to estimate storm surge and the STWAVE model to estimate 

wave height fields throughout the flooded areas. 

14.3.2.3 Mapping of Results

After validating the selected numerical models, the process models storm and sea level scenarios 

and saves the outputs for analysis. These model results constitute a major source of information 

needed to assess exposure and evaluate sensitivity. Pertinent model results, like maximum storm 

surge elevation (or depth), wave height and direction, or overland flow velocity, can be mapped 

in GIS software for use with other relevant spatial layers, like transportation systems, 

demographics, etc. Appropriate examples are found in Choate et al. (2012) and USDOT (2013). 

These results can be presented at discrete times throughout the duration of the model simulation, 

or simply as the maximum values of each quantity. Examples of each include the time history of 

wave characteristics at a particular location, and the maximum envelope of water (MEOW), 

respectively. 

A unique mapping of the damaging coastal processes can be done for each storm and sea level

scenario considered. The effects of sea level on these processes, and the subsequent exposure 

to them, can be evaluated through direct comparisons to the historical, or model hindcast, results. 

Furthermore, the degree to which various scenarios modify the degree of exposure can be 

determined through direct comparisons between them. 

Maps of the damaging coastal processes can be used to assess exposure in different ways. For 

example, maps of storm surge can be used to determine the spatial coverage and extent of 

inundation within the study area; maps of wave characteristics can be used to determine if, and 

to what degree, a particular asset will experience wave forces and overtopping; and maps of 

velocity can be used to determine hydrodynamic loads, scour, and erosion of embankments. 

Accordingly, practitioners may consider use of these data and maps to evaluate sensitivity when 

appropriate (e.g. USACE 2002, ASCE 2016, and this manual). 

Caution should be exercised in the development of maps based on hydrodynamic model results, 

particularly those used to reference elevations. Hydrodynamic model results, like water levels, 

are commonly referenced to a vertical tidal datum (e.g. mean sea level). However, ground surface 

elevations and GIS layers containing transportation infrastructure are generally referenced to a 

vertical survey datum like NAVD. The difference between elevations referenced to either datum 

may be significant and should be accounted for prior to the mapping exercise. Model results 

referenced to a vertical tidal datum can be transformed to almost any vertical survey datum using 

NOAA’s free software program VDATUM (http://vdatum.noaa.gov/). More information about tidal 

and survey datums is found in Section 4.2.2 Tidal and Survey Datums. 

14.3.3 Level of Effort 3: Gulf of Mexico and South Atlantic Coast 

The process of conducting a Level 3 study is almost identical to that of a Level 2 study. This type 

of assessment includes identification of the damaging processes; selection of storm and sea level

scenarios; numerical modeling of the selected scenarios; and assessment of exposure, and 

perhaps sensitivity, based on model outputs. However, the fundamental difference between the 

Level 2 and Level 3 approaches is the ability to assign probability and risk to scenario outcomes 

(see Section 3.1.2 Coastal Storm Flood Frequencies). A Level 3 study will consider hundreds of 

unique storm and sea level scenarios, each having their own assigned probability of occurrence, 

as compared to the one or two historical storms used in a Level 2 study. As a result, the level of 

effort, difficulty, cost, and duration of study are all likely to be higher for a Level 3 study. 

The major steps for developing the information needed to perform a Level of Effort 3 exposure 

assessment for the Gulf of Mexico and Atlantic Coasts are summarized in Table 4.3. The 
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suggested methodology of a Level 3 study is very similar to the process used by FEMA to develop 

coastal flood hazard maps, or FIRMs and similar to the procedures used by the USACE in the 

Coastal Hazards System (USACE 2019) and plan formulation of large coastal projects. Specific 

guidance and details about how the FEMA studies are conducted are available in FEMA (2003). 

Table 14.3. Exposure assessment steps for level of effort 3: Gulf of Mexico/South Atlantic 

Coast.

Step Activity

1 Identify damaging coastal processes of interest (e.g. storm surge, waves, etc.)

2 Select and characterize storm and RSLR scenarios of interest

3 Select, develop, and prepare appropriate numerical modeling tools

4 Validate and/or calibrate the models through hindcast simulations and analysis

5 Incorporate RSLR scenarios into numerical model simulations

6 Perform model simulations of selected storm and sea level scenarios

7 Derive probabilities associated with water levels, wave heights, velocity, etc.

8 Map the damaging coastal processes for each return period (probability) of interest

9 Evaluate exposure and sensitivity of transportation infrastructure for each return 

period

10 Estimate risk as a function of scenario probability and time-interval considered

The most significant differences between a Level 2 and Level 3 study, where storm surge and 

waves are the primary damaging coastal processes of interest, are related to the characterization 

of storm and sea level scenarios, as well as their results. Instead of modeling only one or two 

notable historical storms, dozens to hundreds of synthetic storms may be developed and 

modeled. The parameters of these synthetic storms are initially based on historical storms that 

impacted the study area, but are altered to yield hundreds, and possibly even thousands, of 

unique storm events. The storm’s track, landfall location, forward speed, central pressure, 

maximum winds, and radius to maximum winds (i.e. size) are typically the parameters that are 

adjusted to generate the unique storm scenarios. Each of the synthetic storms is also assigned a 

probability of occurrence based on its unique parameters. By doing so, the return period of each 

storm is known. FEMA has already performed this exercise in the process of updating flood 

hazard maps in many US coastal states and their counties, including many in this region. These 

storm characteristics, probabilities, and the associated model results are archived and stored by 

FEMA. 

While previous FEMA coastal flood hazard studies represent excellent resources for the 

characterization of storm and surge frequencies they do not incorporate or account for the effects 

of sea level rise. Appropriate sea level scenarios could be incorporated into those synthetic storm 

scenarios, but the associated probabilities would generally be unknown. Until recently, most 

published sea level rise projections did not have associated probabilities. The IPCC (2007) 

published sea level rise projections for 2100 with 95 percent confidence intervals (2.5 percent 

probability of being equaled or exceeded) but those estimates did not fully account for the effects 

of ice sheet melting in Greenland or Antarctica. Houston (2013) provides updated sea level rise 

projections for the period 1990 – 2100 that attempted to appropriately account for these 

contributions as well as thermal expansion and the melting of glaciers and ice caps. The global 

mean sea level rise (GMSLR) projections for the period 1990 – 2100, as determined by Houston 

(2013) at the 5 percent, 50 percent, and 95 percent confidence intervals, are 0.59 ft, 1.6 ft, and 

2.7 ft, respectively. Houston (2013) also describes the steps to determine sea mean level rise 
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projections at other confidence intervals. Note, however, that these are global sea level rise 

projections and do not account for local subsidence or regional variations in the distribution of the 

global mean (see Section 4.1 Sea Levels). For a fully probabilistic characterization of relative sea 

level rise, the probabilities associated with local subsidence rates would also have to be known, 

or at least assumed to be constant and stationary. 

Currently no probabilities are associated with future storm intensification. Furthermore, there are 

currently no probabilities associated with future increases or decreases in landfalling storm 

frequency. There is some agreement, though, that the global frequency of all storms will decrease 

with a corresponding increase in the frequency of major storms (Knutson et al. 2010). Therefore, 

it may not be appropriate to assume that the probabilities of intensified storms will be equivalent 

to their present-day equivalent. This remains an area of needed research. 

The inherent benefits in modeling hundreds of unique storm and sea level rise scenarios, each 

with their associated probability, also represent serious obstacles. The level of effort, time, and 

cost to perform such a large number of model simulations may be prohibitive. 

After all storm scenarios have been modeled, the probabilities of each process of interest (e.g. 

water levels, wave heights, velocity, and shoreline change) should be derived from the model 

results. These probabilities are generally extracted from all model results at every geographic 

node or grid point in the computational domain. This is representative of the process that 

delineates a coastal flood plain for a specific return period event, as in the 100-year and 500-year 

flood plains. Statistical approaches for determining event probabilities are briefly described in 

Section 13.1 and can also be found in most hydraulic engineering texts. 

In contrast to a Level 2 study where outcomes are based on specific scenarios, the outcomes of 

a Level 3 study are based on the probabilities of all scenarios considered. Therefore, the model 

results capture the frequent, low-intensity events, as well as the infrequent, high-intensity events, 

and everything in between. Once the values of relevant processes are determined and associated 

with specific probabilities (i.e. 100-year water level, 100-year wave height, etc.), risk can be 

assigned as a function of the project duration, life cycle, or for specific planning horizons. 

As in the Level 2 study, maps of storm water levels, waves, velocity, and possibly shoreline 

change are anticipated to be the primary products of the model simulations. Unlike the Level 2 

study where maps of each process are generated for each scenario, the relevant coastal 

processes will be mapped only at desired return periods. Exposure assessment can then be 

generalized in terms of probability and risk, as can sensitivity analyses. 

14.4 Mid-Atlantic and New England Coast

This section outlines the levels of effort, existing tools and data, appropriate models, and 

methodologies for determining exposure to extreme events along the Mid-Atlantic and New 

England Coasts. In this region of the US, tropical storms, hurricanes, and extratropical storms are 

the predominant extreme events, and the storm surge and waves they generate are the important 

coastal processes of interest (see Figure 14.4 for some New England shorelines). Other 

processes of interest in this region include tides, episodic shoreline change, and runoff to coastal 

waters. A multi-level approach for evaluating exposure to extreme events and sea level along 

these coasts is described in the subsections that follow. 

14.4.1 Level of Effort 1: Mid-Atlantic and New England Coast 

The suggested methodology for performing a Level 1 study in this region is essentially the same 

as that described above in Section 14.3.1. The extreme events in this region are similar with the 

exception of extratropical storms. Accordingly, the damaging coastal processes of interest for this 
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region are also basically the same: storm surge and waves generated by strong storms. The 

major steps for developing the information needed to perform a Level of Effort 1 exposure 

assessment along the Mid-Atlantic and New England Coast are summarized in Table 14.4. 

The coastal flood hazard maps produced by FEMA (https://msc.fema.gov) in this region are 

developed similar to those for the Gulf of Mexico and South Atlantic Coasts and give the same 

types of information. The flood hazard maps in this region show the 100-year return period still 

water elevations, relative to a survey datum, for different flood zones. In some locations, the BFE 

is also provided to show the elevation of probable wave attack. Detailed information about the 

mapping procedures is available in FEMA (2003). Additional guidance for mapping hazards in 

sheltered waters, like coastal embayments, is provided in FEMA (2008).

This region of the US corresponds to FEMA Regions 1, 2, 3, and part of 4 on the Atlantic Coast. 

With the exception of northern Maine, flood map modernization has either been completed 

or is ongoing for most coastal counties along this coast. Many of the coastal counties that 

were impacted by Hurricane Sandy in 2012 were in the process of updating their coastal flood 

hazard maps. In limited cases, communities adopted “Advisory Base Flood Elevations” (ABFEs) 

after Sandy. Unlike the BFEs listed on published FEMA FIRMs, these ABFEs are not based on a 

standard deterministic or probabilistic protocol. ABFEs have been based on storm-specific surge 

or high water mark elevations plus some increment, e.g. 1 to 3 ft, of freeboard. In general, the 

published FEMA FIRMs for this region should be used instead of ABFEs in the Level 1 study, 

unless there is accepted technical logic to use the ABFEs. 

Local rates of relative sea level rise and their future projections should be considered in a Level 

1 study. All coastal areas in this region are experiencing relative sea level rise, with the MidAtlantic region having rates 2 to 3 times higher than those along the New England coast. As 

previously described in Section 14.3.1, the USACE (2011) material on sea level change can be 

used along with their web-based tool (https://cwbi-app.sec.usace.army.mil/rccslc/slcc_calc.html) 

in order to develop appropriate relative sea level rise rates and projections for a specific 

study area. 

One issue that remains unclear for this region of the US is the degree of nonlinearity associated 

with the effects of sea level rise on storm surge. All published studies documenting this 

effect were conducted in the Gulf of Mexico. The offshore bathymetry and topography of the MidAtlantic and New England coasts are very different than those in the Gulf of Mexico, and they 

likely play an important role in determining how storm surge is affected by higher sea levels. 

This is a topic for future research and caution should be exercised when applying the multiplier of 

1.5 suggested in Section 14.3.1. 

14.4.2 Level of Effort 2: Mid-Atlantic and New England Coast 

The process for conducting a Level 2 study in this region is almost identical to the one 

presented in Section 14.3.2, Level of Effort 2: Gulf of Mexico and South Atlantic Coasts. 

Exceptions include the selection and characterization of storm scenarios, as well as some of the 

physical processes that should be included in numerical model simulations. The general 

methodologies for conducting the study and assessing exposure are essentially the same as 

before and they are not repeated but the differences are outlined below. The major 

steps for developing the information needed to perform a Level of Effort 2 exposure 

assessment along the Mid-Atlantic and New England Coast are summarized in Table 14.5. 
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Table 14.4. Exposure assessment steps for level of effort 1: Mid-Atlantic/New England Coast 

Step Activity

1 Obtain appropriate FEMA flood map elevations for study area

2 Choose desired RSLR scenarios

3 Modify sea level change increments to account for nonlinearity

4 Add the result of Step 3 to the elevations obtained in Step 1

5 Obtain appropriate ground surface elevation maps for study area

6 Subtract ground elevations (Step 5) from values found in Step 4 to obtain flood 

depth

7 Multiply flood depths (Step 6) by 0.8 to determine maximum wave height

8 Multiply wave heights (Step 7) by 0.75, add to Step 4 for wave crest elevations

9 Use Equations 4.1 and/or 4.2 to estimate flood flow velocity

10 Consider shoreline retreat and erosion based on historical trends in study area

11 Map the damaging coastal processes (results from Steps 6-9)

12 Evaluate exposure of transportation infrastructure 

Figure 14.4. Portland Head Lighthouse, Maine (2016)
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Table 14.5. Exposure assessment steps for level of effort 2: Mid-Atlantic/New England Coast.

Step Activity

1 Identify damaging coastal processes of interest (e.g. storm surge, waves, tides, runoff, 

etc.)

2 Select and characterize storm and RSLR scenarios of interest

3 Select, develop, and prepare appropriate numerical modeling tools

4 Validate and/or calibrate the models through hindcast simulations and analysis

5 Incorporate RSLR scenarios into numerical model simulations

6 Perform model simulations of selected storm and sea level scenarios

7 Map the damaging coastal processes for each scenario

8 Evaluate exposure and sensitivity of transportation infrastructure for each scenario

Extreme events in the Mid-Atlantic and New England coast include tropical storms, hurricanes, 

and strong extratropical storms. Sometimes, combinations of these events are also possible as 

demonstrated by Hurricane Sandy in 2012. The selection of an appropriate extreme event or 

storm scenario might include historical storms of record, low-intensity storms producing notable 

impacts, or perhaps a synthetic storm that hasn’t even occurred. For example, researchers at 

Stevens Institute and Rutgers University considered a model scenario that combined some 

properties of Tropical Storm Irene (2011), which produced significant precipitation, with the size 

and winds of Hurricane Sandy (2012). A separate scenario simulated the effects of Sandy making 

landfall at the same time as the astronomical high tide, which altered the spatial coverage and 

extent of flooding. 

Since the extreme events in this region are not limited solely to tropical cyclones, simply knowing 

the historical storm parameters will generally not provide enough information to recreate the 

meteorological forcing. Instead, measured or reanalyzed meteorological data should be obtained 

or created for the selected storm scenarios. 

Most coastal circulation and wave models accept meteorological input in terms of wind (or wind 

stress) and pressure fields. An excellent source of meteorological measurements in coastal areas 

is provided by the NOAA National Data Buoy Center (http://www.ndbc.noaa.gov/). These 

platforms commonly record and archive wind speed, wind direction, and atmospheric pressure at 

intervals ranging from 6 minutes to 1 hour. While these platforms are robust, they do occasionally 

fail during extreme storm events, and sometimes their spacing is too coarse to resolve all 

characteristics of the meteorological forcing. The NOAA National Climatic Data Center 

(http://www.ncdc.noaa.gov/) provides fully reanalyzed meteorological data at a range of spatial 

scales appropriate for use in numerical models. If these two options do not provide suitable 

information, there are companies that sell detailed ocean weather data. 

Once the desired storms or storm parameters have been selected, future changes in those storms 

could be incorporated. A storm’s maximum winds, pressures, size, and precipitation are 

parameters of interest, as well as storm speed, track and landfall location. Appropriate regional 

and local relative sea level rise rates and projections can be developed as described in Section

4.1 and elsewhere in this manual. Changes in surface roughness for areas inundated by sea level 

rise can be incorporated into the selected numerical models. 

A Level 2 exposure assessment in this region of the US calls for original numerical modeling of 

the selected storm and sea level scenarios. As described previously, the selected scenarios may 

need a few to tens of simulations. The numerical models should be selected, developed, and 

implemented in a manner that ensures the relevant physical coastal processes are being 
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simulated. For example, the contribution of wave setup to storm surge can become significant in 

this region of the US due to the offshore bathymetry. Numerical models, or combinations of 

models, that incorporate this additional forcing for storm surge should be used. 

The effects of tides and runoff from coastal watersheds on storm surge and waves are 

especially important in the Mid-Atlantic and New England coasts and can be considered in 

model simulations. For example, developing a model scenario that considers the peaks of the 

local storm surge and stream flow hydrographs occurring simultaneously with the astronomical 

high tide will produce the maximum likely spatial coverage and depth of flooding. The increased 

flooding will subsequently allow wave impacts to reach further inland. Therefore, both the 

astronomical tides and stream flows should be included as boundary conditions in model 

simulations. Tidal characteristics are available at NOAA tide gage 

locations (https://tidesandcurrents.noaa.gov/). The USGS provides historical stream flow data 

and statistics (https://waterdata.usgs.gov/usa/nwis/rt). 

If it is not possible to explicitly model the astronomical tides, one may consider an additional offset 

of the water level corresponding to the local mean higher high water (MHHW) elevation. 

Tidal datum information can be obtained from NOAA (https://tidesandcurrents.noaa.gov/). 

This is analogous to the process of changing the water level to account for higher future sea 

levels. The procedures for validating the numerical models, performing the simulations, 

interpreting the results, and mapping exposure are similar to those presented earlier. 

14.4.3 Level of Effort 3: Mid-Atlantic and New England Coast

A Level 3, risk-based, exposure assessment in this region of the US would be performed in a 

manner similar to that described in above in Section 14.3.3, Level of Effort 3: Gulf of Mexico and 

South Atlantic Coast. The primary regional differences have been described in the previous two 

sections. Those differences include the selection and characterization of storms to include both 

tropical and extratropical events; incorporating regionally appropriate sea level rise values; 

prescribing appropriate boundary conditions like astronomical tides and stream flows; and using 

numerical models that explicitly account for wave effects on storm surge. The major steps for 

developing the information needed to perform a Level of Effort 3 exposure assessment along the 

Mid-Atlantic and New England Coast are summarized in Table 14.6. 

In addition to modeling the storm surge and waves for each of the probability-based storm and 

sea level scenarios in a Level 3 study, predictions of storm-induced shoreline change may be 

important for this region of the US. Shoreline change could be estimated using approximations 

like the “FEMA 540” rule; simple models like EDUNE and SBEACH; or sophisticated models like 

XBEACH, DELFT3D, MIKE 21, or Beach-fx (see Section 6.4). 

Both nor’easters and hurricanes cause coastal infrastructure damage along the 

north Atlantic US coast.
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Table 14.6. Exposure assessment steps for level of effort 3: Mid-Atlantic/New England Coast

Step Activity

1 Identify damaging coastal processes of interest (e.g. storm surge, waves, tides, runoff, 

etc.)

2 Select and characterize storm and RSLR scenarios of interest

3 Select, develop, and prepare appropriate numerical modeling tools

4 Validate and/or calibrate the models through hindcast simulations and analysis

5 Incorporate RSLR scenarios into numerical model simulations

6 Perform model simulations of selected storm and sea level scenarios

7 Derive probabilities associated with water levels, wave heights, velocity, etc.

8 Map the damaging coastal processes for each return period (probability) of interest

9 Evaluate exposure and sensitivity of transportation infrastructure for each return 

period

10 Estimate risk as a function of scenario probability and time-interval considered

14.5 Great Lakes

This section outlines the levels of effort, existing tools and data, appropriate models, and 

methodologies for determining exposure to extreme events in the Great Lakes region. In this 

region of the US, frontal storms and extratropical storms are the predominant extreme events, 

and the storm surge and waves they generate are the important coastal processes of interest

(see Figure 14.5). Other processes of interest in this region include changing lake levels, seiching, 

meteotsunamis, and bluff erosion. A comprehensive coastal flood study of the Great Lakes region 

can be found at http://www.greatlakescoast.org. A multi-level approach for evaluating exposure 

to extreme events and lake levels along the Great Lakes coasts is described in the subsections 

that follow. Each approach is described in terms of the existing technical resources available for 

the region. Some specific comments regarding how these methodologies may be affected by 

results of the ongoing flood study are provided. 

14.5.1 Level of Effort 1: Great Lakes Coast

As in other regions of the US, a Level 1 exposure assessment in the Great Lakes might use 

existing FEMA flood hazard maps, or FIRMs, to determine the depth and spatial coverage of 

inundation (https://msc.fema.gov). The major steps for developing the information needed to 

perform a Level of Effort 1 exposure assessment on the Great Lakes Coast are summarized in

Table 14.7. 

Most of the developed and populated shorelines of the Great Lakes have been mapped by FEMA. 

These coastal areas are served by FEMA Regions 2, 3, and 5. There are considerable reaches 

of shoreline along Lakes Superior, Michigan, and Huron that remain unmapped. 
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Table 14.7. Exposure assessment steps for level of effort 1: Great Lakes Coast

Step Activity

1 Obtain appropriate FEMA flood map elevations for study area

2 Choose desired lake level change scenarios that account for seasonal variability 

3 Subtract (or add) the result of Step 2 from (to) the elevations obtained in Step 1

4 Obtain appropriate ground surface elevation maps for study area

5 Subtract ground elevations (Step 4) from values found in Step 3 to obtain flood depth

6 Multiply flood depths (Step 6) by 0.8 to determine maximum wave height

7 Multiply wave heights (Step 7) by 0.75, add to Step 4 for wave crest elevations

8 Use Equations 14.1 and/or 14.2 to estimate flood flow velocity

9 Consider shoreline retreat and erosion based on historical trends in study area

10 Map the damaging coastal processes (results from Steps 6-9)

11 Evaluate exposure of transportation infrastructure 

The methodology and resources for performing a Level 1 exposure assessment in the Great 

Lakes region are essentially the same as those for the regions already described with one 

significant exception. In the Great Lakes region, we may experience a decrease in lake levels, 

not an increase (see Section 4.5, Lake Water Level Fluctuations). However, the projected lake 

level decreases are similar in magnitude to observed long-term oscillations (see Section 4.5). 

Lake levels also exhibit seasonal variability that should be considered. Historical and real time 

lake level data can be obtained from the NOAA Great Lakes Environmental Research Laboratory 

web site (http://www.glerl.noaa.gov/). Return period lake levels can be obtained from the USACE

coastal storm database (https://chswebtool.erdc.dren.mil/). The database, named CSTORM-DB, 

is being populated with return period storm, water level, and wave statistics derived from modeling 

and statistical analysis as portions of the Great Lakes Flood Study are completed. 

Falling lake levels will alter how one evaluates exposure, sensitivity, and vulnerability in the Great 

Lakes region. The spatial coverage and depth of inundation are likely to decrease with falling lake 

levels. The wave impacts will generally be of a similar magnitude but will impact areas at lower 

elevations than present-day. This may expose foundations, embankments, revetments, and other 

structures to very high forces not accounted for in their design. Furthermore, falling lake levels 

may lead to undercutting of the shoreline and/or bluffs as highly erodible soil layers are directly 

impacted by wave action. For these reasons, consideration should be given to the extent of wave 

runup in the various scenarios. 

Great Lakes water levels also exhibit season fluctuations that will be superimposed on the 

projected future trend. Therefore, as lake levels fall there will be times throughout the year when 

water levels and wave impacts occur at elevations below and above the annual mean lake level. 

Accounting for these seasonal fluctuations may make it necessary to consider a range of potential 

elevations that will be exposed to the damaging physical coastal processes. 
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Figure 14.5. Chicago waterfront (2018)

When using the existing FEMA flood hazard maps for the Great Lakes in a Level 1 analysis, the 

stated still water elevations and base flood elevations could be incrementally shifted downward 

to reflect falling lake levels. More information about the expected lake level fluctuations on the 

Great Lakes can be found at the “Great Lakes Integrated Sciences + Assessments” web site 

(http://glisa.msu.edu/index.php). 

In addition to the potential for falling lake levels, the application of the Level 1 methodology in this 

region is unique because there is no existing literature on the potential nonlinear relationship 

between the effect of falling sea or lake levels on storm surge and waves. Therefore, a multiplier 
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is not used for these locations. Further research is needed to quantify and describe these potential 

effects. 

The estimation of shoreline change, dune erosion, and bluff erosion may not be possible using 

simple rules or historical trends. If they are of concern, some simple numerical modeling may be 

performed. Appropriate information can be found on the Great Lakes Coastal Flood Study web 

site. A specific example includes use of the one-dimensional model CSHORE (Johnson et al.

2012) to estimate waves, water levels, and beach profile evolution. The Great Lakes Coastal 

Flood Study web site also provides links to oblique aerial photography, LiDAR bathymetry and 

topographic elevations, and geodatabases that will be useful for studies in this region of the US. 

14.5.2 Level of Effort 2: Great Lakes Coast 

A Level 2 exposure assessment or sensitivity analysis will use many of the same steps, and use 

many of the same models, described above for the other regions of the US. The major steps for 

developing the information needed to perform a Level of Effort 2 exposure assessment for the 

Great Lakes Coast are summarized in Table 14.8. As applied to the Great Lakes region, a Level 

2 assessment will still need identification of damaging physical coastal processes; selection and 

development of relevant storm and lake level scenarios; selection and validation of numerical 

models; model simulations of the desired scenarios; and mapping of exposure. The primary 

differences in application of the Level 2 approach presented earlier is related to identification of 

the physical processes and the storm scenarios that create them. 

Extreme meteorological events in this region are characterized by frontal systems, extratropical 

storms, the remnants of tropical low-pressure systems, and combinations thereof. Similar to other 

regions of the US, these extreme events produce wind-generated surge and waves that cause 

inundation and damaging wave forces to reach further inland and at higher elevations. In rare 

cases, these weather systems can produce seiching of the lakes. In even rarer cases, extreme 

weather systems have generated meteotsunamis that cause damage to infrastructure at higher 

elevations. These physical processes, as well as their effects on the shoreline, are modified in 

the winter seasons by the presence of lake ice. As the extent and duration of ice coverage in the 

Great Lakes, so too will the physical processes and their impacts be modified. 

Table 14.8. Exposure assessment steps for level of effort 2: Great Lakes Coast

Step Activity

1 Identify damaging coastal processes of interest (e.g. storm surge, waves, bluff 

erosion, etc.)

2 Select and characterize storm and lake level scenarios of interest

3 Select, develop, and prepare appropriate numerical modeling tools

4 Validate and/or calibrate the models through hindcast simulations and analysis

5 Incorporate lake level scenarios into numerical model simulations

6 Perform model simulations of selected storm and lake level scenarios

7 Map the damaging coastal processes for each scenario

8 Evaluate exposure and sensitivity of transportation infrastructure for each 

scenario

The selection of appropriate storm scenarios in the Great Lakes region can be treated as in other 

regions of the US. Extreme events of interest may include either infrequent but intense historical

storms that have had significant impacts within the study area, or perhaps a frequent, low-intensity 
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historical storm that had notable impacts. As in other regions, combinations of historical storm 

parameters may be considered in order to generate a synthetic storm. 

Selection of an appropriate extreme event for a study area may also be a function of the damaging 

physical processes it produces. For example, if the inundation and high flood velocity attributed 

to a meteotsunami are of concern, then a storm event capable of (or known to have) producing 

one should be modeled. 

As in other Level 2 assessments, a few storm scenarios representative of the study area, or 

reflective of the processes of interest, should be considered and later modified to account for the 

expected impacts of lake levels in the region. Development of the meteorological forcing for the 

numerical model simulations will take some effort. The meteorological forcing can be 

reconstructed from measured winds and pressures in the study area, or from reanalyzed 

environmental data. Note here that in addition to winds and pressures, precipitation, 

temperatures, and lake ice coverage may also be important. Real-time and historical

meteorological and hydraulic measurements can be obtained from the NOAA Great Lakes 

Environmental Research Laboratory (http://www.glerl.noaa.gov/data/). The NOAA National 

Climatic Data Center (http://www.ncdc.noaa.gov/), NOAA National Centers for Environmental 

Prediction (NCEP) (http://www.ncep.noaa.gov/), and NOAA NCEP Climate Forecast System 

Reanalysis (http://cfs.ncep.noaa.gov/cfsr/) serve as excellent sources for reanalyzed 

meteorological data. The Great Lakes Coastal Flood Study web site should also be consulted.

The relevant lake levels scenarios to consider for the Great Lakes region are those that will 

significantly impact storm surge, waves, and shoreline change; and those that can be readily 

incorporated into hydrodynamic models. Here, the selected storm scenarios should be simulated 

on future lake levels, which are projected to be lower than their present-day elevations. The effects 

of lake ice coverage and duration may also be simulated by decreasing the spatial ice coverage 

and/or increasing the number of ice-free days for storms occurring in the winter months. Ice 

coverage is generally accounted for as an increase in friction or drag coefficients in wind-stress 

formulations. Note that the model should be capable of accepting spatially variable coefficients 

for this to be possible. 

Future lake levels can be incorporated into the selected models as a reduction in the mean lake 

level position: a negative offset of the present-day lake level. If such an offset is not possible in 

the numerical model, then all topographic and bathymetric elevations can be shifted upward 

(positive) by an equivalent amount. Either way, the net effect is that the lake levels will be lower 

relative to the land surface. Caution should be exercised when using the latter approach, as direct 

comparisons of model results to existing data referenced to its original vertical datum will not be 

possible. In this case, the elevations of either the modeled results or the existing infrastructure 

data should be altered. Preference should be given to correcting modeled results at the 

completion of the model study, but prior to exposure mapping, to avoid potential errors. 

The modeling of storm and lake levels scenarios, as well as the validation of selected numerical 

models, should be performed in a manner similar to what was described in previous sections. 

However, the spatial coverage of the general circulation (storm surge) model should receive 

special attention in this region. An entire lake should likely be modeled in order to properly 

simulate wind surge and waves. If the specific study area falls within Lakes Michigan or Huron, 

both lakes should be modeled to capture the coupling between them. The coupling may not impact 

Frontal-passage storms can cause significant infrastructure damage on the 

Great Lakes.
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wind-wave generation as much as it will the lake water levels, but if the contributions of wave 

setup to the total water level are of interest then they should be accounted for in this manner. 

Upon completion of the storm and lake levels scenario simulations, exposure of infrastructure to 

water levels, waves, velocity, and shoreline change should be mapped using a methodology 

consistent with descriptions provided in previous sections of this manual. As in other Level 2 

assessments, exposure maps of each process will be generated for each scenario considered 

and are expected to be the primary products of the study. The maps can also be used to evaluate 

sensitivity of specific infrastructure to the relevant physical processes and the effects of lake levels

on them. 

14.5.3 Level of Effort 3: Great Lakes Coast 

The suggested methodology for performing a Level 3 assessment in the Great Lakes region 

differs from that described for the Gulf and Atlantic Coasts. In the Great Lakes region, probabilities 

are associated with the storm response, e.g. resultant water level, rather than assigned to storm 

characteristics, e.g. wind speed. This methodology is what FEMA refers to as the “storm response 

method” or “total water level method.” The storm response method is more fully described in 

FEMA (2005). Additional guidance for mapping hazards in sheltered waters, like embayments, is 

available in FEMA (2008). The major steps for developing the information needed to perform a 

Level of Effort 3 exposure assessment in the Great Lakes region are summarized in Table 14.9. 

The storm response method and original numerical modeling are being applied in the 

ongoing Great Lakes Coastal Flood Study to develop new maps that communicate risk related 

to the wind surge, wave, wave runup, and shoreline erosion hazards. These hazards will be 

communicated at probabilities defined by the 10-, 50-, 100-, and 500-year return period intervals. 

These correspond to the 10 percent, 2 percent, 1 percent, and 0.2 percent annual chance of 

exceedance, respectively. 

Table 14.9. Exposure assessment steps for level of effort 3: Great Lakes Coast

Step Activity

1 Identify damaging coastal processes of interest (e.g. storm surge, waves, tides, 

bluff erosion)

2 Select and characterize relevant lake levels scenarios (i.e. lower lake levels)

3 Select, develop, and prepare appropriate numerical modeling tools

4 Validate and/or calibrate the models through hindcast simulations and analysis

5 Model and derive hazard probabilities using the storm response method or use 

existing hazard values at design return periods as model boundary conditions 

6 Incorporate lake levels scenarios into numerical model simulations 

7 Perform model simulations of selected storm and lake levels scenarios 

8 Map the damaging coastal processes for each return period (probability) of 

interest

9 Evaluate exposure and sensitivity of transportation infrastructure for each 

return period

10 Estimate risk as a function of scenario probability and time-interval considered

In the storm response method, event probabilities like return period water levels and waves, are 

generated through a combination of traditional frequency analysis, when measurements are 

available, and hindcast numerical modeling when they are not. Storm response records are 
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reconstructed by considering peak values over specific thresholds. The storms that produced the 

peak values are simulated in hindcast mode to recreate the data needed to perform the frequency 

analysis. 

Some probabilistic data are available for the Great Lakes through the USACE WIS and CSTORMDB web data portals. These data are archived at a number of “save points” along the coast. The 

spacing of the save points is generally adequate to resolve detailed wave transformation and 

shoreline processes. A number of companion documents are also available through the study 

web site. 

If the exposure assessment calls for the development of new or different hazard probabilities, 

then the modeling and frequency analysis inherent to the storm response method should be 

performed first. Such modeling would use the application of lake-wide surge and wave models to 

generate the data necessary to perform a frequency analysis. 

The amount of modeling needed to generate probabilistic hazard values can be substantially 

reduced if existing data are available and used in the study. In that case, the number of model 

scenarios is a function of the number of scenarios considered and/or the number of locations 

where the modeling is performed. For a Level 3 exposure assessment, the relevant lake levels

scenarios should consider future lake levels that are lower than their present-day elevations. The 

desired return period water levels can then be incrementally lowered for each scenario and 

applied as boundary conditions in one- and/or two-dimensional process models. The 

corresponding return period wave characteristics can be applied in a similar manner, as can any 

meteorological forcing. 

Appropriate numerical models for simulating storm surge, waves, and shoreline change in this 

region are similar to those listed elsewhere in this manual. In order to generate the probabilistic 

data necessary to perform a Level 3 assessment, the use of one- and/or two-dimensional models 

is suggested. Application of two-dimensional models may reduce the total number of simulations 

as they may resolve the entire study area; however, they typically take much more time to 

complete. As an alternative, a number of one-dimensional process models can be applied at 

desired transects, or shore-perpendicular reaches, within the study area. A specific example 

would be the application of CSHORE with the appropriate return period wave and water level 

values prescribed as boundary conditions. 

A benefit of the storm response method is that probabilities are assigned to the hazards, not the 

storms that generate them. Therefore, the model results will inherently be probabilistic in nature 

and eliminating a need to derive probabilities from model results. This constitutes a significant 

difference from the Level 3 methodology described in previous sections. However, keep in mind 

that in order for the results of the lake levels scenarios to be fully applied in a risk framework, the 

probabilities associated with future lake levels would also have to be known. Such information is 

not currently available and the subsequent limitations should be acknowledged and preferably 

accounted for as part of the Level 3 assessment. 

Upon completion of the scenario simulations, mapping of exposure and sensitivity analyses can 

be performed in a manner similar to what has been described in previous sections of this manual. 

Particular attention should be given to the fact that as lake levels fall, areas not previously exposed 

to direct wave attack and high velocity flows will become vulnerable. As a consequence, sensitivity 

analyses in the Great Lakes region should include structure stability and foundation integrity. 

14.6 Pacific Coast & Islands – Storms

This section specifically outlines the levels of effort, existing tools and data, appropriate models, 

and methodologies for determining exposure to extreme events along the Pacific Coast, including 
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Alaska, Hawaii and the other islands in this region of the US, extratropical storms and tsunamis 

are the predominant extreme events (see Figure 14.6 for types of Pacific Coast shorelines). 

Although rare, southern California and Hawaii have occasionally experienced tropical storms and 

hurricanes. 

This section focuses on storms and their impacts. The coastal processes of interest in this region 

are water levels, large waves, wave setup, wave runup, tides, storm- induced shoreline change, 

bluff erosion, and runoff from coastal watersheds. Water levels and storm intensity in this 

region are strongly influenced by changes in the Pacific Decadal Oscillation (PDO) and El 

Nino-Southern Oscillation (ENSO) processes. A multi-level approach for evaluating exposure 

to extreme storm events with sea levels along these coasts is described. An approach for 

assessing exposure to tsunami hazards is provided in Section 14.7. 

14.6.1 Level of Effort 1: Pacific Coast – Storms 

Similar to other regions of the US, a Level 1 exposure assessment in this region might use existing 

FEMA flood hazard maps, or FIRMs, to determine the depth and spatial coverage of inundation 

(https://msc.fema.gov) at the 1 percent or 0.2 percent annual chance of flooding. Most of the 

developed and populated shorelines of Alaska, California, Hawaii, Oregon, and Washington have 

been mapped by FEMA. Vast reaches of coastline in Alaska remain unmapped, as well as some 

isolated areas in other states. These coastal areas are served by FEMA Regions 9 and 10.

The major steps for developing the information needed to perform a Level of Effort 1 exposure 

assessment for the Pacific Coast are summarized in Table 14.10. The process for developing 

flood hazard maps in this region of the US is essentially the same as that described in Section 

14.5 for the Great Lakes region. The storm response method, or total water level method, is 

applied since the most extreme storm events are non-tropical and, therefore, difficult to assign 

probabilities to. Therefore, the 1 percent and 0.2 percent flood hazards delineated on the FEMA 

maps are derived from measurements or model hindcasts describing the response of coastal 

water levels to historical storms over a 30-, 40-, or 50-year period. More information about the 

mapping procedures and zones is available in FEMA (2005). 

Table 14.10. Exposure assessment steps for level of effort 1: Pacific Coast

Step Activity

1 Obtain appropriate FEMA flood map(s) for study area

2 Choose desired RSLR scenarios (i.e. magnitude and direction)

3 Subtract or add the result of Step 2 from the still water elevations obtained in Step 1

4 Obtain appropriate ground surface elevation maps for study area

5 Subtract ground elevations (Step 4) from values found in Step 3 to obtain flood depth

6 Multiply flood depths (Step 5) by 0.8 to determine maximum wave height

7 Multiply wave heights (Step 6) by 0.75, add to Step 3 for wave crest elevations

8 Use Equations 14.1 and/or 14.2 to estimate flood flow velocity

9 Consider shoreline retreat and bluff erosion based on historical trends or modeling

10 Map the damaging coastal processes 

11 Evaluate exposure of transportation infrastructure

However, the magnitude and direction of sea levels are highly variable in some areas. Most of 

California, Hawaii, Oregon, and Washington are experiencing RSLR having rates on the order of 

0 to 2 ft per century. Isolated portions of northern California, northern Oregon, and northern 
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Washington are experiencing relative sea level fall on the order of 0 to -1 ft per century. In Alaska, 

the relative sea level rates vary greatly from +1 ft/century to -6 ft/century. 

The selection of an appropriate sea level change scenario will be a function of the study area 

location and regional extent. It may not be possible to cover large spatial regions in this type of 

exposure assessment if the sea level change rates are highly variable over the study area. 

The contribution of wind-driven surge to the total water level is much smaller in this region due to 

the steep coastal bathymetry and narrow continental shelf. However, the contribution of wave 

setup and wave runup is much more significant than in other regions. The nonlinear coupling 

between sea level change and wave characteristics is not well understood and only limited 

examples from the Gulf coast exist that quantify those effects. For example, the study of Smith et 

al. (2010) in Louisiana found the increase in wave heights to be equal to or less than the 

incremental change in sea level. Since only limited information is available, and the potential 

nonlinear coupling along the Pacific Coast is currently unknown, a multiplier on sea level change 

should not be used. 

Separate shoreline change modeling may be desired to capture the effects of sea levels on beach 

profile evolution and bluff erosion. FEMA (2005) offers recommendations in this area. 

Most of the data resources presented in previous sections will also be useful for exposure 

assessments in these regions. Specific examples include the NOAA Coastal Services Center 

Digital Coast web data portal (http://coast.noaa.gov/digitalcoast/), the NOAA National 

Geophysical Data Center (http://www.ngdc.noaa.gov/mgg/bathymetry/relief.html), the USGS 

National Map Viewer (http://viewer.nationalmap.gov/viewer/), and the USACE WIS database 

(http://www.usace.army.mil/). 

Figure 14.6. Laguna Beach, California 
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14.6.2 Level of Effort 2: Pacific Coast – Storms

The suggested methodology for a Level 2 study in this region is very similar to the process 

described previously for the Great Lakes. The major steps for developing the information needed 

to perform a Level of Effort 2 exposure assessment for the Pacific Coast are summarized in Table 

14.11. The major differences here include the need to account for tides, water level fluctuations, 

and relative sea level rise (or fall) that is location-specific. Most of the same steps outlined in 

previous Level 2 assessments and most of the data resources and models described earlier will 

also apply here. 

Table 14.11. Exposure assessment steps for level of effort 2: Pacific Coast.

Step Activity

1 Identify damaging coastal processes of interest (e.g. water levels, wave runup, 

erosion)

2 Select and characterize storm and RSLR scenarios of interest

3 Select, develop, and prepare appropriate numerical modeling tools

4 Validate and/or calibrate the models through hindcast simulations and analysis

5 Incorporate RSLR scenarios into numerical model simulations

6 Perform model simulations of selected storm and sea level scenarios

7 Map the damaging coastal processes for each scenario

8 Evaluate exposure and sensitivity of transportation infrastructure for each scenario

The selection and characterization of extreme storm events and sea level for this region will be 

somewhat different from others. As in other regions, a range of possible intensities informs the 

selection of appropriate storm events. However, for statistical robustness it is also important to 

consider the timing of the events with the highest astronomical tides of the year (locally referred 

to as king tides). Because of the large tide range in Alaska, it may also be relevant to consider 

wave impacts at lower elevations, similar to the Great Lakes region. Contributions of runoff from 

coastal watersheds can also be considered in the selection of storm events or the development 

of synthetic storms. 

Sea level rise is expected to lead to greater inundation, along with broader expanses of possible 

wave impacts, during astronomical high tides for even mild extratropical storms. Once the 

selected extreme storm event scenarios have been selected, sea level can be incorporated by 

considering future sea level projections at desired time intervals or planning targets. Of the 

selected storm and sea level rise scenarios, at least one should consider an historical storm on 

present-day sea levels to serve as a hindcast scenario for model validation. 

The selection of appropriate numerical models should be based on the relevant processes of 

interest. Numerical models that simulate astronomical tides, waves, wave setup, wave runup, and 

shoreline change along the Pacific Coast should be considered. Although wind surge is not a 

significant component of the total water level along the Pacific Coast, it will typically be accounted 

for in most general circulation models used to simulate the astronomical tides. Such models are 

generally capable of accepting stream flows as non-oscillatory, or discharge, boundary conditions 

as well. Changes in land surface roughness could be incorporated into the selected numerical 

models as areas are affected by sea level change. 
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In some cases, it may be necessary to supplement the tide, wave, and shoreline modeling with 

other process-based modeling. For example, additional modeling may be needed to capture wave 

transformations and runup in sheltered waters. These procedures are outlined in FEMA (2008). 

The use of equations may also be appropriate and necessary, as in the case of estimating wave 

overtopping rates. The USACE (2002) and Pullen et al. (2007) provide information on these 

situations. 

14.6.3 Level of Effort 3: Pacific Coast – Storms

The major steps for developing the information needed to perform a Level of Effort 3 exposure 

assessment for the Pacific Coast are summarized in Table 14.12. The regionally appropriate 

processes, storm events, and scenarios described in earlier sections are accounted for and 

applied here. 

Probabilistic flood hazards should be based on a storm response method that accounts for the 

frequency of all relevant processes. Much of this analysis is part of the FEMA California Coastal 

Analysis and Mapping Project: Open Pacific Coast study. As new areas are studied in this region, 

the probabilistic hazard data will become part of the USACE CSTORM-DB. Return period wave 

and wind characteristics are available at many save points along the Pacific, Alaskan, and 

Hawaiian coasts. See the USACE WIS Pacific database at (http://www.usace.army.mil/). 

Table 14.12. Exposure assessment steps for level of effort 3: Pacific Coast.

Step Activity

1 Identify damaging coastal processes of interest (e.g. water levels, waves, erosion)

2 Select and characterize relevant scenarios (e.g. sea levels, runoff)

3 Select, develop, and prepare appropriate numerical modeling tools

4 Validate and/or calibrate the models through hindcast simulations and analysis

5 Model and derive hazard probabilities using the storm response method or use 

existing hazard values at desired return periods as model boundary conditions

6 Incorporate RSLR scenarios into numerical model simulations

7 Perform model simulations of selected sea level scenarios

8 Map the damaging coastal processes for each return period (probability) of interest

9 Evaluate exposure and sensitivity of transportation infrastructure for each return 

period

10 Estimate risk as a function of scenario probability and time-interval considered

The selection and application of numerical models for this region should reflect the characteristics 

of the processes to be modeled. As previously mentioned, astronomical tides, water level 

fluctuations related to ENSO and PDO, wave transformation, wave runup, wave overtopping, and 

runoff from coastal watersheds should be simulated to determine the probabilistic flood, wave, 

and erosion hazards. Many of the numerical models described earlier could be applied 

appropriately in this region. One modeling system unique to this region is the USGS Coastal 

Storm Modeling System, or CoSMoS. The system is capable of simulating water levels, waves, 

coastal erosion and inundation. In addition to providing real-time forecasts of these processes, 

Storm wave runup is a significant contributor to infrastructure damage on the 

Pacific coast.
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CoSMoS has also been successfully applied to recreate historical storms and synthetic scenarios 

(http://walrus.wr.usgs.gov/coastal_processes/cosmos/). 

Application of the CoSMoS system to desired return period and sea level scenarios could provide 

the probabilistic hazard values needed to perform a Level 3 risk-based exposure assessment in 

this region. Such detailed assessments will be greatly facilitated as additional flood hazard studies 

are completed by FEMA and USACE along the Pacific Coast. 

14.7 Pacific Coast & Islands – Tsunamis 

This section broadly outlines suggested levels of effort, existing tools and data, appropriate 

models, and methodologies for determining exposure to tsunami events along the Pacific Coast, 

including Alaska and Hawaii. Specific literature for the determination of tsunami hazards is 

relatively new and only exists in limited forms. Probabilistic tsunami hazard assessments are even 

more limited, but a number of pilot studies are ongoing on the Pacific Coast. At the time of the 

preparation of this document, a multi-state and federal study is developing bridge design 

guidelines for the estimation of tsunami loads on highway bridges (Transportation Pooled Fund 

Program 2019). 

This section broadly outlines suggested levels of effort, existing tools and data, appropriate 

models, and methodologies for determining exposure to tsunami events along the Pacific coast, 

as well as the coasts of Alaska and Hawaii (see Figure 14.7 for Hawaii shoreline). Specific 

literature for the determination of tsunami hazards is relatively new and only exists in limited forms. 

Probabilistic tsunami hazard assessments are even more limited, but a number of pilot studies 

are ongoing on the Pacific Coast. Engineering guidance for the estimation of tsunami loads is 

presented in ASCE (2016). The multi-level approach for evaluating exposure to tsunami events 

and sea level along these coasts is described in general terms in this section. 

14.7.1 Level of Effort 1: Pacific Coast – Tsunamis 

An appropriate Level 1 study of exposure in this region may be based upon existing tsunami 

inundation maps. Such maps are available for nearly all coastal areas of Alaska, California, 

Hawaii, Oregon, and Washington. Most of these inundation maps can be obtained through the 

National Tsunami Hazard Mitigation Program web site (https://nws.weather.gov/nthmp/). The 

Pacific Northwest Seismic Network web site also contains links to many tsunami inundation maps 

and associated products (http://www.pnsn.org), as does the NOAA Center for Tsunami Research 

(http://nctr.pmel.noaa.gov/index.html). The following sites provide access or links to the maps for 

each state: 

• Alaska: http://www.dggs.alaska.gov/

• California: http://www.quake.ca.gov/gmaps/WH/tsunamimaps.htm

• Hawaii: http://www.honolulu.gov/demhazards/tsunamimaps.html

• Oregon: http://www.oregongeology.org/tsuclearinghouse/pubs-inumaps.htm

• Washington: http://www.dnr.wa.gov/Publications/ger_tsunami_inundation_maps.pdf

Unlike FEMA flood hazard maps, tsunami inundation maps do not provide the information needed 

to estimate flood depth. This is because tsunami inundation maps show only the spatial coverage 

of inundation that would result from the most probable worst-case tsunami (Bernard et al. 1996). 

The concept of a “still water elevation” does not necessarily apply for the description of tsunami 

hazards. A tsunami is a long period wave and not a wind-driven storm surge. As a result, the flood 

moves as a wave having a large amplitude, long period, and high speed. These properties allow 
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the wave to reach a considerable distance inland, demonstrate considerable wave runup, and 

translate into very high flood velocities and hydrodynamic loads. 

Tsunami inundation maps cannot be directly used to determine the spatial extent and depth of 

flooding, or expected flood velocity. A new tool for estimating some of these properties, based on 

existing tsunami inundation maps and land surface elevations, can be very useful for Level 1 

assessments of the tsunami hazard (Kriebel et al. 2017). The methodology applies an Energy 

Grade Line concept to describe the maximum expected flood depth, flow velocity, and momentum 

flux along chosen shore-perpendicular transects. The concept is somewhat similar to standard 

hydraulic equations for calculating water surface profiles that are available in most hydraulic texts. 

At this time, there are no widespread materials or processes for the assessment of tsunami 

hazards. The reader is directed to Gonzalez et al. (2003) and FEMA (2012) for additional 

information on understanding and assessing tsunami hazards in this region. For those sources, 

Table 14.13 summarizes suggested steps for performing a Level of Effort 1 exposure assessment 

to tsunamis with sea level. 

Table 14.13. Exposure assessment steps for level of effort 1: tsunamis.

Step Activity

1 Obtain appropriate tsunami inundation maps for study area

2 Choose and incorporate desired sea level change scenarios

3 Apply an energy-based concept to estimate flood depth and velocity along a transect

4 Map the damaging coastal processes 

5 Evaluate exposure of transportation infrastructure 

The potential nonlinear effect of future sea levels on tsunami propagation and transformation are 

likely insignificant, but the degree to which wave runup will be affected is currently unknown. 

Therefore, a multiplier should not be applied to the sea level change scenario. 

Incorporating probabilistic tsunami hazards into FEMAs Risk Management, Assessment, and 

Planning (Risk MAP) program is the subject of pilot studies in Crescent City, California (Tsunami 

Pilot Study Working Group 2006; and Gonzalez et al. 2009) and Seaside, Oregon (CGS 2013). 

The probabilistic tsunami hazard maps for Seaside, Oregon have been developed and 

communicate flood risk for the 1 percent and 0.2 percent annual chance events. These levels 

were chosen to be consistent with existing FEMA National Flood Insurance Program guidance. 

14.7.2 Level of Effort 2: Pacific Coast – Tsunamis

A suggested Level 2 study for mapping the exposure of transportation infrastructure to tsunami 

inundation is based on original numerical modeling of selected tsunamigenic events under 

appropriate sea level scenarios. Here, a tsunamigenic event could be a seismic event or landslide. 

The selection of an appropriate tsunami event could include consideration of those that have 

previously impacted the study area, or tsunamigenic events producing a credible worst-case 

scenario. Relevant sea level scenarios could include sea level rise or fall depending on the 

location of the study area and its corresponding sea level trends and projections. 

Once the selected tsunami event and sea level scenarios have been developed, their effects can 

be simulated using an appropriate numerical model. A number of complex numerical models are 

Tsunamis have damaged coastal highways and bridges in Hawaii.
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being applied to the study, and forecasting, of tsunamis. Most of these modern tsunami models 

simulate three mostly independent processes: tsunami generation (i.e. earthquake), transoceanic 

propagation, and inundation of dry land. While the first and last processes are somewhat 

challenging to simulate, modeling the propagation of the tsunami is comparatively easy. In fact, 

tsunami waves propagate as surface gravity waves and undergo many of the same transformation 

processes as other ocean waves. The NOAA Center for Tsunami Research 

(http://nctr.pmel.noaa.gov/index.html) provides an overview of current tsunami forecasting and 

research efforts. 

The NOAA Center for Tsunami Research specifically uses the Method of Splitting Tsunami 

(MOST) model (Titov and Synolakis, 1997; Titov and Gonzalez, 1997). The MOST model system 

is a suite of numerical models capable of simulating the three processes of tsunami evolution. 

The model has been extensively validated against previous tsunami events and is actively being 

implemented in tsunami forecast systems around the US. 

Figure 14.7. Wainiha River, on the north shore of the island of Kauai, Hawaii, is an area with an 

extremely high tsunami hazard and a history of tsunami-induced bridge damage 

There are numerous other tsunami models with capabilities similar to MOST, specifically the 

ability to model the tsunami propagation and inundation of dry land. Some of the more commonly 

used tsunami models include the TSUNAMOS model developed by Dr. Patrick Lynett and 

colleagues (http://coastal.usc.edu/plynett/TSUNAMOS/index.html); the GeoClaw model of Berger 

et al. (2011); the Cornell Multi-grid Coupled Tsunami (COMCOT) model of Liu et al. (1998); the 

TUNAMI model(s) documented in UNESCO (1997); the JRC Tsunami Model of Annunziato and 

Best (2005); and special applications of DELFT3D described in Gelfenbaum et al. (2006, 2007) 

and more recently Sasaki et al. (2012). 
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Summarizing and synthesizing such literature, Table 14.14 provides potential steps for developing 

the information needed to perform a Level of Effort 2 exposure assessment related to tsunami 

hazards and sea level. 

Table 14.14. Exposure assessment steps for level of effort 2: tsunamis

Step Activity

1 Identify damaging processes of interest (e.g. flood depth, velocity, runup)

2 Select and characterize tsunamigenic and sea level scenarios of interest

3 Select, develop, and prepare appropriate numerical modeling tools

4 Validate and/or calibrate the models through simulation of historical tsunami events

5 Incorporate sea level scenarios into numerical model simulations

6 Perform model simulations of selected tsunami and sea level scenarios, including the 

potential ground subsidence from subduction zone rupture events

7 Map the damaging coastal processes for each scenario

8 Evaluate exposure and sensitivity of transportation infrastructure for each scenario

The procedures for mapping exposure to tsunami events and sea level rise will be similar to other 

coastal hazards in other regions of the United States. One particular exception may be the 

communication of inundation as flood depths as opposed to still water elevations. The expected 

results of the model studies will be maps of inundation coverage and depth, flow velocity, and 

possibly momentum flux. As in other Level 2 studies, these maps are anticipated to be the primary 

products of the mapping effort. Exposure assessments and sensitivity analyses can be performed 

using the maps of each hazard for each scenario considered. Methods for evaluating tsunami 

loads and effects are presented in ASCE (2016). 

14.7.3 Level of Effort 3: Pacific Coast - Tsunamis

Probabilistic tsunami hazard analysis is a relatively new concept. Tsunami hazards are often 

defined by the inundation resulting from a probable worst-case scenario for a particular area. 

However, there are no probabilities associated with such scenarios or the tsunami response. An 

accepted framework for the probabilistic description of tsunami hazards has been established and 

is consistent with seismic hazard analysis (Geist and Parsons, 2006). Application of the 

probabilistic analysis technique for California is described in Thio et al. (2010). 

A suitable methodology for performing a probabilistic tsunami hazard analysis is described in 

Gonzalez et al. (2009). While the methodology was applied to a specific location, it could be 

replicated and used as the basis of a probabilistic Level 3 exposure assessment. The 

methodology was used to derive the 100- and 500-year tsunami inundation areas and elevations. 

Application and use of a numerical model (MOST), as well as the derivation of exceedance values, 

are described in Gonzalez et al. (2009). A similar technique is currently being used to develop 

probabilistic tsunami hazard maps for Crescent City, California (CGS 2013). 

The probabilistic values of tsunami inundation should be determined through a joint probability 

analysis that considers both the probability of the tsunamigenic event (i.e. return period of the 

source event of each specified magnitude) and the flood exceedance (Gonzalez et al. 2009). The 

development of tsunami flood hazard maps currently defines flood levels for the 100- and 500-

year return period events. Consideration could be given to the inundation from other return period 

events as well. For consistency with ASCE-7 seismic hazard analysis, 2,500-year Tsunami design 

map is offered by ASCE-7 (https://asce7tsunami.online/). The Transportation Pooled Fund 

research TPF-5(307) (Transportation Pooled Fund Program 2019) produced 1,000-year tsunami 
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inundation map for the participating States (AK, CA, HI, OR, WA). It also produced areal coverage 

of tsunami design parameter map for California and part of Oregon, which include maximum depth 

and maximum velocity in the inundated area (1,000-year return period) that can be directly used 

in tsunami load formulas developed in the same project. These maps include the effect of potential 

ground subsidence in the subduction zone events. 

As in other exposure assessments for the Pacific Coast, appropriate sea level scenarios should 

be incorporated into the probabilistic tsunami modeling. The most relevant sea level rise scenario 

is likely to be the local rate or future projections of relative sea level rise (RSLR). These changes 

in sea level can be incorporated into the numerical models using water level offsets as described 

previously. It may be possible to assign probabilities to sea level change scenarios using local 

rates or projections of subsidence in conjunction with the probability-based estimates of global 

eustatic sea level rise provided in Houston (2013). 

Suggested steps for developing the information needed to perform a Level of Effort 3 exposure 

assessment for tsunami hazards with sea level are summarized in Table 14.15. 

Table 14.15. Exposure assessment steps for level of effort 3: tsunamis

Step Activity

1 Identify damaging processes of interest (e.g. flood depth, velocity, runup)

2 Select and characterize tsunamigenic and sea level scenarios of interest

3 Select, develop, and prepare appropriate numerical modeling tools

4 Validate and/or calibrate the models through simulation of an historical event

5 Incorporate sea level scenarios into numerical model simulations

6 Perform deep water propagation simulations of selected seismic source and sea level 

scenarios to obtain probability distribution of offshore wave height

7 Derive the joint probability, for multiple tsunami sources/magnitudes, of offshore wave 

height

8 Disaggregate the source events based on the offshore wave height associated with 

the return period of interest and identify reasonable number of governing source 

events for shoaling analysis

9 Map the damaging tsunami processes for each return period (probability) of interest

10 Evaluate exposure and sensitivity of transportation infrastructure for each return 

period

11 Estimate risk as a function of scenario probability and time-interval considered

14.8 Vulnerability Assessment Case Studies

This section highlights three vulnerability assessment case studies as examples that have used 

analysis methods similar to those outlined in this chapter above. Coastal engineers with 

specialized training and experience in coastal modeling were included on each of the study 

teams for these case studies – even the Level 1 analysis. The results document both the 

vulnerability of these transportation assets to extreme coastal events today and how that 

vulnerability will increase with future sea level rise: 

1. Section 14.8.1 is a Level 1 case study that uses a form of basic inundation mapping (with 

modifications to approximate increased flooding from wave effects) for different storm and 

tide conditions with increased sea levels. 
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2. Section 14.8.2 is a Level 2 case study that uses original, high-resolution storm surge and 

wave modeling and mapping with increased sea levels and storm strengths. 

3. Section 14.8.3 is a Level 3 case study that uses original, high-resolution surge modeling 

and mapping to develop detailed maps of the probabilistic coastal inundation and flooding 

levels, including depth-frequency curves at specific locations, with increased sea levels. 

14.8.1 Level 1 Case Study: Adapting to Rising Tides - San Francisco Bay

This section describes a case study of mapping the level of flooding exposure of transportation 

infrastructure to coastal extreme events and sea level in a region where coastal storms during El 

Niño episodes are the dominant destructive storms. This case study is the technical inundation 

mapping component of the “Adapting to Rising Tides: Transportation Vulnerability and Risk 

Assessment” pilot project (ART 2011a). It is an example of a Level 1 study using the terminology 

outlined in this chapter. The approach was a form of basic inundation mapping to assess the 

depth of inundation along the Alameda County shoreline of San Francisco Bay (see Figure 14.8). 

The purpose was to inform a vulnerability rating of transportation assets in the study area under 

future sea level rise scenarios considering different storm and tide conditions. Intended as support 

planning, the analyses were not intended to represent or replace detailed engineering analyses. 

14.8.1.1 Background

This case study was one component of the multi-agency ART study to enable transportation 

planners in the San Francisco Bay regions to improve vulnerability and risk assessment practices 

and to help craft effective adaption strategies. Lead agencies involved in the study included the 

San Francisco Bay Conservation and Development Commission, the Metropolitan Transportation 

Commission, the California Department of Transportation (District 4) and the FHWA. The project 

stakeholder groups included twenty local, state, and federal agencies and governments. The 

overall goal of the ART project was to increase the preparedness and resilience of Bay Area 

communities to sea level rise impacts while protecting ecosystem and community services (ART 

2011a). 

This portion of the project was a pilot planning project on a sub-regional scale to test the FHWA 

Risk Assessment Model (a predecessor to Figure 14.2). The sub-region studied was the 

approximately 20-mile long Alameda County bay shoreline. The study evaluated potential 

shoreline impacts, vulnerabilities, and risks; it identified adaptation strategies; and it developed 

adaptation planning tools and resources. This case study focuses on the methodology developed 

to produce the inundation maps for the pilot study (ART 2011b) and is not a summary of the 

broader ART study. 

Case studies can demonstrate how to quantitatively assess the vulnerability of 

coastal highways
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Figure 14.8. Shoreline Drive, Alameda, California on San Francisco Bay (2018)

14.8.1.2 Storm Selection and Sea Level Scenarios 

The ART study focused on sea level rise because of its potential to cause harm. It could damage 

residential, commercial, and industrial structures in low-lying areas near the shoreline as well as 

important habitats and wildlife resources. Two sea level rise scenarios were selected based on a 

review of the literature. The first scenario was an increase of 16 inches consistent with a high-end 

estimate for mid-century and the second scenario was an increase of 55 inches consistent with a 

midrange estimate for the end of the century (ART 2011b). 

Three storm/tide conditions were evaluated for each of the sea level rise scenarios: a high 

tide (MHHW), a 100-year storm water level (still water level), and the 100-year storm water 

level increased by some effects of wind waves. These three storm/tide conditions were 

selected to represent a reasonable range of potential coastal flood levels. The high tide inundation 

is representative of the area that would be subject to frequent or permanent tidal inundation. The 

100-year flood is representative of the area subject to flooding and wave damage in extreme 

storms. Thus, there were a total of six scenarios evaluated - the two sea level rise scenarios 

combined with three storm/tide conditions. 

14.8.1.3 Inundation Mapping Approach

The method used to assess the vulnerability to sea level was a modified form of the basic 

inundation mapping (bathtub) approach. Six inundation maps were developed corresponding to 

the two sea level rise scenarios with each of the three storm/tide conditions. The maps were 

developed by estimating the water level of interest and then comparing that elevation to the 

existing upland topography to determine depth of flooding at all locations across the study area. 

The elevations of the water levels (MHHW, 100-year storm, and 100-year with some increase for 

wave effects) used for this inundation mapping were selected based on extensive modeling of the 

bay. 
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This pilot study was able to use results of two much more extensive modeling efforts focused on 

developing similar estimates around San Francisco Bay: a recently completed USGS modeling 

effort and an ongoing FEMA modeling effort to remap the coastal flood plain around the bay. The 

FEMA effort used a high-resolution hydrodynamic model, MIKE-21, which is comparable to 

ADCIRC and also includes a wave model. Estimates of the MHHW and the 100-year storm 

elevation were taken from the USGS modeling effort at 13 locations along the shore of Alameda 

County for use in this pilot study inundation mapping effort. These MHHW elevations ranged from 

+6.11 ft to +6.85 ft (NAVD) with the values consistently increasing farther to the south. This 

compares with a MHHW elevation at the Presidio of +5.83 ft (NAVD). This variation of high tides 

throughout San Francisco Bay is well-known and results from the bay’s response to tidal waves. 

The tide is amplified in the bay and the amplification increases moving south from the Golden 

Gate area. The estimated 100-year storm still water elevations at those 13 Alameda County 

locations were also obtained from the USGS study results. Those values ranged from +9.2 ft to 

+10.42 ft (NAVD) and the variation was not consistent spatially along the coast. 

This pilot study also then adjusted the 100-year still water levels upward an increment to account 

for the elevation of wave crests in depth-limited situations. These resulting elevation estimates 

were referred to as “wind waves” in the pilot study. This adjustment was essentially a professional 

coastal engineering judgment by the consultant team based on the available modeling results and 

consistent with the project purpose of a general screening-level tool. Inundation was then mapped 

as if the still water level was at that higher elevation and referred to as the “potential wind-wave 

zone.” This adjustment is not standard coastal engineering practice. As stated in the report, the 

physics of overland wave propagation into the flooded areas is not modeled (ART 2011b). 

Similarly, this vulnerability mapping effort also includes a non-standard definition of “overtopping” 

based on whether the elevation of the shoreline feature along the bay is lower than the vertical 

elevation of the water surface. 

14.8.1.4 Inundation Maps

Inundation maps were generated in the San Francisco Bay pilot study by overlaying the flood 

water elevations on detailed maps of the existing upland topography. The maps illustrate the 

potential for coastal flooding in relationship to transportation assets under the six scenarios 

discussed above (2 sea level rise scenarios with MHHW, 100-year flood levels, and 100-year 

flood levels including the “potential wind-wave zone”). 

Examples of the inundation maps from the pilot study showing the inundation areas estimated for 

the scenario of 16 inches of sea level rise with the 100-year storm levels adjusted upward to 

account for the “potential wind-wave zone.” are provided in Figure 14.9 and Figure 14.10. Figure 

14.9 shows the entire study area and Figure 14.10 shows the same information in more detail for 

the northernmost portion of the study area. The maps show the major transportation assets as 

well as the flooded areas. The extent of flooding is shown by the yellow hatching. Specific 

transportation assets are identified in Figure 14.10. Also shown on are the estimated flood depths. 

These mapping results were used to develop metrics of the level of exposure and vulnerability of 

the transportation assets (ART 2011a, ART 2011b). 

San Francisco Bay’s Adapting to Rising Tides (ART) study is an example of a 

Level 1 vulnerability analysis
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Figure 14.9. Inundation map of Alameda County for 100-year storm flood with 16 inches of sea 

level rise and additional elevation for wave effects (from ART 2011b)
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Figure 14.10. Detailed inundation map of the northern portion of Alameda County for the 100-

year storm flood with 16 inches of sea level rise and additional elevation for wave effects (from 

ART 2011b)
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14.8.2 Level 2 Case Study: The Gulf Coast 2 Study - Mobile, Alabama

This section describes a case study assessing the exposure of transportation infrastructure to 

coastal extreme events and sea level in a region where hurricanes are the dominant destructive 

storms. The approach is a scenario-based analysis using high-resolution coastal modeling. Storm 

surge flood elevation and wave height maps were simulated for different possible future sea level 

rise (SLR) and storm scenarios. The results are used to assess the vulnerability of road, bridge, 

tunnel, railway, port, and airport facilities as part of a larger comprehensive study. This case study 

is an example of a Level 2 type analysis. 

14.8.2.1 Background

The USDOT investigated the impacts of sea level on transportation systems in the central Gulf 

Coast region in a multi-year study (USDOT 2013). This study had two primary phases. Phase 1 

(completed in 2008) examined the impacts of sea level on transportation infrastructure at a 

regional scale. Kafalenos et al. (2008) mapped flooding of transportation facilities from Galveston, 

Texas to Mobile, Alabama by future sea levels. The exposure was determined by identifying areas 

with ground elevations below the future sea level rise projections/scenarios. Phase 2 was focused 

on a much smaller study area, Mobile County, Alabama. Phase 2, called the “Gulf Coast 2” or 

“GC2” study, is a more detailed study that developed and applied more complex methods for 

assessing sea level exposure and vulnerability. 

This case study is a portion of the second major task, GC2: Task 2, using scenario-based 

modeling of hurricanes with sea level rise. Additional detail on the study methods and results can 

be found in that task study report (Choate et al. 2012). Some of the model results developed in 

this case study were included as a graphic in the 2014 NCA (Melillo et al. 2014). 

14.8.2.2 Approach

The GC2: Task 2 study used storm surge and wave models with future sea level rise scenarios. 

The results are maps of wave heights and water depths at transportation asset locations. 

Hurricane impacts of surge and waves are the most important coastal process for this study area. 

Mobile County is susceptible to hurricane storm surge damage because of its location along the 

northern Gulf coast. The largest city in the study area, the City of Mobile, is in the northwest corner 

of Mobile Bay, a large, very shallow estuary with a very wide inlet to the Gulf of Mexico, Mobile 

Pass. Storm surge in the northern portions of the bay can be up to 6 ft higher than in the southern 

end of the bay. The north end of the bay is also where most of the transportation systems (roads, 

bridges, ports, and rails) are located. The area is frequently impacted by tropical storms and 

hurricanes. It has been impacted by at least one named storm in 15 of the last 20 years. Thus, 

proper estimation of hurricane impacts is vital for quantifying exposure to flooding and wave 

damage with sea level in Mobile, Alabama. 

The storm surge model was validated by comparing simulated results with actual surge 

measurements for historical storms. The models were then used to simulate surge and waves 

under a variety of storm and sea level scenarios discussed below. The resulting water levels, 

velocities, and wave heights were mapped to quantify exposure within a GIS framework. The 

selection of storm and sea level scenarios and descriptions of the modeling approaches and 

validation are provided in the following sections. 

14.8.2.3 Storm Selection and Sea Level Scenarios

Storms (in this case, hurricanes) and sea level scenarios were selected to bracket a reasonable 

range of expected conditions. The selected storms included a moderate strength hurricane and 

an extremely powerful hurricane. Sea level rise scenarios consistent with the projections 
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presented in Section 4.1 were considered. The storms and sea level scenarios were selected to 

address two main questions: 

1. What are the implications of a moderate hurricane striking the region under a scenario of 

increased sea levels? 

2. What are the implications of a strike by a larger hurricane than the region has 

experienced in recent history? 

Characteristics of two hurricanes which had notable impacts in Mobile County - Hurricane 

Georges (1998) and Hurricane Katrina (2005) - were used to address these questions. Hurricane 

Georges was used to evaluate the effects of a moderate storm and Katrina was used to evaluate 

the effects of a larger, rarer storm. Hurricane Georges made landfall near Biloxi, MS as a Category 

2 hurricane. Hurricane Katrina, one of the most destructive hurricanes in US history, made landfall 

near Buras, Louisiana as a Category 3 hurricane and then again near the Mississippi-Louisiana 

state line (over 100 miles from Mobile, Alabama). The use of these historical storms, or more 

correctly, the use of the characteristics of these historical storms (intensity, size, and track) was 

entered into the numerical models under the assumption that future storms will, in general, have 

similar characteristics as past storms. Additional scenarios included changes in both the storm 

track and the storm intensity. 

The storm and sea level scenarios modeled in this case study are summarized in Table 14.16. 

The first two scenarios, with no sea level rise, were considered primarily for model validation 

(discussed later) and comparison purposes. However, their results show that the area is very 

exposed to extreme events today. All investigators agree that global sea levels will rise in the next 

century (Section 4.1 Sea Levels). The selected sea level rise scenarios were 0.98 ft (30 cm), 2.5 

ft (75 cm), and 6.6 ft (200 cm). These values were selected based on consideration of the 

published sea level rise projections as well as local vertical land movement estimates (see Choate 

et al. 2012). 

Storm track and intensity were also modified in some scenario simulations to address the 

second question. The historical storm track of Hurricane Katrina was shifted eastward such that 

the strongest onshore, northerly hurricane winds would be blowing up the length of Mobile Bay. 

This was done without modifying the forward speed of the storm or the rate of decay as the storm 

moved over the new landfall location. 

Intensification of hurricane winds was considered. Some investigators have suggested a chance 

of a stronger storm than Katrina at any time. Storm intensification was simulated with storm winds 

and central pressure using information found in Knutson and Tuleya (2004) and Knutson et al.

(2010). The wind speeds in Hurricane Katrina were increased by 6.5 percent. This value 

represents an average of the intensification range (+2 percent to +11 percent) suggested in 

Knutson et al. (2010) for hurricane wind intensification by 2100. The central pressure deficit (i.e. 

difference between ambient and storm central pressure) was decreased by 13 percent, which is 

consistent with the results of modeling in Knutson and Tuleya (2004), as well as information

provided in Knutson et al. (2010). 

In the final two scenarios listed in Table 14.16, the 150 mph maximum wind speeds of Katrina 

measured out away from land in the Gulf of Mexico were maintained in the computer simulations 

until the storm made landfall. More explanation of the selection of the scenarios summarized in 

Table 14.16 can be found in the GC2: Task 2 study report (Choate et al. 2012). 

Mobile’s Gulf Coast 2 study is an example of a Level 2 vulnerability 

assessment.
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Table 14.16. Scenarios modeled in GC2 (modified from Choate et al. 2012)

Scenario Name

Sea Level 

Rise? Track Shift? Intensification?

Georges No No No

Katrina No No No

Georges+30 0.98 ft (30 cm) No No

Georges+75 2.5 ft (75 cm) No No

Georges+200 6.6 ft (200 cm) No No

Katrina+75 2.5 ft (75 cm) No No

Katrina-Shift No Yes No

Katrina-Shift+75 2.5 ft (75 cm) Yes No

Katrina-Shift-Intensity+75 2.5 ft (75 cm) Yes Higher winds and 

reduced central 

pressure

Katrina-Shift-MaxWind No Yes Maximum Winds

Katrina-Shift-MaxWind+75 2.5 ft (75 cm) Yes Maximum Winds

14.8.2.4 Storm Surge Modeling

Storm surge and the resulting inundation for each scenario were modeled with the Advanced 

Circulation model ADCIRC. The ADCIRC model is capable of simulating tides, instream flows, 

hurricane wind fields, and the subsequent water levels and velocities at discrete locations 

continuously over a specified duration (Luettich et al. 1992; Westerink et al. 1994). The ADCIRC 

model is currently used as one of many tools to predict storm surge elevations in the development 

of flood insurance rate maps for FEMA (see Section 4.3.2 Storm Surge Modeling and see Webb 

2017). 

Tides were not included in the surge simulations in this case study since the average tide range 

(about 1.2 ft) is much less than storm surge in the study area. Storm durations were modeled over 

a 2.5- to 3.0-day period prior to landfall. Storm histories were adapted from the official NOAA 

National Hurricane Center (NHC) storm database archives that give storm coordinates and 

characteristics at specified times. The internal wind models of ADCIRC were used to simulate the 

hurricane wind fields using these basic storm characteristics. 

In the model scenarios considering sea level rise, the water level everywhere in the 

computational domain was increased by an amount equal to the sea level rise scenario.

This procedure is analogous to lowering the earth surface elevations by an equivalent amount. 

The ADCIRC model has an option for specifying such adjustments without having to alter the 

digital elevation model. 

The modeling in the GC2 study has been referred to previously in this manual (Section 4.3.2

Storm Surge Modeling). The finite element mesh used in this case study is shown in Figure 4.18. 

The mesh has 446,459 nodes and 866,496 triangular mesh elements and includes the entire Gulf 

of Mexico, Caribbean Sea, and much of the western North Atlantic Ocean. To account for upland 

surge flooding a seamless topographic-bathymetric mesh surface was developed for coastal 

Alabama as shown in Figure 4.19. Simulated water levels were saved frequently (e.g. each 

minute) at some discrete locations for comparison with tide gage measurements during the model 

validation process (see Figure 14.12). Simulated water levels and flow velocities were saved at 

three-hour increments for every mesh node for the duration of the simulations. Other model output 
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includes the recording of maximum water levels and velocities at each mesh node over the entire 

simulation. A map of the maximum envelope of water (MEOW) for each scenario was developed. 

Figure 14.11 is an example of estimated surge depths across the study area for the scenario of a 

moderate hurricane like Hurricane Georges with 2.5 ft (75 cm) of sea level rise. Storm surge depth 

and inundation maps were generated by combining the ADCIRC model high water results with a 

GIS-based grid of ground surface elevations. 

The implication of Figure 14.11 is that large portions of the study area near the bays and tidal 

creeks in the study area will be inundated. The modeling approach shows which specific areas 

and assets will be inundated. These areas include 33 miles of “critical” roads, 114 miles of rails, 

and 78 percent of port facilities. These values varied by scenario, as expected, with up to 117 

miles of critical roads, 154 miles of rails, 100 percent of port facilities, and the downtown airport 

being inundated in the most extreme event scenarios. More examples of the maximum storm 

surge results can be found in the GC2: Task 2 study report (Choate et al. 2012). 

The level of exposure and vulnerability resulting from extreme events today is one of the notable 

conclusions that can be drawn from this case study. For example, the road and rail inundation 

mileage results presented for the scenario with 2.5 ft (75 cm) of sea level rise are only slightly 

higher than those for the same storm with today’s sea level (see Table 25 of Choate et al. 2012). 

Similar results have been found in other vulnerability assessment studies (NJTPA 2011). The 

damage experienced in extreme events in the past decade, including Hurricane Sandy and 

Hurricane Katrina, is consistent with and validates this conclusion. The modeling methodology 

outlined in this case study provides a way to quantitatively address the level of exposure and 

vulnerability to both extreme events and sea level rise. 

14.8.2.5 Validation of Storm Surge Model

The storm surge model was validated as part of this Mobile, Alabama case study investigation 

before simulating the impacts of sea level on storm surge. The ADCIRC model and mesh were 

validated by hindcasting Hurricanes Georges and Katrina. Hindcasting is the use of a model to 

simulate a past time period, i.e. not a forecast. It is a common way to validate that a hydrodynamic 

model, like ADCIRC, and the project-specific input mesh are working correctly. An example of this 

model’s hindcast of surge elevation during Hurricane Katrina is shown in Figure 4.20 (Section 

4.3.2). The modeled surge matches the actual storm surge well. 

Time histories of simulated water levels were compared to NOAA tide gage measurements within 

and close to the study area. An example comparison of simulated and measured storm surge 

hydrographs is shown in Figure 14.12 for the Katrina hindcast. Such comparisons show the 

model's ability to faithfully recreate the time-dependent nature of storm surge. The simulated peak 

magnitude, the single most important result, matches the measured value well. The simulated 

peak occurs earlier than the measured peak and the simulated surge does not model the prestorm setup well (the day before landfall). Such inconsistencies are not unusual (see Webb 2017 

for more discussion of model validations). 
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Figure 14.11. Modeled storm surge depths in Mobile County, Alabama for the scenario of 

Hurricane Georges conditions with 2.5 ft (75 cm) of future sea level rise (from Choate et al. 

2012) 
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Figure 14.12. Validation of ADCIRC surge estimates by comparison with a tide gage at Dauphin 

Island, Alabama

A comparison of simulated and measured high water marks (HWMs) was also performed for each 

storm hindcast to evaluate the potential spatial variability of model errors throughout the study 

area. An example is provided in Figure 14.13 for the Hurricane Katrina hindcast. The values 

shown represent the difference in magnitude between simulated and measured maximum still 

water elevations. Comprehensive documentation of HWMs is available for many recent storms. 

However, the reporting of HWMs involves subjective determinations that make direct comparisons 

with simulated water levels difficult. 

Additional analysis of the storm surge hydrographs and HWM comparisons was performed to 

evaluate a range of possible model errors. Calculated model errors are shown in Table 14.17. 

The root-mean-square (RMS) error provides an error magnitude based on direct comparisons 

between modeled and measured data over time, in the case of storm surge hydrographs, and in 

space for the HWM comparisons. An additional error estimate was expressed as the "Percent of 

Peak," which is the ratio of the RMS error to the measured maximum water level. 

Whether the validation is adequate is a matter of professional judgment. The agreement between 

model and measured surge shown here was acceptable for the purposes and scope of this case 

study evaluation. The model was then used for the sea level simulations as discussed. 
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Figure 14.13. Validation of ADCIRC storm surge estimates by comparison with measured high 

water marks

Table 14.17. Validation analysis of ADCIRC model storm surge estimates

Metric RMS Error (ft) Percent of Peak (%)

78 High Water Marks 1.98 13.2

Hydrographs at Pensacola, FL 1.20 19.8

Hydrographs at Dauphin Island, AL 1.00 16.4

14.8.2.6 Wave Modeling

Storm waves were modeled with the Steady-State Spectral Wave model STWAVE. The STWAVE 

model simulates the generation and transformation of waves over variable bathymetry (Smith et 

al. 2001). The model estimates wave characteristics at every grid point in the model domain 

assuming that waves have come to a steady state. 
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The input included the ADCIRC surge simulations, measured winds in Mobile Bay, and measured 

waves in the Gulf of Mexico. The computational domain for the wave model was developed from 

a portion of the same digital elevation model used in the ADCIRC simulations but focused on a 

much smaller area including the bay and offshore into the Gulf. The resulting grid provided wave 

characteristics at a regular spacing of about 300 ft (100 m). This resolution was sufficient for 

subsequent exposure and sensitivity analyses in this case study. 

Storm waves were simulated for the scenarios outlined in Table 14.16 using the maximum surge 

depths from the ADCIRC simulation. Input winds for wave generation in the bay were the 

maximum observed wind speed and direction. Boundary input waves were measured wave 

characteristics in the Gulf of Mexico during the peak of each historical storm. The results from 

each wave model scenario included estimates of significant wave height, peak wave period, 

dominant wave direction, and wave breaking throughout the model domain. 

No direct measurements of waves were available within the study area to validate the STWAVE 

model predictions. Direct measurements of wave characteristics are rarely available close to the 

coast where comparisons are most useful. However, the model has been extensively validated 

and those results are available in the published literature. 

Figure 14.14 shows an example of the significant wave heights estimated by the STWAVE model. 

This example is the scenario of a moderate hurricane (Georges) with 2.5 ft (75 cm) of sea level 

rise. Essentially, these are the waves riding on the surge shown in Figure 4.20. The wave heights 

are much larger in the Gulf of Mexico but the model estimates the regeneration of waves across 

Mobile Bay and into the areas inundated by storm surge. More examples of the wave field results 

corresponding to the scenarios evaluated in Table 14.16 can be found in the GC2: Task 2 study 

report (Choate et al. 2012). 

The significant wave heights at one downtown Mobile location will increase from 2 ft (0.6 m) for a 

moderate hurricane today to over 4.3 ft (1.3 m) for some of the more severe scenarios. This 

increase is particularly important because most buildings will be severely damaged or destroyed 

with wave heights around 1 to 3 ft. 

Results like Figure 14.11 and Figure 14.14 can be examined to quantify the exposure of specific 

transportation assets to the primary damaging mechanisms: storm surge and waves. Thus, this 

type of Level 2 scenario-based analysis provides tools for exposure and vulnerability 

assessments. In summary, model-based analysis, like this case study from GC2, can provide 

quantitative estimates of exposure which account for the complex physics of coastal storm surge 

and wave propagation under different sea level scenarios. 

14.8.3 Level 3 Case Study: Central Artery Project, Boston, Massachusetts

This section summarizes a case study of probabilistic, high-resolution modeling of storm surge, 

waves and tides with future sea level rise in the Boston, Massachusetts area. The case study is 

based on the work of Bosma et al. (2015) and more specific details can be found there and in 

Douglas et al. (2016). 
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Figure 14.14. Modeled wave heights for the scenario of Hurricane Georges conditions with 2.5 ft 

(75 cm) of future sea level rise (from Choate et al. 2012)
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This Boston case study is a comprehensive example of a Level 3 approach to vulnerability 

assessment because it:

• establishes a methodology for using regional storm characterizations produced through 

rigorous and defensible applications of science and engineering including sea level rise, 

• uses high-resolution tide, surge, and wave models for mapping the extent and coverage 

of inundation in coastal areas, 

• models the dynamic response of flooding to sea level rise as compared to simply 

estimating the static response (i.e. not the “bathtub” assumption), 

• maps, in a probabilistic fashion, the extent of inundation under both tidal and storm 

conditions including future sea level rise, and

• quantifies the probabilities of flooding (including flood levels, flood pathways, and flood 

durations) for site-specific transportation assets. 

The results demonstrate some of the advantages of the more complex methods of coastal 

modeling compared to simpler methods. In some parts of the Boston region, the simpler methods 

underpredict flooding as compared with the more comprehensive analysis methods. In many 

cases, a “bathtub” approach overpredicts inundation where flooding will not occur and also 

misidentifies dry area that would actually be inundated. The more complex, probabilistic modeling 

provides key information such as the flood durations and pathways that simpler methods normally 

would not include. Such key information can result in cost savings in the planning and design of 

adaptations (Bosma 2016). 

Results from the more complex modeling can also be used to test the effectiveness of various 

engineering designs and adaptations. For example, green living shoreline alternatives (e.g. 

restoration of dune, wetlands, and reefs), as well as traditional grey infrastructure (e.g. seawalls), 

can be simulated individually or in combination. 

Level 3 analysis, like that presented in this Boston vulnerability assessment, may be justified for 

the planning and design of many high-value US transportation assets along the coast which are 

sensitive to damage in coastal storms (e.g. low bridges, tunnel portals, etc.). 

14.8.3.1 Background

This Boston study demonstrates how modeling can estimate the effects of relative sea level rise 

(RSLR) on the levels of coastal storm flooding and waves at a very high resolution across a broad, 

highly urbanized area with a focus on transportation assets. The Central Artery/Tunnel system 

through downtown coastal Boston, Massachusetts is a vital link in the urban regional 

transportation network and is composed of more than 160 lane-miles (more than half of them in 

tunnels), 6 interchanges and 200 bridges. As one of the most valuable components of the state’s 

transportation infrastructure, its maintenance, protection and enhancement are a priority. 

The two main objectives of this study were to assess the vulnerability of the Central Artery/Tunnel 

system to RSLR and extreme storm events, and to investigate and present adaptation options to 

reduce identified vulnerabilities. This study was jointly funded by MassDOT and the FHWA but a 

key priority for this project was to develop products that, to the degree possible, are useful to other 

Boston agencies and stakeholders who are also doing adaptation work. 

The Boston Central Artery/Tunnel case study is an example of a Level 3 

vulnerability assessment.
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14.8.3.2 Storm Selection and Sea Level Scenarios

Both tropical (i.e. hurricanes) and extra-tropical (i.e. Nor’easters) storm conditions were evaluated 

in the modeling used in this case study. While hurricanes are intense, fast moving storms that 

have a significant impact on coastal communities, they are not as common in the northeast US 

as Nor’easters. Historical water level records and historical meteorological records were used to 

identify a set of Nor’easters to be simulated in the model. In addition to storm intensity and 

direction, the timing of a storm relative to the tidal cycle can be an important consideration along 

the Atlantic Coast. The timing of the peak hurricane surge is very important while the timing of the 

peak Nor’easter surge has little effect on maximum water levels. This is because hurricanes tend 

to be fast moving systems, hence the likelihood of peak surge occurring at the same time as peak 

high tide is relatively low when compared to Nor’easters, which typically last for 24 hours or more. 

A set of about 400 representative synthetic storms were combined with a set of about 200 

Nor’easters. The probability of flooding due to both hurricanes and Nor’easters was estimated by 

developing composite probability distributions for flooding (see Bosma et al. 2015 for details). 

Two GMSLR scenarios were evaluated to bracket the potential future sea level rise outcomes for 

the Boston Harbor area. The selected GMSLR estimates were taken from Parris et al. (2012). 

The actual GMSLR values used in the coastal hydrodynamic modeling were 0, 0.6 and 3.2 ft 

above present levels: 

• 0 ft of GMSLR represents present-day conditions (of 2013 in this study),

• 0.6 ft of GMSLR represents 2030 conditions, and 

• 3.2 ft of GMSLR represents both a “high” scenario for 2070 and an “intermediate-high” 

scenario for 2100. 

The GMSLR heights were adjusted for local subsidence of 0 in, 0.74 in, and 2.5 in for 2013, 2030, 

and 2070/2100 respectively. 

Thus, the total RSLR scenarios modelled in the Boston vulnerability assessment were:

• 0 ft for 2013,

• 0.68 ft for 2030, and

• 3.4 ft for 2070/2100. 

14.8.3.3 Coastal Hydrodynamics Modeling

The hydrodynamic models, ADCIRC to simulate storm surge, circulation, and tide; and SWAN to 

simulate waves were coupled to include the joint effects of wave setup. Both the ADCIRC 

circulation model and the SWAN wave model are implemented on an unstructured mesh with 3 

levels of nested meshes (see Section 3.1 Grids, Meshes and Nesting of Webb 2017 for an 

explanation of these terms). 

Three resolutions of the mesh are used in the modeling: 

• a regional-scale mesh which includes much of the north Atlantic, which is a previously 

validated model mesh used in numerous FEMA, NOAA, USACE studies, 

• a mesh providing an intermediate level of resolution to transition from the regional-scale 

mesh to the highly resolved mesh needed along the MA coastline, and 

• a site-specific fine mesh of sufficiently high resolution to ensure that all important 

topographic and bathymetric features that influence flow dynamics within the Boston 

Central Artery/Tunnel system were captured. 
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The site-specific, fine mesh was developed to delineate the centerlines and banks of waterways 

within the model domain using a rather painstaking, manual method. In some areas, the resolution 

of the model is approximately 10 ft. Figure 14.15 shows how highly-resolved the mesh is in a 

portion of the downtown area. 

Figure 14.15. Example of the fine, highly-resolved hydrodynamic model mesh used in the 

Boston Central Artery/Tunnel vulnerability assessment study (from Douglas 2015; permission to 

use image provided by MassDOT)

14.8.3.4 Calibration and Validation of the Storm Surge Model

The models used in the Boston case study project (ADCIRC, SWAN) are rooted in sound science 

and utilize standard governing equations of water motion. However, the propagation of water 

through a unique geographic setting results in site-specific variations that may need adjustment 

of model parameters, called calibration, to more accurately represent the real-world system. For 

example, in an urban landscape, an area consisting of numerous buildings will influence flow 

differently than a marsh, which will influence flow differently than a parking area, which will 

influence flow differently than an estuary. For these types of cases, it is reasonable to adjust 

parameters, such as frictional factors, within accepted bounds to better represent the water 

propagation. The Boston model was calibrated using both normal tidal conditions and a 

representative storm event for the northeast US. The “Blizzard of 1978” was used for the storm 

calibration. That storm was a slow-moving Nor’easter on Feb 6-7, 1978. It caused record-setting 

flood levels from Provincetown, Massachusetts to eastern Maine. 
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The calibrated model was then validated using another historical storm, the “Perfect Storm of 

1991.” This validation used the same model parameters (e.g. bottom friction, diffusivity, etc.) as 

used to simulate the Blizzard of 1978. The model validation also represents a different type of 

storm. While the calibration event (Blizzard of 1978) was a purely extra-tropical event, the 

validation event (Perfect Storm of 1991) was a hybrid of a tropical and extra-tropical event (see 

Webb 2017 for an explanation of the calibration and validation steps in proper coastal 

hydrodynamic modeling). 

Finally, the calibrated model was used to simulate the full range of storm events (both hurricanes 

and Nor’easters) and sea level rise conditions using a Monte Carlo statistical approach which 

accounted for tidal variation. 

14.8.3.5 Inundation Mapping

An objective of the study was to assess the vulnerability of the Central Artery/Tunnel system to 

RSLR and extreme storm events. One way this was achieved was by mapping coastal flooding 

in a quantified, probabilistic framework. 

Figure 14.16 is one example of the inundation maps developed. The color scale shows the 

probability of flooding in these areas ranges from the 0.1%-risk storm (i.e. the 1,000-year storm) 

to the 100% risk level (i.e. the bay or the ocean that is always flooded). This example is for 

2070/2100 conditions, i.e. with 3.4 ft of RSLR from present-day conditions. 

Figure 14.17 shows another, more focused, example of a flood mapping product from the Boston 

vulnerability assessment study. The probability of flooding in the area of Christopher Columbus 

Park near Long Wharf is shown on an overlay of an oblique aerial image. This example is for 

2030 conditions, i.e. with 0.68 ft of RSLR from present-day conditions. 

14.8.3.6 Adaption Engineering Implications

The results of the modeling used in the Boston case study can inform a wide variety of engineering 

decisions related to adaptions. The flood levels at the specific location of transportation assets 

are quantified with depth-frequency curves (e.g. 2-year to 1,000-year return period depths). The 

changes in these curves with sea level rise are also estimated. 

Many other highly detailed, site-specific analyses are results of the modeling. One example is 

demonstrated in Figure 14.18 and Figure 14.19. Specifically, these figures are focused on the 93 

Granite Ave. site in Milton, Massachusetts. This location is currently home to the MassDOT Fuel

Depot Complex and is also being considered for the potential future residence of the primary 

MassDOT maintenance facility. As such, this location represents an important site for MassDOT 

both from a current operational perspective and from an engineering design and future use 

perspective. The two figures evaluate the site in the present day (2013 for this study) and the 

near-term future (2030). 

Figure 14.18 shows the present day, i.e. 2013, flooding for this area. The dashed black line shows 

the parcel of interest, while the solid black lines show the existing structures. The map shows the 

present-day flooding depths corresponding to the 1% flooding probability level (areas of 1% 

probability or greater). Depths of flooding are generally small for present-day, with depth of water 

in the parcel of approximately 6 inches and restricted to the southern parking area and the two 

southern buildings. Accessibility to the site (via Granite Ave.) remains viable for the 1% return 

period water level in present-day conditions. 
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Figure 14.16. Example of the inundation map products from the Boston Central Artery/Tunnel 

Vulnerability Study. The exceedance probability of coastal flooding in 2070/2100 (from Bosma 

et al. 2015; permission to use image provided by MassDOT)

Figure 14.17. Example of coastal flood mapping results from the Boston Central Artery/Tunnel 

vulnerability study focused on the area around Long Wharf and Christopher Columbus Park 

(from Bosma 2019; permission to use image provided by MassDOT)
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Figure 14.18 also shows the residence time of the flooding and flood pathways to the site. The 

residence time gives an indication of how long the flooding is expected to last for the 1% 

probability. This type of information can only be obtained from a coastal hydrodynamic model 

such as those described here for a Level 2 or Level 3 analysis. For present-day (2013), the 

residence time of the flooding is 7.33 hours. In other words, the flooding remains at the site for 

7.33 hours before it recedes (and peaks at 0.5 ft). The figure also shows the two local flood 

pathways that influence the area. The flood pathway to the north originates in a small marsh creek 

that allows water to propagate landward and flood into the local neighborhood and road system. 

The flood pathway to the south is the low-lying wetland area that connects further to the south to 

the Neponset River. 

Potential adaptations could consider local measures (e.g. raising the elevations of the buildings 

on the parcel, flood proofing structures, local on-site berms or walls) or more regional approaches 

(e.g. berms, tide gates, flood walls, etc.) at the source of the flooding for the area that would not 

only serve to protect the 93 Granite Ave. site, but also other infrastructure (e.g. roads, homes, 

etc.). 

Looking forward to 2030, Figure 14.19 presents the depths for the 1% flooding probability, which 

have now increased to an average of 1.5 ft for a good portion of the parcel, while also showing 

inhibited accessibility to the site via Granite Ave. The entire parcel has depths of at least 0.5 ft, 

and depths reach 2 ft. The residence time is now 10 hours indicating access to the site would be 

unavailable for that length of time. The pathways of flooding remain the same and I-93 is at a high 

enough elevation to remain unaffected in this area, as well as to provide a barrier to flooding. This 

increased risk of flooding at this location gives an indication that, at minimum, careful engineering 

approaches and planning should be taken if the primary maintenance facility is to be relocated to 

this site. 
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Figure 14.18. Model results showing depth for a 1% flooding probability in 2013 at the 93 

Granite Ave. location, as well as residence time and local flood pathways (from Bosma et al.

2015; permission to use image provided by MassDOT)
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Figure 14.19. Model results showing depth for a 1% flooding probability in 2030 at the 93 

Granite Ave. location, as well as residence time and local flood pathways (from Bosma et al. 

2015; permission to use image provided by MassDOT)
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Chapter 15 - Adaptation Strategies for Coastal 

Highways

The best coastal highway infrastructure adaptation strategies for coping with extreme events and 

future sea level rise are most likely forms of coastal engineering and planning already utilized for 

resilience today. Because society has long lived along the coast, coastal engineering is an ancient 

field. However, the challenges facing today’s engineers and planners are greater than ever. 

Infrastructure strategies will need to confront the on-going societal migration to coastal areas and 

sea levels projected to rise to levels society has never experienced. 

This chapter briefly summarizes the potential coastal damage mechanisms and adaptation 

strategies for highways in the coastal environment to extreme events and sea level rise. Many of 

the technical issues summarized in this chapter are discussed in more detail elsewhere in this 

document. 

15.1 Typical Coastal Damage Mechanisms

This section discusses some typical coastal damage mechanisms including damage to roadways, 

bridges, rails, and tunnels:

• roadway damage by wave attack,

• roadway and railway damage by coastal “weir-flow,”

• roadway damage by bluff erosion and shoreline recession,

• bridge deck damage by waves on surge,

• structure damage by wave runup,

• loss of function due to more frequent and extensive flooding

• tunnel and road damage by overtopping, and

• damage by tsunamis. 

The vulnerability of coastal transportation assets to these damage mechanisms will increase with 

sea level rise (Jacobs et al. 2018b). There is limited information on the details of the “sensitivity” 

levels for most of these coastal damage mechanisms in the coastal engineering community. More 

research is needed in this area to improve our ability to assess the vulnerability of our existing 

coastal transportation system. 

15.1.1 Roadway Damage by Wave Attack

Many coastal roads are on constructed embankments or natural bluffs that can be eroded by 

wave action during extreme events. These embankments are often damaged to the extent that 

the roadway pavement is undermined and damaged. Embankments used as approaches to 

coastal bridges are particularly susceptible to this wave attack damage in extreme coastal events 

as illustrated in Figure 15.1. In this case, the peak storm surge allowed waves to attack an 

embankment not designed for those wave conditions. Note the undamaged bridge over a coastal 

water body in the background and the undermined pavement in the foreground. 

This susceptibility of the approaches occurs in areas where they are at higher elevations than 

most of the roadway due to waterway clearance issues. Apparently, the higher elevation subjects 
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these portions of the embankment to direct action of larger wave heights while nearby lowerelevation embankments are submerged beneath much of the wave action (Douglass et al. 2004). 

The damage can be complete or partial depending on the duration of the storm surge and the 

design of the embankment. The damage is common on the side of the embankment exposed to 

large waves during the coastal storm. 

The sensitivity of roadways to wave attack depends on the storm characteristics at the specific 

location (surge level and duration, wave heights, etc.) as well as the embankment 

condition/design (grass, exposed sand, slope protection, etc.). The single most important storm 

parameter is likely wave height at the embankment. For very small waves, embankments 

designed for rainfall events will survive coastal storms for the duration of a storm surge. In other 

words, the embankment will be inundated but not damaged if there are only very small waves and 

very small flow velocities. However, if wave heights are of any significant height, say H > 0.5 ft, 

most soil embankments will erode. Chapter 7 summarizes proper design of coastal revetments 

for wave attack. The fundamental tool is Hudson’s equation for sizing the revetment armor stone 

as a function of wave height, slope, etc. Hudson’s equation is based on a low level of damage (5 

percent of the armor stones moving during design conditions). The USACE Coastal Engineering 

Manual (USACE 2002) presents methods to evaluate higher percentage levels of revetment 

damage as well as other methods for designing coastal revetments. 

Figure 15.1. Partial embankment damage caused by waves on storm surge during Hurricane 

Ivan (Pensacola, Florida, 2004) 

Embankments used as approaches to coastal bridges are particularly 

susceptible to wave attack damage
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15.1.2 Roadway and Railway Damage by Coastal “Weir-Flow”

Storm surge waters flowing across the road and down an embankment can cause damage as 

illustrated in Figure 15.2. In this photograph of US 98 (Okaloosa Island, Florida) taken in 2005, 

the Gulf of Mexico is to the right and storm surge elevation is just slightly greater than the roadway 

elevation. This is the coastal “weir-flow” damage mechanism described in detail in Chapter 10, 

Highway Overwashing. Much of this damage to road pavements is on the landward side of 

the road and not the seaward side of the road. Waves on the flowing surge waters can increase 

the scour potential on the landward side of the road significantly. Two related damage 

mechanisms occur as storm surge recedes: weir-flow damage on the seaward side of the 

roadway as surge waters flow back out to sea late in the storm, and scour of the embankment 

caused by flow parallel to the road as water moves to “breaches” or lower spots in the road as 

the storm surge recedes. 

Figure 15.2. Example of the coastal weir-flow damage mechanism as it occurs (photograph 

provided by FDOT, circa 2005) 

The sensitivity of roadways to damage from the weir-flow damage mechanism depends on both 

the hydraulics and the embankment conditions (slopes, elevations, cover). However, there is very 

little information on the rate of damage of roadway embankments to the coastal weir-flow damage 

mechanism. Both laboratory tests and field observations indicate that embankment shoulders of 

unvegetated loose sand (common in many coastal areas) erode with even minimal overtopping 

flow (see Section 10.2.1). Vegetated shoulders and embankments made of compacted sub-soils 
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will erode slower and thus may experience less than total pavement damage if the storm 

conditions, particularly surge duration, are mild enough. 

Railway embankments can experience the same damage mechanisms as roadway 

embankments: direct wave attack and the weir-flow mechanism. Figure 15.3 shows damage to a 

railway embankment which occurred during Hurricane Katrina. The “sediment’ here was not 

unconsolidated sand but unconsolidated gravel (track “ballast”). The incoming or rising-limb storm 

surge flowed across the tracks from left to right in this photograph. The damage was primarily due 

to the weir-flow mechanism as the storm surge elevations exceeded the rail elevations. 

Figure 15.3. Railway embankment damage caused primarily by the coastal weir-flow damage 

mechanism during (Mississippi, 2005)

15.1.3 Roadway Damage by Bluff Erosion and Shoreline Recession

Roadways along many coasts are threatened and damaged by coastal bluff erosion and shoreline 

recession. Usually, the damage occurs during an extreme event like a hurricane or northeaster. 

However, often, shoreline erosion had previously increased the vulnerability of the roadway due 

to other longer-term processes. Figure 15.4 shows an example of a highway (North Carolina SH 

12, the Atlantic Ocean is to the left in the photograph) damaged in the latter stages of Hurricane 

Ida (2009). The roadway is located along a receding shoreline. That recession had progressively 

increased the vulnerability of this highway prior to the hurricane. 

Understanding the underlying geomorphological framework can be valuable for the highway 

engineer. This type of damage will continue to occur as sea levels rise. Essentially this barrier 

island is rolling over itself as a form of migration toward the mainland in response to RSLR. The 

roadway is now on the beach face. These phenomena are long-term geological processes that 

impact coastal highways and should be considered in design through engineering or siting. The 
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rate of bluff and shoreline erosion at different locations is a function of the site-specific geology 

and coastal processes. Chapter 8 discusses general considerations in this situation. Some states 

have developed detailed maps of the coastal hazards of bluff erosion which can be used as a 

basis for evaluating the increased vulnerability to extreme events and sea level rise (see e.g. 

Revell et al. 2011). 

Figure 15.4. Pavement damage due to waves and surge in an extreme event (North Carolina 

SH 12, 2009; permission to use photograph provided by Dave Henderson)

15.1.4 Bridge Deck Damage by Waves on Surge

Bridges over coastal waters can be severely damaged and destroyed by wave-induced loads if 

storm surge allows the wave to strike the bridge deck (Figure 15.5). Section 11.2 discusses this 

damage mechanism, loads induced by repeated individual waves striking the bridge 

superstructure. 

Figure 15.5 shows two bridges destroyed by wave loads in Hurricane Katrina. The bridge on the 

left (US 90) was a wider replacement for the bridge on the right (which was used for fishing only 

for years prior to Katrina). The newer bridge was designed at about the same elevation as the 

older bridge despite the older bridge being severely damaged in Hurricane Camille in 1969. This 

sequence of events implies that: 

• a correlation between surge/wave height and vulnerability exists (note the undamaged 

spans at higher elevations on the left bridge), 

• bridges should have a service life at least of 75-years; building a replacement bridge at 

the same low chord elevation as the damaged bridge resulted in a service life of only 36 

years, 
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• design standards likely will evolve as we learn more about exposure and vulnerability 

along the coast (Indeed, the Guide Specifications of AASHTO 2008 were developed 

specifically for this situation shortly after Hurricane Katrina), and

• sea level rise will complicate these correlations. 

Figure 15.5. Two bridges destroyed by wave loads in Hurricane Katrina (Biloxi Bay, Mississippi)

Sensitivity to this damage mechanism depends primarily on the wave heights which can strike 

the structure during the peak of the storm and the relative elevation of the storm surge as 

compared to the bridge elevation (see Chapter 11). Most bridge decks can survive wave-induced 

loads if just the crests of the largest wave heights are striking the bridge deck. Most damage 

occurs when the storm surge (still water level) elevation is at or slightly above that of the low chord 

of the bridge deck. At this condition, the full waves can strike the rigid structures and the loads 

can be extremely high with each individual wave. Laboratory experiments have found the wave 

loads to be as much as three times the dead weight of bridge decks when the storm surge (SWL) 

is very near or just above the bridge deck elevation (Douglass et al. 2007). 

Evaluating the vulnerability of bridge decks to extreme events and sea level calls for estimating 

the effect of storm surge, wave heights and sea level rise on the wave loads. Sensitivity of specific 

bridges to wave-induced loads can be evaluated using available methods for estimating those 

loads and comparing those loads with the structural resistance (weight and connections) to those 

loads (e.g. FHWA 2017a). Sea level rise will increase the vulnerability of many existing 

coastal bridges requiring more research into the methods for estimating and reducing waveinduced loads for vulnerability assessment and adaptation planning. Wave loads on bridge decks 

are extremely sensitive to the storm surge elevation and thus extremely sensitive to sea level rise 

(see Chapter 14). 
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15.1.5 Structure Damage by Wave Runup 

Wave runup is the uprush of a wave’s water up a beach (Section 5.5) or structure (Section 7.5). 

Wave runup causes high velocity flows in individual waves to flow above the storm surge still 

water elevation where it can cause structural damage and scour. FEMA (2011b) and USACE 

(2002) provide information for estimating loads on structures (buildings) in the coastal flood plain. 

These loads include breaking and broken wave-induced loads, hydrostatic and buoyancy loads, 

and flow-induced hydrodynamic loads. Much of the load information is a function of local depth 

and/or local, depth-limited, wave height. 

The sensitivity of damage caused by wave runup to extreme events and sea level can be 

evaluated by estimating the effects of storm surge, wave effects of storm surge, wave heights and 

sea level rise on the loads and the extent of runup. In particular, the damage caused by wave 

runup will increase as sea levels rise and storm surge increases. 

15.1.6 Tunnel and Road Damage by Overtopping

Hurricane Sandy flooded some highway and transit tunnels when the storm surge elevation 

exceeded the elevation of the tunnel entrances, or portals. The basic damage mechanism of 

overtopping is inundation resulting from flow over the portal walls and down entrances. Tools to 

estimate the hydraulics of such flows are available in hydraulic engineering textbooks. Several 

specific damage mechanisms include damage to electrical systems, ventilation systems, and 

blockage of the tunnel itself. The sensitivity of a specific tunnel to this damage is related to the 

storm characteristics and the tunnel portal design. The relative elevations of the storm surge and 

the portal walls are important. A form of a detention pond analysis can be used to evaluate the 

internal level of flooding in a tunnel for specific storms. Often a small amount of flood water will 

not damage tunnels as they are designed for some rainwater drippage off vehicles and cleaning. 

Tunnels may use pumping system to remove these water sources; however, the larger quantities 

of surge into the portals may overwhelm the capacities of these pumps (if the event hasn’t already 

resulted in a loss of power to those pumps). 

A related issue for some tunnels and many coastal highways is flooding due to wave 

overtopping. Wave overtopping is water splashing over a seawall when runup exceeds the 

elevation of the top of the wall. Wave overtopping onto coastal roads is fairly common during 

storms in many parts of the US. Two important aspects of overtopping are the time-averaged 

volumetric rate of overtopping and the intensity or force of a single wave overtopping event (see 

Section 7.5). Overtopping rate is a function of freeboard (the elevation difference between the 

storm surge still water level and the wall crest) and wave height. It is particularly sensitive to storm 

surge level in extreme events and thus, will be extremely sensitive to sea level rise. 

15.1.7 Damage by Tsunamis

Tsunamis damage transportation assets through extreme hydrodynamic loads, scour, and debris 

impact loads. The hydrodynamic forces include hydrostatic forces, buoyant forces, drag forces, 

surge or impact forces (Nistor et al. 2010). Bridges and roadways on embankments both suffer 

damage in tsunamis. The 2011 Japan tsunami destroyed bridges through a variety of damage 

mechanisms including hydrodynamic loads removing bridge decks (much like the hurricanes in 

the southeastern US) or damaged substructure. Some lower elevation bridge decks survived 

while higher elevation decks did not (Chock et al. 2013). Many bridge approach embankments 

were damaged by scour due to water flowing over the embankment (Yashinsky 2011), which 

appears to be very similar to the weir-flow damage mechanism common in storm surge previously 

described. 
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At the time of the preparation of this document, a multi-state and federal study is developing 

bridge design guidelines for the estimation of tsunami loads on highway bridges (Transportation 

Pooled Fund Program 2019). 

15.2 Adaptation Strategies for Coastal Highway Infrastructure

This section discusses possible adaptation strategies for coastal transportation infrastructure to 

extreme events and sea level rise. Adaptation means preparing for the impacts on the nation’s 

transportation infrastructure and systems. Specifically, it refers to planning, designing, 

constructing, operating, or maintaining transportation infrastructure while incorporating 

consideration of extreme events and sea level rise. Adaptation strategies may include providing 

protective countermeasures intended to prevent, delay or reduce the severity of problems related 

to extreme events and sea level. 

Adaptation strategies to respond to coastal infrastructure problems related to extreme events and 

sea level rise may be categorized as follows: 

• Manage and maintain 

• Increase redundancy 

• Protect 

• Accommodate 

• Relocate 

Combinations of these strategies may also be employed. The next five sub-sections discuss these 

types of strategies related to highways in the coastal environment. 

15.2.1 Manage and Maintain

One category of adaptation strategies is to maintain existing infrastructure for optimal 

performance and manage the response to extreme events through advanced preparation.

Examples include: 

• Increase and target maintenance activities (e.g. culvert cleaning, sand sweeping). 

• Relocate movable assets prior to forecast storm events (e.g. moving maintenance 

vehicles and equipment to less vulnerable areas). 

• Enhance and practice emergency procedures including vulnerable asset closures. 

• Stockpile and strategically place fuel, temporary bridges, and road construction materials 

for quick deployment. 

Coastal SDOT’s already use these, and other, forms of specialized management and 

maintenance to prepare for - and in response to - coastal storms. These procedures have been 

developed internally and in coordination with other federal, state and local entities to maintain and 

restore service in response to coastal storms and flooding. 

15.2.2 Increase Redundancy

Another general category of adaptation strategy is to increase the redundancy of the system. 

This means ensuring that transportation services provided by infrastructure can be supplied by 

other means/alternatives. Examples include: 
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• Identify and enhance, as appropriate, alternative roads, routes, or modes to serve 

transportation needs during times of compromised service (e.g. enhanced ferry service). 

• Consider constructing or enhancing closely spaced roads versus one road. 

Coastal transportation organizations often address the redundancy issues such as ferries in 

planning-level activities in the wake of or in preparation for storm events. 

15.2.3 Protect

A common category of adaptation strategies is to protect the existing system. The goal is 

to reduce or eliminate damage by providing protective physical barriers to extreme events. This 

may include “hard” structures such as seawalls and “soft” strategies such as nature-based 

solutions. Protective strategies keep water away from infrastructure or provide resistance to the 

damaging forces of water and waves. Examples include: 

• New or enlarged seawalls, bulkheads, revetments, etc. (hard structures to provide barriers 

or resistance to damaging forces), 

• Beach nourishment (soft strategy to move water away from infrastructure),

• Dune restoration and vegetation (soft strategies to provide barriers and resistance to 

damaging forces), or

• Living shorelines along sheltered coasts (combination of hard and soft 

structures/strategies to increase resistance to damaging forces). 

“Hard” engineering structures have been used to protect many coastal transportation assets. 

These include seawalls, bulkheads, revetments, breakwaters, groins, and jetties. These 

structures protect many miles of coastal highways today. The functional design purpose of 

different types of coastal structures is explained in Chapter 7. That material on the structural 

design of coastal revetments can be easily adapted for future RSLR. USACE (2002) presents 

more information on the design of seawalls and revetments. The most common successful type 

of coastal revetment for wave attack is a rubble-mound structure consisting of an engineered 

slope or pile of rocks (or other armor units). Because the rocks can move, there is an inherent 

resilience in these structures as compared with completely rigid structures. As sea level rises, 

these solutions should be considered in more situations to protect assets that are not now 

threatened. Many highway revetments were built in immediate response to a specific storm but 

the vulnerability prior to the storm had been increasing for years due to longer term processes of 

shoreline recession and sea level rise. Also, existing seawall and revetment structures may have 

to be modified to withstand higher wave conditions as sea level rises. This may include increasing 

their height and include placement of new layers of rock to strengthen existing seawalls as well 

as construction of new seawalls (Ewing 2018). 

“Soft” engineering includes most of the nature-based solutions discussed in Chapter 8 such as 

beach nourishment, dune restoration, and marsh creation. The general goal is to use 

engineering that more closely mimics the natural systems in the vicinity. Beach 

nourishment is the direct placement of large amounts of sand on the beach to widen the beach 

(see Chapter 8). Beach nourishment is a commonly used adaptation to sea level rise and has 

been successfully used to stabilize recessional shorelines in many places in the US (NRC 1995, 

Douglass 2002, Houston 2018). Beach nourishment with sand dune restoration also reduces 

damage to landward infrastructure. For example, beach communities in New Jersey with 

nourishment and constructed sand dunes suffered significantly less damage in Hurricane Sandy 

than other nearby communities (Houston and Dean 2013). Beach nourishment and dune 

restoration was used extensively along the New Jersey and New York shore in response to 
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Hurricane Sandy to support the long-term sustainability of the coastal ecosystem and 

communities as well as to reduce the economic costs and risks associated with future storm 

events (US Congress 2013). 

Modern coastal engineering shoreline stabilization solutions often combine beach nourishment 

with coastal structures. These solutions are “hybrid” solutions using components of the “soft” and 

“hard” approach. The primary purpose of the structure is to retain the sand. Some of these “hybrid” 

solutions emulate natural geomorphology features such as pocket beaches between headlands 

where the sand nourishment is placed between constructed headland breakwaters (see Chapter 

8). During Hurricane Sandy, the presence of a relic stone seawall buried beneath a nourished 

dune and beach reduced wave forces by a factor of two, effectively protecting the upland 

infrastructure, including a coastal highway (Smallegan et al. 2016). 

The term “soft engineering” can also include “green” infrastructure engineering, or “nature-based 

solutions, such as wetland creation and living shorelines. Living shorelines are combinations of 

structure, vegetation, and sand (in some cases) to stabilize the shoreline and provide nearshore 

habitat to allow native species of flora and fauna to flourish. They are particularly attractive 

alternatives to traditional bulkhead or revetment structures along sheltered bay and river 

shorelines. More research is needed on the engineering of these green coastal infrastructure 

approaches. 

The FHWA developed “Nature-Based Solutions for Coastal Highway Resilience: An 

Implementation Guide” (Webb et al. 2019). The FHWA designed this implementation guide to 

help transportation practitioners understand how and where nature-based solutions can be used 

to improve the resilience of coastal roads and bridges. 

15.2.4 Accommodate

Another category of adaptation strategy is to accommodate. The infrastructure is modified or 

redesigned to better coexist in the future environment. Examples include: 

• Increasing bridge deck elevations and strengthen bridge structures,

• Lowering roadway profiles to allow overwash without pavement damage during extreme 

events, or

• Raising tunnel portal walls to reduce likelihood of flooding. 

Careful assessment of the vulnerability of the infrastructure to extreme events and sea level is 

needed to develop cost-effective accommodation strategies. An understanding of magnitudes, 

probabilities, and uncertainties of projected stressors such as sea level rise and extreme events 

is particularly important when considering this strategy for the long-term extension of the design 

life of infrastructure assets. Accommodation strategies may also be used for short- or mediumterm design horizons. 

Increased elevation is the only proven adaptation option for coastal bridges subject to 

wave attack during extreme events. Several of the major bridges destroyed by hurricanes in 

the southeastern US were replaced with new bridges elevated much higher to avoid those wave 

loads in extreme events. These included: 

• I-10 bridge over Escambia Bay near Pensacola, Florida,

Modern coastal engineering solutions often combine some “hard” structures 

with some “soft” or nature-based approach.
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• I-10 bridge over Lake Pontchartrain near Slidell, Louisiana,

• US 90 bridge over Bay Saint Louis, Mississippi (see Figure 15.6), and

• US 90 bridge over Biloxi Bay, Mississippi. 

Figure 15.6. New, much higher, US 90 bridge across Bay St. Louis, Mississippi built after 

Hurricane Katrina (June 14, 2007) 

Another oft-discussed option is to increase the connection strength to the bridge substructure. 

This approach, however, will transfer those loads to the substructure and foundation, so care 

should be taken to evaluate wave-induced load failure mechanisms such as negative bending of 

the deck, pile bending or shear failure, failure of the pile to bent cap connections, and possible 

soil failure around the foundation (Douglass et al. 2006a, Robertson et al. 2011, FHWA 2017a). 

More research is needed on wave-induced loads for use in design of bridges at low enough 

elevations to be inundated by storm surge and waves. 

Increased elevation is an adaption option for seawalls that protect roadways and tunnel 

entrances. Increasing the elevation of the roadway is an option for those roads that flood because 

of wave runup and overtopping and are already protected by seawalls or revetments. Temporary 

doors or storm surge barriers have been used at some tunnel portals for many years and the 

further development of these has been suggested as an adaptation for coastal tunnels that can 

flood in extreme events. More research into effective tunnel options is needed. 

15.2.5 Relocate

The final category of adaptation strategies is relocation (see Section 8.4). The goal is to lessen 

or eliminate exposure to coastal stressors by relocating infrastructure away from the coastline. 

This could be in conjunction with disinvestment, repurposing, abandoning or removing existing 

exposed infrastructure. Examples include: 
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• Relocating infrastructure further inland away from the coastline,

• Repurposing or reclassifying paved road to all-terrain vehicle road, or

• Reconditioning a damaged vehicular bridge to serve as a pedestrian bridge or fishing pier. 

Relocation or abandonment has long been a common response to shoreline recession and bluff 

erosion. Recession and erosion are natural processes which become a problem primarily when 

infrastructure is threatened. For example, storm-related bluff erosion led to a relocation of a 

portion of the coast-parallel California 1, the Pacific Coast Highway, in April 2013. Chapter 8 

discusses other roads that have been relocated in the past several decades. This includes one 

section of a coast-parallel road in Texas that was abandoned after storm damage in 1989. At that 

location, relocation of the roadway landward away from the Gulf of Mexico was constrained by 

the ecological value of the coastal wetlands in a National Wildlife Refuge. Similar issues, the 

ecological value of natural wetland and barrier island habitat, led to consideration of relocating 

miles of NC 12 from that barrier island onto a bridge in the bay (FHWA 2010). Portions of coastal 

road have already been relocated in response to storm damage as shown in Figure 15.7. At some 

locations, relocation is constrained by private property. 

The USACE (2014) presents that agency’s guidance concerning adaptation strategies for 

incorporating sea level change in the planning of coastal works. It addresses a number of 

concerns that transportation organizations face with coastal infrastructure including discussions 

about appropriate time horizons and ranges of scenarios for planning. 

In alignment with 23 CFR § 515 (Asset Management Plans), for transportation assets vulnerable 

to extreme events and sea level rise, a SDOT might consider all the adaptation options, including 

combinations thereof: manage and maintain, increase redundancy, protect, accommodate, and 

relocate, as part of their life-cycle planning and risk identification efforts. For example, some 

combination of relocation, accommodation (lowering of roadway), and protection (dune 

construction and a buried revetment on the embankment) have been proposed and constructed 

for some coast parallel roads that have been damaged by frequent overwashing such as shown 

in Figure 15.7. This approach is a reasonable adaptation to sea level in these situations. These 

types of decisions have been made throughout history along the coast in both real-world 

applications and theoretical analyses. They will likely have to be made more often by coastal 

engineers in coordination with the transportation engineering community as sea levels rise in the 

coming decades. 
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Figure 15.7. Example of coastal highway relocation in Florida (SH 399: the dashed line shows 

the previous highway location: permission to use image provided by the Gulf Islands National 

Seashore)
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Appendix – Workshop Participants

Peer exchange meetings and workshops were held during the development of this 3rd edition. 

The participation by these professionals is gratefully acknowledged. 

A one-day peer exchange meeting in March 2018 in Washington, DC provided initial input into 

the overall revisions for this 3rd edition. Participants in that peer exchange included Mary Cialone, 

Elizabeth Habic, Maria Honeycutt, David Kriebel, Matt Lauffer, Patrick Lynette, Mark Osler,

Elizabeth Sciaudone, and William Sweet.

A day and a half meeting at the Turner-Fairbank Hydraulic Research Center in May 2017 focused 

on coastal highway flooding issues with emphasis on the combined effects of sea level rise and 

rainfall runoff. Participants in that workshop included Elizabeth Habic, Rick Renna, and Carlton 

Spirio. 

A series of four one-day workshops were held in March and April 2018 focused on emerging 

issues associated with sea level rise. The workshops were facilitated by FHWA and hosted by 

SDOTs. These workshops informed the content of Chapter 9 Increased Flooding due to Relative 

Sea Level Rise. Participants included:

• Hanover, Maryland: Brian Cox, LaTonya Gilliam, Gina Goettler, Robert Graff, Elizabeth 

Habic, Matt Lauffer, Joy Liang, Peter Mattejat, Andrew McDaniel, Michael Michalski, 

Karuna Pujara, William Seiger, Eileen Singleton, Steve Sisson, Ryan White, and Russ 

Yurek, 

• Tallahassee, Florida: Jenna Bowman, Jennifer Carver, Kevin Claridge, Samantha 

Danchuk, Mario Dominguez, Katey Earp, Whitney Gray, Rick Jenkins, Chase Knight, 

Kenneth Konyha, Abderrahmane Maamar-Tayeb, Steve Nolan, Jayantha Obeysekera, 

James Poole, Carlos Ribbeck, Ricardo Salazar, Carlton Spirio, and Georgio Tachiev, 

• Olympia, Washington: Luke Assink, Craig Boone, William Fletcher, Peggy Fox, Andy 

Haub, Julie Heilman, John Himmel, Mike Knapp, Casey Kramer, Sharon Love, Mark 

Mobbs, Gregor Myhr, Simon Page, Carol Lee Roalkvam, Jeff Rudolph, Steve Russel, Alex 

Steely, James Struthers, Babbak Talebi, Theresa Turpin, and Susan Wimberly

• Boston, Massachusetts: Michael Arpino, Stephen Bartha, David Belange, Peter Collette, 

Teresa Crean, Jason Dvelis, Hanan Fouad, Mark Griffin, Charlie Hebson, Mike Hogan, 

Kirsten Howard, Jennifer Jacobs, Julia Knisel, Julie LaBranche, Timothy Mallette, Rick 

McCullough, Ellen Mecray, Norm Miller, Steven Miller, Timothy Paris, and Daniel Vieira. 
Appendix – Workshop Participants HEC-25 3rd ed.

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