Guidelines for Design

of Structures for Vertical 

Evacuation from Tsunamis 

Second Edition 

FEMA P-646 / April 2012 

FEMA
 

FEMA P-646 / April 2012

Guidelines for Design of Structures for 

Vertical Evacuation from Tsunamis

Second Edition

Prepared by

APPLIED TECHNOLOGY COUNCIL

201 Redwood Shores Pkwy, Suite 240

Redwood City, California 94065

www.ATCouncil.org

Prepared for

FEDERAL EMERGENCY MANAGEMENT AGENCY 

National Earthquake Hazard Reduction Program 

 

NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION

National Tsunami Hazard Mitigation Program

Michael Mahoney, FEMA Project Officer

Robert D. Hanson, FEMA Technical Monitor

ATC MANAGEMENT AND OVERSIGHT 

Christopher Rojahn (Project Executive)

William L. Coulbourne (Project Manager)

Jon A. Heintz (Project Quality Control Monitor)

William T. Holmes (Project Tech. Monitor)

PROJECT MANAGEMENT COMMITTEE

(SECOND EDITION)

Ian N. Robertson (Project Technical Director)

Gary Chock

John Hooper

Timothy J. Walsh

Harry Yeh

PROJECT MANAGEMENT COMMITTEE 

Steven Baldridge

Frank Gonzalez

John Hooper

Ian N. Robertson

Timothy J. Walsh

Harry Yeh

PROJECT REVIEW PANEL

John Aho

George Crawford

Richard Eisner

Lesley Ewing

Michael Hornick

Christopher P. Jones

Chris Jonientz-Trisler

Marc L. Levitan

George Priest

Charles W. Roeder

Jay Wilson
Notice

Any opinions, findings, conclusions, or recommendations expressed in this publication do not necessarily 

reflect the views of the Federal Emergency Management Agency (FEMA), the National Oceanic & 

Atmospheric Administration (NOAA), or the Applied Technology Council (ATC). Additionally, neither 

ATC, FEMA, NOAA, nor any of their employees, makes any warranty, expressed or implied, nor 

assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any 

information, product, or process included in this publication. Users of information in this publication 

assume all liability arising from such use.

Cover Images: Photographs showing various examples of potential vertical evacuation structures. Clockwise from top left: (1)

designated vertical evacuation building in Kesennuma Port, Japan, where numerous residents found safe refuge at the roof level

during the 2011 Tohoku tsunami; (2) sports complex where large numbers of people could gain easy access to elevated concourse 

and seating levels; (3) multi-level cast-in-place reinforced concrete parking garage in Biloxi, Mississippi, that survived storm 

surge inundation during Hurricane Katrina; and (4) earthen mound with ramp access to a safe elevation. Photographs provided 

courtesy of Ian Robertson, University of Hawaii at Manoa and Magnusson Klemencic Associates, Seattle, Washington.
Foreword

This publication was equally funded by the National Oceanic and 

Atmospheric Administration (NOAA), which leads the National Tsunami 

Hazard Mitigation Program (NTHMP) and by the Federal Emergency 

Management Agency (FEMA), which is responsible for the implementation 

portion of the National Earthquake Hazard Reduction Program (NEHRP). 

FEMA initiated this project in September 2004 with a contract to the Applied 

Technology Council. The project was undertaken to address the need for 

guidance on how to build a structure that would be capable of resisting the 

extreme forces of both a tsunami and an earthquake. This question was 

driven by the fact that there are many communities along our nation’s west 

coast that are located on narrow spits of land and are vulnerable to a tsunami 

triggered by an earthquake on the Cascadia subduction zone, which could 

potentially generate a tsunami of 20 feet in elevation or more within 20 

minutes. Given their location, it would be impossible to evacuate these 

communities in time, which could result in a significant loss of life. Many 

coastal communities subject to tsunami located in other parts of the country 

also have the same potential problem. In these cases, the only feasible 

alternative is vertical evacuation, using specially design, constructed and 

designated structures built to resist both tsunami and earthquake loads. 

The significance of this issue came into sharp relief with the December 26, 

2004 Sumatra earthquake, the Indian Ocean Tsunami, and the March 11, 

2011 Tohoku Japan Tsunami. While these events resulted in a tremendous 

loss of life, this would have been even worse had not many people been able 

to take shelter in multi-story reinforced concrete buildings or been able to get 

to high ground sites after the tsunami warning was delivered. Without 

realizing it, these survivors were demonstrating the concept of vertical 

evacuation from a tsunami.

This publication presents the following information:

• General information on the tsunami hazard and its history;

• Guidance on determining the tsunami hazard, including the need for 

tsunami depth and velocity on a site-specific basis; 

• Different options for vertical evacuation from tsunamis;

FEMA P-646 Foreword iii
• Determining tsunami and earthquake loads and structural design criteria 

necessary to address them; and,

• Structural design concepts and other considerations.

This is the second edition of FEMA P-646, originally published in June 

2008. In this second edition revisions were made throughout the document, 

but particularly to the following items:

• Inclusion of observations and lessons learned from the March 11, 2011 

Tohoku tsunami;

• Revision and enhancement of the debris impact expression to remove 

over-conservatism in the prior edition; and

• Updating of all reference documents to the most current version.

FEMA also issued a companion document in 2009, FEMA P-646A, Vertical 

Evacuation from Tsunamis: A Guide for Community Officials, that presents 

information on how the use of this design guidance can be encouraged and 

adopted at the State and local levels.

FEMA is grateful to the original Project Management Committee of Steve 

Baldridge, John Hooper, Ian Robertson, Tim Walsh, and Harry Yeh. We are 

also grateful to the Project Review Committee, the members of which are 

listed at the end of the document, and to the staff of the Applied Technology 

Council. The updates included in this second edition were made thanks to 

Gary Chock, John Hooper, Ian Robertson, Tim Walsh, and Harry Yeh. Their 

hard work has provided this nation with a first document of its kind, a 

manual on how citizens may for the first time be able to survive a tsunami, 

one of the most terrifying natural hazards known. 

– Federal Emergency Management Agency

iv Foreword FEMA P-646
Preface

In September 2004 the Applied Technology Council (ATC) was awarded a 

“Seismic and Multi-Hazard Technical Guidance Development and Support” 

contract (HSFEHQ-04-D-0641) by the Federal Emergency Management 

Agency (FEMA) to conduct a variety of tasks, including one entitled 

“Development of Design and Construction Guidance for Special Facilities 

for Vertical Evacuation from Tsunamis,” designated the ATC-64 Project. 

This project included a review of available international research and stateof-the-practice techniques regarding quantification of tsunami hazard and 

tsunami force effects. 

In 2008, this work resulted in the publication of the FEMA P-646 report, 

Guidelines for Design of Structures for Vertical Evacuation from Tsunamis, 

providing technical guidance and approaches for tsunami-resistant design, 

identification of relevant tsunami loads and applicable design criteria, 

development of methods to calculate tsunami loading, and identification of 

architectural and structural system attributes suitable for use in vertical 

evacuation facilities. In 2009, the companion FEMA P-646A report, Vertical 

Evacuation from Tsunamis: A Guide for Community Officials, was released 

providing information on how to use vertical evacuation design guidance at 

the state and local government levels.

Following its publication in 2008, FEMA P-646 was used in conceptual 

design studies as part of tsunami evacuation planning in Cannon Beach, 

Oregon. It was also used in ongoing research related to the development of 

Performance-Based Tsunami Engineering conducted at the University of 

Hawaii at Manoa, under the George E. Brown, Jr. Network for Earthquake 

Engineering Simulation (NEES). Based on findings from these activities, 

FEMA initiated a follow-up contract, designated the ATC-79 Project, to 

review the design guidance contained in FEMA P-646, and to consider 

updates, if needed, based on this new information. 

As a result of this review, selected revisions were deemed necessary. 

Technical updates contained in this Second Edition of the FEMA P-646 

report are related to: (1) inclusion of observations and lessons learned from 

the March 11, 2011 Tohoku tsunami; (2) revision of the debris impact 

expression to remove over-conservatism deemed to be present in the prior 

edition; (3) additional explanation of the definition of tsunami elevation as it 

FEMA P-646 Preface v
relates to runup elevation used in tsunami force equations; and (4) update of 

reference documents to the most current version.

ATC is indebted to the members of the ATC-79 Project Team responsible for 

the technical development of this Second Edition of the FEMA P-646 report. 

The Project Management Committee, including Ian Robertson (Project 

Technical Director), Gary Chock, John Hooper, Tim Walsh, and Harry Yeh, 

reviewed new technical information relative to guidance contained in the 

original report, and decided on the necessary updates. 

ATC remains indebted to the members of the ATC-64 Project Team who 

participated in the development of the original FEMA P-646 report. The 

Project Management Committee, consisting of Steven Baldridge (Project 

Technical Director), Frank Gonzalez, John Hooper, Ian Robertson, Tim 

Walsh, and Harry Yeh, were responsible for the development of the technical 

criteria, design guidance, and related recommendations. Technical review 

and comment at critical developmental stages were provided by the Project 

Review Panel, consisting of Christopher Jones (Chair and ATC Board 

Representative), John Aho, George Crawford, Richard Eisner, Lesley Ewing, 

Michael Hornick, Chris Jonientz-Trisler, Mark Levitan, George Priest, 

Charles Roeder, and Jay Wilson. The affiliations of all individuals who 

participated in the development of the original and second edition reports are 

provided in the list of Project Participants.

ATC also gratefully acknowledges the input and guidance provided by 

Michael Mahoney (FEMA Project Officer), Robert Hanson (FEMA 

Technical Monitor), William Holmes (ATC Project Technical Monitor), 

William Coulbourne for ATC project management, and Peter N. Mork for 

ATC report production services.

Jon A. Heintz Christopher Rojahn

ATC Director of Projects ATC Executive Director

vi Preface FEMA P-646
Table of Contents

Foreword........................................................................................................ iii

Preface .............................................................................................................v

List of Figures................................................................................................ xi

List of Tables.............................................................................................. xvii

1. INTRODUCTION................................................................................1

1.1 Objectives and Scope ...................................................................1

1.2 Deciding to Construct a Vertical Evacuation Structure................2

1.2.1 Tsunami Hazard versus Risk...........................................2

1.2.2 Decision-Making and Design Process.............................2

1.3 Limitations ...................................................................................4

1.4 Organization.................................................................................5

2. BACKGROUND ..................................................................................7

2.1 General .........................................................................................7

2.1.1 Historic Tsunami Activity...............................................7

2.1.2 Behaviors and Characteristics of Tsunamis ..................10

2.2 Tsunami Effects on Buildings....................................................16

2.2.1 Historic Data on Tsunami Effects.................................16

2.2.2 Observations from the Indian Ocean Tsunami..............19

2.2.3 Observations from the Tohoku Japan Tsunami.............25

2.2.4 Observations from Hurricane Katrina ...........................31

2.2.5 Implications for Tsunami-Resistant Design ..................34

3. TSUNAMI HAZARD ASSESSMENT .............................................37

3.1 Current Tsunami Modeling and Inundation Mapping................37

3.2 The NOAA Tsunami Program: Forecast Modeling and

Mapping .....................................................................................38

3.3 The National Tsunami Hazard Mitigation Program: Credible

Worst-Case Scenarios.................................................................41

3.4 The FEMA Map Modernization Program: Probabilistic

Tsunami Hazard Assessments....................................................44

3.5 Limitations in Available Modeling and Mapping Products.......46

3.6 Hazard Quantification for Design of Tsunami Vertical 

Evacuation Structures.................................................................47

3.7 Recommendations to Improve Tsunami Hazard Assessment ....49

4. VERTICAL EVACUATION OPTIONS .........................................51

4.1 Vertical Evacuation Considerations...........................................51

4.1.1 Single-Purpose Facilities...............................................51

4.1.2 Multi-Purpose Facilities................................................52

4.1.3 Multi-Hazard Considerations........................................53

4.2 Vertical Evacuation Concepts....................................................53

4.2.1 Existing High Ground ...................................................53

FEMA P-646 Table of Contents vii
4.2.2 Soil Berms .................................................................... 54

4.2.3 Multi-Story Parking Garages........................................ 55

4.2.4 Community Facilities.................................................... 55

4.2.5 Commercial Facilities................................................... 56

4.2.6 School Facilities ........................................................... 58

4.2.7 Existing Buildings ........................................................ 58

5. SITING, SPACING, SIZING, AND ELEVATION 

CONSIDERATIONS......................................................................... 59

5.1 Siting Considerations................................................................. 59

5.1.1 Warning, Travel Time, and Spacing ................................ 59

5.1.2 Ingress and Vertical Circulation ...................................... 61

5.1.3 Consideration of Site Hazards ......................................... 62

5.2 Sizing Considerations................................................................ 64

5.2.1 Services and Occupancy Duration ................................... 65

5.2.2 Square Footage Recommendations from Available

Sheltering Guidelines....................................................... 65

5.2.3 Recommended Minimum Square Footage for 

Short-Term Refuge from Tsunamis................................. 67

5.3 Elevation Considerations........................................................... 67

5.4 Size of Vertical Evacuation Structures...................................... 68

6. LOAD DETERMINATION AND STRUCTURAL DESIGN 

CRITERIA ............................................................................................ 69

6.1 Currently Available Structural Design Criteria ......................... 69

6.1.1 Current U.S. Codes, Standards, and Guidelines........... 69

6.1.2 Summary of Current Design Requirements.................. 72

6.1.3 Limitations in Available Flood Design Criteria

Relative to Tsunami Loading........................................ 73

6.2 Performance Objectives............................................................. 74

6.2.1 Tsunami Performance Objective .................................. 75

6.2.2 Seismic Performance Objectives.................................. 76

6.3 Earthquake Loading................................................................... 76

6.3.1 Near-Source-Generated Tsunamis................................ 77

6.3.2 Far-Source-Generated Tsunamis.................................. 77

6.4 Wind Loading ............................................................................ 78

6.5 Tsunami Loading....................................................................... 78

6.5.1 Key Assumptions for Estimating Tsunami Load 

Effects........................................................................... 79

6.5.2 Hydrostatic Forces........................................................ 80

6.5.3 Buoyant Forces............................................................. 82

6.5.4 Hydrodynamic Forces................................................... 83

6.5.5 Impulsive Forces........................................................... 85

6.5.6 Floating Debris Impact Forces...................................... 86

6.5.7 Damming of Accumulated Waterborne Debris............ 90

6.5.8 Uplift Forces on Elevated Floors.................................. 91

6.5.9 Additional Retained Water Loading on Elevated 

Floors............................................................................ 93

6.6 Combination of Tsunami Forces................................................ 94

6.6.1 Tsunami Force Combinations on the Overall

Structure........................................................................ 94

6.6.2 Tsunami Force Combinations on Individual

Components.................................................................. 96

viii Table of Contents FEMA P-646
6.7 Load Combinations....................................................................97

6.8 Member Capacities and Strength Design Considerations ..........98

6.9 Progressive Collapse Considerations .........................................99

6.9.1 Department of Defense Methodology ...........................99

6.9.2 General Services Administration Methodology ..........101

7. STRUCTURAL DESIGN CONCEPTS AND ADDITIONAL 

CONSIDERATIONS .......................................................................103

7.1 Attributes of Tsunami-Resistant Structures..............................103

7.2 Structural Considerations for Tsunami Load Effects...............103

7.2.1 Foundation / Scour Design Concepts..........................104

7.2.2 Breakaway Wall Concepts ..........................................105

7.3 Concepts for Modifying and Retrofitting Existing 

Structures..................................................................................107

7.4 Permitting and Quality Assurance for Vertical 

Evacuation Structures...............................................................108

7.4.1 Permitting and Code Compliance................................108

7.4.2 Peer Review.................................................................108

7.4.3 Quality Assurance / Quality Control...........................109

7.5 Planning Considerations for Vertical Evacuation Structures...109

7.6 Cost Considerations for Vertical Evacuation Structures..........110

APPENDIX A – VERTICAL EVACUATION STRUCTURE 

EXAMPLES FROM JAPAN ..........................................................113

APPENDIX B – COMMUNITY DESIGN EXAMPLE..........................119

B.1 Site 1 Example: Escape Berm ..................................................121

B.2 Site 2 Example: Multi-Use Structure .......................................124

APPENDIX C – EXAMPLE CALCULATIONS....................................129

C.1 Inundation Depth......................................................................130

C.2 Hydrostatic and Buoyant Forces ..............................................130

C.3 Hydrodynamic and Impulsive Forces.......................................132

C.4 Impact Force.............................................................................132

C.5 Damming Effect of Waterborne Debris ...................................134

C.6 Hydrodynamic Uplift Forces....................................................135

APPENDIX D – BACKGROUND INFORMATION ON IMPACT 

LOAD CALCULATIONS...............................................................137

D.1 Available Models for Impact Loads.........................................137

D.2 Summary and Discussion .........................................................142

APPENDIX E – MAXIMUM FLOW VELOCITY AND 

MOMENTUM FLUX IN THE TSUNAMI RUNUP ZONE ........145

E.1 Flow Velocity...........................................................................145

E.2 Momentum Flux.......................................................................148

GLOSSARY................................................................................................151

REFERENCES...........................................................................................161

PROJECT PARTICIPANTS....................................................................173

FEMA P-646 Table of Contents ix

List of Figures

Figure 1-1 Decision-making and design process for vertical 

evacuation structures..............................................................3

Figure 2-1 Maximum computed tsunami amplitudes (in centimeters) 

in the Indian Ocean..............................................................11

Figure 2-2 Schematic diagrams of the vertical displacement 

resulting from subduction-type fault dislocation: 

(a) rupture zone located far offshore; and (b) rupture 

zone adjacent to coastline with coastal subsidence..............12

Figure 2-3 Tide gage records (in meters) for the 2004 Indian Ocean 

tsunami at: (a) Ta Phao Noi, Thailand, showing the 

leading depression wave; and (b) Tuticorin, India, 

showing the leading elevation wave ....................................13

Figure 2-4 Measured runup heights of the 1993 Okushiri tsunami 

along Inaho Coast, demonstrating that runup height 

varies significantly between neighboring areas...................14

Figure 2-5 Sketch of a bore and photo of the 1983 Nihonkai-Chubu 

Tsunami showing the formation of a bore offshore.............14

Figure 2-6 Sketch of a surge and photo of the 1983 Nihonkai-Chubu 

Tsunami showing the formation of a surge..........................15

Figure 2-7 A sequence of photos of the 1983 Nihonkai-Chubu 

Tsunami showing surge flooding from tsunami runup ........15

Figure 2-8 Degrees of building damage vs. tsunami runup height........16

Figure 2-9 Scotch Cap Lighthouse destroyed by the 1946 Aleutian 

Tsunami ...............................................................................17

Figure 2-10 Total destruction of a group of wood-frame houses in 

Aonae Village, Okushiri Island, Japan ................................18

Figure 2-11 Beach houses with varying levels of damage in 

El Popoyo, Nicaragua (1992 Nicaragua Tsunami) ..............18

Figure 2-12 Damage caused by impact from waterborne debris 

(fishing boat) in Aonae, Japan (1993 Okushiri 

Tsunami)..............................................................................19

Figure 2-13 Examples of reinforced concrete structures that survived 

the 1993 Okushiri Tsunami..................................................19

Figure 2-14 Damaged masonry beach house in Devanaanpattinam, 

India (2004 Indian Ocean Tsunami) ....................................20

Figure 2-15 Example of surviving reinforced concrete mosque in 

Uleele, Banda Aceh .............................................................21

FEMA P-646 List of Figures xi
Figure 2-16 Examples of waterborne debris from the 2004 Indian 

Ocean Tsunami.................................................................... 21

Figure 2-17 Damage to non-engineered concrete columns due to 

debris impact ....................................................................... 22

Figure 2-18 Damage to corner column due to debris damming.............. 22

Figure 2-19 Scour around shallow spread footing in Khao Lak 

area ...................................................................................... 23

Figure 2-20 Uplift damage to precast concrete floor panels and 

harbor piers.......................................................................... 23

Figure 2-21 Examples of structural collapse due to strong ground 

shaking in Banda Aceh prior to tsunami inundation ........... 24

Figure 2-22 Scene of near-total devastation of Minamisanriku.............. 25

Figure 2-23 Minamisanriku designated coastal evacuation building –

note tsunami trace on sign................................................... 26

Figure 2-24 Exterior elevator and stairway access to large roof 

evacuation area protected by 2 meter high braced guard 

fence on Minamisanriku coastal evacuation building ......... 27

Figure 2-25 Potential evacuation earth mound at West end of Sendai 

Port...................................................................................... 27

Figure 2-26 Surviving and damaged reinforced concrete buildings in 

Minamisanriku .................................................................... 28

Figure 2-27 Collapsed top floor of reinforced concrete building with 

steel truss roof ..................................................................... 29

Figure 2-28 Overturned cold storage building in Onagawa.................... 30

Figure 2-29 Overturned steel-framed office building in Onagawa......... 30

Figure 2-30 Overturned three-story commercial building on mat 

foundation ........................................................................... 31

Figure 2-31 Pass Christian office building with cast-in-place concrete 

pan 

joist floor system that suffered non-structural damage 

at first two floors but no structural damage......................... 32

Figure 2-32 Progressive collapse of upper floors of a parking garage 

due to damage in lower level columns from impact of 

an adjacent barge-mounted casino ...................................... 33

Figure 2-33 Failure of prestressed piles due to damming effect of 

shipping container............................................................... 33

Figure 2-34 Negative bending failure of a prestressed double-tee 

floor system due to uplift forces.......................................... 34

Figure 3-1 Coastal sites for site-specific tsunami inundation models 

for the Tsunami Forecasting System................................... 39

xii List of Figures FEMA P-646
Figure 3-2 Tsunami inundation modeling products for Seattle, 

Washington ..........................................................................41

Figure 3-3 Tsunami inundation map for Seattle, Washington 

produced and published by the state of Washington, 

using modeling products as guidance ..................................42

Figure 3-4 Yaquina Bay, Oregon tsunami inundation map with 

three inundation lines...........................................................43

Figure 3-5 Tsunami elevations with a 90% probability of not being 

exceeded in 50 years............................................................45

Figure 3-6 The 500-year tsunami map for Seaside, Oregon, 

depicting maximum wave heights that are met or 

exceeded at an annual probability of 0.2% ..........................46

Figure 4-1 Soil berm combined with a community park at 

Sendai Port, Japan................................................................54

Figure 4-2 Cast-in-place reinforced concrete parking garage in 

Biloxi, Mississippi after hurricane Katrina..........................55

Figure 4-3 Sports complex. Designed for assembly use, this 

type of structure can accommodate circulation and 

service needs for large numbers of people...........................56

Figure 4-4 Hotel and convention complex. Meeting rooms, 

ballrooms, and exhibit spaces located above the tsunami 

inundation elevation can be used to provide areas of 

refuge. ..................................................................................57

Figure 4-5 Residential apartment building in Kamaishi, Japan, with 

designated refuge area at or above the fourth level .............57

Figure 4-6 Evacuation map for Waikiki, Hawaii, indicating use of 

existing buildings for vertical evacuation............................58

Figure 5-1 Vertical evacuation refuge locations considering travel 

distance, evacuation behavior, and naturally occurring 

high ground..........................................................................62

Figure 5-2 Site hazards adjacent to vertical evacuation structures........63

Figure 6-1 Seismic performance objectives linking building 

performance levels to earthquake hazard levels ..................75

Figure 6-2 Three types of coastal inundation where the tsunami 

elevation (TE) at a site of interest could be less than,

equal to, or greater than the ultimate inland runup 

elevation (R).........................................................................80

Figure 6-3 Comparison between numerical modeling (blue line) 

and field measurement of run-up (white dots) and flow 

elevations (blue dots) at Pago Pago Harbor, American 

Samoa ..................................................................................80

Figure 6-4 Hydrostatic force distribution and location of 

resultant................................................................................82

FEMA P-646 List of Figures xiii
Figure 6-5 Buoyant forces on an overall building with watertight 

lower levels ......................................................................... 83

Figure 6-6 Hydrodynamic force distribution and location of 

resultant............................................................................... 84

Figure 6-7 Hydrodynamic impulsive and drag forces on 

components of a building subjected to inundation by 

a tsunami bore ..................................................................... 86

Figure 6-8 Waterborne debris impact force .......................................... 87

Figure 6-9 Maximum flow velocity of depth, d, at the ground 

elevation, z, and maximum runup elevation, R ................... 89

Figure 6-10 A definition sketch for upward buoyant force exerted 

on an elevated floor............................................................. 92

Figure 6-11 Gravity loads exerted on an elevated floor with water 

retained by exterior walls during rapid drawdown.............. 94

Figure 6-12 Impulsive and drag forces applied to an example 

building ............................................................................... 95

Figure 6-13 Debris dam and drag forces applied to an example

building ............................................................................... 96

Figure 6-14 Tie force strategy in a frame structure............................... 100

Figure 6-15 Detailing of reinforcing steel for potential loss of a 

supporting column............................................................. 101

Figure 6-16 Missing column strategy ................................................... 102

Figure 7-1 Effect of breakaway walls on waves................................. 106

Figure A-1 Successful designated vertical evacuation building in 

Kesennuma Port, Japan ..................................................... 114

Figure A-2 Life-Saving Tower ............................................................ 114

Figure A-3 Nishiki Tower.................................................................... 115

Figure A-4 Refuge at Shirahama Beach Resort................................... 116

Figure A-5 Tsunami refuge in Kaifu, Japan ........................................ 117

Figure A-6 Berm constructed for tsunami refuge in Aonae, Japan ..... 117

Figure A-7 Aonae Elementary School. Upper floor is intended

for use as tsunami refuge space......................................... 118

Figure B-1 Hypothetical sketch of example community showing 

potential vertical evacuation structure sites and 

evacuation routes............................................................... 119

Figure B-2 Example community inundation map ............................... 120

Figure B-3 Example community inundation flow velocity map ......... 121

Figure B-4 Example escape berm design ............................................ 122

Figure B-5 Example escape berm plan layout..................................... 123

xiv List of Figures FEMA P-646
Figure B-6 Example escape berm section ............................................123

Figure B-7 Example escape berm rear elevation..................................124

Figure B-8 Example gymnasium..........................................................125

Figure B-9 Example gymnasium plan..................................................126

Figure B-10 Example gymnasium elevation..........................................126

Figure C-1 Definition sketch for example calculations........................129

Figure C-2 Condition resulting in buoyant forces................................131

Figure D-1 Ranges of duration of impact.............................................142

Figure E-1 Maximum flow velocity of depth, d, at the ground 

elevation, z, and maximum runup elevation, R..................147

FEMA P-646 List of Figures xv

List of Tables

Table 2-1 Qualitative Tsunami Hazard Assessment for U.S. 

Locations......................................................................................9

Table 2-2 Comparison of Relative Time and Loading Scales for 

Various Coastal Hazard Phenomena ..........................................10

Table 5-1 Tsunami Sources and Approximate Warning Times..................60

Table 5-2 Maximum Spacing of Vertical Evacuation Structures 

Based on Travel Time ................................................................61

Table 5-3 Square Footage Recommendations – ICC-500 Standard on 

the Design and Construction of Storm Shelters .........................66

Table 5-4 Square Footage Recommendations – FEMA 361 Design 

and Construction Guidance for Community Shelters.................66

Table 5-5 Square Footage Recommendations – American Red Cross 

Publication No. 4496..................................................................66

Table 6-1 Mass and Stiffness of Some Waterborne Floating Debris..........88

Table 7-1 Approximate Scour Depth as a Percentage of Flow 

Depth, d ....................................................................................105

Table B-1 Design Elevations for Areas of Refuge ....................................120

Table B-2 Tsunami Flow Velocity at Each Site........................................121

FEMA P-646 List of Tables xvii

Chapter 1

Introduction

1.1 Objectives and Scope

Tsunamis are rare events often accompanied by advance warning. As such, 

strategies for mitigating tsunami risk have generally involved evacuation to 

areas of naturally occurring high ground outside of the tsunami inundation 

zone. Most efforts to date have focused on the development of more 

effective warning systems, improved inundation maps, and greater tsunami 

awareness to improve evacuation efficiency. 

In some locations, high ground may not exist, or tsunamis triggered by local 

events may not allow sufficient warning time for communities to evacuate 

low-lying areas. Where horizontal evacuation out of the tsunami inundation 

zone is neither possible nor practical, a potential solution is vertical 

evacuation into the upper levels of structures designed and detailed to resist 

the effects of a tsunami. 

The focus of this document is on structures intended to provide protection 

during a short-term high-risk tsunami event. Such facilities are generally 

termed refuges. A vertical evacuation refuge from tsunamis is a building or 

earthen mound that has sufficient height to elevate evacuees above the level

of tsunami inundation, and is designed and constructed with the strength and 

resiliency needed to resist the effects of tsunami waves.

This document is a resource for engineers, architects, state and local 

government officials, building officials, community planners, and building 

owners who are considering the construction and operation of tsunamiresistant structures that are intended to be a safe haven for evacuees during a 

tsunami event. It provides guidance on the design and construction of 

structures that could be used as a refuge for vertical evacuation above rising 

waters associated with tsunami inundation, and includes specific 

recommendations on loading, configuration, location, operation, and 

maintenance of such facilities. It is intended for use in areas of the United 

States that are exposed to tsunami hazard, but that should not preclude the 

use of this guidance for facilities located in other areas exposed to similar 

hazards.

A Vertical Evacuation Refuge 

from Tsunamis is a building or 

earthen mound that has sufficient 

height to elevate evacuees above 

the level of tsunami inundation, 

and is designed and constructed 

with the strength and resiliency 

needed to resist the effects of 

tsunami waves.

FEMA P-646 1: Introduction 1
1.2 Deciding to Construct a Vertical Evacuation Structure

Many factors influence the decision to construct a vertical evacuation 

structure, including: 

• the likelihood of a region being affected by a tsunami event, 

• the potential consequences of a tsunami event (e.g., damage, injury, and 

loss of life), 

• the elements of a local emergency response plan, including available 

evacuation alternatives,

• the planned and potential uses for a refuge facility, and 

• the cost of constructing a tsunami-resistant structure. 

1.2.1 Tsunami Hazard versus Risk

Hazard is related to the potential for an event to occur, while risk is related to 

consequences, given the occurrence of an event. Tsunami hazard is a 

measure of the potential for a tsunami to occur at a given site. It is also a 

measure of the potential magnitude of site-specific tsunami effects, including 

extent of inundation, height of runup, flow depth, and velocity of flow. 

Tsunami risk is a measure of the consequences given the occurrence of a 

tsunami, which can be characterized in terms of damage, loss of function, 

injury and loss of life. Risk depends on many factors including vulnerability 

and population density. 

Similar to other hazards (e.g., earthquake and wind) structural design criteria 

for tsunami effects are based on relative tsunami hazard. The decision to 

build a vertical evacuation structure, however, may ultimately be based on 

real or perceived risk to a local population as a result of exposure to tsunami 

hazard. 

1.2.2 Decision-Making and Design Process

A flowchart outlining the decision-making and design process for vertical 

evacuation structures is shown in Figure 1-1. 

Given a known or perceived tsunami threat in a region, the first step is to 

determine the severity of the tsunami hazard. This involves identification of 

potential tsunami-genic sources and accumulation of recorded data on 

tsunami occurrence and runup. Chapter 3 provides guidance on the 

assessment of tsunami hazard, which can include a probabilistic assessment 

considering all possible tsunami sources, or a deterministic assessment 

considering the maximum tsunami that can reasonably be expected to affect a 

site. Once potential tsunami sources are identified, and the level of tsunami 

Tsunami Hazard is a measure of 

the potential for a tsunami to occur 

at a given site.

Tsunami Risk is a measure of 

the consequences given the 

occurrence of a tsunami, which can 

be characterized in terms of 

damage, loss of function, injury 

and loss of life.

2 1: Introduction FEMA P-646
hazard is known, site-specific information on the extent of inundation, height 

of runup, flow depth, and velocity of flow is needed. Some of this 

information may be obtained from available tsunami inundation maps, where 

they exist; however site-specific tsunami inundation studies should be 

performed to obtain reliable estimates of tsunami flow characteristics at the 

site of the proposed vertical evacuation structure.

Figure 1-1 Decision-making and design process for vertical evacuation 

structures.

Given the tsunami hazard and extent of inundation, the potential risk of 

damage, injury, and loss of life in the region must then be evaluated. Explicit 

evaluation of tsunami risk is beyond the scope of this document, and will 

depend on a number of different factors including the presence of a tsunami 

warning system, existence of a local emergency response plan, availability of 

FEMA P-646 1: Introduction 3
various evacuation alternatives, vulnerability of the existing building stock, 

and locations of existing short- and long-term shelter facilities. The 

feasibility of evacuation to existing areas of refuge, as well as the tsunamiresistance of these areas, must be considered. Vertical evacuation structures 

will likely be most useful when there is not enough time between the tsunami 

warning and tsunami inundation to allow a community to evacuate out of the 

inundation zone or to existing areas of high ground. In most cases this will 

be in communities at risk for near-source-generated tsunamis.

Where the risk to a coastal community is deemed to be unacceptably high, 

vertical evacuation can be a possible solution for mitigating tsunami risk. 

Chapter 4 outlines a number of potentially viable options for design and 

construction of vertical evacuation structures. 

Implementation of vertical evacuation requires a distribution of structures 

throughout the community that are suitable for providing refuge from the 

effects of tsunami inundation, and that are appropriately sized for the 

population. Chapter 5 provides guidance on locating and sizing vertical 

evacuation structures.

Once the decision to utilize vertical evacuation is made, structures must be 

designed and constructed to be tsunami-resistant. Loading and other criteria 

for the design of vertical evacuation structures are provided in Chapters 6 

and 7. The 2012 International Building Code, Appendix M, may be adopted 

by local jurisdictions that have a tsunami hazard and that regulate the design 

and construction of structures placed in high-risk or high-hazard areas. 

1.3 Limitations

This document is a compilation of the best information available at the time 

of publication. It provides guidance for design and construction of vertical 

evacuation structures that is currently not available in other design guides, 

building codes, or standards. It is not intended to supersede or replace 

current codes and standards, but rather to supplement them with guidance 

where none is otherwise provided. It is intended to provide specific 

recommendations and design criteria that are unique to tsunami loading 

conditions for vertical evacuation structures, once the decision has been 

made to build such a structure. It is not intended to mandate or imply that all 

structures in tsunami hazard areas should be made tsunami-resistant using 

these criteria. Such a decision would be cost-prohibitive, especially for lightframe residential structures. 

Vertical evacuation structures designed in accordance with the guidance 

presented in this document would be expected to provide safe refuge under 

4 1: Introduction FEMA P-646
the assumed design conditions. For these structures, multiple design 

assumptions are required, including the intensity of a local earthquake that 

could threaten the structure prior to a tsunami, the flow depths and velocities 

of the design tsunami at the site, and the type of waterborne debris that may 

be characteristic at the site. Maximum loading must therefore be considered 

uncertain, and conservative assumptions should be made, particularly since 

these structures are expected to provide security and safety to the public.

Large damaging tsunamis are rare events, and existing knowledge is based 

on limited historic information. Coastal inundation patterns are based on 

complex combinations of many parameters, and are highly uncertain. 

Proportioning a structure for a design tsunami event does not necessarily 

mean the structure will be able to resist every possible tsunami event. 

Selection of the design tsunami is therefore based on the tsunami hazard in a

region, the risk tolerance of a local community, and economic considerations.

Critical to the design of a vertical evacuation structure is the height of the 

refuge area above the anticipated tsunami flow depth. Even if the structure 

survives inundation, overtopping of the refuge area will result in 

unacceptable loss of life of those who sought refuge in the designated 

evacuation structure. This is clearly unacceptable performance of a vertical 

evacuation refuge and every effort must be taken to avoid this outcome. 

1.4 Organization

This document provides guidance on siting concepts, performance 

objectives, design loads, design concepts, and emergency management issues

that should be considered in locating, designing, and operating vertical 

evacuation structures as a refuge from tsunamis. Examples are presented that 

illustrate how the criteria are used. Information contained in this document is 

organized as follows: 

Chapter 1 defines the scope and limitations for the guidance contained in this 

document. Chapter 2 provides background information on tsunami effects 

and their potential impacts on buildings in coastal communities. Chapters 3

through 7 provide design guidance on characterization of tsunami hazard, 

choosing between various options for vertical evacuation structures, locating 

and sizing vertical evacuation structures, estimation of tsunami load effects, 

structural design criteria, design concepts, and other considerations. 

Appendices A through E provide supplemental information, including 

examples of vertical evacuation structures from Japan, example tsunami load 

calculations, a community design example, development of impact load 

FEMA P-646 1: Introduction 5
equations, and background on maximum flow velocity and momentum flux 

in the tsunami runup zone.

A Glossary defining terms used throughout this document, and a list of 

References identifying resources for additional information, are also 

provided.

6 1: Introduction FEMA P-646
Chapter 2 

Background

2.1 General

Tsunami is a Japanese word meaning “harbor” (tsu) and “wave” (nami). The 

term was created by fishermen who returned to port to find the area 

surrounding the harbor devastated. It is a naturally occurring series of waves 

that can result when there is a rapid, large-scale disturbance of a body of 

water. The most common triggering events are earthquakes below or near 

the ocean floor, but a tsunami can also be created by volcanic activity, 

landslides, undersea slumps, and impacts of extra-terrestrial objects. The 

waves created by this disturbance propagate away from the source. In deep 

water, the waves are gentle sea-surface slopes that can be unnoticeable. As 

the waves approach the shallower waters of the coast, however, the velocity 

decreases while the height increases. Upon reaching the shoreline the waves 

can have hazardous height and force, penetrating inland, damaging 

structures, and flooding normally dry areas. 

In this document, tsunamis are categorized by the location of the triggering 

event and the time it takes the waves to reach a given site. A far-sourcegenerated tsunami is one that originates from a source that is far away from 

the site of interest, and takes 2 hours or longer after the triggering event to 

arrive. A near-source-generated tsunami is one that originates from a source 

that is close to the site of interest, and can arrive within 30 minutes. Sites 

experiencing near-source-generated tsunamis will generally feel the effects 

of the triggering event (e.g., shaking caused by a near-source earthquake). A 

mid-source-generated tsunami is one in which the source is somewhat close 

to the site of interest, but not close enough for the effects of the triggering 

event to be felt at the site. Mid-source-generated tsunamis would be 

expected to arrive between 30 minutes and 2 hours after the triggering event. 

2.1.1 Historic Tsunami Activity

The combination of a great ocean seismic event with the right bathymetry 

can have devastating results, as was brought to the world’s attention by the 

Indian Ocean Tsunami of December 26, 2004 and more recently the Tohoku 

Japan Tsunami of March 11, 2011. The Indian Ocean Tsunami was created 

by a magnitude-9.3 underwater earthquake and devastated coastal areas 

around the northern Indian Ocean. The tsunami took anywhere from 15 

A Tsunami is a naturally occurring 

series of ocean waves resulting from 

a rapid, large-scale disturbance in a 

body of water, caused by 

earthquakes, landslides, volcanic 

eruptions, and meteorite impacts.

A far-source-generated 

tsunami is one that originates from 

a source that is far away from the 

site of interest, and takes 2 hours or 

longer after the triggering event to 

arrive.

A near-source-generated 

tsunami is one that originates from 

a source that is close to the site of 

interest, and arrives within 30 

minutes. The site of interest might 

also experience the effects of the 

triggering event. 

A mid-source-generated 

tsunami is one in which the source 

is somewhat close to the site of 

interest, and would be expected to 

arrive between 30 minutes and 2 

hours after the triggering event.

FEMA P-646 2: Background 7 
minutes to 7 hours to hit the various coastlines it affected. It is estimated that 

the tsunami took over 220,000 lives and displaced over 1.5 million people.

The Tohoku Japan Tsunami was generated by the magnitude 9.0 Great East 

Japan Earthquake and led to inundation heights along the coast of the main 

Japanese island of Honshu that exceeded all historical records for that region. 

Breakwater and seawall defensive systems were overtopped or destroyed in 

almost all communities along the Tohoku coastline, leading to over 19,000 

missing or dead, and extensive damage to ports, buildings, bridges and other 

coastal infrastructure.

Wave propagation times from far-source-generated tsunamis can allow for 

advance warning to distant coastal communities. Near-source-generated 

tsunamis, however, can strike suddenly and with very little warning. The 

1993 tsunami that hit Okushiri, Hokkaido, Japan, for example, reached the 

shoreline within 5 minutes after the earthquake, and resulted in 202 fatalities 

as victims were trapped by debris from the earthquake and unable to flee 

toward higher ground and more secure places.

Although considered rare events, tsunamis occur on a regular basis around 

the world. Each year, on average, there are 20 tsunami-genic earthquake

events, with five of these large enough to generate tsunami waves capable of 

causing damage and loss of life. In the period between 1990 and 1999 there 

were 82 tsunamis reported, 10 of which resulted in more than 4,000 fatalities. 

With the trend toward increased habitation of coastal areas, more populations 

will be exposed to tsunami hazard. 

Relative tsunami hazard can be characterized by the distribution and 

frequency of recorded runups. Table 2-1 provides a qualitative assessment of 

tsunami hazard for regions of the United States that are threatened by 

tsunamis, as it has been characterized by the National Oceanic & 

Atmospheric Administration (NOAA) using the last 200 years of data on 

recorded runups.

Alaska is considered to have the highest potential for tsunami-generating 

events in the United States. Earthquakes along the Alaska-Aleutian 

subduction zone, particularly in the vicinity of the Alaskan Peninsula, the 

Aleutian Islands, and the Gulf of Alaska have the capability of generating 

tsunamis that affect both local and distant sites. The 1964 earthquake in 

Prince William Sound resulted in 122 fatalities, including 12 in California 

and 4 in Oregon. In 1994 a landslide-generated tsunami in Skagway Harbor 

resulted in one death and $21 million in property damage.

8 2: Background FEMA P-646
Table 2-1 Qualitative Tsunami Hazard Assessment for U.S. Locations 

(Dunbar, et. al., 2008)

Region

Hazard Based on Recorded 

Runups

Hazard Based on Frequency 

of Runups 

Atlantic Coast Very low to low Very low

Gulf Coast None to very low None to very low

Caribbean High High

West Coast High High

Alaska Very high or severe Very high

Hawaii Very high or severe Very high

Western Pacific Moderate High

The Cascadia subduction zone along the Pacific Northwest coast poses a 

threat from northern California to British Columbia, Canada. An earthquake 

along the southern portion of the Cascadia subduction zone could create 

tsunami waves that would hit the coasts of Humboldt and Del Norte counties 

in California and Curry County in Oregon within a few minutes of the

earthquake. Areas further north, along the Oregon and Washington coasts,

could see tsunami waves within 20 to 40 minutes after a large earthquake.

Communities along the entire U.S. Pacific coastline are at risk for far-sourcegenerated (trans-Pacific) tsunamis and locally triggered tsunamis. In 

southern California there is evidence that movement from local offshore 

strike-slip earthquakes and submarine landslides have generated tsunamis 

affecting areas extending from Santa Barbara to San Diego. The largest of 

these occurred in 1930, when a magnitude-5.2 earthquake reportedly 

generated a 20-foot-high wave in Santa Monica, California (California 

Geological Survey, 2006).

Hawaii, located in the middle of the Pacific Ocean, has experienced both farsource-generated tsunamis and locally triggered tsunamis (PararasCarayannis, 1968). The far-source 2011 Tohoku Japan Tsunami resulted in 

inundation of a number of coastal communities in Hawaii, causing structural 

and non-structural damage to homes, hotels and small boat harbors. Total 

damages were estimated at $40 million. The most recent near-source 

damaging tsunami in Hawaii occurred in 1975, the result of a magnitude-7.2 

earthquake off the southeast coast of the island of Hawaii. This earthquake 

resulted in tsunami wave heights more than 20 feet and, in one area, more 

than 40 feet. Two deaths and more than $1 million in property damage were 

attributed to this local Hawaiian tsunami (Pararas-Carayannis, 1976).

FEMA P-646 2: Background 9
Although the Atlantic and Gulf Coast regions of the United States are 

perceived to be at less risk, there are examples of deadly tsunamis that have 

occurred in the Atlantic Ocean. Since 1600, more than 40 tsunamis and 

tsunami-like waves have been cataloged in the eastern United States. In 

1929, a tsunami generated in the Grand Banks region of Canada hit Nova 

Scotia, killing 51 people (Lockridge et al., 2002). 

Puerto Rico and the U.S. Virgin Islands are at risk from earthquakes and 

underwater landslides that could occur in the Puerto Rico Trench subduction 

zone. Since 1530, more than 50 tsunamis of varying intensity have occurred 

in the Caribbean. In 1918, an earthquake in this zone generated a tsunami 

that caused an estimated 40 deaths in Puerto Rico. In 1867, an earthquakegenerated tsunami caused damage and 12 deaths on the islands of Saint 

Thomas and Saint Croix. In 1692 a tsunami generated by massive landslides 

in the Puerto Rican Trench reached the coast of Jamaica, causing an 

estimated 2,000 deaths (Lander, 1999). 

2.1.2 Behaviors and Characteristics of Tsunamis

Information from historic tsunami events indicates that tsunami behaviors 

and characteristics are quite distinct from other coastal hazards, and cannot 

be inferred from common knowledge or intuition. The primary reason for 

this distinction is the unique timescale associated with tsunami phenomena. 

Unlike typical wind-generated water waves with periods between 5 and 20 

sec, tsunamis can have wave periods ranging from a few minutes to over 1 

hour (FEMA, 2005). This timescale is also important because of the 

potential for wave reflection, amplification, or resonance within coastal 

features. Table 2-2 compares various coastal hazard phenomena. 

Table 2-2 Comparison of Relative Time and Loading Scales for Various 

Coastal Hazard Phenomena

Coastal Hazard 

Phenomenon

Time scale 

(Duration of 

Loading)

Loading Scale 

(Height of 

Water)

Typical Warning 

Time 

Wind-generated 

waves

Tens of seconds 1 to 2 meters 

typical

Days

Tsunami runup Tens of minutes to 

an hour

1 to 10 meters Several minutes to 

hours

Hurricane storm 

surge

Several hours 1 to 10 meters Several hours to a 

few days

Earthquake shaking Seconds N/A Seconds to none

There is significant uncertainty in the prediction of hydrodynamic 

characteristics of tsunamis because they are highly influenced by the tsunami 

Tsunami wave periods can 

range from a few minutes to over 1 

hour, resulting in an increased 

potential for reflection, 

amplification, or resonance within 

coastal features. 

10 2: Background FEMA P-646
waveform and the surrounding topography and bathymetry. Although there 

are exceptions, previous research and field surveys indicate that tsunamis 

have the following general characteristics:

• The magnitude of the triggering event determines the period of the

resulting waves, and generally (but not always) the tsunami magnitude 

and damage potential (FEMA, 2005). 

• A tsunami can propagate more than several thousand kilometers without 

losing energy.

• Tsunami energy propagation has strong directivity. The majority of its 

energy will be emitted in a direction normal to the major axis of the 

tsunami source. The more elongated the tsunami source, the stronger the 

directivity (Okal, 2003; Carrier and Yeh, 2005). Direction of approach 

can affect tsunami characteristics at the shoreline, because of the 

sheltering or amplification effects of other land masses and offshore 

bathymetry (FEMA, 2005). A numerical example for the 2004 Indian 

Ocean Tsunami is shown in Figure 2-1.

Figure 2-1 Maximum computed tsunami amplitudes (in centimeters) in the Indian Ocean 

(Titov, NOAA Center for Tsunami Research, 

http://nctr.pmel.noaa.gov/indo_1204.html)

• At the source, a tsunami waveform contains a wide range of wave 

components, from short to long wavelengths. Long wave components 

propagate faster than short wave components; therefore, a transoceanic 

tsunami is usually characterized by long-period waves (several to tens of 

FEMA P-646 2: Background 11
minutes). Shorter wave components are left behind and attenuated by 

radiation and dispersion. 

• For a locally-generated tsunami, the first leading wave is often a receding 

water level followed by an advancing positive heave (an elevation wave). 

This may not be the case if the coastal ground subsides by co-seismic 

displacement. For far-source-generated tsunamis, the leading wave is 

often an elevation wave. This trend may be related to the pattern of sea 

floor displacement resulting from a subduction-type earthquake, shown 

schematically in Figure 2-2. Figure 2-3 shows a leading depression wave 

measured at a tide gage station in Thailand during the 2004 Indian Ocean 

Tsunami, in contrast with a leading elevation wave measured at the 

southern end of India.

(a)

(b)

Figure 2-2 Schematic diagrams of the vertical displacement resulting from subduction-type 

fault dislocation: (a) rupture zone located far offshore; and (b) rupture zone 

adjacent to coastline with coastal subsidence (Geist, 1999).

12 2: Background FEMA P-646
(a)

(b)

Figure 2-3 Tide gage records (in meters) for the 2004 Indian Ocean 

tsunami at: (a) Ta Phao Noi, Thailand, showing the leading 

depression wave; and (b) Tuticorin, India, showing the leading 

elevation wave.

• Tsunamis are highly reflective at the shore, and capable of sustaining 

their motion for several hours without dissipating energy. Typically 

several tsunami waves attack a coastal area, and the first wave is not 

necessarily the largest. Sensitive instrumentation can detect tsunami 

activity for several days. 

• Tsunami runup height varies significantly in neighboring areas. The 

configuration of the continental shelf and shoreline affect tsunami 

impacts at the shoreline through wave reflection, refraction, and 

shoaling. Variations in offshore bathymetry and shoreline irregularities 

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

5 6 7 8 9 10 11 12 13 14 15

elevation (m)

time (hours)

-0.5

0.0

0.5

1.0

1.5

2.0

25 26 27 28 29 30 31 32 33 34 35

Elevation (m)

time (hours)

Tsunami runup heights vary

significantly in neighboring areas

due to variations in offshore 

bathymetry that can increase or 

decrease local tsunami impacts.

FEMA P-646 2: Background 13
can focus or disperse tsunami wave energy along certain shoreline 

reaches, increasing or decreasing tsunami impacts (FEMA, 2005). 

Figure 2-4 shows significant variation in runup heights measured along 

the northwest coastline of Okushiri Island.

 

Figure 2-4 Measured runup heights of the 1993 Okushiri tsunami along Inaho Coast, demonstrating 

that runup height varies significantly between neighboring areas.

• The majority of eyewitness accounts and visual records (videos and 

photographs) indicate that an incident tsunami will break offshore 

forming a bore or a series of bores as it approaches the shore. A 

turbulent bore is defined as a broken wave having a steep, violently 

foaming and turbulent wave front, propagating over still water of a finite 

depth, as shown in Figure 2-5. These broken waves (or bores) are 

considered relatively short waveforms (although still longer than windgenerated waves) riding on a much longer main heave of the tsunami. 

Such bore formations were often observed in video footage of the 2004 

Indian Ocean Tsunami and the 2011 Tohoku Japan Tsunami.

 

Figure 2-5 Sketch of a bore and photo of the 1983 Nihonkai-Chubu Tsunami showing the formation of 

a bore offshore (photo from Knill, 2004).

14 2: Background FEMA P-646
• After a bore reaches the shore, the tsunami rushes up on dry land in the 

formation of a surge, as shown in Figure 2-6. In some cases, especially 

when a long-wavelength, leading-elevation, and far-source-generated 

tsunami attacks land on a steep slope, the runup can be characterized as a 

gradual rise and fall of water (i.e., surge flooding) as shown in Figure 

2-7. The impact of the 1960 Chilean tsunami at some Japanese localities 

and the 1964 Alaska tsunami at the town of Port Alberni, Canada are 

classic examples of surge flooding. 

 

Figure 2-6 Sketch of a surge and photo of the 1983 Nihonkai-Chubu Tsunami showing the 

formation of a surge (photo courtesy of N. Nara).

 

Figure 2-7 A sequence of photos of the 1983 Nihonkai-Chubu Tsunami showing surge flooding from 

tsunami runup (photo courtesy of S. Sato).

FEMA P-646 2: Background 15
2.2 Tsunami Effects on Buildings

Damage studies from historic tsunami events, the 2004 Indian Ocean 

Tsunami and the 2011 Tohoku Japan Tsunami, and storm surge associated 

with Hurricane Katrina in 2005 have provided information on the response of 

the built environment to devastating tsunamis and coastal flooding. 

Although there is considerable damage to, and often total destruction of, 

residential and light-framed buildings during extreme coastal flooding, there 

are also numerous examples of mid- to high-rise engineered structures that 

survived tsunami inundation. 

Structural damage from tsunamis can be attributed to: (1) direct hydrostatic 

and hydrodynamic forces from water inundation; (2) impact forces from

water-borne debris; (3) fire spread by floating debris and combustible 

liquids; (4) scour and slope/foundation failure; and (5) wind forces induced 

by wave motion. 

2.2.1 Historic Data on Tsunami Effects

Studies of damage from historic tsunamis have shown that building 

survivability varies with construction type and tsunami runup height (Yeh et 

al., 2005). Figure 2-8 shows data on damage for various types of construction 

resulting from the 1993 Okushiri Tsunami and earlier tsunamis. 

 For a given tsunami height, wood frame construction experienced 

considerably more damage and was frequently destroyed, while reinforced 

concrete structures generally sustained only minor structural damage. Recent 

data, including those of the 2004 Indian Ocean Tsunami, support this 

conclusion. 

Figure 2-8 Degrees of building damage vs. tsunami runup height. Marks filled in black are 

data from the 1993 Okushiri tsunami; hollow marks are data from previous 

tsunami events (adapted from Shuto, 1994, Yeh, et al., 2005). 

There are numerous examples of 

mid- to high-rise engineered 

structures that have survived 

tsunami inundation.

16 2: Background FEMA P-646
Note that the total destruction of one concrete structure is identified in Figure 

2-8. This structure was the lighthouse at Scotch Cap, Unimak Island. The 

Scotch Cap lighthouse is shown in Figure 2-9, before and after the 1946 

Aleutian Tsunami. There is some question as to how well the lighthouse was 

constructed, but it is possible that its destruction was the result of a wave 

breaking directly onto the structure, which was located right at the shoreline. 

The breaking wave could have been equivalent to a “collapsing” breaker, one 

of the classifications of wave breakers used in coastal engineering (Wiegel, 

1964) that occurs at shorelines with steeply sloping beaches.

 

Figure 2-9 Scotch Cap Lighthouse destroyed by the 1946 Aleutian Tsunami.

The 1993 Okushiri Tsunami completely destroyed the entire town of Aonae. 

Figure 2-10 shows bare concrete foundations typically observed as remnants 

of wood-frame residential construction after the tsunami. 

The 1992 Nicaragua Tsunami event provided other examples of variations in 

the performance of different structures. Figure 2-11 shows severe scour and 

complete destruction of a grade-level wood-frame house (left), and survival 

of an elevated wood frame and a grade-level rigid masonry structure (right). 

All three houses were located on a beach berm in the same vicinity, less than 

200 meters apart. 

Building failures have been observed when waterborne debris traveling at 

significant speeds impacts buildings. An example of the destruction caused 

by the impact of water-borne debris from the 1993 Okushiri Tsunami is 

shown in Figure 2-12. The debris in this case was a fishing boat that had 

broken free from its moorings. Waterborne debris is also known to collect 

FEMA P-646 2: Background 17
between structural supports creating a barrier that can significantly increase 

hydraulic forces on the building. 

Figure 2-10 Total destruction of a group of wood-frame houses in Aonae 

Village, Okushiri Island, Japan (1993 Okushiri Tsunami).

 

Figure 2-11 Beach houses with varying levels of damage in El Popoyo, Nicaragua (1992 Nicaragua Tsunami). 

All three houses are in the same vicinity.

18 2: Background FEMA P-646
Figure 2-12 Damage caused by impact from water-borne debris (fishing

boat) in Aonae, Japan (1993 Okushiri Tsunami) (photo courtesy 

J. Preuss).

In contrast to the many failures reported as a result of past tsunamis, many

structures have been observed to survive tsunami inundation. Two structures 

that survived the 1993 Okishiri Tsunami are shown in Figure 2-13. Both are 

two-story reinforced concrete structures, and both were inundated by at least 

3 meters of water. 

 

Figure 2-13 Examples of reinforced concrete structures that survived the 1993 Okushiri Tsunami: vista house 

at Cape Inaho (left); and fish market in Aonae (right) (photo courtesy N. Shuto). 

2.2.2 Observations from the Indian Ocean Tsunami

Damage observed as a result of the 2004 Indian Ocean tsunami confirmed 

observations from historic data on tsunami effects, and provided new 

evidence on observed effects.

FEMA P-646 2: Background 19
Figure 2-14 shows a damaged unreinforced masonry house in 

Devanaanpattinam, India. Foundations experienced severe scour, and the 

rear walls were forced out by hydraulic pressure due to flooding inside the 

house. This type of damage is commonly observed in masonry buildings.

Figure 2-14 Damaged masonry beach house in Devanaanpattinam, India (2004 Indian Ocean Tsunami).

As observed in past tsunamis, numerous engineered buildings survived the 

2004 Indian Ocean Tsunami. In some instances, there was damage to 

structural elements at the lower levels, but seldom to an extent that led to 

total collapse of the structure. One example of a surviving structure is a 

mosque located at the water’s edge in Uleele, Banda Aceh, shown in Figure 

2-15. The inundation depth at the mosque was about 10 m (just under the 

roof line), and the surrounding town was destroyed. The mosque suffered 

significant damage but was still standing. 

Dalrymple and Kriebel (2005) commented that the survival of many hotel 

buildings in Thailand was due in part to the relatively open nature of the first 

floor construction, so that “these buildings suffered little structural damage as 

the force of the tsunami broke through all of the doors and windows, thus 

reducing the force of the water on the building itself.”

The 2004 Indian Ocean Tsunami provided additional evidence of the effects 

of waterborne debris impact and scour on structural elements. Examples of 

waterborne debris included fishing boats and vehicles (Figure 2-16). 

Damage to structural elements of non-engineered reinforced concrete 

buildings was attributed to impact from such debris (Figure 2-17). Examples 

are also evident where debris damming resulted in damage to structural 

members (Figure 2-18). An example of observed scour below a shallow 

foundation is shown in Figure 2-19. From a review of available data taken 

by various survey teams, it appears that the maximum scour depth measured 

onshore was 3m in Khao Lak, Thailand.

20 2: Background FEMA P-646
Figure 2-15 Example of surviving reinforced concrete mosque in Uleele, 

Banda Aceh (photo courtesy J. Borerro).

 

Figure 2-16 Examples of waterborne debris from the 2004 Indian Ocean Tsunami (photos courtesy of M. 

Saatcioglu, A. Ghobarah and I. Nistor, CAEE, 2005).

A noteworthy structural failure encountered in the 2004 Indian Ocean 

Tsunami was uplift of precast concrete panels in buildings and docks (Figure 

2-20). Uplift forces were sufficient to lift the concrete panels and break 

attachments between the panels and the supporting members. These failures 

cannot be explained by buoyancy effects alone, which reduce net downward 

gravity forces by the volume of water displaced. Net uplift forces sufficient 

to fail these elements have been attributed to additional buoyancy effects due 

to trapped air and vertical hydrodynamic forces caused by the rising water. 

FEMA P-646 2: Background 21
 

Figure 2-17 Damage to non-engineered concrete columns due to debris impact (photos courtesy of M. 

Saatcioglu, A. Ghobarah and I. Nistor, CAEE, 2005).

Figure 2-18 Damage to corner column due to debris damming (photo courtesy of M. Saatcioglu, A. Ghobarah 

and I. Nistor, CAEE, 2005).

22 2: Background FEMA P-646
Figure 2-19 Scour around shallow spread footing in Khao Lak area (Dalrymple and 

Kriebel, 2005).

 

Figure 2-20 Uplift damage to precast concrete floor panels and harbor piers (photo courtesy of M. 

Saatcioglu, A. Ghobarah and I. Nistor, CAEE, 2005).

Also, lack of adequate seismic capacity led to a number of collapses of 

multistory reinforced concrete buildings in Banda Aceh and other areas near

the epicenter of the magnitude-9.3 earthquake that triggered the tsunami 

(Figure 2-21). These collapses occurred prior to inundation by tsunami 

waves, and highlight the importance of providing adequate seismic resistance 

in addition to tsunami resistance in regions where both hazards exist.

FEMA P-646 2: Background 23
 (a) Beam-column connection failures

(b) Soft story failure

Figure 2-21 Examples of structural collapse due to strong ground shaking in Banda 

Aceh prior to tsunami inundation (photos courtesy of M. Saatcioglu, A. 

Ghobarah and I. Nistor, CAEE, 2005).

24 2: Background FEMA P-646
2.2.3 Observations from the Tohoku Japan Tsunami

Along the Tohoku coast, tsunami inundation height was in the range of 5 to 

30+ meters. In general, light-frame residential construction subject to about a 

story height or more of inundation will collapse. In this event, complete 

collapse of residential light-frame construction occurred in nearly 100% of 

all affected areas extending to the edge of the inundation limit. In 

commercial and industrial areas, 75-95% of the low rise buildings collapsed, 

with the higher collapse rate occurring as tsunami height reached the upper 

range (Figure 2-22). In these inundated coastal zones, buildings taller than 5 

stories were uncommon. Despite this devastation, there were a number of 

these multi-story buildings that survived the tsunami without loss of 

structural integrity of their vertical load carrying system or foundation. In 

fact, a significant proportion of the surviving buildings did not appear to have 

significant structural damage. This provides some encouragement regarding 

the potential resilience of larger modern buildings having robust seismic 

designs and scour and uplift-resistant foundations, even when subjected to 

tsunami inundation greater than that for which they were designed. 

Figure 2-22 Scene of near-total devastation of Minamisanriku (photo 

courtesy of I. Nistor, ASCE, 2012). 

Under a 2005 Japanese Cabinet Office guideline, buildings to be designated 

as tsunami shelters should be made of concrete or other similarly robust 

materials. They should be at least three stories high in areas where flood 

levels are predicted to reach two meters, or at least four stories high if flood 

FEMA P-646 2: Background 25
levels are predicted to reach three meters. The 18 municipal governments in 

Aomori, Iwate, Miyagi and Chiba prefectures had designated a total of 88 

buildings as vertical evacuation sites.

Figure 2-23 and Figure 2-24 show the designated evacuation area on the roof 

of a coastal building in Minamisanriku. This building was built as a 

residential structure, but with specific vertical evacuation attributes as part of 

the design. Access to the roof level evacuation area was provided by external 

elevator and staircase accessible without entering the rest of the building. 

The evacuation area measured a total of 660 square meters and was 

surrounded by a well-braced 2 meter high guard fence. Even though this 

building was overtopped by 0.7 meters, those who sought refuge on the roof 

survived the tsunami.

Figure 2-23 Minamisanriku designated coastal evacuation building – note tsunami trace on sign (photo 

courtesy of I. Robertson, ASCE, 2012).

Unfortunately, many of the designated vertical evacuation buildings were not 

tall enough for the flow depths encountered during this tsunami. An

unknown number of people who sought refuge in these structures did not 

survive the inundation, even though the structures remained intact. It is 

therefore paramount that structures designated for vertical evacuation refuge 

be tall and strong enough to keep the refugees safe even during tsunami 

events that exceed the maximum considered event.

26 2: Background FEMA P-646
Figure 2-24 Exterior elevator and stairway access to large roof evacuation area protected by 2 meter high 

braced guard fence on Minamisanriku coastal evacuation building (photo courtesy of I. Robertson, 

ASCE, 2012).

Figure 2-25 shows a man-made earth mound in a park area at the West end of 

Sendai port that was only inundated to about half its height, allowing 

considerable area for refuge in an otherwise flat region. Similar mounds near 

the coastline in Natori were overtopped during the tsunami so would not 

have been suitable as evacuation sites. Only limited erosion was observed on 

the flanks of these earth mounds indicating that this concept can work, 

provided the evacuation site on the top of the mound is well above the 

inundation level.

Figure 2-25 Potential evacuation earth mound at West end of Sendai Port 

(photo courtesy of I. Robertson, ASCE, 2012).

As observed in prior tsunamis, the Tohoku Japan Tsunami created all loading 

and effects including hydrostatic forces, hydrodynamic forces, debris 

damming and debris impact forces, and scour effects.

Any of these effects alone, or in combination with the others, was observed 

to cause structural failures to low- to mid-rise building components of any 

structural material. Building performance was not guaranteed simply by 

generic choice of structural material and structural system. Lateral strength 

and element resistance to impact were critical to avoid local damage, while 

FEMA P-646 2: Background 27
resistance to progressive collapse was effective at preventing local member 

failures from precipitating disproportionate structural collapse.

A number of low-rise reinforced concrete buildings in Minamisanriku 

survived complete inundation (Figure 2-26). Many of these buildings had 

solid concrete walls facing the ocean, exposing them to the maximum 

possible hydrodynamic loading. A nearby reinforced concrete building with 

shear walls framing the lower two floors, and concrete cantilever columns 

supporting a steel truss roof, suffered complete collapse of the top story 

(Figure 2-27). The large quantity of trees as debris in the flow, and the 

susceptibility of cantilever columns to flexural failure, likely contributed to 

this failure.

Figure 2-26 Surviving and damaged reinforced concrete buildings in 

Minamisanriku (photo courtesy of I. Robertson, ASCE, 2012).

The harbor town of Onagawa experienced a tsunami surge of approximately 

18+ meters that overtopped nearly all buildings in the area except for those 

on a central hillside. Outflow velocities following this initial tsunami run-up 

were particularly high. Despite this, many low-rise steel and concrete

buildings survived. Among the failed structures were more than a half-dozen 

overturned and displaced whole buildings, nearly structurally intact from 

foundation to roof. These buildings were either floated by hydrostatic forces 

and carried away, or overturned by hydrodynamic forces of the tsunami 

inflow or outflow, or a combination of both effects. The contribution of these 

28 2: Background FEMA P-646
effects to the failures depended on the degree of openness of the building 

structures.

Figure 2-27 Collapsed top floor of reinforced concrete building with steel 

truss roof (photo courtesy of I. Nistor, ASCE, 2012).

One illustration is a two-story reinforced concrete cold storage building,

which had refrigerated storage on the ground floor and the refrigeration 

equipment on the second floor. Due to this function, the building consisted of 

a closed concrete shell except for doors and a few second floor windows for 

its administrative room and ventilation. Hydrostatic buoyancy lifted the 

building off its pile foundation, which did not have tensile capacity, and 

carried it over a low wall before being deposited about 15 meters inland from 

its original location (Figure 2-28).

Other overturned concrete and steel buildings were sufficiently open to 

relieve hydrostatic uplift but were still toppled by hydrodynamic forces of the 

incoming or returning flow. A four-story structural steel moment resisting 

frame lost many of its lightweight precast concrete cladding panels and had 

numerous window openings (Figure 2-29). Nevertheless, the building’s spuncast hollow precast piles were sheared off or extracted from the ground, and 

the office building displaced by about 15 meters.

Figure 2-30 shows a three-story reinforced concrete building frame with 

shear walls on a 0.9 meter-thick mat foundation which overturned toward 

Onagawa bay during the tsunami return flow.

FEMA P-646 2: Background 29
Figure 2-28 Overturned cold storage building in Onagawa (photo courtesy 

of G. Chock, ASCE, 2012).

Figure 2-29 Overturned steel-framed office building in Onagawa (photo 

courtesy of G. Chock, ASCE, 2012).

30 2: Background FEMA P-646
Figure 2-30 Overturned three-story commercial building on mat foundation 

(photo courtesy of I. Robertson, ASCE, 2012). 

2.2.4 Observations from Hurricane Katrina

The storm surge along the Mississippi Gulf coast was estimated to have been 

between 25 and 28 feet during Hurricane Katrina (FEMA 548, 2006). This 

resulted in extensive inundation of low-lying coastal regions from New 

Orleans, Louisiana to Mobile, Alabama. 

While hurricane storm surge and tsunami inundation both result in coastal 

flooding, the characteristic behavior of this flooding can be quite different. 

Hurricane storm surge typically inundates coastal areas for a longer duration 

(several hours) with repeated pounding from wave action and gusting winds. 

Tsunami inundation generally takes place over a shorter time period (tens of 

minutes) with rapidly changing water levels and sweeping currents. Because 

of these differences, extrapolation of conclusions from hurricane storm surge 

effects to tsunami inundation effects is necessarily limited. In spite of these 

differences, however, observations from Hurricane Katrina appear to support 

many of the effects documented with tsunami inundation and the conclusions 

drawn from historic tsunami data. 

The worst storm surge in Hurricane Katrina was experienced between Pass 

Christian and Biloxi along the Mississippi coast, and thousands of lightframed single- and multi-family residences were totally destroyed or badly 

damaged by this surge (FEMA 549, 2006). However, consistent with 

Observations from Hurricane 

Katrina appear to support many of 

the effects documented with 

tsunami inundation and the 

conclusions drawn from historic 

tsunami data.

FEMA P-646 2: Background 31
observations from past tsunamis, most multi-story engineered buildings 

along the coastline survived the surge with damage limited to nonstructural 

elements at the lower levels (Figure 2-31).

Figure 2-31 Pass Christian office building with cast-in-place concrete pan joist floor system that suffered nonstructural damage at first two floors but no structural damage (Hurricane Katrina, 2005).

Also consistent with past tsunami observations, Hurricane Katrina illustrated 

the effects of debris impact and damming. At the parking garage structure 

shown in Figure 2-32, impact from a barge-mounted casino failed a lower 

level column resulting in progressive collapse of the surrounding portions of 

the structure. In Figure 2-33, damming effects were significant enough to 

fail a series of prestressed concrete piles at a construction site when a 

shipping container lodged between the piles and blocked the surge flow.

Similar to uplift failures observed in the 2004 Indian Ocean Tsunami, uplift 

loading applied to the underside of floor systems is blamed for the collapse 

of elevated floor levels in numerous engineered structures. Parking garages 

constructed of precast prestressed concrete double-tee sections, like the one 

shown in Figure 2-34, were susceptible to upward loading caused by 

additional buoyancy forces from air trapped below the double-tee sections 

and upward hydrodynamic forces applied by the surge and wave action. 

Although most failures of this type did not result in collapse of the entire 

structure, loss of floor framing can lead to column damage, increased 

unbraced lengths, and progressive collapse of a disproportionate section of 

the building.

32 2: Background FEMA P-646
Figure 2-32 Progressive collapse of upper floors of a parking garage due to damage in 

lower level columns from impact of an adjacent barge-mounted casino

(Hurricane Katrina, 2005).

FEMA P-646 2: Background 33
Figure 2-33 Failure of prestressed piles due to damming effect of shipping container

(Hurricane Katrina, 2005).

Figure 2-34 Negative bending failure of a prestressed double-tee floor system due to uplift forces (Hurricane 

Katrina, 2005). 

2.2.5 Implications for Tsunami-Resistant Design 

Building survivability varies with construction type and tsunami runup 

height. While observations from past tsunamis show that certain types of 

construction are largely destroyed by high-velocity water flow, there is much 

evidence that appropriately designed structural systems can survive tsunami 

inundation with little more than nonstructural damage in the lower levels, 

and can continue to support the levels of a building above the flood depth. 

This enables consideration of vertical evacuation as a viable alternative when 

horizontal evacuation out of the inundation zone is not feasible. 

Observed effects from historic tsunami data, the 2004 Indian Ocean 

Tsunami, the 2011 Tohoku Japan Tsunami, and supporting evidence from 

extreme storm surge flooding associated with Hurricane Katrina result in the 

following implications for tsunami-resistant design: 

• Vertical evacuation structures must be tall enough to ensure safety of 

those seeking refuge even if the tsunami event exceeds the maximum 

There is much evidence that 

appropriately designed structural 

systems can survive tsunami 

inundation. 

This enables consideration of 

vertical evacuation as a viable 

alternative when horizontal 

evacuation out of the inundation 

zone is not feasible.

34 2: Background FEMA P-646
considered tsunami. They should be well-engineered reinforced concrete 

or steel-frame structures.

• In the case of near-source generated tsunami hazards, vertical evacuation 

structures must be designed for seismic loading in addition to tsunami 

load effects.

• Vertical evacuation structures should be located away from the wave 

breaking zone.

• Impact forces and damming effects from waterborne debris are 

significant and must be considered.

• When elevated floor levels are subject to inundation, uplift forces from 

added buoyancy due to trapped air and vertical hydrodynamic forces on 

the floor slab must be considered.

• Scour around the foundations must be considered.

Because of uncertainty in the nature of water-borne debris and the potential 

for very large forces due to impact, progressive collapse concepts should be 

employed in the design of vertical evacuation structures to minimize the 

possibility of disproportionate collapse of the structural system.

FEMA P-646 2: Background 35

Chapter 3

Tsunami Hazard Assessment

Tsunami hazard in a particular region is a combination of the presence of a 

geophysical tsunami source, exposure to tsunamis generated by that source, 

and the extent of inundation that can be expected as a result of a tsunami 

reaching the site. The consequences of that hazard to the population of a 

coastal community are a function of the time it takes a tsunami to propagate 

from a source to the site, maximum flood depth, maximum current velocity, 

integrity of the built environment, and the ability to evacuate to areas of 

refuge. 

Inundation is a complex process influenced by many factors. These include 

the source characteristics that determine the nature of the initially generated 

waves, the bathymetry that transforms the waves as they propagate to the 

shoreline, the topography traversed, the structures and other objects 

encountered, and the temporal variation in bathymetry, topography, 

structures and other objects caused by the impact of successive waves. In 

general, the physics of tsunami inundation is time-dependent, threedimensional, and highly nonlinear. 

Modeling of tsunami inundation is a key component of tsunami hazard 

assessment. Progress has been made in the development of modeling tools, 

but theory is still under development. This chapter provides an overview of 

currently available modeling tools and associated products available through 

nationally-coordinated efforts such as the National Oceanic and Atmospheric 

Administration (NOAA) Tsunami Program and the U.S. National Tsunami 

Hazard Mitigation Program (NTHMP).

3.1 Current Tsunami Modeling and Inundation Mapping

Site-specific inundation models and model-derived products, including maps, 

are essential for reliable tsunami hazard assessment. The NOAA Tsunami 

Program and the NTHMP are engaged in closely related modeling efforts. 

The NOAA Tsunami Program is focused on the development of the NOAA 

Tsunami Forecast System (Titov and Synolakis, 2005). The NTHMP Hazard 

Assessment effort is working on the development of inundation maps for 

emergency management programs (González, et al., 2005a). Both efforts are 

fundamentally dependent on tsunami numerical modeling technology.

Modeling of tsunami inundation is a 

key component of tsunami hazard 

assessment. Current efforts to 

characterize tsunami hazard 

include:

The NOAA Tsunami Program: 

Forecast Modeling and Mapping

The National Tsunami Hazard 

Mitigation Program: Credible WorstCase Scenarios

The FEMA Map Modernization 

Program: Probabilistic Tsunami 

Hazard Assessments

FEMA P-646 3: Tsunami Hazard Assessment 37
Tsunami modeling studies generally result in products that include a spatial 

mapping of the model output in either static or animated form. Primary 

tsunami wave parameters include the amplitude η(x,y,t) and associated 

current velocity components u(x,y,t) and v(x,y,t). A Geographic Information 

System (GIS) database of these output parameters and associated input data 

(e.g., model computational grids and source parameters) can be used to 

derive parameters such as flood depth, velocity, acceleration, and momentum 

flux. 

3.2 The NOAA Tsunami Program: Forecast Modeling and 

Mapping

As part of the Tsunami Forecasting System, NOAA is developing sitespecific inundation models at 75 sites shown in Figure 3-1. The National 

Center for Tsunami Research (NCTR) at the Pacific Marine Environmental 

Laboratory (PMEL) in Seattle, Washington, has the primary responsibility 

for this forecast modeling and mapping effort. The first step at each site is 

the development of a Reference Model using a grid with the finest resolution 

available, followed by extensive testing against all available data to achieve 

the highest possible accuracy. The second step is development of the Standby 

Inundation Model (SIM), which is used as the forecast model. This is done 

through modification of the grid to optimize for speed, yet retain a level of 

accuracy that is appropriate for operational forecast and warning purposes. 

The NCTR employs a suite of tsunami generation, propagation, and 

inundation codes developed by Titov and Gonzalez (1997). On local spatial 

scales, nonlinear shallow water (NSW) equations are solved numerically. 

Propagation on regional and transoceanic spatial scales requires equations 

that are expressed in spherical coordinates. Propagation solutions are 

obtained by a numerical technique that involves a mathematical 

transformation known as splitting (Titov, 1997). Consequently, this suite of 

models has become known as the Method of Splitting Tsunamis (MOST) 

(Tang, 2009) model. 

Because life and property are at stake when tsunami warnings are issued, 

NOAA requires that models used in the Tsunami Forecasting System meet 

certain standards (Synolakis, 2006). Among the requirements are:

• Peer-reviewed publication. A peer-reviewed article must be published 

that documents the scientific and numerical essentials of the model and 

includes at least one model comparison study using data from an 

historical tsunami.

38 3: Tsunami Hazard Assessment FEMA P-646
Figure 3-1 Coastal sites for site-specific tsunami inundation models for the 

Tsunami Forecasting System.

FEMA P-646 3: Tsunami Hazard Assessment 39
• Benchmarking. The model must be tested against other peer models in a 

benchmark workshop, and the results documented in a report. The 

National Science Foundation has supported two tsunami inundation 

modeling benchmark workshops (Yeh, et al., 1996; Liu, et al., 2006). 

The National Tsunami Hazard Mitigation Program (NTHMP) supported 

another benchmarking workshop in 2011 (NTHMP MMS Tsunami 

Inundation Model Validation Workshop, 3-28-2011 to 4-1-2011, Texas 

A&M Galveston campus) for which a peer-reviewed proceedings volume 

is in preparation (NTHMP, 2012).

• Operational assessment. Important factors to be assessed include the 

model speed, accuracy, special operating environment needs, ease of use, 

and documentation.

Models meeting these requirements include the ADvanced CIRCulation 

(ADCIRC) model (Luettich and Westerink, 1991, 1995a, and 1995b; Myers 

and Baptista, 1995), hydrodynamic models of Kowalik and Murty (1993a, 

1993b) as applied and field-checked against observed inundation in Alaska 

by Suleimani and others (2002a; 2002b), and the MOST model (Titov and 

Synolakis, 1998). 

The MOST model has been extensively tested against laboratory 

experimental data and deep-ocean and inundation field measurements, and 

by successful modeling of benchmarking problems through participation in 

NSF-sponsored tsunami inundation model benchmark workshops. 

As of June 2008, reference inundation models and forecast models have been 

completed using the MOST model for seven sites in Alaska, four sites in 

Washington, three sites in Oregon, five sites in California, seven sites in 

Hawaii, one site in North Carolina, and one site in South Carolina. Planned 

and completed sites are shown in Figure 3-1. 

The primary function of these models is to provide NOAA Tsunami Warning 

Centers with real-time forecasts of coastal community inundation before and 

during an actual tsunami event. However, these site-specific inundation 

models can be applied to inundation modeling studies and the creation of 

inundation parameter databases, digital products, and maps specifically 

tailored to the design process. 

Models provisionally validated in the most recent benchmark workshop 

include the Alaska Tsunami Forecast Model from the West Coast and Alaska 

Tsunami Warning Center, the Alaska Tsunami Model from the University of 

Alaska, SELFE from the Oregon Health Sciences Institute, FUNWAVE, 

from the Universities of Delaware and Rhode Island, THETIS from the 

40 3: Tsunami Hazard Assessment FEMA P-646
Université de Pau et des Pays de l’Adour and University of Rhode Island, 

BOSZ and NEOWAVE from the University of Hawaii, TSUNAMI3D from 

University of Alaska and Texas A&M Galveston, GeoClaw from the 

University of Washington, and the MOST model from the Pacific Marine 

Environmental Laboratory. Most of these models are described elsewhere but 

the benchmark validation documentation is in preparation.

3.3 The National Tsunami Hazard Mitigation Program: 

Credible Worst-Case Scenarios

State mapping efforts performed as part of the National Tsunami Hazard 

Mitigation Program (NTHMP) are based on credible worst-case scenarios. 

Credible worst-case scenario maps are based on a geophysical tsunami 

source that can be scientifically defended as a worst-case scenario for a 

particular region or community, and a tsunami inundation model simulation 

for that scenario. The simulation output becomes the basis for maps that 

typically display maximum inundation depth and maximum current speed or 

velocity. Example worst-case scenario inundation model results for Seattle, 

Washington are shown in Figure 3-2. These products are provided to state 

geotechnical scientists, who then produce official state inundation maps such 

as the one for Seattle, Washington shown in Figure 3-3. 

Figure 3-2 Tsunami inundation modeling products for Seattle, Washington. 

Left panel: zoned estimates of maximum inundation depth. 

Right panel: zoned estimates of maximum current (Titov, et al., 

2003).

FEMA P-646 3: Tsunami Hazard Assessment 41
Figure 3-3 Tsunami inundation map for Seattle, Washington produced and 

published by the state of Washington, using modeling products 

as guidance (Walsh et al., 2003).

These maps are considered essential for effective disaster planning and 

development of emergency management products and programs. They guide 

the development of evacuation maps, educational and training materials, and 

tsunami mitigation plans. By 2004, the NTHMP Hazard Assessment 

component had completed 22 inundation mapping efforts and 23 evacuation 

maps covering 113 communities and an estimated 1.2 million residents at 

risk (González, et al., 2005a). 

There are variations in state products because each state differs in its 

geophysical setting and the resulting tsunami regime including legislative 

goals, policies, agency structure, mission, scientific and technical 

infrastructure, and financial status. Differences between state mapping 

products include the following: 

• Although most credible worst-case scenarios are based on seismic 

sources, maps generated for Alaska and California also include landslide 

sources in the tsunami hazard assessment. 

• Oregon inundation maps, like the one for Yaquina Bay shown in Figure 

3-4, display three inundation lines to depict the uncertainty in the hazard 

posed by tsunamis from the local Cascadia subduction zone. 

42 3: Tsunami Hazard Assessment FEMA P-646
• In addition to worst-case scenarios, maps in Alaska also depict 

inundation from a number of locally-generated scenario tsunamis.

Figure 3-4 Yaquina Bay, Oregon tsunami inundation map with three 

inundation lines (Priest et al., 1997a; Priest et al.,1997b).

Detailed tsunami inundation simulations for credible worst-case scenarios 

can also be used to derive parameters such as flood depth, velocity, 

acceleration, and momentum flux, which are used to calculate forces for 

tsunami-resistant design. These data are archived with the state government 

hazard mapping agencies, cooperating academic institutions, and the NOAA 

Pacific Marine Environmental Laboratory. Currently, a central archive for 

all state mapping products does not exist. However, existing maps and 

FEMA P-646 3: Tsunami Hazard Assessment 43
reports are available for viewing, download, or purchase from the following 

state web sites:

• Alaska: http://www.dggs.dnr.state.ak.us/pubs/publisher/dggs

• California: http://www.conservation.ca.gov/cgs/geologic_hazards

/Tsunami/Inundation_Maps/Pages/Index.aspx

• Oregon: http://www.oregongeology.com/sub/publications/IMS/ims.htm

• Washington:

http://www.dnr.wa.gov/Publications/ger_tsunami_inundation_maps.pdf

3.4 The FEMA Map Modernization Program: Probabilistic 

Tsunami Hazard Assessments

On the regional scale, FEMA (1997) presents a probabilistic estimate of the 

tsunami hazard for the West coast, Alaska, and Hawaii (Figure 3-5). On the 

local scale, FEMA Flood Insurance Rate Maps (FIRMs) present area-specific 

flooding scenarios for 100-year and, occasionally, 500-year events (i.e., 

events with a 1% and a 0.2% annual probability of exceedance, respectively). 

The FIRMs provide a basis for establishing flood insurance premiums in 

communities that participate in the National Flood Insurance Program 

(NFIP), which is administered by FEMA. These maps were based on tsunami 

hazard assessment methods developed prior to 1990. To evaluate the 

underlying methodologies used to assess tsunami and other coastal flooding 

hazards, FEMA formed focused study groups for each of the flooding 

mechanisms. The Tsunami Focused Study Group found that the current 

treatment of tsunami inundation is inadequate, and recommended a joint 

NOAA/U.S. Geological Survey (USGS) pilot study to develop an appropriate 

methodology for Probabilistic Tsunami Hazard Assessments (PTHA) that 

could be used to update FIRMs (Chowdhury, et al., 2005). 

In the joint NOAA/USGS/FEMA Seaside, Oregon Tsunami Pilot Study 

(Tsunami Pilot Study Working Group, 2006), USGS and academic 

colleagues developed a database of near- and far-field tsunami sources 

associated with a specified probability of occurrence, while NOAA 

developed a corresponding database of inundation model results based on the 

sources. The resulting PTHA methodology integrates hydrodynamics, 

geophysics, and probability theory to meet specific FEMA actuarial needs, 

and now represents the current state of the art in tsunami hazard assessment 

for emergency management and engineering design. 

44 3: Tsunami Hazard Assessment FEMA P-646
Figure 3-5 Tsunami elevations with a 90% probability of not being 

exceeded in 50 years (FEMA, 1997).

The 500-year maximum tsunami wave height map for Seaside, Oregon 

shown in Figure 3-6 is an example of the type of product that can be 

generated by such a study. The resulting GIS database of all model inputs, 

outputs, and related data can be used to conduct in-depth, site-specific 

probabilistic studies of tsunami hazard for design of vertical evacuation 

structures.

FEMA P-646 3: Tsunami Hazard Assessment 45
Figure 3-6 The 500-year tsunami map for Seaside, Oregon, depicting 

maximum wave heights that are met or exceeded at an annual 

probability of 0.2% (Tsunami Pilot Study Working Group, 

2006).

3.5 Limitations in Available Modeling and Mapping 

Products

The quality, content, and availability of currently available modeling and 

mapping products are limited. Quality varies considerably and, in many 

cases, cannot be assessed because standard modeling and mapping 

procedures have not been adopted. Most maps do not provide estimates of 

currents, so their content is often inadequate for use in design. Digital model 

products are generally not available to derive the more relevant parameters 

needed for calculation of forces on structures. Availability of information is 

limited because a central repository for maps and other model products does 

not exist. 

46 3: Tsunami Hazard Assessment FEMA P-646
Limitations in bathymetric and topographic databases are being addressed 

through coordination of NOAA, USGS, and NTHMP to improve the 

coverage, quality and availability of the data, but this is an ongoing effort. 

3.6 Hazard Quantification for Design of Tsunami Vertical 

Evacuation Structures

Given a known or perceived tsunami threat in a region, the first step is to 

determine the severity of the tsunami hazard. This involves identification of 

potential tsunami-genic sources and accumulation of recorded data on 

tsunami occurrence and runup. This can include a probabilistic assessment 

considering all possible tsunami sources, or a deterministic assessment 

considering the maximum tsunami that can reasonably be expected to affect a 

site. 

Once potential tsunami sources are identified, and the severity of the tsunami 

hazard is known, site-specific information on the extent of inundation, height 

of runup, and velocity of flow is needed. Some of this information can be 

obtained from available tsunami inundation maps, where they exist; 

otherwise site-specific tsunami inundation studies must be performed. In the 

absence of available maps or site-specific inundation studies, analytical 

solutions can be used to estimate tsunami inundation parameters for 

preliminary or approximate design. Analytical solutions for flow velocity, 

depth, and momentum flux are provided in Chapter 6 and Appendix E. 

In this document, the design tsunami event is termed the Maximum 

Considered Tsunami (MCT). There is, however, no firm policy or 

methodology for setting a Maximum Considered Tsunami at a specified 

hazard level. For the design criteria contained within this document, it is 

anticipated that the hazard level corresponding to the Maximum Considered 

Tsunami will be consistent with a 2500-year return period. The hazard level 

for tsunamis is therefore similar to the return period associated with the 

Maximum Considered Earthquake used in seismic design. However, the 

Maximum Considered Tsunami is not defined to be the same as the 

Maximum Considered Earthquake because the tsunami source may be distant 

rather than local.

Existing tsunami hazard assessments in some areas may be adequate for the 

design of vertical evacuation structures. Even if published hazard maps do 

not include velocity and depth information, the underlying modeling might. 

Where the NTHMP has been producing tsunami inundation maps (Alaska, 

California, Hawaii, Oregon, Puerto Rico, and Washington), the state hazard 

assessment team (http://nthmp.pmel.noaa.gov) will provide details of the 

Tsunami hazard can be 

characterized by:

(1) a probabilistic assessment 

considering all possible tsunami 

sources; or 

(2) a deterministic assessment 

considering the maximum tsunami 

that can reasonably be expected to 

affect a site.

The Maximum Considered 

Tsunami (MCT) is the design 

tsunami event. For site-specific 

tsunami hazard assessments, the 

Maximum Considered Tsunami 

should be developed using the 

Deterministic Maximum Considered 

Earthquake as the initial condition 

of the tsunami model.

FEMA P-646 3: Tsunami Hazard Assessment 47
appropriate modeling parameters and can either perform the assessment or 

provide a referral.

For site-specific tsunami hazard assessments, the Maximum Considered 

Tsunami should be developed using the tsunami-genic seismic events 

determined from a probabilistic hazard analysis. At a minimum, this analysis 

should also be checked for a Deterministic MCE for a near-source-generated 

tsunami in the United States evaluated as the largest potentially tsunamigenic earthquake reported in the “Quaternary Fault and Fold Database of the 

United States” http://earthquake.usgs.gov/regional/qfaults/. 

Where the greatest threat is from a far-source-generated tsunami, selection of 

a Maximum Considered Tsunami is more difficult. At a minimum, it should 

be based on the largest event recorded in the National Geophysical Data 

Center (NGDC) Historical Tsunami Database (http://www.ngdc.noaa.gov/

hazard/tsu_db.shtml) with allowance for the limited accuracy, quantity, and 

period of time covered by the historic record. It should also consider the 

largest earthquakes likely in all regions that have generated historic tsunamis 

affecting the site being considered. The NOAA forecast modeling program 

may be able to model a Maximum Considered Tsunami for these cases using 

the reference inundation models that have higher resolution and larger 

computational domains rather than the tsunami inundation models used for 

real-time forecasting. 

Tsunami inundation modeling is not routinely available commercially, but is 

performed by a number of organizations including government laboratories 

(USGS, NOAA, Los Alamos National Laboratory), selected universities 

(Cornell University, Oregon Health and Science University, Texas A&M 

Galveston, University of Hawaii, University of Alaska Fairbanks, University 

of Rhode Island, University of Southern California, University of 

Washington), and some consulting companies. An extensive bibliography of 

past tsunami-related research in modeling is available in Wiegel (2005, 

2006a, 2006b, and 2008). The NTHMP has suggested some minimum 

guidelines regarding tsunami hazard mapping and modeling. 

• Models should meet the benchmark standards (Synolakis and others, 

2007), which were recently updated at the benchmarking workshop in 

Galveston (Horillo and others, in preparation). 

• Digital Elevation Models (DEMs) used to develop modeling grids near 

shore should be at a resolution of at least 1/3 arc second, or about ten 

meters, but should not be smaller than the spacing of the source 

topographic data unless necessary to resolve important morphologic 

features. 

48 3: Tsunami Hazard Assessment FEMA P-646
• DEMs should be based on the most accurate digital elevation model 

available. Lidar is becoming increasingly available and can achieve

vertical accuracy of <1 foot.

• Model runtime should be sufficient to capture the maximum inundation 

and drawdown of the tsunami simulation.

• The computational grid developed from the DEM should be fine enough 

that any topographic or bathymetric feature that has an impact on 

inundation should be represented by more than three grid cells.

• The computational grid domain should be large enough to capture all 

important tsunami wave dynamics.

• A vertical datum of Mean High Water should be used to capture tidal 

conditions or an alternative maximum flooding condition should be used 

in modeling for tsunamis in lakes. 

It should be noted that the above recommendations do not include modeling 

for tsunamis induced by landslides, volcanoes, or meteorite impacts.

3.7 Recommendations to Improve Tsunami Hazard 

Assessment

Similar to design for other hazards, a desirable goal for tsunami-resistant 

design of vertical evacuation structures is to achieve a uniform level of safety 

across all communities subjected to tsunami risk. ASCE 7, Minimum Design 

Loads for Buildings and Other Structures, is based on achieving structural 

reliability performance goals using probabilistic definitions of all hazards. In 

seismic and wind design, the starting point is probabilistic mapping of 

earthquake and wind hazard. The hazard is further refined by considering 

local effects such as soil type for seismic design, and topographic effects for 

wind design. Similar concepts can be used for tsunami design. Essential 

tools for tsunami hazard assessment are tsunami inundation models, maps, 

and comprehensive databases of tsunami inundation parameters. 

Although more difficult for the public to interpret since they do not represent 

a single selected scenario, probabilistic maps for tsunami hazard can be made 

and are needed for reliable design of tsunami-resistant structures for uniform 

risk (Geist and Parsons, 2006). The probabilistic approach also provides a 

means to account for uncertainty. Probabilistic analysis of tsunamis is also 

important in explicitly defining the probability associated with individual 

deterministic scenarios.

FEMA P-646 3: Tsunami Hazard Assessment 49

Chapter 4 

Vertical Evacuation Options

A vertical evacuation refuge from tsunamis is a building or earthen mound 

that has sufficient height to elevate evacuees above the level of tsunami 

inundation, and is designed and constructed with the strength and resiliency 

needed to resist the effects of tsunami waves. Vertical evacuation refuges

can be stand-alone or part of a larger facility. They can be single-purpose 

refuge-only facilities, or multi-purpose facilities in regular use when not 

serving as a refuge. They can also be single-hazard (tsunami only) or multihazard facilities. 

In concept, these options are applicable to new or existing structures, but it 

will generally be more difficult to retrofit an existing structure than to build a 

new tsunami-resistant structure using these criteria. This chapter describes 

the features of different vertical evacuation options that are available, and 

provides guidance to assist in choosing between various options. 

It should be stressed that evacuation to high ground is always preferred 

where access to nearby high ground exists. This provides the option for 

refugees to move to even higher ground if the tsunami inundation is greater 

than anticipated, something that may not be possible in an evacuation 

building or earthen mound because of the height limitation of the refuge. 

4.1 Vertical Evacuation Considerations

Vertical evacuation structures can be intended for general use by the 

surrounding population, or by the occupants of a specific building or group 

of buildings. Choosing between various options available for vertical 

evacuation structures will depend on emergency response planning and needs 

of the community, the type of construction and use of the buildings in the 

immediate vicinity, and the project-specific financial situation of the state, 

municipality, local community, or private owner considering such a 

structure.

4.1.1 Single-Purpose Facilities

The tsunami hazard assessment and inundation study may show that the best 

solution is to build new, separate (i.e., stand-alone) facilities specifically 

designed and configured to serve as vertical evacuation structures. Potential 

advantages of single-purpose, stand-alone facilities include the following: 

In concept, vertical evacuation 

options are applicable to new or 

existing structures, but it will 

generally be more difficult to 

retrofit an existing structure than to 

build a new tsunami-resistant 

structure using these criteria.

Vertical evacuation facilities can be 

single-purpose, multi-purpose, or 

multi-hazard facilities. 

FEMA P-646 4: Vertical Evacuation Options 51
• They can be sited away from potential debris sources or other site 

hazards. 

• They do not need to be integrated into an existing building design or 

compromised by design considerations for potentially conflicting usages. 

• They are structurally separate from other buildings and therefore not 

subject to the potential vulnerabilities of other building structures. 

• They will always be ready for occupants and will not be cluttered with 

furnishings or storage items associated with other uses. 

• Single-purpose, stand-alone structures will likely be simpler to design, 

permit, and construct because they will not be required to provide normal 

daily accommodations for people. They can have simplified prototypical 

structural systems, resulting in lower initial construction costs. 

One example of a single-purpose facility is a small, elevated structure with 

the sole function of providing an elevated refuge for the surrounding area in 

the event of a tsunami. A possible application for such a facility would 

include low-lying residential neighborhoods where evacuation routes are not 

adequate, and taller safer structures do not exist in the area. 

4.1.2 Multi-Purpose Facilities

A coastal community may not have sufficient resources to develop a singlepurpose tsunami vertical evacuation structure or a series of structures, so 

creative ways of overcoming economic constraints are required. Possible 

solutions include co-location of evacuation facilities with other communitybased functions, co-location with commercial-based functions, and economic 

or other incentives for private developers to provide tsunami-resistant areas 

of refuge within their developments. The ability to use a facility for more 

than one purpose provides immediate possibility for a return on investment 

through daily business or commercial use when the structure is not needed as 

a refuge. 

Multi-purpose facilities can also be constructed to serve a specific need or 

function in a community, in addition to vertical evacuation refuge. Examples 

include elevated man-made earthen berms used as community open spaces. 

In downtown areas or business districts, they can be specially constructed 

private or municipal parking structures incorporating tsunami resistant 

design. On school campuses, vertical evacuation facilities could serve as 

gymnasiums or lunchrooms on a daily basis. In residential subdivisions, they 

can be used as community centers. 

52 4: Vertical Evacuation Options FEMA P-646
4.1.3 Multi-Hazard Considerations

Communities exposed to other hazards (e.g., earthquakes, hurricanes) may 

choose to consider the possible sheltering needs associated with these other

hazards, in addition to tsunamis. This could include allowances for different 

occupancy durations, consideration of different post-event rescue and 

recovery activities, and evaluation of short- and long-term medical care

needs. 

Designing for multiple hazards requires consideration of the load effects that 

might be unique to each type of hazard. This can pose unique challenges for 

the resulting structural design. For example, the structural system for vertical 

evacuation structures exposed to near-source-generated tsunamis will likely 

need to be designed for seismic hazards. Such a structure might include 

break-away walls or open construction in the lower levels to allow water to 

pass through with minimal resistance. Open construction in the lower levels 

of a multi-story structure are contrary to earthquake engineering practice to 

avoid soft or weak stories in earthquake-resistant construction. Proper design 

and construction will need to include special consideration by the structural 

engineer of these and other potential conflicting recommendations. 

4.2 Vertical Evacuation Concepts

To provide refuge from tsunami inundation, vertical evacuation solutions 

must have the ability to receive a large number of people in a short time 

frame and efficiently transport them to areas of refuge that are located above 

the level of flooding. Potential vertical evacuation solutions can include 

areas of naturally occurring high ground, areas of artificial high ground 

created through the use of soil berms, new structures specifically designed to 

be tsunami-resistant, or existing structures demonstrated to have sufficient 

strength to resist anticipated tsunami effects. 

Nonstructural systems and contents located in the levels below the 

inundation depth should be assumed to be a total loss if the design tsunami 

occurs. If the building is required to remain functional in the event of a 

disaster, the loss of lower level walls, nonstructural systems, and contents 

should be taken into account in the design of the facility and selection of 

possible alternative uses. 

4.2.1 Existing High Ground

Naturally occurring areas of high ground may be able to be utilized or 

modified to create a refuge for tsunami vertical evacuation. Large open areas 

offer easy access for large numbers of evacuees with the added advantage of 

avoiding the possible apprehension about entering a building following an 

Vertical evacuation structures can be 

soil berms, parking garages, 

community facilities, commercial 

facilities, school facilities, or existing 

buildings. 

FEMA P-646 4: Vertical Evacuation Options 53
earthquake. In addition, most coastal communities have educated their 

populations to “go to high ground” in the event of a tsunami warning. The 

topography of the existing high ground should be evaluated for the potential 

of wave runup or erosion. Some modification of the existing topography may 

be required to address these issues. 

4.2.2 Soil Berms

If natural high ground is not available, a soil berm can be constructed to raise 

the ground level above the tsunami runup height, as shown in Figure 4-1. 

Although care must be taken to protect the sides of the soil berm from the 

incoming and outgoing tsunami waves, this option can be relatively costeffective in comparison to building a stand-alone structure. The height of the 

berm must be sufficient to avoid becoming inundated, and the slope of the 

sides must allow for ingress. A maximum ramp slope in the range of one

foot vertical rise to four feet horizontal run (1 in 4) is recommended. Soil 

berms have the added benefit that they are immune to damage from large 

debris strikes such as shipping containers, barges and ships, making them 

suitable for locations near port facilities (Figure 4-1). 

Figure 4-1 Soil berm combined with a community park at Sendai Port, 

Japan. Concrete lining on the ocean face can deflect incoming 

waves while sloped sides provide for quick access. Graphic in 

the lower right side illustrates where the evacuation berm is 

located in Sendai Port. 

54 4: Vertical Evacuation Options FEMA P-646
4.2.3 Multi-Story Parking Garages

Parking garages are good candidates for use as vertical evacuation structures. 

Similar to the example shown in Figure 4-2, most parking garages are open 

structures that will allow water to flow through with minimal resistance. 

They can also be open for pedestrian access at any time of the day or night. 

Interior ramps allow ample opportunity for ingress, and easy vertical 

circulation to higher levels within the structure. Parking garages can also be 

used to provide additional community amenities on the top level, including 

parks, observation decks, and sports courts. They are also obvious revenuegenerating facilities, especially in areas that attract large numbers of tourists. 

Parking garages, however, tend to be constructed using low-cost, efficient 

structural systems with minimal redundancy. If designed with higher 

performance objectives in mind, and if subjected to additional code review 

and construction inspection by local jurisdictions, parking garages could be 

effective vertical evacuation structures. 

Figure 4-2 Cast-in-place reinforced concrete parking garage in Biloxi, 

Mississippi after Hurricane Katrina. Open structural systems 

allow water to pass through with minimal resistance, and 

interior ramps allow for easy ingress and vertical circulation.

4.2.4 Community Facilities

Vertical evacuation structures could be developed as part of other 

community-based needs such as community centers, recreational facilities, 

sports complexes, libraries, museums, and police or fire stations. One such 

FEMA P-646 4: Vertical Evacuation Options 55
example is shown in Figure 4-3. When not in use as a refuge, facilities such 

as these can be useful for a variety of functions that enhance the quality of 

life in a community. When choosing alternative uses for a vertical 

evacuation facility, consideration should be given to potential impacts that

other uses might have on the vertical evacuation function. Potential negative 

impacts could include clutter that could become debris that disrupts ingress. 

Limited access after regular operating hours would make it difficult to use a 

facility for evacuation from a tsunami that could occur at any time of the day 

or night. Priority should be given to uses with complementary functions, 

such as accommodations for large numbers of people and 24-hour access. 

4.2.5 Commercial Facilities

Vertical evacuation structures could be developed as part of business or other 

commercial facilities including multi-level hotels, restaurants, or retail 

establishments, as shown in Figure 4-4. For example, if the refuge area is 

part of a hotel complex, meeting rooms, ballrooms, and exhibit spaces that 

are located above the tsunami inundation elevation could be used to provide 

refuge when the tsunami occurs. The apartment building shown in Figure 4-5 

was used successfully as a vertical evacuation structure during the Tohoku 

tsunami. Exterior stairs provided 24 hour access to the upper floors 

designated as the evacuation refuge.

Figure 4-3 Sports complex. Designed for assembly use, this type of 

structure can accommodate circulation and service needs for 

large numbers of people. 

56 4: Vertical Evacuation Options FEMA P-646
Figure 4-4 Hotel and convention complex. Meeting rooms, ballrooms, and 

exhibit spaces located above the tsunami inundation elevation can 

be used to provide areas of refuge.

Figure 4-5 Residential apartment building in Kamaishi, Japan, with designated 

refuge area at or above the fourth level.

FEMA P-646 4: Vertical Evacuation Options 57
4.2.6 School Facilities

Similar to community facilities, public and private school facilities have the 

benefit of providing useful and essential services to the communities in 

which they reside. Ongoing construction of schools provides an opportunity 

and potential funding mechanism for co-located tsunami vertical evacuation 

structures. This has the added benefit of possible additional public support 

for projects that increase the safety of school-age children. Obviously these 

buildings must be tall enough or sited on high ground so that they are useful 

as tsunami refuge areas.

4.2.7 Existing Buildings

Historic damage patterns suggest that many structures not specifically 

designed for tsunami loading can survive tsunami inundation and provide 

areas of refuge. It is possible that some existing structures could serve as 

vertical evacuation structures or could be made more tsunami-resistant with 

only minor modifications. An assessment of both the functional needs and 

potential structural vulnerabilities would be required to determine if an 

existing building can serve as a vertical evacuation structure. 

In some situations, providing some level of protection is better than none. 

An example of this concept is shown in Figure 4-6. In a tsunami evacuation 

map for Waikiki, it is noted that “structural steel or reinforced concrete 

buildings of six or more stories provide increased protection on or above the 

third floor”, and are identified as potential areas of refuge.

Figure 4-6 Evacuation map for Waikiki, Hawaii, indicating use of existing 

buildings for vertical evacuation. 

58 4: Vertical Evacuation Options FEMA P-646
Chapter 5 

Siting, Spacing, Sizing, and 

Elevation Considerations

Tsunami risk is unique in that some communities may be susceptible to farsource-generated tsunamis (longer warning time), near-source-generated 

tsunamis (shorter warning time), or both. Far-source-generated tsunamis 

generally allow sufficient warning time so that emergency response plans can 

be based on evacuation out of the inundation zone. Near-source-generated 

tsunamis may not allow sufficient time for evacuation, so emergency 

response plans may need to include vertical evacuation refuge. This chapter 

provides guidance on how to locate vertical evacuation refuges within a 

community, and how to determine the size of a vertical evacuation structure. 

5.1 Siting Considerations

Vertical evacuation structures should be located such that all persons 

designated to take refuge can reach the structure within the time available 

between tsunami warning and tsunami inundation. Travel time must also 

take into consideration vertical circulation within the structure to levels 

above the tsunami inundation elevation. Structures located at one end of a 

community may be difficult for some users to reach in a timely fashion. 

Routes to the structure should be easily accessible and well-marked.

Location of vertical evacuation structures within a community should take 

into account potential hazards in the vicinity of a site that could jeopardize 

the safety of the structure, and should consider that natural behaviors of 

persons attempting to avoid coastal flooding. 

5.1.1 Warning, Travel Time, and Spacing 

The West Coast and Alaska Tsunami Warning Center (WC/ATWC) in 

Alaska, and the Pacific Tsunami Warning Center (PTWC) in Hawaii monitor 

potential tsunamis, and warn affected populations of an impending tsunami. 

Table 5-1 summarizes approximate warning times associated with the 

distance between a tsunami-genic source and the site of interest. A farsource-generated tsunami originates from a source that is far away from the 

site, and could have 2 hours or more of advance warning time. A nearsource-generated tsunami originates from a source that is close to the site, 

Vertical evacuation 

structures should be 

located such that all persons 

designated to take refuge 

can reach the structure within 

the time available between 

tsunami warning and tsunami 

inundation. 

FEMA P-646 5: Siting, Spacing, Sizing, and Elevation Considerations 59
and could have 30 minutes or less of advance warning time. Sites 

experiencing near-source-generated tsunamis will generally feel the effects 

of the triggering event (e.g., shaking caused by a near-source earthquake), 

and these effects will likely be the first warning of the impending tsunami. A 

mid-source-generated tsunami is one in which the source is somewhat close 

to the site of interest, but not close enough for the effects of the tsunami 

generating event to be felt at the site. Mid-source-generated tsunamis would 

be expected to have between 30 minutes and 2 hours of advance warning 

time.

Table 5-1 Tsunami Sources and Approximate Warning Times

Location of Source Approximate Warning Time (t)

Far-source-generated tsunami t > 2 hrs

Mid-source-generated tsunami 30 min < t < 2 hrs

Near-source-generated tsunami t < 30 min

Consideration must be given to the time it would take for designated

occupants to reach a refuge. To determine the maximum spacing of tsunami 

vertical evacuation structures, the critical parameters are warning time and 

ambulatory capability of the surrounding community. Once maximum 

spacing is determined, size must be considered, and population becomes an 

important parameter. Sizing considerations could necessitate an adjustment 

in the number and spacing of vertical evacuation structures if it is not feasible 

to size the resulting structures large enough to accommodate the surrounding 

population at the maximum spacing. Sizing considerations are discussed in 

Section 5.2.

The average, healthy person can walk at approximately 4-mph. Portions of 

the population in a community, however, may have restricted ambulatory 

capability due to age, health, or disability. The average pace of a mobilityimpaired population can be assumed to be about 2-mph.

Assuming a 2-hour warning time associated with far-source-generated 

tsunamis, vertical evacuation structures would need to be located a maximum 

of 4 miles from any given starting point. This would result in a maximum 

spacing of approximately 8 miles between structures. Similarly, assuming a

30 minute warning time, vertical evacuation structures would need to be 

located a maximum of 1 mile from any given starting point, or 2 miles 

between structures. Shorter warning times would require even closer 

spacing. Table 5-2 summarizes maximum spacing of vertical evacuation 

structures based on travel time associated with a mobility-impaired 

population. 

Recommended maximum 

spacing of vertical 

evacuation structures 

depends on warning time, 

ambulatory speed, and the 

surrounding population 

density.

60 5: Siting, Spacing, Sizing, and Elevation Considerations FEMA P-646
Table 5-2 Maximum Spacing of Vertical Evacuation Structures Based on 

Travel Time 

Warning Time

Ambulatory 

Speed Travel Distance Maximum Spacing

2 hrs 2 mph* 4 miles 8 miles

30 min 2 mph* 1 mile 2 miles

15 min 2 mph* 1⁄2 mile 1 mile

* Based on the average pace for a mobility-impaired population

5.1.2 Ingress and Vertical Circulation

Tsunami vertical evacuation structures should be spaced such that people 

will have adequate time not only to reach the structure, but to enter and move 

within the structure to areas of refuge that are located above the anticipated 

tsunami inundation elevation. 

Increased travel times may need to be considered if obstructions exist, or 

could occur, along the travel or ingress route. Unstable or poorly secured 

structural or architectural elements that collapse in and around the entrance, 

or the presence of contents associated with the non-refuge uses of a structure, 

could potentially impede ingress. Allowance for parking at a vertical 

evacuation refuge may decrease travel time to the refuge, but could 

complicate access when the potential traffic jams are considered. 

Stairs or elevators are traditional methods of ingress and vertical circulation

in buildings, especially when designated users have impaired mobility. 

Ramps, such as the ones used in sporting venues, however, can be more 

effective for moving large numbers of people into and up to refuge areas in a 

structure. Estimates of travel time may need adjustment for different 

methods of vertical circulation. Disabled users may need to travel along a 

special route that accommodates wheelchairs, and those with special needs 

may require assistance from others to move within the structure. 

When locating vertical evacuation structures, natural and learned behaviors 

of evacuees should be considered. Most coastal communities have educated 

their populations to “go to high ground” in the event of a tsunami warning. 

Also, a natural tendency for evacuees will be to migrate away from the shore. 

Vertical evacuation structures should therefore be located on the inland side 

of evacuation zones and should take advantage of naturally occurring 

topography that would tend to draw evacuees towards them. Figure 5-1 

illustrates an arrangement of vertical evacuation structures in a community 

based on these principles.

FEMA P-646 5: Siting, Spacing, Sizing, and Elevation Considerations 61
Figure 5-1 Vertical evacuation refuge locations considering travel distance, 

evacuation behavior, and naturally occurring high ground. 

Arrows show anticipated vertical evacuation routes.

5.1.3 Consideration of Site Hazards

Special hazards in the vicinity of each site should be considered in locating 

vertical evacuation structures. Potential site hazards include breaking waves, 

sources of large waterborne debris, and sources of waterborne hazardous 

materials. When possible, vertical evacuation structures should be located 

away from potential hazards that could result in additional damage to the 

structure and reduced safety for the occupants. Due to limited availability of 

possible sites, and limitations on travel and mobility of the population in a 

community, some vertical evacuation structures may need to be located at 

sites that would be considered less than ideal. Figure 5-2 illustrates adjacent 

site hazards that could exist in a typical coastal community. 

Potential site hazards 

include breaking waves, 

sources of large waterborne 

debris, and sources of 

waterborne hazardous 

materials.

62 5: Siting, Spacing, Sizing, and Elevation Considerations FEMA P-646
Figure 5-2 Site hazards adjacent to vertical evacuation structures

(numbered locations). Arrows show anticipated vertical 

evacuation routes.

Wave breaking takes place where the water depth is sufficiently finite. In the 

design of usual coastal structures (e.g., breakwaters, seawalls, jetties), critical 

wave forces often result from breaking waves. In general, tsunamis break 

offshore. In the case of very steep terrain, however, they can break right at 

the shoreline, which is known as a collapsing breaker. 

Forces from collapsing breakers can be extremely high and very uncertain. 

Location of vertical evacuation structures within the tsunami wave-breaking 

zone poses unknown additional risk to the structure. While the possibility of 

tsunami wave breaking at an on-shore location is not zero, it is considered to 

be very rare. For these reasons, recommended sites for vertical evacuation 

structures are located inland of the wave-breaking zone, and wave breaking 

forces are not considered in this document. 

In Figure 5-2, vertical evacuation structures are located some distance inland 

from the shoreline. Structure No. 1 is located adjacent to a harbor and 

container terminal. Impact forces from ships, barges, boats, and other 

FEMA P-646 5: Siting, Spacing, Sizing, and Elevation Considerations 63
waterborne debris have the potential to become very large. Locations with 

additional sources of large, possibly buoyant debris increase the chances of 

impact by one or more waterborne missiles, and increase the potential risk to 

the structure. If possible, it would be better if this structure was sited away 

from the harbor and container terminal. If there is no alternative location 

available to serve this area of the community, this structure would need to be 

designed for potential impact from the shipping containers and boats likely to 

be present during tsunami inundation.

Structure No. 2 is located off to the side of the harbor and adjacent to a 

parking lot. This structure would need to be designed for debris consistent 

with the use of the parking lot and surrounding areas, which could include 

cars, trucks, and recreational vehicles.

Structure No. 3 is immediately adjacent to a gas station. In past tsunamis, 

ignition of flammable chemicals or other floating debris has resulted in 

significant risk for fire in partially submerged structures. Depending on the 

potential for fuel leakage from this station in the event of a tsunami (or a 

preceding earthquake), this structure would need to be designed with fire 

resistive construction and additional fire protection.

Structure No. 4 is adjacent to a waterfront park facility. This location can be 

ideal, as the potential for waterborne debris can be relatively low. Possible 

hazards could include debris from park structures, naturally occurring 

driftwood, or larger logs from downed trees. This area has a higher potential 

for tourists and visitors unfamiliar with the area. It would require additional 

signage to inform park users what to do and where to go in the event of a 

tsunami warning.

Structure No. 5 is adjacent to an emergency response facility. Co-locating at 

such facilities can provide opportunities for direct supervision by lawenforcement and monitoring and support of refuge occupancies by other 

emergency response personnel. 

At two locations, Structure No. 6 is intended to aid evacuees in taking 

advantage of naturally occurring high ground. 

5.2 Sizing Considerations

Sizing of a vertical evacuation structure depends on the intended number of 

occupants, the type of occupancy, and the duration of occupancy. The 

number of occupants will depend on the surrounding population and the 

spacing and number of vertical evacuation structures located in the area. 

64 5: Siting, Spacing, Sizing, and Elevation Considerations FEMA P-646
Duration of occupancy will depend on the nature of the hazard and the 

intended function of the facility. 

5.2.1 Services and Occupancy Duration

A vertical evacuation structure is typically intended to provide a temporary 

place of refuge during a tsunami event. While tsunamis are generally 

considered to be short-duration events (i.e., pre-event warning period and 

event lasting about 8 to 12 hours), tsunamis include several cycles of waves. 

The potential for abnormally high tides and coastal flooding can last as long 

as 24 hours. 

A vertical evacuation structure must provide adequate services to evacuees 

for their intended length of stay. As a short term refuge, services can be 

minimal, including only limited space per occupant and basic sanitation 

needs. Additionally, a vertical evacuation structure could be used to provide 

accommodations and services for people whose homes have been damaged 

or destroyed. As a minimum, this would require an allowance for more 

space for occupants, supplies, and services. It could also include 

consideration of different post-event rescue and recovery activities, and 

evaluation of short- and long-term medical care needs. Guidance on basic 

community sheltering needs is not included in this document, but can be 

found in FEMA 361, Design and Construction Guidance for Community 

Shelters (FEMA, 2000a). 

Choosing to design and construct a vertical evacuation structure primarily for 

short-term refuge, or to supply and manage it to house evacuees for longer 

periods of time, is an emergency management issue that must be decided by 

the state, municipality, local community, or private owner. 

5.2.2 Square Footage Recommendations from Available 

Sheltering Guidelines 

Square footage recommendations are available from a number of different 

sources, and vary depending on the type of hazard and the anticipated 

duration of occupancy. The longer the anticipated stay, the greater the 

minimum square footage recommended. 

A shelter for mostly healthy, uninjured people for a short-term event would 

require the least square footage per occupant. A shelter intended to house 

sick or injured people, or to provide ongoing medical care, would require 

more square footage to accommodate beds and supplies. For longer duration 

stays, even more square footage is needed per occupant for minimum privacy 

and comfort requirements, and for building infrastructure, systems, and 

services needed when housing people on an extended basis. 

FEMA P-646 5: Siting, Spacing, Sizing, and Elevation Considerations 65
Table 5-3, Table 5-4 and Table 5-5 summarize square footage 

recommendations contained in International Code Council/National Storm 

Shelter Association, ICC-500, Standard on the Design and Construction of 

Storm Shelters (ICC/NSSA, 2007), FEMA 361 Design and Construction 

Guidance for Community Shelters (FEMA, 2000a), and American Red Cross 

Publication No. 4496, Standards for Hurricane Evacuation Shelter Selection

(ARC, 2002).

Table 5-3 Square Footage Recommendations – ICC-500 Standard 

on the Design and Construction of Storm Shelters (ICC/NSSA, 

2007)

Hazard or Duration

Minimum Required Usable Floor 

Area in Sq. Ft. per Occupant

Tornado

Standing or seated 

Wheelchair

Bedridden

5

10

30

Hurricane 

Standing or seated 

Wheelchair

Bedridden

20

20

40

Table 5-4 Square Footage Recommendations – FEMA 361 Design 

and Construction Guidance for Community Shelters

(FEMA, 2000a)

Hazard or Duration

Recommended Minimum Usable 

Floor Area in Sq. Ft. per Occupant

Tornado 5

Hurricane 10

Table 5-5 Square Footage Recommendations – American Red Cross 

Publication No. 4496 (ARC, 2002)

Hazard or Duration

Recommended Minimum Usable 

Floor Area in Sq. Ft. per Occupant

Short-term stay (i.e., a few days) 20

Long-term stay (i.e., days to weeks) 40

The number of standing, seating, wheelchair, or bedridden spaces should be 

determined based on the specific occupancy needs of the facility under 

66 5: Siting, Spacing, Sizing, and Elevation Considerations FEMA P-646
consideration. When determining usable floor area, ICC-500 includes the 

following adjustments to gross floor area: 

• Usable floor area is 50 percent of gross floor area in shelter areas with 

concentrated furnishings or fixed seating.

• Usable floor area is 65 percent of gross floor area in shelter areas with 

un-concentrated furnishings and without fixed seating.

• Usable floor area is 85 percent of gross floor area in shelter areas with 

open plan furnishings and without fixed seating. 

5.2.3 Recommended Minimum Square Footage for Short-Term 

Refuge from Tsunamis

For short-term refuge in a tsunami vertical evacuation structure, the duration 

of occupancy should be expected to last between 8 to 12 hours, as a 

minimum. Because tsunami events can include several cycles of waves, 

there are recommendations that suggest evacuees should remain in a tsunami 

refuge until the second high tide after the first tsunami wave, which could 

occur up to 24 hours later. 

Based on square footage recommendations employed in the design of 

shelters for other hazards, the recommended minimum square footage per 

occupant for a tsunami refuge is 10 square feet per person. It is anticipated 

that this density will allow evacuees room to sit down without feeling overly 

crowded for a relatively short period of time, but would not be considered 

appropriate for longer stays that included sleeping arrangements. This 

number should be adjusted up or down depending on the specific occupancy 

needs of the refuge under consideration.

5.3 Elevation Considerations

In order to serve effectively as a vertical evacuation structure, it is essential 

that the area of refuge be located well above the maximum tsunami 

inundation level anticipated at the site. Determination of a suitable elevation 

for tsunami refuge must take into account the uncertainty inherent in 

estimation of the tsunami runup elevation, possible splash-up during impact 

of tsunami waves, and the anxiety level of evacuees seeking refuge in the 

structure. Unfortunately a number of designated evacuation structures in 

Japan were inundated during the Tohoku tsunami, leading to loss of life of 

many of the refugees. To account for this uncertainty, the magnitude of 

tsunami force effects is determined assuming a maximum tsunami runup 

elevation that is 30% higher than values predicted by numerical simulation 

modeling or obtained from tsunami inundation maps. Because of the high 

Recommended minimum 

square footage is 10 

square feet per occupant.

FEMA P-646 5: Siting, Spacing, Sizing, and Elevation Considerations 67
consequence of potential inundation of the tsunami refuge area, it is 

recommended that the elevation of tsunami refuge areas in vertical 

evacuation structures include an additional allowance for freeboard above 

this elevation. 

The recommended minimum freeboard is one story height, or 10 feet (3 

meters) above the tsunami runup elevation used in tsunami force 

calculations. The recommended minimum elevation for a tsunami refuge 

area is, therefore, the maximum tsunami runup elevation anticipated at the 

site, plus 30%, plus 10 feet (3 meters). This should be treated as an absolute 

minimum, with additional conservatism strongly encouraged. 

5.4 Size of Vertical Evacuation Structures

Given the number and spacing of vertical evacuation structures, and the 

population in a given community, the minimum size can be determined based 

on square footage recommendations for the intended duration and type of

occupancy. Consideration of other functional needs, such as restrooms,

supplies, communications, and emergency power, should be added to the 

overall size of the structure.

Given the maximum tsunami runup elevation anticipated at the site, the 

minimum elevation of the area of refuge within a vertical evacuation 

structure can be determined based on minimum freeboard recommendations. 

Recommended minimum

refuge elevation is the 

maximum anticipated tsunami 

runup elevation, plus 30%, 

plus 10 feet (3 meters).

68 5: Siting, Spacing, Sizing, and Elevation Considerations FEMA P-646
Chapter 6

Load Determination and 

Structural Design Criteria

This chapter summarizes current code provisions as they may relate to 

tsunami load effects, describes intended performance objectives for vertical 

evacuation structures, specifies equations for estimating tsunami forces, and 

provides guidance on how tsunami forces should be combined with other 

effects.

6.1 Currently Available Structural Design Criteria

Very little guidance is provided in currently available structural design codes, 

standards, and guidelines on loads induced by tsunami inundation. 

Established design information focuses primarily on loads due to rising water 

and wave action associated with riverine flooding and storm surge. While 

little specific guidance was provided prior to this publication, the 

presumption heretofore had been that available flood design standards were 

to be adapted for designing for tsunami load effects. Therefore, it is 

important to understand those standards and how they differ from tsunami 

conditions. 

6.1.1 Current U.S. Codes, Standards, and Guidelines

International Building Code. The International Code Council International 

Building Code (ICC, 2012) Section 1612 Flood Loads, Section 1804 

Excavation, Grading and Fill, and Appendix G Flood Resistant Construction 

provides information on flood design and flood-resistant construction 

including by reference to ASCE/SEI Standard 24-05, Flood Resistant Design 

and Construction (ASCE 24, 2006a). Appendix M: Tsunami Generated 

Flood Hazard, provides tsunami regulatory criteria for those communities 

that have a recognized tsunami hazard and have developed and adopted a 

map of their Tsunami Hazard Zone, and is focused on keeping critical and 

high risk structures out of the tsunami inundation zone. However, buildings 

are permitted within the Tsunami Hazard Zone if designed as a Vertical 

Evacuation Refuge complying with the FEMA P-646 Guidelines or if 

designed to resist without collapse the hydrostatic, hydrodynamic, debris 

accumulation and impact, and scour effects of the Maximum Considered 

Very little guidance is provided 

in currently available structural 

design codes, standards, and 

guidelines on loads induced by 

tsunami inundation. 

Established design information 

focuses primarily on loads due 

to rising water and wave action 

associated with riverine 

flooding and storm surge.

FEMA P-646 6: Load Determination and Structural Design Criteria 69
Tsunami. Appendices G and M are non-mandatory unless adopted by a local 

jurisdiction having authority.

ASCE/SEI Standard 24-05. The American Society of Civil 

Engineers/Structural Engineering Institute (ASCE/SEI) Standard 24-05 

Flood Resistant Design and Construction (ASCE, 2006a) provides minimum 

requirements for flood-resistant design and construction of structures located 

in flood-hazard areas. Topics include basic requirements for flood-hazard 

areas, high-risk flood-hazard areas, coastal high-hazard areas, and coastal A 

zones. This standard was formulated for compliance with FEMA National 

Flood Insurance Program (NFIP) floodplain management requirements. 

ASCE/SEI Standard 7-10. ASCE/SEI Standard 7-10 Minimum Design 

Loads for Buildings and Other Structures (ASCE, 2010) provides 

expressions for forces associated with flood and wave loads on specific types 

of structural components. Chapter 5 of this standard, Flood Loads, covers 

important definitions that relate to flooding and coastal high-hazard areas 

related to tides, storm surges, and breaking waves. (In 2016 it is anticipated 

that a new Chapter 6, Tsunami Loads and Effects, will be added.) 

FEMA P-55 Coastal Construction Manual. The fourth edition of the 

FEMA P-55 Coastal Construction Manual (FEMA, 2011) includes 

discussion of coastal seismic and tsunami loads. This Manual was developed 

to provide design and construction guidance for low-rise (less than three 

stories), one- and two-family residential structures built in coastal areas 

throughout the United States. The Coastal Construction Manual addresses 

seismic loads for coastal structures, and contains expressions for flood loads, 

wave loads, and load combinations for specific types of structural 

components. 

The Manual also provides general information on tsunami hazard. Section 

3.3.3 states that:

“Tsunamis are long-period water waves generated by undersea shallowfocus earthquakes or by undersea crustal displacements (subduction of 

tectonic plates), landslides, or volcanic activity. Tsunamis can travel 

great distances, undetected in deep water, but shoaling rapidly in coastal 

waters and producing a series of large waves capable of destroying 

harbor facilities, shore protection structures, and upland buildings ... 

Coastal construction in tsunami hazard zones must consider the effects of 

tsunami runup, flooding, erosion, and debris loads. Designers should 

also be aware that the “rundown” or return of water to the sea can also 

damage the landward sides of structures that withstood the initial runup.”

70 6: Load Determination and Structural Design Criteria FEMA P-646
The Manual also notes that tsunami effects at a particular site will be 

determined by the following four basic factors:

• the magnitude of the earthquake or triggering event,

• the location of the triggering event,

• the configuration of the continental shelf and shoreline, and

• the upland topography.

This Manual contains a warning statement in Chapter 8 that “This Manual 

does not provide guidance for estimating flood velocities during tsunamis. 

The issue is highly complex and site-specific. Designers should look for 

model results from tsunami inundation or evacuation studies.”

With regard to designing to resist tsunami loads, Section 8.6 of the Manual

states that:

“Tsunami loads on residential buildings may be calculated in the same 

fashion as other flood loads; the physical processes are the same, but the 

scale of the flood loads is substantially different in that the wavelengths 

and runup elevations of tsunamis are much greater than those of waves 

caused by tropical or extratropical cyclones ... When the tsunami forms 

a borelike wave, the effect is a surge of water to the shore. When this 

occurs, the expected flood velocities are substantially higher than in nontsunami conditions ... and if realized at the greater water depths, would 

cause substantial damage to all buildings in the path of the tsunami.”

Although authors of the Coastal Construction Manual conclude that it is 

generally not feasible or practical to design normal structures to withstand 

tsunami loads, it should be noted that this study was for conventional single 

family residential construction, and did not take into account the possibility 

of special design and construction details that would be possible for vertical 

evacuation structures and other larger buildings. 

City and County of Honolulu Building Code. The City and County of 

Honolulu Building Code (CCH, 2007), Chapter 16, Article 11, provides 

specific guidance for “structural design of buildings and structures subject to 

tsunamis” in Section 16-11.5(f). The loading requirements in this section are 

based on a January 1980 Dames & Moore report, Design and Construction 

Standards for Residential Construction in Tsunami-Prone Areas in Hawaii, 

specifically Appendix A, Proposed Building Code Amendments. Drag forces 

were based on a non-bore velocity of flow in feet per second roughly 

estimated as equal in magnitude to the depth in feet of water at the structure 

(inconsistent with a Froude number assumption that would relate to the 

FEMA P-646 6: Load Determination and Structural Design Criteria 71
square root of the depth). The report states that “The adequacy of this 

approach ... has not been satisfactorily examined.” However, at the same 

time prescriptive forces on walls were based on a bore flow velocity of 

2 gh . Rough estimates are also given for anticipated scour around piles and 

piers based on distance from the shoreline and the soil type at the building 

site. However, the basis for these scour values is not documented. These 

provisions have not been updated since they were first adopted in the 1980’s, 

and are now largely archaic and primarily for historical reference.

6.1.2 Summary of Current Design Requirements

Coastal areas that are subject to high-velocity wave action from storms or 

seismic sources are designated Coastal High Hazard V-Zones (ASCE, 2010). 

In ASCE 7-2010 Chapter 5, Flood Loads, areas inland of Coastal V-Zones 

that are subject to smaller waves caused by storm surges, riverine flooding, 

seiches or tsunamis are designated Coastal A-Zones (ASCE, 2010). 

However, the Coastal Construction Manual defines the Coastal V-Zone as 

“an area subject to high-velocity wave action from storms or tsunamis”, and 

the Coastal A-Zone as an area “in which the principal source of flooding is 

coastal storms, and where the potential base flood wave height is between 1.5 

and 3.0 feet.”

In design for coastal flooding due to storm surge or tsunamis, buildings or 

structures are proportioned to resist the effects of coastal floodwaters. Design 

and construction must be adequate to resist the anticipated flood depths, 

pressures, velocities, impact, uplift forces, and other factors associated with 

flooding, as defined by the code. 

Habitable space in building structures must be elevated above the regulatory 

coastal storm flood elevation by such means as posts, piles, piers, or shear 

walls parallel to the expected direction of flow. Spaces below the base flood 

elevation must be free from obstruction. Walls and partitions in a coastal 

high-hazard area are required to break away so as not to induce excessive 

loads on the structural frame. 

The effects of long-term erosion, storm-induced erosion, and local scour are 

to be included in the design of foundations of buildings or other structures in 

coastal high-hazard areas. Foundation embedment must be far enough below 

the depth of potential scour to provide adequate support for the structure. 

Scour of soil from around individual piles and piers must be provided for in 

the design. Shallow foundation types are not permitted in V-Zones unless 

the natural supporting soils are protected by scour protection, but are 

permitted in A-Zones subject to stability of the soil and resistance to scour. 

72 6: Load Determination and Structural Design Criteria FEMA P-646
The main building structure must be adequately anchored and connected to 

the elevating substructure system to resist lateral, uplift, and downward 

forces.

6.1.3 Limitations in Available Flood Design Criteria Relative to 

Tsunami Loading

Although many of the hydrostatic and hydrodynamic loading expressions in 

the above-referenced codes, standards and guidelines are well-established, 

there are significant differences between tsunami inundation and riverine or 

storm surge flooding. For a typical tsunami, the water surface fluctuates near 

the shore with amplitude that may range from several meters to over 10 

meters during a period of a few minutes to tens of minutes. A major 

difference between tsunamis and other coastal flooding is increased flow 

velocity for tsunamis, which results in significant increases in velocityrelated loads on structural components. Application of existing loading 

expressions to tsunami loading conditions requires an estimate of the tsunami 

flood depth and velocity, neither of which is provided with accuracy by the 

above referenced information on flood and tsunami design. 

Although impact of floating debris is required to be considered by the codes 

discussed in this chapter, impact force produced by a change in momentum is 

dependent on estimates of the debris mass, velocity, and the time taken for 

the mass to decelerate. No accommodation is made for added mass of the 

water behind the debris, or the potential for damming if debris is blocked by 

structural components. More significant forms of debris, such as barges, 

fishing boats, and empty storage tanks may need to be considered for 

tsunamis, depending on the location of the building under consideration. The 

size, mass, and stiffness of this type of debris are not considered in currently 

available criteria.

No consideration is given to upward loads on the underside of structures or 

components that are submerged by the flood or tsunami flow. These vertical 

hydrodynamic loads, different from buoyancy effects, are considered by the 

offshore industry in design of platforms and structural members that may be 

submerged by large waves. 

There are two primary scour mechanisms that occur during a tsunami event. 

Shear-induced scour is similar to that observed during storm surge flooding, 

and consists of soil transport due to the flow velocity. Liquefaction-induced 

scour results from rapid drawdown as the water recedes. Without sufficient 

time to dissipate, pore pressure causes liquefaction of the soil resulting in 

substantially greater scour than would otherwise occur. Although the codes 

Although many of the 

hydrostatic and 

hydrodynamic loading 

expressions in currently 

available codes, standards 

and guidelines are wellestablished, there are 

significant differences 

between tsunami inundation 

and riverine or storm surge 

flooding.

FEMA P-646 6: Load Determination and Structural Design Criteria 73
discussed in this chapter require consideration of scour, little guidance (other 

than rough estimates) is given as to the potential extent of scour. 

6.2 Performance Objectives

While specific performance objectives for various forms of rare loading can 

vary, acceptable structural performance generally follows a trend 

corresponding to:

• little or no damage for small, more frequently occurring events;

• moderate damage for medium-size, less frequent events; and

• significant damage, but no collapse for very large, rare events.

In the case of earthquake hazards, model building codes, such as the 

International Building Code, implicitly assign seismic performance 

objectives to buildings based on their inherent risk to human life (e.g., very 

large occupancies) or their importance after an earthquake (e.g., emergency 

operation centers or hospitals). Buildings and other structures are classified 

into Risk Categories I through IV, in order of increasing risk to human life or 

importance, and code prescriptive design criteria are correspondingly 

increased, with the intention of providing improved performance. For Risk 

Category IV, design rules are intended to result in a high probability of 

buildings remaining functional after moderate shaking, and experiencing 

considerably less damage than normal buildings in very rare shaking.

Currently available performance-based seismic design procedures are 

intended to explicitly evaluate and predict performance, instead of relying on 

the presumed performance associated with prescriptive design rules. 

However, performance-based design is an emerging technology and the 

targeted performance cannot be delivered with 100% certainty. The current 

standard-of-practice for performance-based seismic design contained in 

ASCE/SEI 41-06 Seismic Rehabilitation of Existing Buildings (ASCE, 

2006b) defines discrete performance levels with names intended to connote 

the expected condition of the building: Collapse, Collapse Prevention, Life 

Safety, Immediate Occupancy, and Operational. Seismic performance 

objectives are defined by linking one of these building performance levels to 

an earthquake hazard level that is related to the recurrence interval (return 

period) and the intensity of ground shaking, as shown in Figure 6-1. 

74 6: Load Determination and Structural Design Criteria FEMA P-646
Figure 6-1 Seismic performance objectives linking building performance 

levels to earthquake hazard levels (adapted from SEAOC, 1995). 

When determining performance objectives for natural hazards, the most 

difficult issue is deciding how rare (or intense) the design event should be. 

For seismic design in the United States, this issue has been resolved through 

the adoption of a national earthquake hazard map defining the risk-target 

Maximum Considered Earthquake (MCE) and the intensity of shaking 

associated with such an event (ASCE, 2010). 

6.2.1 Tsunami Performance Objective

In this document, the design tsunami event is termed the Maximum 

Considered Tsunami (MCT). Unfortunately, there are no standardized 

national maps available for defining this hazard. In addition, due to the 

complexity of the tsunami hazard, which must consider near and distant 

tsunami-genic sources and highly uncertain relationships between earthquake 

events and subsequent tsunami, as of 2011 no firm policy has been 

established in the code defining a methodology for setting a Maximum 

Considered Tsunami at a consistent hazard level. Current methods for 

tsunami hazard assessment are described in Chapter 3.

Vertical evacuation structures designed in accordance with the guidance 

presented in this document would be expected to provide a stable refuge 

when subjected to a design tsunami event consistent with the Maximum 

Considered Tsunami identified for the local area. 

In general, the Maximum Considered Tsunami will be a rare, but realistic 

event with large potential consequences, generally to be taken as having a 

The Tsunami Performance 

Objective includes the 

potential for significant 

damage while maintaining a 

reliable and stable refuge 

when subjected to the 

Maximum Considered Tsunami. 

Most structures would be 

expected to be repairable, 

although the economic viability 

of repair will be uncertain. 

FEMA P-646 6: Load Determination and Structural Design Criteria 75
collapse prevention design equivalent of a 2% probability of being exceeded 

in a 50-year period or a 2500 year average return period (similar to the 

probability level of seismic criteria). Consistent with the general trend of 

acceptable performance for “Maximum Considered” loadings, the 

performance of vertical evacuation structures in this event would include the 

potential for significant damage while maintaining a reliable and stable 

refuge above the inundation height, although the economics of repair versus 

replacement will be uncertain, depending on the specifics of the situation 

including the magnitude of the actual event, interaction with the local 

bathymetry, and the design and construction of the facility. 

6.2.2 Seismic Performance Objectives 

The performance objective for vertical evacuation structures subjected to 

seismic hazards should be consistent with that of code-defined essential 

facilities such as hospitals, police and fire stations, and emergency operation 

centers. Following the prescriptive approach in the International Building 

Code, vertical evacuation structures are assigned to Risk Category IV, 

triggering design requirements that provide enhanced performance relative to 

typical buildings for normal occupancies. 

In the specific case of earthquakes generating a near source tsunami, design 

for enhanced performance is necessary to assure that the structure is still 

usable for a tsunami following a local seismic event. To obtain a higher level 

of confidence that a vertical evacuation structure will achieve enhanced 

seismic performance, the design developed by prescriptive code provisions

can be evaluated using currently available performance-based seismic design 

techniques and verification analyses. Utilizing the approach in ASCE/SEI 

41-06, the performance objective for code-defined essential facilities should 

be at least Immediate Occupancy performance for the Design Basis 

Earthquake (DBE) and Life Safety performance for the Maximum 

Considered Earthquake (MCE). 

6.3 Earthquake Loading

The recommended basis for seismic design of vertical evacuation structures 

is the International Building Code, which references ASCE/SEI 7-10 

Minimum Design Loads for Buildings and Other Structures for its seismic 

requirements. These requirements are based on the NEHRP Recommended 

Provisions for Seismic Regulations for New Buildings and Other Structures

(FEMA, 2004a) and additional information provided in the Commentary

(FEMA, 2004b). Vertical evacuation structures should be designed using 

rules for Risk Category IV buildings. 

Seismic Performance 

Objectives are consistent 

with the code-defined 

performance of essential 

facilities such as hospitals, 

police and fire stations, and 

emergency operation 

centers.

76 6: Load Determination and Structural Design Criteria FEMA P-646
The recommended basis for seismic evaluation and rehabilitation of existing 

buildings that are being considered for use as vertical evacuation structures is 

the SEI/ASCE Standard 31-03 Seismic Evaluation of Existing Buildings

(ASCE, 2003b), using the Immediate Occupancy performance objective, and 

ASCE/SEI Standard 41-06 Seismic Rehabilitation of Existing Buildings, 

using the performance objectives specified in Section 6.2.2. 

6.3.1 Near-Source-Generated Tsunamis

A vertical evacuation structure located in a region susceptible to near-sourcegenerated tsunamis is likely to experience strong ground shaking 

immediately prior to the tsunami. As a properly designed essential facility, it 

is expected that sufficient reserve capacity will be provided in the structure to 

resist the subsequent tsunami loading effects. The reserve capacity of the 

structure, which will be some fraction of the original, needs to be evaluated. 

It is recommended that the condition of the structure after the Design Basis 

Earthquake (DBE) be used to determine the adequacy for tsunami loading. If 

inadequate, the resulting design would then need to be modified as necessary 

to address tsunami effects. For areas that are subject to near-sourcegenerated tsunamis, this sequential loading condition will clearly control the 

design of the structure. To help ensure adequate strength and ductility in the 

structure for resisting tsunami load effects, Seismic Design Category D, as 

defined in ASCE/SEI 7-10, should be assigned to the structure, as a 

minimum.

A properly designed essential facility is also expected to have improved 

performance of non-structural components including ceilings, walls, light 

fixtures, fire sprinklers, and other building systems. For evacuees to feel 

comfortable entering a vertical evacuation structure following an earthquake, 

and remaining in the structure during potential aftershocks, it is important 

that visible damage to both structural and non-structural components be 

limited. Particular attention should be focused on non-structural components 

in the stairwells, ramps, and entrances that provide access and vertical 

circulation within the structure. 

6.3.2 Far-Source-Generated Tsunamis

Although a vertical evacuation structure is not likely to experience 

earthquake shaking directly associated with a far-source tsunami, seismic 

design must be independently included as dictated by the seismic hazard that 

is present at the site. Even in regions of low seismicity, however, it is 

recommended that Seismic Design Category D be assigned to the structure, 

A vertical evacuation structure 

located in a region susceptible 

to near-source-generated 

tsunamis is likely to 

experience strong ground 

shaking immediately prior to 

the tsunami.

FEMA P-646 6: Load Determination and Structural Design Criteria 77
as a minimum, to help ensure adequate continuity, strength, and ductility for 

resisting tsunami load effects.

6.4 Wind Loading

The recommended basis for wind design of a vertical evacuation structure is 

the International Building Code, which references ASCE/SEI 7-10 Minimum 

Design Loads for Buildings and Other Structures for the majority of its wind 

requirements. In many locations affected by tsunami risk, earthquake 

loading will likely govern over wind loading, but this is not necessarily true 

for all regions. 

At locations where wind loading controls the design, the use of special 

seismic detailing for structural components should be considered. It is 

recommended that Seismic Design Category D be assigned to the structure, 

as a minimum, to help ensure adequate strength and ductility for resisting 

tsunami load effects.

6.5 Tsunami Loading

The following tsunami load effects should be considered for the design of 

vertical evacuation structures: (1) hydrostatic forces; (2) buoyant forces; (3) 

hydrodynamic forces; (4) impulsive forces; (5) debris impact forces; (6) 

debris damming forces; (7) uplift forces; and (8) additional gravity loads 

from retained water on elevated floors. 

In this document, wave-breaking forces are not considered in the design of 

vertical evacuation structures. In general, tsunamis break offshore, and 

vertical evacuation structures should be located some distance inland from 

the shoreline. The term ‘wave-breaking’ is defined here as a plunging-type 

breaker in which the entire wave front overturns. When waves break in a 

plunging mode, the wave front becomes almost vertical, generating an 

extremely high pressure over an extremely short duration. Once a tsunami 

wave has broken, it can be considered as a bore because of its very long 

wavelength. Further justification for not considering wave-breaking forces 

can be found in Yeh (2008).

Wave-breaking forces could be critical for vertical evacuation structures 

located in the wave-breaking zone, which is beyond the scope of this 

document. If it is determined that a structure must be located in the wavebreaking zone, ASCE/SEI 7-10 Minimum Design Loads for Buildings and 

Other Structures and the Coastal Engineering Manual, EM 1110-2-1100,

(U.S. Army Coastal Engineering Research Center, 2008) should be consulted 

for additional guidance on wave-breaking forces. 

Tsunami Load Effects 

include: 

(1) hydrostatic forces; 

(2) buoyant forces; 

(3) hydrodynamic forces; 

(4) impulsive forces; 

(5) debris impact forces; 

(6) debris damming forces; 

(7) uplift forces; and 

(8) additional gravity loads 

from retained water on 

elevated floors.

78 6: Load Determination and Structural Design Criteria FEMA P-646
6.5.1 Key Assumptions for Estimating Tsunami Load Effects

Tsunami load effects are determined using the following key assumptions:

• Tsunami flows consist of a mixture of sediment and seawater. Most 

suspended sediment transport flows do not exceed 5% sediment 

concentration. Based on an assumption of vertically averaged sedimentvolume concentration of 5% in seawater, the fluid density of tsunami 

flow should be taken as 1.1 times the density of freshwater, or ρs = 1,100 

kg/m3

 = 2.13 slugs/ft3. 

• Tsunami flow depths vary significantly depending on the threedimensional bathymetry and topography at the location under 

consideration. Figure 6-2 shows three possible scenarios where 

topography could affect the relationship between maximum tsunami 

elevation, TE, at a particular location and the ultimate inland runup 

elevation, R. For the loading expressions presented in this chapter, it is 

assumed that Figure 6-2b applies, that is TE = R. These expressions may 

be adjusted if numerical simulations of tsunami inundation provide more 

appropriate estimates of TE at the location being considered. 

• There is significant variability in local tsunami runup heights, based on 

local bathymetry and topographic effects, and uncertainty in numerical 

simulations of tsunami inundation. Based on empirical judgment from 

past tsunami survey data, it is recommended that the design runup 

elevation, R, be taken as 1.3 times the predicted maximum runup 

elevation, R*, to envelope the potential variability in the estimates of 

modeling. The inundation elevation from the runup point back towards 

the shoreline would then be scaled by the same factor. Figure 6-3 shows 

a typical numerical prediction (Yamazaki et al., 2011) made for the 2009 

Samoa Tsunami, which demonstrates that the 1.3 safety factor for 

uncertainty is realistic. 

FEMA P-646 6: Load Determination and Structural Design Criteria 79
Figure 6-2 Three types of coastal inundation where the tsunami elevation 

(TE) at a site of interest could be less than, equal to, or greater 

than the ultimate inland runup elevation (R) 

Figure 6-3 Comparison between numerical modeling (blue line) and field measurement of 

run-up (white dots) and flow elevations (blue dots) at Pago Pago Harbor, 

American Samoa (Yamazaki et al, 2011).

6.5.2 Hydrostatic Forces

Hydrostatic forces occur when standing or slowly moving water encounters a 

structure or structural component. This force always acts perpendicular to the 

surface of the component of interest. It is caused by an imbalance of 

c)

80 6: Load Determination and Structural Design Criteria FEMA P-646
pressure due to a differential water depth on opposite sides of a structure or 

component. Hydrostatic forces may not be relevant to a structure with a 

finite (i.e., relatively short) breadth, around which the water can quickly flow 

and fill in on all sides. Hydrostatic forces are usually important for long 

structures such as sea walls and dikes, or for evaluation of an individual wall 

panel where the water level on one side differs substantially from the water 

level on the other side.

Hydrostatic and buoyant forces must be computed when the ground floor of a 

building is watertight, or is sufficiently insulated and airtight to prevent or 

delay the intrusion of water. In this situation, the hydrostatic force should be 

evaluated for individual wall panels. The horizontal hydrostatic force on a 

wall panel can be computed using Equation 6-1:

2

max

1

2 F p A gbh h cw s = = ρ , (6-1)

where pc is the hydrostatic pressure, Aw is the wetted area of the panel, ρs is 

the fluid density including sediment (1100 kg/m3 = 2.13 slugs/ft3

), g is the 

gravitational acceleration, b is the breadth (width) of the wall, and hmax is the 

maximum water height above the base of the wall at the structure location. If 

the wall panel with height hw is fully submerged, then the horizontal 

hydrostatic force can be written as Equation 6-2:

ρ

  = = −     max 2

w

h cw s w

h F p A g h bh (6-2)

where hmax is the vertical difference between the design tsunami elevation R

and the base elevation of the wall at the structure, zw, as shown in Equation 6-

3: 

max = − =− 1.3 * w w h R z Rz (6-3)

where R* is the estimated maximum inundation elevation at the structure 

from a detailed numerical simulation model, or the runup elevation at 

maximum horizontal penetration of the tsunami from available tsunami 

inundation maps. The design runup elevation, R, is taken as 1.3 times the 

predicted maximum runup elevation, R*. The moment about the base of the 

wall can be evaluated using the line of action of the hydrostatic force 

resultant, as shown in Figure 6-4.

FEMA P-646 6: Load Determination and Structural Design Criteria 81
Figure 6-4 Hydrostatic force distribution and location of resultant.

6.5.3 Buoyant Forces

Buoyant or vertical hydrostatic forces will act vertically through the centroid 

of the displaced volume on a structure or structural component subjected to 

partial or total submergence. The total buoyant force equals the weight of 

water displaced. Buoyant forces on components must be resisted by the 

weight of the component and any opposing forces resisting flotation. 

Buoyant forces are a concern for structures that have little resistance to 

upward forces (e.g., light wood frame buildings, basements, empty tanks 

located above or below ground, swimming pools, components designed 

considering only gravity loads). 

For a watertight structure, the total buoyant force is given by Equation 6-4: 

F gV b s = ρ (6-4) 

where ρs is the fluid density including sediment (1100 kg/m3

 = 2.13 

slugs/ft3

), and V is the volume of water displaced by the building, i.e., the 

volume below the level of hmax as determined by Equation 6-3. Buoyant 

forces on an overall building are shown in Figure 6-5. If there is insufficient 

building weight to resist buoyant forces, tension piles may be used to 

increase the resistance to flotation, but reduction in pile side friction due to 

anticipated scour around the tops of the piles must be considered.

82 6: Load Determination and Structural Design Criteria FEMA P-646
Figure 6-5 Buoyant forces on an overall building with watertight lower 

levels.

6.5.4 Hydrodynamic Forces

When water flows around a structure, hydrodynamic forces are applied to the 

structure as a whole and to individual structural components. These forces 

are induced by the flow of water moving at moderate to high velocity, and 

are a function of fluid density, flow velocity and structure geometry. Also 

known as drag forces, they are a combination of the lateral forces caused by 

the pressure forces from the moving mass of water and the friction forces 

generated as the water flows around the structure or component. 

Hydrodynamic forces can be computed using Equation 6-5: 

= ρ 2

max

1 ( ) 2

F C B hu d sd (6-5) 

where ρs is the fluid density including sediment (1100 kg/m3

 = 2.13 

slugs/ft3

), Cd is the drag coefficient, B is the breadth of the structure in the 

plane normal to the direction of flow (i.e. the breadth in the direction parallel 

to the shore), h is flow depth, and u is flow velocity at the location of the 

structure. For forces on components, B is taken as the width of the 

component. The drag coefficient may be conservatively taken as Cd = 2.0; 

the actual value is shape-, orientation-, and size-dependent. The resultant 

hydrodynamic force is applied approximately at the centroid of the wetted 

surface of the component, as shown in Figure 6-6. 

R

DATUM

DESIGN RUNUP HEIGHT

Building

Weight

Fb

Pile Tension

Total Displaced

Volume, V

hmax

FEMA P-646 6: Load Determination and Structural Design Criteria 83
Figure 6-6 Hydrodynamic force distribution and location of resultant.

The combination hu

2 represents the momentum flux per unit mass per unit 

width. Note that (hu

2

)max does not equal hmax u

2

max. The maximum flow 

depth, hmax, and maximum flow velocity, umax, at a particular site may not 

occur at the same time. The hydrodynamic forces should be based on the 

parameter (hu

2

)max, which is the maximum momentum flux per unit mass per 

unit width occurring at the site at any time during the tsunami. 

The maximum value of (hu

2

) can be obtained by running a detailed numerical 

simulation model or acquiring existing simulation data. The numerical 

model in the runup zone must be run with a very fine grid size to ensure 

adequate accuracy in the prediction of hu

2

. 

When numerical simulation data are not available, the value (hu

2

)max can be 

roughly estimated based on information in the inundation map, using 

Equation 6-6:

( )     = −+        

2

2 2

max 0.125 0.235 0.11 z z hu gR R R (6-6)

where g is the acceleration due to gravity, R is the design runup elevation 

taken as 1.3 times the maximum runup elevation, R*, and z is the ground 

elevation at the base of the structure. To use this formula, the sea level datum 

must be consistent with that used in the inundation maps. 

The basis of Equation 6-6 is described in Appendix E. Although this 

classical analytical solution is based on one-dimensional nonlinear shallowwater theory for a uniformly sloping beach, with no lateral topographical 

variation and no friction, the maximum value of (hu2

) obtained from 

Equation 6-6 can be used for: (1) preliminary design; (2) approximate design 

in the absence of other modeling information; and (3) to evaluate the 

reasonableness of numerical simulation results. 

84 6: Load Determination and Structural Design Criteria FEMA P-646
R* and z can be obtained from tsunami inundation maps. Because of 

uncertainties in modeling tsunami inundation, it is recommended that 

numerically predicted values of (hu2

) should be compared with the values 

computed using Equation 6-6 to determine reasonableness. 

6.5.5 Impulsive Forces

Impulsive forces are caused by the leading edge of a surge of water 

impacting a structure. Ramsden (1993) performed comprehensive 

experiments on impulsive forces. Laboratory data show no significant initial 

impact force (impulse force) in dry-bed surges, but an “overshoot” in force 

was observed in bores that occur when the site is initially flooded. The 

maximum overshoot is approximately 1.5 times the subsequent 

hydrodynamic force, consistent with some, but not all, of the independent 

laboratory data obtained by Arnason (2005). Further analysis of the 

conditions for the occurrence of this effect and high-speed video of similar 

test cases suggests it occurs when the surge depth to object width ratio is 

small so that a transient amount of additional “ponded” water depth 

accumulates against the forward side of the object before being eventually 

relieved by flowing around the sides. Since impact momentum increases 

with the sudden slam of the steep front of a bore (Yeh, 2007), the lack of 

overshoot in dry-bed surge can be attributed to the relatively mild slope of 

the front profile of the water surface. If the runup zone is flooded by an 

earlier tsunami wave, subsequent waves could impact buildings in the form 

of a bore. 

For conservatism and especially for structural wall elements of significant 

width it is recommended that the impulsive forces be taken as 1.5 times the 

hydrodynamic force, as shown in Equation 6-7: 

=1.5 F F s d (6-7) 

Impulsive forces may act on members at the leading edge of the tsunami 

bore, while hydrodynamic forces will certainly act on all members that have 

already been passed by the leading edge, as shown in Figure 6-7. 

FEMA P-646 6: Load Determination and Structural Design Criteria 85
Figure 6-7 Hydrodynamic impulsive and drag forces on components of a 

building subjected to inundation by a tsunami bore.

6.5.6 Floating Debris Impact Forces

The impact force from waterborne debris (e.g., floating driftwood, lumber, 

boats, shipping containers, automobiles, buildings) can be a cause of building 

damage. Unfortunately, it is difficult to estimate this force accurately. 

Background information on the development of the recommended impact 

force calculation is provided in Appendix D. 

The debris impact force can be estimated using Equation 6-8, which is a 

more direct generalized form of the ASCE 7 Chapter 5 equation for debris 

impacts during riverine flooding, without the reduction factors for random 

orientation: 

F u km c i =1.3 (1 ) max d + (6-8) 

where 

1.3 is the Importance Coefficient for Risk Category IV 

structures that is specified by ASCE 7 Chapter 5 for debris 

impacts,

umax is the maximum flow velocity carrying the debris at the 

site (the debris is conservatively assumed to be moving at 

the same speed as the flow), except for debris rolling along 

the bottom where the velocity may be reduced by 50%, 

c is a hydrodynamic mass coefficient which represents the 

effect of fluid in motion with the debris (see Table 6-1). 

This coefficient depends on the size, shape, and orientation 

of the object with respect to the flow direction. Note that it 

h

z

max R

DATUM

DESIGN RUNUP HEIGHT

Fs,c2

Fs,b2

Fd,c1 Fs,c1

Fd,b2

Fd,c2

F - Impulsive forces on columns and beams at leading edge of bore

d,c1

s,c1

F - Drag forces on columns and beams behind leading edge of bore

c1 and c2 - Columns at first and second levels. b2 - Beams at second level

86 6: Load Determination and Structural Design Criteria FEMA P-646
no longer represents the traditional added-mass term derived 

from potential flow hydrodynamics (see Appendix D).

k is the effective net combined stiffness of the impacting 

debris and impacted structural element(s) deformed by the 

impact (i.e. 1/k = 1/ks + 1/kd). In this equation, the net 

stiffness is utilized to implicitly incorporate the impact 

duration to stop the debris. If the impact is large enough to 

cause inelastic behavior in the structure, this should be 

considered in determining the effective stiffness.

md is the mass of the debris.

Unlike other forces, impact forces are assumed to act locally on a single 

member of the structure at the elevation of the water surface, as shown in 

Figure 6-8. The probability of two or more simultaneous debris strikes is 

assumed to be low enough that it can be ignored.

Figure 6-8 Waterborne debris impact force.

Debris impact forces should be evaluated considering the location of the 

vertical evacuation structure and potential debris in the surrounding area. 

For example, it is likely that floating debris would consist primarily of 

driftwood, logs and pier pilings for most coastal towns, whereas for some 

large port areas, the debris could be shipping containers. Locations near 

yacht marinas or fishing harbors should consider possible impact from boats 

that break their moorings.

Use of Equation 6-8 requires the mass, hydrodynamic mass coefficient, and 

stiffness properties of the debris. Approximate values of md, c, and kd for 

common waterborne debris are listed in Table 6-1. The mass of contents in 

the shipping containers should only be included if they are rigidly attached to 

the container to prevent sliding during impact. Stiffness values for 20-ft 

standard shipping containers were determined using the secant stiffness 

corresponding to 25mm displacement for containers modeled numerically 

(Peterson and Naito, 2012). Values for the 20-ft heavy shipping containers 

z

R

DATUM

DESIGN RUNUP HEIGHT

W Fi

d

FEMA P-646 6: Load Determination and Structural Design Criteria 87
were increased by the proportion of container weight, while those for the 40-

ft containers were adjusted based on differences in framing section 

properties. Mass and stiffness properties for other types of debris should be 

derived or estimated as part of the design process. 

Table 6-1 Mass and Stiffness of Some Waterborne Floating Debris 

Type of Debris

Mass (md )

in kg

Hydrodynamic

Mass Coefft. 

(c)

Debris Stiffness 

(kd ) in N/m

Lumber or Wood Log – oriented

longitudinally

450 0 2.4 x 106 *

20-ft Standard Shipping Container –

oriented longitudinally

2200 

(empty)

0.30 85 x106 **

20-ft Standard Shipping Container –

oriented transverse to flow

2200 

(empty)

1.00 80 x106 **

20-ft Heavy Shipping Container –

oriented longitudinally

2400 

(empty)

0.30 93 x106 **

20-ft Heavy Shipping Container –

oriented transverse to flow

2400 

(empty)

1.00 87 x106 **

40-ft Standard Shipping Container –

oriented longitudinally

3800 

(empty)

0.20 60 x106

40-ft Standard Shipping Container –

oriented transverse to flow

3800 

(empty)

1.00 40 x106

* Haehnal and Daly, 2002; ** Peterson and Naito, 2012 

The magnitude of the debris impact force depends on mass and velocity. 

Smaller (lighter) debris requiring little or no draft to float can travel at higher 

velocities than larger (heavier) debris requiring much larger depths to float. 

Use of maximum flow velocity without consideration of the depth required to 

float large debris would be unnecessarily conservative. The appropriate 

maximum flow velocity umax for a given flow depth can be obtained by 

running a detailed numerical simulation model or by acquiring existing 

simulation data. It is noted, however, that numerical predictions of flow 

velocities are less accurate than predictions of inundation depths, and the grid 

size for numerical simulations in the runup zone should be very fine in order 

to obtain sufficient accuracy in velocity predictions. Because of the 

uncertainty involved in even ‘accurate’ numerical simulations, it is suggested 

that a margin of safety be applied to the computed flow velocity, depending 

on the level of confidence in the numerical model simulations.

When a suitable numerical simulation model is unavailable, the maximum 

flow velocity carrying lumber or a wooden log (with essentially no draft) can 

be estimated using the analytical solution for tsunami runup on a uniformly 

sloping beach with no lateral topographical variation, given by Equation 6-9:

88 6: Load Determination and Structural Design Criteria FEMA P-646
  = −     max 2 1 z u gR R . (6-9)

where g is the acceleration due to gravity, R is the design runup height that is 

1.3 times the ground elevation R* at the maximum tsunami penetration, and z

is the ground elevation at the structure (the datum must be at the sea level). 

Background information on the development of this equation is provided in 

Appendix E.

For a shipping container or other similar large debris with draft d, the ratio of 

the draft d to the maximum runup height R can be computed, and Figure 6-9

can be used to estimate the maximum flow velocity. Draft d can be 

estimated using Equation 6-10:

ρ =

s f

W d

g A (6-10)

where W is the weight of the debris, ρs is the fluid density including sediment 

(1100 kg/m3 = 2.13 slugs/ft3

), g is the acceleration due to gravity, and Af is 

the cross-sectional area parallel to the water surface such that the product d ×

Af represents the volume of water displaced by the debris. 

Figure 6-9 Maximum flow velocity of depth, d, at the ground elevation, z, 

and maximum runup elevation, R. The bottom curve represents 

the lower limit of maximum flow velocity.

Based on the appropriate curve for d/R, and ratio between the elevation of the 

structure relative to the design runup elevation (z/R), Figure 6-9 will provide 

an estimate of the maximum flow velocity. It should be understood that 

FEMA P-646 6: Load Determination and Structural Design Criteria 89
Figure 6-9 is based on an analytical solution valid only for the flow in the 

vicinity of the runup tip on a uniformly sloping beach, with no lateral 

topographical variation, and no friction. Computed values may differ from 

the actual velocities, and additional engineering evaluation and judgment 

should be considered. Background information on the development of 

Figure 6-9 is provided in Appendix E. 

Impacts by Floating Vehicles. The impact of vehicles has been studied and 

codified for the case of vehicles impacting safety guardrails in parking 

structures. Vehicles are designed to resist impacts with significant inelastic 

deformation in order to reduce the forces experienced by passengers. It is 

recommended that the prescriptive code force of 6,000 lbs. used for safety 

barriers in parking structures be utilized to consider this effect on structural 

members immersed during the tsunami (ASCE 7, 2010). Alternatively, a 

work-energy approach similar to that discussed in Appendix D can be used. 

6.5.7 Damming of Accumulated Waterborne Debris

The damming effect caused by accumulation of waterborne debris can be 

treated as a hydrodynamic force enhanced by the breadth of the debris dam 

against the front face of the structure. Equation 6-11 is a modification of 

Equation 6-5 to include the breadth of the debris dam: 

= ρ 2

max

1 ( ) 2

F C B hu dm s d d (6-11) 

where ρs is the fluid density including sediment (1100 kg/m3 = 2.13 

slugs/ft3

), Cd is the drag coefficient, Bd is the breadth of the debris dam, h is 

flow depth, and u is flow velocity at the location of the structure. It is 

recommended that the drag coefficient be taken as Cd = 2.0. 

The maximum momentum flux per unit width (hu2

)max should be obtained by 

running a detailed numerical simulation model or acquiring existing 

simulation data. If no numerical simulation results are available, an estimate 

of (hu2

)max can be determined using Equation 6-6. 

Since debris damming represents an accumulation of debris across the 

structural frame, the total debris damming force will likely be resisted by a 

number of structural components, depending on the framing dimensions and 

the size of debris dam. The debris damming force, Fdm, should be assumed to 

act as a uniformly distributed load over the extent of the debris dam. It 

should be assigned to each resisting structural component by an appropriate 

tributary width, and distributed uniformly over the submerged height of each 

resisting component. The recommended minimum debris dam width is the 

larger of Bd = 40 feet (or 12 m), representing a sideways shipping container, 

90 6: Load Determination and Structural Design Criteria FEMA P-646
or a full structural bay width. The effects of debris damming should be 

evaluated at various locations on the structure to determine the most critical 

location. In addition, it has been observed that internal building contents 

may generate accumulated debris dammed against the exterior wall. The 

exterior wall may have partially failed to allow water flow, but structural 

studs and girts may be capable of holding contents in, thus generating 

hydrodynamic drag forces on the captured internal debris as the water flows 

through the structure. Accordingly, a full structural bay of debris dam is the 

minimum recommended width. 

6.5.8 Uplift Forces on Elevated Floors

Uplift forces will be applied to floor levels of a building that are submerged 

by tsunami inundation. In addition to standard design for gravity loads, these 

floors must also be designed to resist uplift due to buoyancy and 

hydrodynamic forces. When computing the buoyant forces on a floor slab, 

consideration must be given to the potential for increased buoyancy due to 

the additional volume of water displaced by air trapped below the floor 

framing system. In addition, exterior walls at the upper floor level will 

exclude water until their lateral resistance is exceeded by the applied 

hydrostatic pressure. This can significantly increase the displaced volume of 

water contributing to the buoyancy, as shown in Figure 6-10. 

The total upward buoyant force exerted on a floor system can be estimated 

using Equation 6-12: 

F gA h b s fb = ρ (6-12) 

where ρs is the fluid density including sediment (1100 kg/m3 = 2.13 

slugs/ft3

), g is the acceleration due to gravity, Af is the area of the floor panel 

or floor framing component, and hb is the water height displaced by the floor 

(including potentially entrapped air). The value of hmax indicated in Figure 6-

10 should be determined using Equation 6-3. 

The upward buoyant force per unit area exerted to the floor system can be 

estimated using Equation 6-13: 

f gh b sb = ρ (6-13) 

FEMA P-646 6: Load Determination and Structural Design Criteria 91
Figure 6-10 A definition sketch for upward buoyant force exerted on an 

elevated floor.

Hydrodynamic forces can also act vertically on floor slabs. During rapid 

inundation, rising water will apply uplift to the soffit of horizontal structural 

components, adding to the buoyancy uplift. The presence of structural walls 

and columns in a building will obstruct the tsunami flow passing through the 

building, and recent experiments have shown that this can result in 

significant uplift forces on the floor slab immediately in front of the 

obstruction. It is recommended that the building structural layout be 

designed to minimize obstruction of tsunami flow through the lower levels of 

the building.

Until further research results become available, the total uplift force on the 

floor system can be estimated using Equation 6-14:

= ρ

1 2

2

F C Au u us fv (6-14)

where Cu is a coefficient (taken as 3.0), ρs is the fluid density including 

sediment (1100 kg/m3 = 2.13 slugs/ft3

), Af is the area of the floor panel or 

floor framing component, and uv is the estimated vertical velocity or water 

rise rate (adapted from American Petroleum Institute, 1993). 

The hydrodynamic uplift per unit area can be determined from Equation 

6-15:

= ρ

1 2

2 u u sv f Cu (6-15)

Unless a detailed hydrodynamic study is performed, the value of uv for the 

condition of sloping terrain below the building can be estimated using 

Equation 6-16:

h

h

α

h

u s

max

b

f A = BxL

F

b

92 6: Load Determination and Structural Design Criteria FEMA P-646
u u v = tanα (6-16) 

where u is the horizontal flow velocity corresponding to a water depth, hs

equal to the elevation of the soffit of the floor system, and α is the average 

slope of grade at the site, as shown in Figure 6-10. Using the maximum 

horizontal flow velocity, umax, in Equation 6-15 would be unnecessarily 

conservative since it may not correspond to a flow depth equal to the floor 

soffit elevation. The maximum horizontal velocity u in Equation 6-16 can 

also be estimated using Figure 6-9 by replacing d/R with hs/R. 

6.5.9 Additional Retained Water Loading on Elevated Floors

During drawdown, water retained on the top of elevated floors, as shown in 

Figure 6-11, will apply additional gravity loads that can exceed the loads for 

which the floor system was originally designed. The depth of water retained, 

hr, will depend on the maximum inundation depth at the site, hmax, and the 

lateral strength of the wall system at the elevated floor. It should be assumed 

that the exterior wall system will be compromised at some point so that water 

will inundate submerged floor levels. Because of the rapid rate of 

drawdown, it is likely that much of this water will be retained in the upper 

levels (at least temporarily) resulting in significant additional gravity load on 

the floor system. The maximum potential downward load per unit area, fr, 

can be estimated using Equation 6-17: 

f gh r sr = ρ (6-17) 

where ρs is the fluid density including sediment (1100 kg/m3 = 2.13 

slugs/ft3

), g is the acceleration due to gravity, and hr is the maximum 

potential depth of water retained on the elevated floor determined using 

Equation 6-18: 

= −≤ r max 1 bw hh hh (6-18) 

where hmax is the maximum inundation level predicted at the site, h1 is the 

floor elevation above grade, and hbw is the maximum water depth that can be 

retained before failure of a significant portion of the wall due to internal 

hydrostatic pressure of the retained fluid.

For elevated floors without walls (such as a parking structure with open 

guardrails) water may remain on elevated floors until it has had time to drain 

off the structure. Drainage systems should be provided to ensure that the 

weight of retained water does not exceed the live load for which the floor is 

designed if the floor is necessary for structural stability.

FEMA P-646 6: Load Determination and Structural Design Criteria 93
Figure 6-11 Gravity loads exerted on an elevated floor with water retained 

by exterior walls during rapid drawdown.

6.6 Combination of Tsunami Forces

Not all tsunami load effects will occur simultaneously, nor will they all affect 

a particular structural component at the same time. This section describes 

combinations of tsunami forces that should be considered for the overall 

structure and for individual structural components. Other potential 

combinations should be considered as needed, based on the particular siting, 

structural system, and design of the structure under consideration. 

6.6.1 Tsunami Force Combinations on the Overall Structure

Tsunami forces are combined on the overall structure as follows:

• Uplift due to buoyancy, Fb, and hydrodynamic uplift, Fu, have the effect 

of reducing the total dead weight of a structure, which may impact the 

overturning resistance. Buoyancy and hydrodynamic uplift appropriate 

for the design inundation level should be considered in all load 

combinations. 

• Impulsive forces, Fs, are very short duration loads caused by the leading 

edge of a surge of water impinging on a wall-like structure. As the surge 

passes through a structure, impulsive forces will be applied sequentially 

to all structural components, but not at the same time. Once the leading 

edge of the surge has passed a structural component, it will no longer 

experience the impulsive force, but rather a sustained hydrodynamic drag 

force, Fd. The total horizontal hydrodynamic force on a structure will 

therefore be a combination of impulsive forces on members at the 

leading edge of the surge, and drag forces on all previously submerged 

members behind the leading edge. Figure 6-12 shows how this 

hr

hmax

h1

u

Fr

Not all tsunami load effects 

will occur simultaneously, nor 

will they all affect a particular 

structural component at the 

same time.

94 6: Load Determination and Structural Design Criteria FEMA P-646
combination would apply to a building with multiple columns and shear 

walls. The worst case lateral load will likely occur when the leading 

edge of the surge fully impacts the most closed off section of the 

building. 

• Debris impact forces, Fi, are short duration loads due to impact of large 

floating objects with individual structural components. Since large 

floating objects are not carried by the leading edge of the surge, the

effect of debris impact is combined with hydrodynamic drag forces, Fd, 

but not impulsive forces, Fs. Although many floating objects may impact 

a building during a tsunami event, the probability of two or more impacts 

occurring simultaneously is considered small. Therefore, only one 

impact should be considered to occur at any point in time. Both the 

individual structural component and the overall structure must be 

designed to resist the impact force in combination with all other loads 

(except impulsive forces).

• Debris damming has the effect of increasing the exposed area for 

hydrodynamic loading. The debris damming force, Fdm, should be 

considered to act in the most detrimental location on a structure while 

hydrodynamic forces act on all other components of the structure. Figure 

6-13 shows typical debris dam locations that could be considered in 

conjunction with drag forces on all other submerged structural 

components. It is conservative to ignore any shielding effect provided by 

the debris dam for components downstream of the dam. 

Figure 6-12 Impulsive and drag forces applied to an example building.

FEMA P-646 6: Load Determination and Structural Design Criteria 95
Figure 6-13 Debris dam and drag forces applied to an example building. 

• Breakaway walls are not part of the structural support of the building, 

and are intended, through design and construction, to fail under specific 

lateral loading. If lower level infill walls are designed as breakaway 

walls, the maximum lateral load will be the load at which the walls will 

“fail,” and the overall structure, as well as the structural components 

supporting these walls, must be designed to resist this failure load. 

Guidance on the design of break-away walls is provided in Chapter 7.

• Design of floor systems to withstand the effects of potential retained 

water, Fr, can be performed independently of the lateral loading on the 

structure. 

6.6.2 Tsunami Force Combinations on Individual Components

Tsunami forces are combined on individual structural components (e.g., 

columns, walls, and beams), as follows: 

• Impulsive force, Fs, applicable to wall and pier structural elements due to 

the leading edge of the tsunami bore, for maximum hu2

. 

• Hydrodynamic drag force, Fd, plus debris impact, Fi, at the most critical 

location on the member, for maximum hu2

. 

• Debris damming, Fdm, due to a minimum 40-foot wide or structural bay 

width debris dam causing the worst possible loading on the member, for 

maximum hu2

. 

96 6: Load Determination and Structural Design Criteria FEMA P-646
• Hydrostatic pressure, Fh, on walls enclosing watertight areas of a 

structure, for maximum h.

For uplift on floor framing components, the following combinations of 

tsunami forces should be considered:

• Buoyancy, Fb, of submerged floor framing components including the 

effects of entrapped air and upturned beams or walls, for maximum h.

• Hydrodynamic uplift, Fu, due to rapidly rising flood waters, for flow 

velocity at a depth equal to the soffit of the floor system, hs.

• Maximum uplift case: The larger of the above uplift loads combined with 

90% dead load and zero live load on the floor system, for design against

uplift failure of floor slabs, beams, and connections.

For downward load on floor framing components due to retained water, the 

following force combination should be considered:

• Downward load due to water retained by exterior walls, fr, combined 

with 100% dead load.

6.7 Load Combinations

The load combinations presented herein are based on the guidance given in 

the Commentary of ASCE/SEI 7-10 Minimum Design Loads for Buildings 

and Other Structures (ASCE, 2010), but are modified from those used in 

Section 2.5, Load Combinations for Extraordinary Events, of ASCE/SEI 

Standard 7-10. The modification is based on the presumption that only the 

refuge floor areas will be occupied during a tsunami event. They have been 

reviewed in the development of this document, but have not been extensively 

studied. They should be considered in addition to all other load 

combinations required by the current building code in effect, or Section 2 of 

ASCE/SEI 7-10.

Tsunami forces that will act on the entire structure and on individual 

structural components should be calculated in accordance with Section 6.5 

and Section 6.6. The resulting member forces (Ts) should then be combined 

with gravity load effects using the following Strength Design Load 

Combinations:

Load Combination 1: 1.2D + 1.0Ts + 1.0LREF + 0.25L

Load Combination 2: 0.9D + 1.0Ts

Tsunami Load 

Combinations should be 

considered in addition to all 

other load combinations 

provided by the current 

building code in effect, or 

ASCE/SEI 7-05.

FEMA P-646 6: Load Determination and Structural Design Criteria 97
where D is the dead load effect, Ts is the tsunami load effect, LREF is the live 

load effect in the refuge area (assembly loading), and L is the live load effect 

outside of the refuge area.

A load factor of 1.0 is used in conjunction with tsunami forces calculated in 

accordance with this document for the following reasons: (1) it is anticipated 

that the tsunami hazard level corresponding to the Maximum Considered 

Tsunami will be consistent with the 2500-year return period associated with 

the Maximum Considered Earthquake used in seismic design; and (2) 

potential variability in tsunami runup elevations is explicitly considered by 

applying a 30% increase to runup elevations used in tsunami force 

calculations. 

Load Combination 1 considers the refuge area in the vertical evacuation 

structure to be fully loaded with assembly live load (i.e., 100 psf). The 

assembly live load represents a practical upper limit for the maximum 

density of evacuees standing in the refuge area. In combination with tsunami 

inundation, it is expected that all other floor areas will experience a reduced 

live load equal to 25% of the design live load. This reduced live load is 

consistent with live load reductions used in combination with earthquake 

forces. When gravity load effects oppose tsunami load effects, Load 

Combination 2 applies.

No additional importance factor, I, is applied to tsunami loads in this 

document. These design guidelines have been developed specifically for 

tsunami evacuation structures, and the critical nature of these structures has 

been considered throughout.

Seismic loads are not considered to act in combination with tsunami loads. 

While aftershocks are likely to occur, the probability that an aftershock will 

be equivalent in size to the design level earthquake, and will occur at the 

same time as the maximum tsunami loading, is considered to be low. 

However, since seismic design in the U.S. does utilize post-elastic ductility, 

seismically damaged components may have less available ductility for a 

subsequently arriving local tsunami. 

6.8 Member Capacities and Strength Design 

Considerations

Model building code provisions and engineering standards for Strength 

Design, also known as Load and Resistance Factored Design (LRFD), 

provide material-specific member capacity calculations and strength 

reduction factors for various force actions and different structural 

components. Until further research shows otherwise, it is recommended that 

Member Capacities and 

Strength Reduction 

Factors should be applied 

to design for tsunami 

loading in the same way 

they are currently applied to 

design for earthquake and 

wind loading.

98 6: Load Determination and Structural Design Criteria FEMA P-646
capacity calculations and strength reduction factors be applied to design for 

tsunami loading in the same way they are currently applied to design for 

earthquake and wind loading. 

6.9 Progressive Collapse Considerations

Reducing the potential for disproportionate (i.e., progressive) collapse due to 

the loss of one or more structural components will increase the likelihood 

that a vertical evacuation structure will remain standing if a column is 

severely damaged due to waterborne debris. The decision to include 

progressive collapse considerations in the design for a particular structure 

will depend on the site and the nature of the debris that could potentially 

impact the structure. Because the potential exists for localized severe 

damage due to debris impact, design for progressive collapse prevention is 

strongly encouraged. In the United States, primary design approaches for 

progressive collapse include measures to implement “tie force”, “enhanced 

local resistance” and “alternative load path” mitigation measures. For 

essential facility occupancies including emergency shelters, the Department 

of Defense requires the application of all three measures. The General 

Services Administration requires the alternative load path design technique to 

span over a missing vertical load carrying column or wall element.

6.9.1 Department of Defense Methodology

The Department of Defense (DOD) has adopted occupancy-dependent 

requirements for progressive collapse prevention to address the potential for 

progressive collapse in the design of facilities using UFC 4-023-03, Design 

of Buildings to Resist Progressive Collapse (DOD, 2009). For Risk Category 

IV, the designer provides: 

1. Internal, peripheral, and vertical tie force capacities so that the building 

is mechanically tied together to enhance the development of alternative 

load paths. 

2. Enhanced Local Resistance of the first two stories on the building 

perimeter, with flexural capacities of columns and walls increased by 

factors of 2 and 1.5, respectively, over the design flexural strength 

determined from the alternative load path procedure. The shear 

capacities of these elements shall be greater than the flexural capacities. 

For design of vertical evacuation structures it is proposed that these 

measures be applied to all levels anticipated to be submerged by the 

tsunami, but not less than the first two stories.

FEMA P-646 6: Load Determination and Structural Design Criteria 99
3. Alternative Load Path to enable the structure to bridge over vertical loadbearing elements that are notionally removed one at a time along the 

exterior.

The tie force strategy is illustrated in Figure 6-14. 

Figure 6-14 Tie force strategy in a frame structure.

Tension ties in reinforced concrete structures typically consist of continuous 

reinforcing steel in beams, columns, slabs, and walls, as shown in Figure 

6-15. Reinforcement required for tension ties can be provided in whole, or in 

part, by steel already sized to resist other actions, such as shear or flexure. In 

many cases, the quantity of steel provided to resist gravity and lateral forces 

for typical reinforced concrete structures is also sufficient to develop the 

necessary tie forces. 

It is reasonable to check tie force compliance after a structure is initially 

designed for gravity and lateral loading. Ties must be properly spliced and 

adequately anchored at each end in order to develop their full capacity and 

perform as anticipated. Reinforcing steel used as tension ties must have 

lapped, welded, or mechanically joined (Type 1 or Type 2) splices per ACI 

318, Building Code Requirements for Structural Concrete (ACI, 2011). 

Splices should be staggered and located away from joints and regions of high 

stress.

100 6: Load Determination and Structural Design Criteria FEMA P-646
Figure 6-15 Detailing of reinforcing steel for potential loss of a supporting 

column. 

Anchorage is critical to the performance of ties and must be carefully 

assessed, particularly in cases where building layout may be non-typical. 

Seismic detailing should be used to anchor ties to other ties, or at points of 

termination (such as at the perimeter of a building). This includes providing 

seismic hooks and seismic development lengths, as defined in ACI 318. 

6.9.2 General Services Administration Methodology

The General Services Administration (GSA) missing column strategy is an 

independent check performed without consideration of other loads. This 

approach is based on the concept that loss of a single column, in this case due 

to impact from waterborne debris, should not result in progressive collapse of 

the surrounding structural components. 

Current progressive collapse criteria are found in Progressive Collapse 

Analysis and Design Guidelines for New Federal Office Buildings and Major 

Modernization Projects (GSA, 2003). As illustrated in Figure 6-16, this 

strategy requires evaluation of surrounding structural components to continue 

to support anticipated gravity loads in a series of missing column scenarios. 

Live loads on the building are reduced to simulate those in place at the time 

the column is damaged. In the case of vertical evacuation structures, full live 

loads should be considered in the refuge area while reduced live loads can be 

considered elsewhere in the building. 

The missing column approach utilizes plastic design concepts in evaluating 

the capability of surrounding structural components to continue to support 

gravity loads, so some damage in these components is permitted as a result of 

a missing column scenario. Given that waterborne debris is most likely to 

impact an exterior or corner column, missing column scenarios should 

FEMA P-646 6: Load Determination and Structural Design Criteria 101
consider the potential loss of any single exterior column. Loss of interior 

columns need not be considered.

Figure 6-16 Missing column strategy.

102 6: Load Determination and Structural Design Criteria FEMA P-646
Chapter 7

Structural Design Concepts 

and Additional Considerations

This chapter summarizes structural design concepts and other considerations 

relevant to the design of vertical evacuation structures, including retrofit of 

existing structures, permitting, peer review, quality control, planning issues, 

and potential cost impacts.

7.1 Attributes of Tsunami-Resistant Structures

Structural system selection and configuration, from foundation to roof 

framing, can have a significant effect on the ability of a vertical evacuation 

structure to withstand anticipated tsunami, earthquake, and wind loading. 

Many common structural systems can be engineered to resist tsunami load 

effects. 

Structural attributes that have demonstrated good behavior in past tsunamis 

include: (1) strong systems with reserve capacity to resist extreme forces; (2) 

open systems that allow water to flow through with minimal resistance; (3) 

ductile systems that resist extreme forces without failure; and (4) redundant 

systems that can experience partial failure without progressive collapse. 

Systems exhibiting these attributes include reinforced concrete and steel 

moment frame systems, and reinforced concrete shear wall systems. 

7.2 Structural Considerations for Tsunami Load Effects

Foundation design must consider the local effects of scour and liquefaction. 

In many cases foundation support will consist of deep foundations (piles). 

Pile design must consider increased demands due to downdrag and additional 

lateral forces, and increased unbraced pile length due to scour. Potential 

uplift from the overall buoyancy of the structure and overturning moments 

due to hydrodynamic and unbalanced hydrostatic loads need to be accounted 

for in the foundation design. 

Design of individual columns for tsunami lateral loads should be performed 

assuming the appropriate degree of fixity at the column base and at each 

floor level. For example, a reinforced concrete column in a multi-story 

building supported by pile foundations can be assumed fixed at the base and 

Tsunami-Resistant Structures 

have: 

(1) strong systems with reserve 

capacity to resist extreme forces;

(2) open systems that allow water 

to flow through with minimal 

resistance; 

(3) ductile systems that resist 

extreme forces without failure; and 

(4) redundant systems that can 

experience partial failure without 

progressive collapse.

FEMA P-646 7: Structural Design Concepts and Additional Considerations 103
at each floor level. A steel column forming part of a moment-resisting frame 

can be assumed pinned or fixed at the base and at each floor level. 

Column shape is also important. Round columns will result in lower drag 

forces than square or rectangular shapes. In addition, waterborne debris will 

be less likely to fully impact round columns. 

If shear walls are used, the plan orientation of the walls is important. It is 

recommended that the shear walls be oriented parallel to the anticipated 

direction of tsunami flow to reduce associated hydrodynamic forces and 

impact forces from waterborne debris.

Design of reinforced concrete walls for tsunami forces should consider the 

full load on the wall, including hydrodynamic and debris impact forces,

spanning vertically between floor levels. Reinforced concrete beams poured 

integral with the floor will be braced by the slab. Design of beams for 

horizontal tsunami forces should take into account the lateral bracing 

provided by the floor slab. Isolated beams must be designed for horizontal 

shear and bending induced by tsunami loads. 

Floor systems must be designed for the effects of buoyancy and 

hydrodynamic uplift, which will induce shear and bending effects that are 

opposite to those resulting from gravity loads. Even though lower levels of a 

vertical evacuation structure are not intended for use during a tsunami, 

failure could result in damage or collapse of columns supporting upper 

levels, including the tsunami refuge area. 

In structural steel floor systems, lateral torsional buckling of beam bottom 

flanges must be considered when subjected to uplift loading. In reinforced 

concrete floor systems, continuity of reinforcement should be provided in 

beams and slabs for at least 50% of both the top and bottom reinforcement. 

Prestressed concrete floor systems must be carefully checked for buoyancy 

and hydrodynamic uplift effects when submerged. Internal prestressing 

forces used to oppose dead loads add to these effects. Web elements of 

typical prestressed joist systems are susceptible to compression failure under 

uplift conditions, and many typical bearing connections are not anchored for 

potential net uplift forces. Localized damage to the concrete in a prestressed 

floor system can result in loss of concrete compressive capacity, and release 

of the internal prestressing forces. 

7.2.1 Foundation / Scour Design Concepts

Scour around shallow foundations can lead to failure of the supported 

structural element. Foundations consisting of drilled shafts or driven piles 

104 7: Structural Design Concepts and Additional Considerations FEMA P-646
can be designed to avoid this failure; however, they must be able to resist all 

applied loads after scouring has exposed the pile cap and top of the shafts or 

piles.

Dames and Moore (1980) suggests that scour depth is related to distance 

from the shoreline and soil type. As indicated in Table 7-1, scour depth is 

estimated as a percentage of the maximum tsunami flow depth, d. 

Table 7-1 Approximate Scour Depth as a Percentage of Flow Depth, d

(Dames and Moore, 1980)

Soil Type

Scour depth (% of d)

(Shoreline Distance < 300 feet) 

Scour depth (% of d)

(Shoreline Distance > 300 feet) 

Loose sand 80 60

Dense sand 50 35

Soft silt 50 25

Stiff silt 25 15

Soft clay 25 15

Stiff clay 10 5 

Observations after the Indian Ocean Tsunami indicate that scour can occur 

significantly farther inland than 300 feet from the shoreline. Scour depths of 

10 to 13 feet (3 to 4 meters) were observed in locations of high velocity flow 

during the Tohoku tsunami. Conservative engineering judgment should be 

exercised in categorizing the soil type at the site into the broad categories 

listed above.

7.2.2 Breakaway Wall Concepts

Solid enclosure walls below the tsunami inundation level will result in large 

tsunami loads on the overall building. These walls will also increase the 

potential for wave scour at grade beams and piles. Non-structural walls 

below the anticipated tsunami flow depth can be designed as breakaway 

walls to limit the hydrostatic, buoyancy, hydrodynamic, and impulsive forces 

on the overall building and individual structural members. Breakaway wall 

requirements are described in detail in the FEMA 55 Coastal Construction 

Manual (FEMA, 2005), which complies with National Flood Insurance 

Program (NFIP) requirements for construction in the mapped V-Zone. 

Breakaway walls can create wave reflection and runup prior to failure as 

indicated in Figure 7-1. 

In accordance with ASCE/SEI Standard 24-05 Flood Resistant Design and 

Construction (ASCE, 2006a), walls, partitions, and connections to the 

structure that are intended to break away are designed for the largest of the 

following loads acting perpendicular to the plane of the wall: 

FEMA P-646 7: Structural Design Concepts and Additional Considerations 105
Figure 7-1 Effect of breakaway walls on waves (FEMA, 2005).

• The wind load specified in ASCE/SEI Standard 7-05 Minimum Design 

Loads for Buildings and Other Structures (ASCE, 2006b).

• The earthquake load specified in ASCE/SEI Standard 7-05.

• 10 psf (0.48kN/m2

).

• Not more than 20 psf (0.6 kN/m2

) unless the design meets the following 

conditions: (1) breakaway wall collapse is designed to result from a flood 

load less than that which occurs during the base flood; and (2) the 

supporting foundation and the elevated portion of the building is 

designed to resist collapse, permanent lateral displacement, and other 

structural damage due to the effects of flood loads in combination with 

other loads.

Standard engineering practice can often result in considerable design 

overstrength, which would be detrimental to a breakaway wall system and 

the supporting structure. Care should be taken to avoid introducing 

unnecessary conservatism into the design. All components, including 

sheathing, siding, and window frame supports, must be considered in 

determining the actual strength of the breakaway wall system, and the 

resulting maximum load on the supporting structure. The most desirable 

fusing mechanism includes failure of the top and side connections while the 

bottom connection remains intact, allowing the wall panel to lay down under 

the tsunami flow without becoming detached and part of the debris flow. 

Metal Stud Walls. Metal stud infill walls are commonly used as part of the 

building envelope. Unless properly galvanized, metal studs will corrode 

rapidly in the coastal environment. Recent lateral load testing of typical 

metal stud wall configurations shows that ultimate failure occurs when the 

studs separate from either the top or bottom tracks. However, the load 

106 7: Structural Design Concepts and Additional Considerations FEMA P-646
required to produce this failure is as much as four times the wind load for 

which the studs were initially designed. It is therefore necessary to introduce 

some sort of a “fuse” at the top track connection to ensure that the wall fails 

at a predictable load. Such a fuse might include a reduced stud section at the 

top of the studs. Testing of fuse mechanisms would be required to verify that 

they have the capacity needed to resist design loads, but will fail at 

predictably higher load levels.

Masonry Walls. Masonry walls are commonly used as enclosures in lower 

levels of larger buildings. They can be restrained with the use of a dowel pin 

fuse system around the top and sides of the wall, without bonded contact to 

the structure. Such a system should be tested to verify that it will fail at 

predictable load levels that exceed design loads. If properly fused, the 

masonry wall will cantilever from the foundation and load will no longer be 

applied to the surrounding structural frame, upon failure of the dowel pins. 

To allow wall failure due to foundation rotation without damage to the 

remaining structure, separation of the wall foundation from the building 

foundation should be considered. 

7.3 Concepts for Modifying and Retrofitting Existing 

Structures 

It may not always be feasible to construct new buildings in an area that 

requires vertical evacuation refuge. Although retrofitting existing buildings 

to perform as a vertical evacuation structure could be expensive and 

disruptive to current users of the building, it may be the most viable option 

available. Existing buildings considered for use as vertical evacuation 

structures should possess the structural attributes listed in Section 7.1 that are 

associated with tsunami-resistant structures, and should be evaluated for 

tsunami load effects in accordance with Chapter 6. In the case of nearsource-generated tsunamis, existing buildings should also be evaluated for 

seismic effects. Because of the importance of vertical evacuation structures, 

and the need for these facilities to function as a refuge when exposed to 

extreme tsunami and seismic loading, reduced loading criteria for existing 

buildings, as is the current state-of-practice for seismic evaluation of existing 

buildings, is not recommended for evaluation of potential tsunami vertical 

evacuation structures. 

The following concepts can be considered in the modification and retrofit of 

existing buildings for use as vertical evacuation structures:

• Roof system. Upgrade roof systems to support additional live loads 

associated with refuge occupancy. Protect or relocate existing building 

functions at the roof level (e.g., mechanical equipment) that would be at 

Existing buildings considered for 

use as vertical evacuation structures 

should possess the attributes of 

tsunami-resistant structures listed in 

Section 7.1

FEMA P-646 7: Structural Design Concepts and Additional Considerations 107
risk or unsafe in the immediate vicinity of high occupancy areas. Modify 

existing roof parapets for fall protection of refuge occupants. 

• Wall system. Consider modifying walls and wall connections in the 

lower levels of the building to perform as breakaway walls to minimize

tsunami hydrostatic, hydrodynamic, and surge forces on the building. 

• Access. Modify ingress into the building and improve vertical 

circulation through the use of new entrances, ramps, and stairs. Consider 

placing access points on the outside of the building for ease of 

construction and high visibility. 

• Potential Debris. Remove or relocate building ground level functions 

that may become potential water-borne debris. 

• Existing hazards at the site. Consider and protect against other hazards 

that might exist at the building site, including other adjacent buildings 

that could collapse, and the presence of hazardous or flammable 

materials near the site. 

7.4 Permitting and Quality Assurance for Vertical 

Evacuation Structures

7.4.1 Permitting and Code Compliance

Before construction begins, all necessary state, local, building, and other 

permits should be obtained. Because model building codes and engineering 

standards do not address the design of a tsunami refuge specifically, design 

professionals should meet with building officials to discuss possible design 

requirements. 

In general, mechanical, electrical, and plumbing systems should be designed

for the normal daily use of the facility, unless otherwise directed by the 

authority having jurisdiction. Designing these systems for the high 

occupancy load that would occur only when the structure is serving as a 

vertical evacuation refuge may not be necessary. 

7.4.2 Peer Review

A vertical evacuation structure is a unique structure that must withstand 

special loads and load combinations. While earthquake, wind, and flood 

loading effects are well understood in the design and permitting process, 

consideration of tsunami load effects includes some new concepts and 

approaches. Considering the importance of vertical evacuation structures 

and the extreme nature of tsunami loading, peer review by a qualified 

individual or team is recommended. 

The unique nature of vertical 

evacuation structures may 

require special allowances for:

(1) permitting and code compliance; 

(2) peer review; and

(3) quality assurance.

108 7: Structural Design Concepts and Additional Considerations FEMA P-646
7.4.3 Quality Assurance / Quality Control

Because a vertical evacuation structure must perform well during extreme 

loading conditions, quality assurance and quality control for the design and 

construction of the structure should be at a level above that for normal 

building construction. Design calculations and drawings should be 

thoroughly scrutinized for accuracy. 

The quality of both construction materials and methods should be ensured 

through the development and application of a quality control program. A 

quality assurance plan should be based on the Special Inspection 

Requirements listed in Chapter 17 of the International Building Code (ICC, 

2006). Special inspections and quality assurance provisions for primary 

seismic- and wind-resisting systems should be applied to tsunami-resisting 

elements of vertical evacuation structures. Exceptions that waive the need 

for quality assurance when elements are prefabricated should not be allowed. 

In addition to the building elements that are normally included special 

inspection programs, the following items require special attention:

• Breakaway walls and their connections to structural components to avoid 

unintended conservatism in construction.

• Other special components or details that are used to minimize tsunamiloading effects.

• Piles, pilecaps and grade beams that will potentially experience the 

effects of scour.

7.5 Planning Considerations for Vertical Evacuation

Structures

In addition to structural design, planning for vertical evacuation facilities 

should consider a number of issues, including access, parking, pets, 

occupancy limitations, and protection of critical functions. 

• Access and Entry. Confusion and panic will occur if evacuees arrive at 

a refuge facility, but cannot enter. Provisions should be made to ensure 

access in the event of a tsunami, while providing adequate security 

during times when the facility is unoccupied. Ideally, a vertical 

evacuation refuge should be configured so that it is always accessible, or 

can be entered without emergency personnel.

• Americans with Disabilities Act (ADA). Vertical evacuation 

structures, when not operating as a refuge, must comply with Federal, 

state, and local ADA requirements and ordinances for the normal daily 

use of the facility. Design of ingress and vertical circulation within a 

Planning for vertical 

evacuation facilities should 

allow for: 

(1) access and entry; 

(2) Americans with Disabilities Act; 

(3) parking; 

(4) pets; 

(5) occupancy limitations; and

(6) protection of critical functions. 

FEMA P-646 7: Structural Design Concepts and Additional Considerations 109
vertical evacuation structure should consider the needs of disabled 

occupants to the extent possible, and the extent required by law, in the 

case of emergency evacuation. Given potential limitations on 

functionality of power sources and vertical conveyance systems (e.g., 

elevators and escalators) in the event of a near-source earthquake, 

disabled occupants may need assistance accessing refuge areas in vertical 

evacuation structures. 

• Parking. Parking at evacuation facilities can be a problem. Traffic 

congestion can adversely affect access to the facility, and parked vehicles 

can become waterborne debris that can damage the structure. Planning 

for vertical evacuation facilities should consider parking limitations. 

• Pets. Refuge facilities are typically not prepared to accommodate pets. 

Many people, however, do not want to leave their pets behind during a 

disaster. Planning should carefully consider the policy regarding pets. 

• Occupancy Limitations. Population density can be non-uniform, and 

can vary by time of day, week, or year. In the event of a tsunami, 

evacuation behavior of the surrounding population may result in an 

unequal distribution of evacuees among available refuge facilities. In 

determining the maximum occupancy for a refuge facility, the time of 

day, day of the week, or season of the year that will result in the largest

number of possible evacuees should be considered. The maximum 

occupancy might need to be increased in order to accommodate 

unexpected additional occupants or visitors in the area. 

• Protection of Critical Functions. A vertical evacuation facility must be 

operational to serve its intended function in the event of a tsunami. 

Functions that are critical for operation as a short-term refuge, 

emergency response, medical care, or long-term sheltering facility must 

be protected from tsunami inundation, or located within the area of 

refuge. These might include emergency power, electrical equipment, 

communications equipment, basic sanitation needs, medical and 

pharmaceutical supplies, and emergency provisions (e.g., food, water, 

and supplies).

7.6 Cost Considerations for Vertical Evacuation 

Structures

Design of vertical evacuation structures for tsunami load effects will require 

more strength, ductility, and robustness than is necessary for normal-use 

structures. As recommended in this document, this can include the use of 

seismic detailing provisions, progressive collapse preventative measures, 

customized breakaway wall details, and deeper foundation systems. As such, 

110 7: Structural Design Concepts and Additional Considerations FEMA P-646
it is expected that structural construction costs will be higher for vertical 

evacuation structures than for other structures. While there are no direct 

comparisons between the cost of a conventional structure versus the cost of a 

tsunami-resistant structure, order-of-magnitude information on potential 

structural construction cost increases can be obtained from currently 

available information. 

Structural costs, however, are only a fraction of total construction costs for a 

building. Depending on the nature of building occupancy and use, structural 

construction costs can range between 5% and 40% of total construction costs. 

Structural costs are a lower percentage of the total for occupancies with 

special uses (e.g., hospitals) requiring more expensive nonstructural systems 

and contents, and are higher percentage of the total for occupancies with 

standard uses (e.g., offices).

Anecdotal evidence from design and construction of essential facilities (e.g., 

hospitals) in California, Oregon, and Washington indicate that the cost 

premium for seismic design requirements associated with essential facilities 

versus ordinary occupancy facilities is on the order of 10% to 20% of 

structural construction costs. This would represent an increase on the order 

of 1% to 8% in terms of total construction costs.

In a recent study funded by the National Institute of Standards and 

Technology (NIST), Engineering Design and Cost Data for Reinforced 

Concrete Buildings for Next Generation Design and Economic Standards for 

Structural Integrity (NIST, 2007), the cost premium for progressive collapseresistant design was on the order of 10% to 20% of structural construction 

costs. Similar to seismic design, this would represent an increase on the 

order of 1% to 8% in terms of total construction costs.

Considering additional allowances for added strength to resist tsunami load 

effects, it is reasonable to expect that a tsunami-resistant structure, including

seismic-resistant and progressive collapse-resistant design features, would 

experience about a 10% to 20% order-of-magnitude increase in total 

construction costs over that required for normal-use buildings. While each 

project will be unique, and relative costs will depend on the specific tsunami 

hazard and site conditions, it should not be assumed that incorporation of 

tsunami-resistant design features in a vertical evacuation structure will be 

cost prohibitive.

Structural construction costs are only 

a fraction of total construction costs 

for a building.

Tsunami-resistant structures could 

experience about a 10% to 20% 

order-of-magnitude increase in total 

construction costs over that required 

for normal-use buildings.

FEMA P-646 7: Structural Design Concepts and Additional Considerations 111

Appendix A

Vertical Evacuation Structure 

Examples from Japan

In Japan there are examples of structures that were designed and constructed 

specifically for the purpose of tsunami refuge. The Government of Japan, 

Director-General for Policy Planning, published Guidelines for Tsunami 

Evacuation Buildings in Japanese in June 1995 (DGPP, 1995). Okada, et al 

(2005) of the Building Center of Japan, Building Technology Research 

Institute, provide an English explanation in SMBTR - Structural Design 

Method of Buildings for Tsunami Resistance, which has been used for design 

of vertical evacuation structures such as the apartment building in 

Minamisanriku, shown in Figure 2-23.

A number of multi-story reinforced concrete and structural steel buildings in 

Japan were designated as vertical evacuation buildings prior to the Tohoku 

tsunami. All performed well structurally, though many were too low for the 

actual inundation depth, resulting in loss of life (Murakami et al, 2012). 

Figure A-1 shows such a building in Kesennuma Port that was successfully 

used by refugees during the tsunami. Over 4000 buildings and other 

structures in Japan are now officially designated for use as vertical 

evacuation refuges (Yomiuri Shimbun, 2012).

Life-Saving Tower: The Life-Saving Tower (Tasukaru Tower) developed 

by Fujiwara Industries Company, Limited, Japan, is shown in Figure A-2. 

This is a simple and economical structure that enables a temporary high 

refuge for evacuees. The structure has a 5.4-meter span between the 

supporting posts, a refuge elevation of 5.8 meters from ground level, and a 

capacity of 50 people. 

FEMA P-646 A: Vertical Evacuation Structure Examples from Japan 113
Figure A-1 Successful designated vertical evacuation building in 

Kesennuma Port, Japan.

Figure A-2 Life-Saving Tower

114 A: Vertical Evacuation Structure Examples from Japan FEMA P-646
Nishiki Tower: The Nishiki Tower, shown in Figure A-3, was constructed 

in the town of Kise, Mie Prefecture, Japan. The five-story, 22-meter tall 

reinforced concrete structure resembles a lighthouse, and has a spiral 

staircase winding up the outside of the building. It was specifically designed 

to serve as a tsunami refuge, but is used for other (non-refuge) purposes on

normal days. The first floor is used for public toilet and storage space for 

fire equipment; the second floor for a meeting room; and the third floor for 

an archival library for natural disasters. The fourth and fifth floors have 73 

square meters of refuge space for evacuees. 

Figure A-3 Nishiki Tower.

Nishiki Tower is a well-engineered structure that is designed to withstand a 

seismic event commensurate to JMA VII on the Japanese earthquake 

intensity scale that is comparable to a MMI XII (modified Mercalli scale). 

The building is founded on a 4-meter deep sand-and-gravel layer, and is 

supported on concrete piles extending 6 meters below grade. The possibility 

of liquefaction is remote, considering the large particle size of the sand-andgravel layer. Elastic design was employed for consideration of tsunami 

forces. Based on historical data from the 1944 Tou-Nankaido Earthquake, a 

design tsunami of 6 meters in height was used for design. It is designed to 

withstand the impact of a 10-ton ship at a velocity of 10 m/sec. This 

FEMA P-646 A: Vertical Evacuation Structure Examples from Japan 115
criterion was based on size of ships moored in the neighboring port. The

intended performance level allows for partial damage of the building without 

incurring loss of life. 

Elevated Shelter at Shirahama Beach Resort: A rather aesthetic tsunami 

refuge was constructed at a beach resort in the town of Shirahama, 

Tokushima Prefecture, shown in Figure A-4. It is designed to accommodate 

700 refugees in the area of 700 square meters. The design inundation 

elevation is 7.5 meters, based on historical data from the 1854 Ansei-Tokai 

Earthquake (M 8.4) and resulting tsunami. With a planned freeboard of 4

meters, the evacuation platform is located at elevation of 11.5 meters. This 

reinforced concrete structure is designed to withstand a maximum base 

acceleration of 780 gal. Because of a potential for soil liquefaction, pipe 

piles were driven approximately 20 meters deep into bedrock. The facility is 

also equipped with a solar-powered lighting system.

Figure A-4 Refuge at Shirahama Beach Resort (photo courtesy of N. Shuto).

Other Tsunami Refuge Structures: There are other structures in Japan 

specifically designed as tsunami refuges. A reinforced concrete structure in 

the town of Kaifu, Tokushima Prefecture, Japan is shown in Figure A-5. An 

artificial high ground (berm), shown in Figure A-6, was constructed in 

Aonae, Okushiri-Island, Japan, where the 1993 tsunami struck the hardest. 

After the 1993 Okushiri Tsunami, Aonae elementary school, shown in Figure 

A-7, was reconstructed as a tsunami resistant structure. The upper floor can 

116 A: Vertical Evacuation Structure Examples from Japan FEMA P-646
be used as a tsunami refuge space. The ground floor of the school is 

constructed with breakaway walls to relieve tsunami forces. 

Figure A-5 Tsunami refuge in Kaifu, Japan.

Figure A-6 Berm constructed for tsunami refuge in Aonae, Japan.

FEMA P-646 A: Vertical Evacuation Structure Examples from Japan 117
Figure A-7 Aonae Elementary School. Upper floor is intended for use as tsunami refuge 

space.

118 A: Vertical Evacuation Structure Examples from Japan FEMA P-646
Appendix B

Community Design 

Example

A hypothetical community is indicated in Figure B-1 below. In this 

appendix, the initial design and configuration of a series of vertical 

evacuation structures is illustrated. 

The community has evaluated public and private sites that might be appropriate 

for construction of new vertical evacuation structures and identified existing 

facilities for possible renovation for use as vertical evacuation structures. This 

evaluation includes consideration of the number of sites required based on 

travel time and population, as discussed in Chapter 5.

Figure B-1 Hypothetical sketch of example community showing potential 

vertical evacuation structure sites and evacuation routes. 

FEMA P-646 B: Community Design Example 119
An assessment of the tsunami inundation depths and flow velocities is 

necessary for assessing tsunami effects within the community and 

determining tsunami design parameters. Predicted tsunami inundation depths 

for this example community are shown in Figure B-2.

Figure B-2 Example community inundation map. Shaded areas show 

various predicted tsunami inundation depth, d. 

In this example community, the area of refuge at each site would need to be 

elevated as indicated in Table B-1.

Table B-1 Design Elevations for Areas of Refuge 

Site

Predicted 

Inundation Depth

Freeboard

(3 meters plus 30%)

Design 

Elevation

Site 1 3 m 3 m + 0.9 m 6.9 m

Site 2 4 m 3 m + 1.2 m 8.2 m

Site 3 3 m 3 m + 0.9 m 6.9 m

Site 4 4 m 3 m + 1.2 m 8.2 m

Site 5 3 m 3 m + 0.9 m 6.9 m

Tsunami inundation depths indicated in Figure B-2 are increased by 30% to 

account for local variability in numerical simulations. An additional

minimum freeboard of 3 meters (or one-story height) is recommended to 

ensure that the area of refuge is not inundated from splash or wave action. 

120 B: Community Design Example FEMA P-646
The velocity at a particular site is affected by the surrounding topography as 

well as natural and man-made obstructions to flow. Predicted flow velocities

for this example community are shown in Figure B-3 and summarized in 

Table B-2.

Figure B-3 Example community inundation flow velocity map. Shaded 

areas show various predicted tsunami flow velocities, u.

Table B-2 Tsunami Flow Velocity at Each Site 

Site Tsunami Flow Velocity

Site 1 9 m/s

Site 2 12 m/s

Site 3 9 m/s

Site 4 12 m/s

Site 5 9 m/s

B.1 Site 1 Example: Escape Berm

Site 1 has several unique conditions to consider. The waterfront in this area 

is somewhat industrial in nature and includes a container terminal facility at 

the harbor. Areas adjacent to the site contain some residential development. 

The evacuation population at this site would include both employees of the 

harbor industrial area and adjacent residences. 

FEMA P-646 B: Community Design Example 121
The community has been struggling with finding ways to address other social 

issues in this area, which have included a lack of recreational facilities for the 

residents, some neglected and deteriorating properties, and a need to

revitalize and enhance the area. At this site a man-made berm, as shown in 

Figure B-4, provides an opportunity to add new public open space in addition 

to vertical evacuation refuge. This solution creates a unique elevated park 

setting for the community, which addresses recreational needs, and provides 

a scenic overlook for the waterfront. 

With a location adjacent to a container terminal facility, there is a potential 

for shipping containers to become waterborne debris. Construction of the

berm utilizing a sheet piles to contain the fill addresses this issue.

Figure B-4 Example escape berm design.

The features of this escape berm, illustrated in Figure B-5, include the 

following:

• Location 1 (Figure B-5). The semicircular configuration was selected to 

help divert tsunami flood waters and potential waterborne debris around 

the facility and away from the access stairs and ramp. The elevated area 

is over 31,000 square feet, and can handle over 3,000 evacuees at 10 

square feet per person. There is sufficient space in the elevated area to 

accommodate a comfort station that could be used for both day to day 

recreational purposes and emergency use.

122 B: Community Design Example FEMA P-646
Figure B-5 Example escape berm plan layout.

• Location 2 (Figure B-5). The ocean facing side of the berm is essentially 

vertical to prevent tsunami flood waters and potential floating debris 

from moving upslope into the area of refuge. Trees and other 

landscaping can be used to hide the vertical face and create an

aesthetically appealing feature.

• Location 3 (Figure B-5). The sides of the berm can be sloped to provide 

additional access to the area of vertical refuge. Care should be taken to 

orientate the slope so that water and debris are not inadvertently 

channeled upslope.

• Locations 4 and 5 (Figure B-5). Stairs and ramps provide primary 

access for both recreational and emergency purposes.

Additional considerations are illustrated in Figures B-6 and B-7 and 

described below. 

Figure B-6 Example escape berm section.

FEMA P-646 B: Community Design Example 123
Figure B-7 Example escape berm rear elevation.

• Location 1 (Figure B-6). Where the elevated area is adjacent to a steep 

drop off, guard rails or walls of appropriate size and height should be 

provided for fall protection. Using a solid wall for the guardrail will 

have the added benefit of providing additional protection from tsunami 

runup or splash onto the area of refuge. Walls can be configured to 

divert splash away from the wall.

• Location 2 (Figure B-6). Materials used to help create the berm will 

need to be constructed deep enough below existing grade to ensure that 

retaining system is not undermined by scour around the perimeter of the 

berm.

• Location 3 (Figure B-7). With sufficient length, both ADA compliant 

ramps and stairs can be provided. This will address both the day to day 

recreational use of the facility as well as emergency evacuation needs. 

Sloped surfaces on the sides of the berm can be used to provide 

additional access, and can also help channel floating debris away from 

the base of the ramps and stairs to minimize the risk of blockage.

B.2 Site 2 Example: Multi-Use Structure

Site 2 is situated on property managed by the school district. The site is 

located adjacent to an existing school and the surrounding area contains a 

combination of residential and business use. The existing school is located 

well within the inundation zone. The waterfront in this area includes an ongrade parking lot that services businesses in the area, and a nearby oceanfront 

park. The evacuation population at this site would include children attending 

the school, neighbors in the adjacent residences, employees of nearby 

businesses, and nearby users of the oceanfront park. 

The school district has had an ongoing need for a covered gymnasium. At 

this site, the community has decided to incorporate the roof of the proposed 

gymnasium into its emergency planning. It is decided that this new structure 

will be designed to meet the requirements for a vertical evacuation structure 

124 B: Community Design Example FEMA P-646
to serve two important community needs. The structure is illustrated in 

Figure B-8.

Located adjacent to an on-grade parking lot, the structure will need to be 

designed for potential impacts from floating vehicles. If the community is 

located in a climate that requires the gymnasium to be enclosed, special 

attention should be paid to the design of the exterior wall system. Walls 

should be detailed as breakaway walls to minimize tsunami loading on the 

overall structure. Otherwise the structure will need to be designed to for the 

corresponding increased hydrostatic, hydrodynamic, and impulse loads.

As a school facility, the building must also be designed to address typical 

health and safety requirements for school facilities in normal use (when not 

serving as a vertical evacuation refuge).

Figure B-8 Example gymnasium.

Features of this multi-use structure, illustrated in Figure B-9 and Figure 

B-10, include the following:

• Location 1 (Figure B-9). The rectangular layout is selected based on the 

gymnasium requirements for the school. The elevated area is over 

10,000 square feet in size, and can handle over 1,000 evacuees at 10 

square feet per person. Using available census information, it has been 

determined that this should be sufficient for the surrounding area this 

facility is intended to serve.

FEMA P-646 B: Community Design Example 125
• Location 2 (Figure B-9). Stair access is designed using a concrete 

encased stair structure that will have its own inherent strength. The 

shape is intended to channel tsunami flow and potential debris away 

from both the structure and the stair system.

• Location 3 (Figure B-9). An additional ADA accessible ramp system is 

considered for a future phase of the project. This could utilize sheet piles 

and fill to further channel tsunami flow and waterborne debris away from 

the structure.

Figure B-9 Example gymnasium plan.

Figure B-10 Example gymnasium elevation.

126 B: Community Design Example FEMA P-646
• Location 4 (Figure B-10). The structural system utilizes a concrete 

moment frame to create an open lower level that will keep hydrodynamic 

loads on the structure to a minimum. This includes using circular shaped 

columns. 

• Location 5 (Figure B-10). Additional strength can be provided in the 

system by using walls that parallel the anticipated direction of the 

tsunami inundation flow. 

• Location 6 (Figure B-9). The stairs structures can be integrated with the 

primary structure to provide additional strength, or they can be made 

structurally independent.

FEMA P-646 B: Community Design Example 127

Appendix C

Example Calculations

A rectangular-shaped tsunami evacuation structure, 10 m wide, is constructed 

at a site 200 m from the shoreline, where the elevation is 4 m from the sea 

level. The local beach slope is 1/50 and there is no significant alongshore 

variation in the topography. The tsunami inundation map indicates the 

elevation R* = 10 m at the maximum inundation point (runup height of 10 m 

at the location 500 m from the shoreline). A log (8.53 m long, 0.35 m in 

diameter, and 450 kg mass) is considered as the design waterborne missile 

for the impact loading. In addition, the impact loading of a 40-ft shipping 

container (40 ft L x 8 ft W x 8-1/2 ft H, or 12.2 m x 2.44 m x 2.59 m) is 

estimated. A definition sketch for these example calculations is provided in 

Figure C-1.

Figure C-1 Definition sketch for example calculations: R* is the maximum 

runup elevation (the maximum inundation distance is 500 m) 

and z is the elevation at the location of the tsunami evacuation 

structure (located 200 m from the shoreline). Two horizontal 

lines represent the initial water level and the maximum

inundation level, respectively.

If a reliable and accurate tsunami inundation numerical model satisfying the 

criteria in Chapter 3 has been used to estimate flow depth and velocity at the 

building location, then the numerical data should be used for the force 

evaluations. At a site of interest, the following parameters should be 

extracted from the numerical simulation: the maximum inundation depth 

hmax, the maximum flow speed of the depth greater than the debris draft umax, 

and the maximum value of the product, (hu)

2

max. The local effects of the 

tsunami flows are difficult to predict due to nonlinear interactions of threedimensional flows. It is recommended that the design inundation elevation 

be increased at the building site by 30% over the computed inundation 

elevation, that the design flow velocity be increased by 15%, and that the 

R* = 10 m

z = 4 m

datum

FEMA P-646 C: Example Calculations 129
momentum flux (hu2

) be increased by 70% over the computed values for 

conservatism. These safety factors are simply a guideline based on the 30% 

error band in modeled tsunami runup heights compared with observed runup 

heights from past tsunami survey data. In practice, the safety factors should 

be determined based on the confidence in accuracy of the numerical 

simulations. Once the design flow parameters are determined, then the 

forces can be calculated using the methods described below. 

The following calculations are for situations where no detailed numerical 

simulation data are available. In such cases, it is assumed that the only 

information available at a site of interest is an inundation map, and the forces 

are intended to be conservatively estimated. 

C.1 Inundation Depth

The recommended design runup height, R, is 30% greater than the predicted 

maximum runup elevation, R*, to account for local amplification and 

uncertainty in the predicted value, i.e., R = 1.3 R* = 13 m. Therefore, the 

design inundation depth at the structure is 13 – 4 = 9 m. A minimum 

freeboard of 3 m (10 ft) or one story height is recommended. If the typical 

floor height is 4 m, the refuge area must be located higher than 9 + 4 = 13 m 

above the ground level. This would imply that the refuge area should be 

located on the 4th floor or higher. Note that when numerical simulation data 

are available, the inundation depth at the site can be obtained directly. 

However, it is still recommended that a 30% safety factor be applied to the 

computed inundation elevation at the site.

C.2 Hydrostatic and Buoyant Forces

It is recommended that all nonstructural walls at the lower levels of the 

building be designed as breakaway walls. In that case, the hydrostatic forces 

and potential uplift of the overall building are significantly reduced. 

However, if the structure, or any portion of the structure, is constructed 

watertight at the lower levels, then the wall panels must be designed for the 

anticipated hydrostatic pressure. The maximum force acting on a wall panel 

of 4 m wide and 3 m tall on the ground floor can be computed using 

Equation 6-2. Since the wall panel on the ground floor is fully submerged:

( )( ) ( )( )

ρ

     

  = ×−+ −    

=

= − +∆ −

3 2 3m 1100 kg m 9.81m sec 1.3 10m (4m 0.5m) 3m 4m 2

906 kN

( ) 2

W

h s W

h F gR z z hb

130 C: Example Calculations FEMA P-646
or an average lateral pressure of

2 906 kN/12 m 75.5 kPa Ph = =

where ∆z is the height at the toe of the wall panel from the ground level, 

assumed to be 0.5 m. Note that the fluid density ρ = 1.1 ρwater is used 

assuming a mixture of seawater and sediment.

With the water level at 9 m at the building location, the first and second 

floors will be submerged. Assuming the nonstructural walls have broken 

away at these two levels, but not yet at the third level, then the uplift due to 

buoyancy acting on the third floor should be evaluated. Assuming plan 

dimensions of 5 m by 5 m for a typical floor panel on the third floor, and a 

floor elevation of 7 m above the ground level, as shown in Figure C-2, then 

the upward buoyant force can be computed using Equation 6-4:

= ρ

= × ×−−

=

3 2 (1100 kg/m )(9.81m/sec )(5 m 5 m)((1.3 10 m 4 m) 7 m)

 540 kN

F gA h b s fb

or an upward pressure of 

2 540 kN/25m 21.6 kPa Ph = =

where hb is the water height displaced by the floor including the effect of air 

trapped below the floor, as shown in Figure C-2.

Figure C-2 Condition resulting in buoyant forces.

7m

9m

2m

5m

FEMA P-646 C: Example Calculations 131
C.3 Hydrodynamic and Impulsive Forces

Hydrodynamic drag and impulse forces are exerted on the building as a 

whole, assuming no breakaway walls at the lower levels. The maximum

value of h u2 at the site can be computed using Equation 6-6, with z = 4 m, R 

= 13 m and g = 9.81 m/sec2

:

( )     = −+ =        

2

2 2 3 2

max 0.125 0.235 0.11 105 m sec z z hu gR R R

Hence, from Equation 6-5 the fluid force is: 

= ρ

=

=

2

max

3 3 2

1 ( ) 2

1 (1100 kg/m )(2.0)(10 m)(105 m /sec ) 2

 1155 kN

F C hu d sd

where B = 10 m (shelter width), and Cd = 2.0. If the worst-case tsunami 

arrives at a previously flooded site, then the tsunami front may form a bore. 

The impulsive force for this condition would be 1.5 times the hydrodynamic 

force, as in Equation 6-7:

F F s d =1.5 =1730 kN

If the nonstructural walls at the lower level are designed to break away 

during a tsunami, then the hydrodynamic drag and impulse forces would be 

computed for all individual structural members (e.g., columns, shear walls) 

and combined as shown in Figure 6-12.

C.4 Impact Force

The maximum flow velocity at the site can be estimated using R = 13 m in 

Equation 6-9:

  = −    

  = −=    

max 2 1

4m 2 (13 m) 1 13.3 m sec. 13m

z u gR R

g

Note that this flow velocity is at the leading tongue of the flow where the 

flow depth is nil. Hence, this value of approximately 48 km/hr (30 mph) will 

be conservative. Using this conservative velocity estimate, the impact force 

due to a floating log can be computed by Equation 6-8, with c= 0, k = 2.4 x 

106 N /m, and m = 450 kg:

132 C: Example Calculations FEMA P-646
max 1.3 (1 ) F u km c i d = +

6 1.3(13.3 m/sec) (2.4 10 N/m)(450 kg)( = ×+1 0)

 568 kN =

This force would be applied locally at the assumed point of impact.

If the assumed draft, d, of the log is 0.25m, then the velocity is evaluated 

using Figure 6-9. Using the ratios ζ = z/R = 0.31, and the flow depth, d/R = 

0.019, at the location of the site:

= max 0.53

2

u

g R

u m max = 0.53 2 9.81 13 8.5 sec ( )( ) =

The impact force is then: 

1.3 (1 ) max F u km c i = d +

6 1.3(8.5 m/sec) (2.4 10 N/m)(450 kg)(1 =×+ 0)

 363 kN =

which is more realistic than the previous estimate (568 kN). The total force 

on the structure at the time of the impact can be determined conservatively 

by combining this impact force with the hydrodynamic drag force 

determined earlier:

F F i d += + = 363 1155 1518 kN

To compute the impact force due to a floating shipping container, the draft, 

d, must be estimated:

d

s h

m g d

ρ gA =

3

(3800 kg) 0.116 m

(1100 kg/m ) (12.2 m 2.44 m)

g

g = = ×

where md is the weight (Table 6-1) and Ah is the cross sectional area of the box 

in the horizontal plane, and the constant g cancels out. Considering the 

configuration of the support frame at the bottom of the container and the large 

horizontal dimension, the container is assumed to float freely at d = 0.5 m. 

The maximum flow velocity that supports a draft, d = 0.5 m, can be found 

from Figure 6-9. At the location of the site, ζ = z/R = 0.31, and the flow 

FEMA P-646 C: Example Calculations 133
depth, d/R = 0.039. Figure 6-9 shows υ = 0.31. Hence, the maximum 

velocity is:

u gR max = = 0.31 2 5.0 m/sec

Note that Figure 6-9 is only valid near the leading tip of the runup, therefore, 

use of a numerical model to estimate inundation flow depth and velocity is 

encouraged for large and heavy debris objects.

With a debris velocity of 5 m/s at impact, the impact force due to a 

longitudinal strike by the shipping container is computed by Equation 6-8 

with c = 0.2, k = 60 x 106 N /m, and md = 3800 kg (Table 6-1):

max 1.3 (1 ) F u km c i d = +

6 1.3(5.0 m/sec) (60 10 N/m)(3800 kg)(1 =× + 0.2)

 3400 kN =

The impact force due to a transverse strike by the shipping container is 

computed by Equation 6-8 with c = 1.0, k = 30 x 106 N /m, and md = 3800 kg

(Table 6-1):

max 1.3 (1 ) F u km c i d = +

6 1.3(5.0 m/sec) (30 10 N/m)(3800 kg)(1 1 =× + )

 3100 kN =

These are large forces compared with the hydrodynamic drag determined 

earlier. They represent a conservative assumption that the container is 

traveling at high velocity and applies a direct strike to the building. Unless 

the site is located adjacent to a container storage yard, this is a low 

probability event. The incorporation of progressive collapse prevention in 

the building design is intended to protect against failure of an exterior 

column or wall element due to this low probability impact. 

C.5 Damming Effect of Waterborne Debris

The damming effect of debris can be computed using Equation 6-11, which 

is readily obtained from the hydrodynamic force computed earlier, 

substituting the recommended debris dam width of 12 m (40 ft):

12 m (1155 kN) 1386 kN 10 m

Fdm

  = ×=    

If the building were wider than 12 m, then the damming effect should be 

considered at various locations as shown in Figure 6-13 to determine the 

134 C: Example Calculations FEMA P-646
worst condition for loading on the structure as a whole, and on individual 

structural elements.

C.6 Hydrodynamic Uplift Forces

The hydrodynamic uplift force can be computed using Equation 6-14. 

Assuming that the water depth at the soffit of the second floor is hs = 3 m, 

and at the location of the shelter site, ζ = z/R = 0.31, and the flow depth, d/R

= hs/R = 0.23, Figure 6-9 shows υ along the limit curve at ζ = 0.31. Hence, 

the maximum velocity is:

u g = = 0.15 2 2.4m sec. R

The vertical velocity can be computed using Equation 6-16, assuming the 

slope at the site is 1/20:

u u v = tanα = (2.4 1 20 0.12 m sec )( ) =

Hence, the hydrodynamic uplift force given by Equation (6-14) is:

( )( )( )( )

= ρ

= ×

=

2

2 3

1

2

1 3 1100 kg m 5m 5m 0.12m sec 2

594 N

F CA u us fv u

which is insignificant for the beach slope assumed in this example. If a 

beach slope of 1/5 is assumed, the hydrodynamic uplift force increases to 

9.5 kN or an uplift pressure of 0.38 kPa on the bottom of the floor slab.

FEMA P-646 C: Example Calculations 135

Appendix D

Background Information on 

Impact Load Calculations

D.1 Available Models for Impact Loads

The impact force from waterborne debris (e.g., floating driftwood, lumber, 

boats, shipping containers, automobiles, and other buildings) can be a cause 

of structural damage or even building destruction. Unfortunately, it is 

difficult to estimate this force accurately. Unlike the other forces, the impact 

force occurs locally at the point of contact when the debris is smaller than the 

building. Impact forces can be assumed to act at or near the water surface 

level when the debris strikes the building. Most available models are based 

on the impulse-momentum concept, in which the impulse of the resultant 

force acting for an infinitesimal time is equal to the change in linear 

momentum:

( ) τ

= = →τ ∫0 I F dt d mu ; 0 (D-1)

where:

I = impulse

F = resultant force

m = mass of waterborne debris

u = velocity of the debris

t = time

For actual computations, a small but finite time, ∆t (not infinitesimal), and 

the average change in momentum are used as an approximation. There is 

significant uncertainty in evaluating the duration of impact, ∆t. The 

following are available formulae for debris-impact force estimation.

Matsutomi (1999). Matsutomi experimentally investigated the impulse 

forces of driftwood. He performed two sets of experiments: one in a small 

water tank and the other for full-scale impact in air. In his small water tank, a 

bore and a surge were generated (a bore is a moving hydraulic jump onto a 

quiescent shallower water in front of it, while a surge is a moving water body 

onto a dry bed). A scaled-down driftwood model was placed 2.5 m upstream 

from the receiving wall. The model driftwood was picked up by the 

generated bore (or surge) and impacted onto the receiving vertical wall. His 

FEMA P-646 D: Background Information on Impact Load Calculations 137
full-scale impact experiments were conducted to compensate for potential 

scale effects in his small-scale experiments. A full-scale log was tied at the 

end of a pendulum and was swung against a stationary stop equipped with a 

load cell. It is noted that this impact condition in the air may significantly 

differ from an actual waterborne case because of the absence of the added 

mass effect of water: prior to the impact, the waterborne debris is carried by 

the surrounding water flow and the momentum of the water may increase or 

decrease the impact force. Matsutomi then compensated for the added mass 

effect with the data obtained from the small-scale water tank experiments. 

Based on a regression analysis of the large amount of data, Matsutomi 

proposed Equation D-2 for the impact force, F:

σ

γ γ

    =          

1.2 0.4

2 1.6 f

M

w w

F u C

D L g D L (D-2)

where: 

γ w = the specific weight of the log, 

D and L = the diameter and the length of the log, 

respectively, 

CM = 1 + Ca, is an inertia coefficient,

Ca is the added mass coefficient based on the displaced fluid 

volume,

u = the velocity of the log at impact, and

σf = the yield stress of the wood. 

Matsutomi recommended σf = 20 × 106 Pa for a wet log. The equation 

applies when driftwood collides at almost right angles with “rigid” structures 

such as reinforced concrete buildings.

From small-scale experimental data, he recommended a value of CM ≈ 1.7 for 

driftwood located at the tip of the inundation flow or strong bore condition, 

and CM ≈ 1.9 for a steady flow or if the log is located behind the tip of the 

inundation flow or strong bore. Note that the recommended values of CM are 

the upper limit when more than 60% of the receiving wall is open and 

permeable. The value of CM is smaller when the receiving wall does not 

allow the flow to pass through. For a solid (impermeable) receiving wall, 

Matsutomi found that CM = 0.5 for a bore and CM = 1.1 for a surging flow. 

Note that in the case of a bore striking an impermeable wall (i.e., no flowthrough), CM is less than unity (= 0.5). This is because the flow reflection at 

the wall actually reduces the impact force. 

138 D: Background Information on Impact Load Calculations FEMA P-646
In spite of a thorough study with a large amount of laboratory data, the 

derived form of Equation D-2 is inconvenient due to the particular choice of 

the scaling parameters, and it is only applicable to driftwood or logs. 

Ikeno et al. (2001, 2003). Laboratory experiments similar to Matsutomi 

(1999) were performed to examine the impact forces of objects other than 

driftwood or logs. They used cylindrical, square column, and sphericallyshaped drift bodies. Note that unlike Matsutomi’s experiments, Ikeno et al. 

only examined the impact onto an impermeable vertical wall. The following 

empirical formula was derived based on small-scale experiments 

(approximately 1/100 model):

 

=  

   

2.5

M

F u SC

g m g DL

(D-3)

where:

S = a constant (equal to 20 for a bore case),

CM = 1 + Ca is the inertia coefficient

Ca is the added mass coefficient based on the displaced fluid 

volume

m = the mass of the drift body.

CM = 0.5 was used regardless of the shape of the objects for a bore impact 

onto an impermeable wall, which was adopted from Matsutomi’s results. For 

a dry-bed surge, Ikeno and Tanaka (2003) suggested S = 5 and CM = 0.8 for 

spherical-shaped objects and CM = 1.5 ~ 2.0 for cylinders and square-shaped 

columns. The results by Ikeno et al. are valid only for the condition of an 

impermeable wall (i.e., the entire incident flow reflects back to the offshore 

direction). This is why the inertia coefficient has a value less than unity.

Haehnel and Daly (2002). At the U.S. Army Cold Regions Research and 

Engineering Laboratory (CRREL), Haehnel and Daly performed experiments

similar to Matsutomi (1999). They considered reduced-scale logs in steady 

flow in a small flume, and prototype logs in a large towing basin. It must be 

noted that the condition in the towing basin differs from the actual impact 

condition of a waterborne object. In the towing basin the water is stationary 

while in the actual condition moving water carries the debris. Instead of the 

impulse-momentum approach, Haehnel and Daly analyzed the data using 

linear dynamic analysis with one degree of freedom. Since the collision 

occurs over a short duration, damping effects are neglected. Assuming the 

overall structural system has a period much greater than the impact duration, 

approximating a rigid structure, the model can be formulated by Equation 

D-4: 

FEMA P-646 D: Background Information on Impact Load Calculations 139
m x +k x = 0 (D-4)

where:

m = the mass of the log, 

x = the summation of the compression of the building and 

the log during impact and rebound, with the dot 

denoting the time derivative, and

k = the effective stiffness associated with both the log and 

the building. 

The effective stiffness of the collision is 1/k =1/ks + 1/kd where ks is the local 

stiffness of the structure at the impact zone; and kd is the stiffness of the 

debris, and other nonstructural elements deformed at impact. The structure 

will be rigid if the structure stiffness is much greater than the stiffness of the 

target zone nonstructural elements or the debris. The structure will also act 

as if it is rigid if the mass of the structure is so great that it does not move 

appreciably in response to the impact. 

Solving Equation D-4 yields the maximum force given by Equation D-5:

F kx u k m max = = Max( ) (D-5)

where: u is the impact velocity.

Based on their laboratory experiments, the effective stiffness k between a log 

and a rigid building was estimated to be 2.4 × 106 N/m. 

Haehnel and Daly demonstrated that the impulse-momentum approach could 

be reduced to the constant-stiffness approach shown in Equation D-5 by 

setting π ∆ =

2

m

t

k (note that, to be consistent to Equation D-4, the force is 

considered a sinusoidal function in time). The work-energy approach can 

also be made equivalent to Equation D-5 by setting the stopping distance as 

= m S u

k . The work-energy approach is an impact force estimation that 

equates the work done on the building with available kinetic energy of the 

floating debris object. Based on their laboratory data, the following formulae 

were suggested by Haehnel and Daly:

Constant-stiffness approach:

= = ≈ Max( ) 1550 F kx u k m u m max (D-6)

Impulse-momentum approach: 

140 D: Background Information on Impact Load Calculations FEMA P-646
π

= ≈ ∆90.9

2 max

um F um

t

(D-7)

Equivalent to ∆t = 0.0173 sec.

Work-energy approach:

=≈ +

∆

2

2 125 8000 max

u m F mu

x

(D-8)

Note that in Equations D-6, D-7, and D-8, the velocity, u, is in m/sec and the 

mass, m, is in kg. It is emphasized that errors associated with the use of a 

towing tank (instead of the realistic condition of a log being carried with 

flow) may be significant in the results by Haehnel and Daly (2002), since the 

added mass effect of water flowing with the debris is not included. To 

include the added mass effect that opposes the direction of acceleration, the 

single degree of freedom equation of motion D-4 above (Equation D-4) 

would be modified to:

mx kx m x + =− a

and therefore,

() 0 m m x kx + += a (D-9)

where ma is the hydrodynamic mass given by;

(displaced volume) m cm c a disp = = ρ

where c is the hydrodynamic mass coefficient. 

Note that this application of hydrodynamic mass to the solution of the 

structural dynamics equation of debris impacts in water no longer represents 

the traditional added-mass term derived from potential flow hydrodynamics. 

However, both are similar transitory impulsive effects and are more related 

to the shape and orientation of the body than a property of its true mass. To 

avoid confusion, the term “hydrodynamic mass” is used in Equation D-9 

instead of the term “added mass.”

Solving Equation D-9 yields the maximum force given by Equation D-10:

max ( ) (1 ) F u k m m u km c = += + a (D-10)

SEI/ASCE Standard 7-10 (ASCE, 2010). ASCE gives the following 

modified design formula based on Equation D-1:

π

= 2∆

muC C C C R I O D B max F

t

(D-11)

where:

FEMA P-646 D: Background Information on Impact Load Calculations 141
m = the water-borne-debris mass, 

u = the impact velocity of the debris, 

CI = the importance coefficient, 

CO = the orientation coefficient, (statistically based)

CD = the depth coefficient, 

CB = the blockage coefficient, 

Rmax = the maximum response ratio for impulsive load, and 

∆t = the impact duration. 

The C coefficients are based on results of laboratory testing and on 

engineering judgment. Rmax is a coefficient to compensate for the effect of the 

degree of flexibility of the building. A single value of the impact duration, ∆t

= 0.03 sec, is recommended (Kriebel, et al, 2000), although there is wide 

variation in the impact duration owing to, for example, the object material 

and deformability, the flow blockage condition, and the flexibility of the 

building element being struck. It is worth noting that the City and County of 

Honolulu Building Code (CCH, 2000) recommends ∆t values for wood 

construction as 1.0 sec, steel construction as 0.5 sec, and reinforced concrete 

as 0.1 sec; these values are unsubstantiated. Furthermore, the FEMA 55 

Coastal Construction Manual (FEMA, 2005) provides ∆t values shown in 

Figure D-1. Such an excessive variation in ∆t could make Equation D-11 

unreliable if used with various prescriptive impact duration values. 

Improved results may result with explicit calculation of the duration per 

π ∆ =

2

m

t

k .

Figure D-1 Ranges of duration of impact (FEMA, 2005). 

D.2 Summary and Discussion

Review of previous work clearly demonstrates the uncertainty of the present 

understanding of waterborne debris-impact forces. The form of Equation D11 exhibits a struggle to obtain an engineering estimate of the forces by 

142 D: Background Information on Impact Load Calculations FEMA P-646
adjusting five coefficients based on engineering judgment, together with a 

single estimate for ∆t. All of the prediction formulae are based on smallscale laboratory data by compensating with the full-scale measurements in 

compromised conditions. For example, Matsutomi’s full-scale data were 

obtained by the impact study in air, and Haehnel and Daly’s data were 

obtained in a towing tank. Since the added mass effect appears important at 

the impact (the impact halts not only the waterborne debris itself but also 

decelerates a portion of the water caused to flow around it), the results 

derived from the compromised experimental conditions may contain 

significant errors.

Even if the impact velocity, u, and the debris mass, m, were given, each 

formula yields a different functional relation to predict the forces, which 

indicates complexity and uncertainty inherent in the problem. For each of the 

available methods, proportionality between the impact force, debris velocity 

and mass are:

Constant-stiffness approach ⇒ F um ∝ ,

Impulse-momentum approach ⇒ F um ∝ ,

Work-energy approach ⇒ ∝ 2 F um , (D-12)

Ikeno and Tanaka (2003) ⇒ ∝ 2.5 n F um , n ≈ 0.58, and

Matsutomi (1999) ⇒ ∝ 1.2 n F um , n ≈ 0.66.

Although Equation D-2 by Matsutomi is based on his substantial analyses of 

a large set of laboratory data, the form of Equation D-2 is physically 

ambiguous in terms of the choice of the scaling parameters, is limited only to 

cylindrical shaped debris, and is inconvenient for use in actual practice. The 

empirical Equation D-3 by Ikeno et al. is based on their small-scale 

laboratory experiments with an impermeable wall; hence, its extrapolation is 

unreliable for most real-world applications. Proper estimates of ∆t and ∆x are 

uncertain for the impulse-momentum and work-energy approaches, 

respectively. The value of the effective constant stiffness, k, should be 

evaluated when using Haehnel and Daly’s Equation D-5. In reality, k is not 

constant; it is likely a function of x during the impact.

Until more comprehensive studies can be made, an effective stiffness 

approach given in Equation D-10, based on Haehnel and Daly, is 

recommended because of its simple but rational formulation. In addition, as 

shown in the foregoing comparisons in Equations D-12, the functional 

relation of m and u to the force F is similar to Matsutomi’s empirical 

Equation D-2, which was derived based on a very large amount of 

experimental data. Considering that Matsutomi’s empirical treatment was 

FEMA P-646 D: Background Information on Impact Load Calculations 143
based on the impulse-momentum approach, the coincidental similarity with 

the constant-stiffness approach provides additional confidence in the 

formulation. With the introduction of the hydrodynamic mass parameter, c, 

the hydrodynamic mass effect is already included in Equation D-10. 

Applying an importance factor of 1.3 per ASCE 7-10 for critical facilities 

results in the recommended impact expression as shown in Equation D-13:

max 1.3 (1 ) F u km c i d = + (D-13)

In this expression, k must be determined along the direction of the impact 

based on the modeled debris (e.g., as mentioned earlier, k = 2.4 × 106 N/m 

was recommended for a log by Haehnel and Daly). Note that a proper 

estimate of k is the key for this method. In reality, k may not be the elastic 

stiffness; it is likely a function of x during inelastic impacts of structural 

significance. Hence, the linearized equation D-4 may be inadequate. 

Engineering judgment and iterative analysis may be necessary to determine 

the most appropriate secant stiffness to be used for a particular magnitude of 

impact. The values for k suggested in Table 6-1 for shipping containers were 

developed based on computer models of standard containers (Peterson and 

Naito, 2012). An added advantage for the use of Equation D-13 is that k is 

not as sensitive as ∆x in the work-energy approaches, which can be shown 

from the fact that ∆x is proportional to 1

k , as discussed earlier.

The hydrodynamic mass coefficient, c, is due to the fact that the decelerating 

body must also momentarily decelerate or disturb some volume of the 

surrounding fluid flow. It depends greatly on the size, shape, and orientation 

of the object with respect to the surge direction. Estimated values for c are 

provided in Table 6-1 for the various debris strike conditions considered.

144 D: Background Information on Impact Load Calculations FEMA P-646
 

 

 

 

 

 

 

 

 

 

 

 

Appendix E
 

Maximum Flow Velocity and 

Momentum Flux in the 

Tsunami Runup Zone 

E.1 Flow Velocity 

For prediction of flow velocities and depths at a site of interest for a given 

design tsunami, the best practice available is to run a detailed numerical 

simulation model with a very fine grid size (less than 10 meters) in the 

tsunami runup zone. Such a numerical model is usually run with a nested 

grid system with a grid size of several kilometers in the abyssal plain, a few 

hundreds of meters on the continental shelf, a few tens of meters near the 

shore, and less than 10 meters in the runup zone. A numerical simulation can 

provide the complete time history of flow velocity and depth at the site of 

interest. 

Alternatively, the use of analytical solutions can be considered. Although 

some simplifications and assumptions must be imposed, the results are useful 

as a guideline for checking the reasonableness of results, or as estimate of 

approximate values in the absence of other information. Available analytical 

solutions are based on one-dimensional, fully nonlinear shallow-water-wave 

theory for the condition with a uniformly sloping beach. With those 

assumptions, the exact solution for the runup of an incident bore was given 

by Shen and Meyer (1963), based on Ho and Meyer (1962). The maximum 

fluid velocity occurs at the leading runup tip as calculated by Equation E-1: 

u = 2 g x tan α , (E-1) 

where: 

α = the beach slope, 

g = the gravitational acceleration, and 

x = the distance from the maximum runup location to the location of 

interest; the location of interest must be above the initial 

shoreline. 

Results indicate that the flow at the leading runup tip moves up the beach 

under gravity, just like a particle with simple energy transfer between its 

kinetic and potential energies. According to Yeh (2006), Equation E-1 

FEMA P-646 E: Maximum Flow Velocity and Momentum Flux in the 

Tsunami Runup Zone 

145 
 

 

 

 

 

 

 

 

 

 

 

provides the upper-limit envelope of the flow velocity for all incident 

tsunami forms. Because a real beach is not uniformly sloped, it is more 

convenient to present Equation E-1 as a function of the ground elevation, 

instead of distance as follows: 

 z 

umax = 2 g R1 −  (E-2)  R 

where: 

R = the ground elevation at the maximum penetration of tsunami 

runup, measured from the initial shoreline, and 

z = the ground elevation of the location of interest, measured from 

the initial shoreline level. 

It is emphasized that the model does not include the effects of friction and 

the maximum flow velocity occurs at the leading runup tip, where the flow 

depth is zero. Since debris requires some finite flow depth in order to float 

(draft), use of Equations E-1 and E-2 to estimate velocity for impact load 

calculations is overconservative. 

Based on Shen and Meyer’s (1963) results, Peregrine and Williams (2001) 

provided the formulae for the temporal and spatial variations in fluid velocity 

and flow depth of the incident bore runup in the vicinity of the leading runup 

tip. With slightly different scaling, Yeh (2007) expressed Peregrine and 

Williams’ formulae for the flow depth and velocity, respectively as follows: 

1 2 η = (2 2τ τ − − 2ζ )

2 

(E-3) 36τ2 

and 

1 2 υ = (τ − 2τ + 2 ζ ) (E-4) 3τ

where, in the above equations: 

d u g z

; ζ = R 2 gR R R η = ; υ = ; τ = t tan α

d = the water depth, 

R = the ground elevation at the maximum penetration of tsunami 

runup, measured from the initial shoreline, 

u = the flow velocity, 

g = the gravitational acceleration, 

α = the beach slope, 

t = the time: 0 when the bore passes at the initial shoreline, and 

146 E: Maximum Flow Velocity and Momentum Flux in the FEMA P-646 

Tsunami Runup Zone 
 

 

 

 

 

 

z = the ground elevation of the location of interest, measured from 

the initial shoreline: this identifies the location of interest along a 

uniformly sloping beach. 

For a given maximum runup penetration, a bore formation should yield the 

fastest flow velocity among all the incident tsunami formations. Gradual 

flooding of non-breaking tsunamis should result in slower flow velocity than 

that caused by the bore runup. Therefore, Equations E-3 and E-4 can be used 

to estimate the maximum flow velocity at a given location for a given flow 

depth. Combining Equations E-3 and E-4 and eliminating τ, Figure E-1 can 

be derived. Each curve in the figure represents the dimensionless flow 

velocity υ versus the location ζ (in terms of ground elevation, z) for a given 

local flow depth, d. This figure can be used to evaluate the maximum flow 

velocity that can carry floating debris with finite draft depth, since draft of 

the debris must be greater than the flow depth to make the debris float. 

Equations E-3 and E-4 are valid only for the flows very close to the leading 

runup tip. Therefore, the velocity estimated for a case with a sufficiently 

large draft depth can be overly conservative. 

Figure E-1	 Maximum flow velocity of depth, d, at the ground elevation, z, 

and maximum runup elevation, R. The bottom curve represents 

the lower limit of maximum flow velocity. 

The bottom curve in Figure E-1 is the lower limit of the maximum flow 

velocity for a given depth, d. Note that the results in Figure E-1 are based on 

the runup condition of uniform incident bore. Local inundation depth of 

other tsunami forms usually exceeds that of a bore runup, and the maximum 

flow velocity is lower than the limit curve in Figure E-1. Hence when a 

floating-debris has a draft that exceeds the flow depth of the bore runup, the 

FEMA P-646 E: Maximum Flow Velocity and Momentum Flux in the 

Tsunami Runup Zone 

147 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

design velocity umax can be estimated conservatively with the lower limit 

curve. 

E.2 Momentum Flux 

When a detailed numerical simulation model is available, the critical values 

of forces can be evaluated directly for a location of interest. The maximum 

value of the product of water depth and the square of flow velocity hu2 is 

needed to compute the hydrodynamic forces. 

Even in the case of no numerical simulation data, the maximum momentum 

flux per unit water mass per unit width hu2 can be estimated conservatively 

once the maximum runup height (or distance) is determined. Using the exact 

solution algorithm, Yeh (2006) developed an envelope curve of hu2 

, 

expressed in Equation E-5: 

hu2 

= x x 0.11( )

2 

+ 0.015( ) (E-5) 2 2 gα f f f

where: 

hu2 = the momentum flux per unit mass per unit width, 

α = the beach slope, 

g = the gravitational acceleration, 

x = the distance from the maximum runup location to the location of 

interest (the location of interest must be above the initial 

shoreline), and 

f = the maximum runup distance. 

Once the maximum runup distance, f, is determined (e.g., from an available 

inundation map), the momentum flux, ρ hu2 per unit breadth at a given 

location x, can be computed by Equation E-5. It is emphasized that Equation 

E-5 is for a uniform beach slope; therefore, some adjustments need to be 

made to evaluate realistic conditions. Because a real beach is not uniformly 

sloped, it is more convenient to express Equation E-5 as a function of ground 

elevation instead of distance, as follows: 

2 hu z   z 2 

= 0.125 − 0.235 + 0.11 (E-6) 2   gR R   R 

where: 

hu2 = the momentum flux per unit mass per unit width, 

g = the gravitational acceleration, 

148 E: Maximum Flow Velocity and Momentum Flux in the FEMA P-646 

Tsunami Runup Zone 
 

 

R = the ground elevation at the maximum penetration of tsunami 

runup, measured from the initial shoreline, and 

z = the ground elevation of the location of interest, measured from 

the initial shoreline: this identifies the location of interest along a 

uniformly sloping beach. 

Although a real beach is not uniformly sloped and tsunami runup is not a 

one-dimensional motion, Figure E-1 and Equations E-2 and E-6 provide an 

analytical basis for runup conditions. 

FEMA P-646 E: Maximum Flow Velocity and Momentum Flux in the 

Tsunami Runup Zone 

149 

Glossary

The following definitions are provided to explain the terms and acronyms 

used throughout this document. Many have been taken directly from the 

FEMA 55, Coastal Construction Manual (FEMA, 2005). 

A

ADA – Americans with Disabilities Act. Law requiring that design 

accommodations be made for persons with certain disabilities.

Armor – Material used to protect slopes from erosion and scour by 

floodwaters, such as riprap, gabions, or concrete.

ASCE – American Society of Civil Engineers.

ATC – Applied Technology Council.

A-Zone – Under the National Flood Insurance Program, the area subject to 

inundation by a 100-year flood where waves are less than 3 feet high 

[designated Zone A, AE, A1-A30, A99, AR, AO, or AH on a Flood 

Insurance Rate Map (FIRM)]. 

B

Base flood – Flood that has a 1% probability of being equaled or exceeded in 

any given year, also known as the 100-year flood.

Base Flood Elevation (BFE) – Elevation of the base flood in relation to a 

specified datum, such as the National Geodetic Vertical Datum or the North 

American Vertical Datum. The Base Flood Elevation is the basis of the 

insurance and floodplain management requirements of the National Flood 

Insurance Program.

Bathymetry – Underwater configuration of a bottom surface of an ocean, 

estuary, or lake. 

Berm – A mound of soil or other earthen material.

Bore – A long, broken wave propagating into a quiescent body of water, with

an abrupt increase in water depth at its front face covered with turbulent, 

tumbling water.

FEMA P-646 Glossary 151
Breakaway wall – Under the National Flood Insurance Program, a wall that 

is not part of the structural support of the building and is intended, through its 

design and construction, to collapse under specific lateral loading forces 

without causing damage to the elevated portion of the building or supporting 

foundation system. Breakaway walls are required by the National Flood 

Insurance Program regulations for any enclosures constructed below the Base 

Flood Elevation beneath elevated buildings in coastal high-hazard areas (also 

referred to as V-Zones). In addition, breakaway walls are recommended in 

areas where floodwaters flow at high velocities or contain ice or other debris. 

Building codes – Regulations adopted by local governments that establish 

standards for construction, modification, and repair of buildings and other 

structures.

Building official – An officer or other designated authority charged with the 

administration and enforcement of the code, or a duly authorized 

representative such as a building, zoning, planning, or floodplain 

management official.

Bulkhead – A wall or other structure, often of wood, steel, stone, or 

concrete, designed to retain or prevent sliding or erosion, and occasionally

used to protect against wave action.

C

CAEE – Canadian Association for Earthquake Engineering.

Cast-in-place concrete – Concrete that is formed, placed, and cured in its 

final location in the structure.

Cladding – Exterior surface of the building envelope.

Coastal A-Zone – The portion of the Special Flood Hazard Area landward 

of a V-Zone or landward of an open coast without mapped V-Zone in which 

the principal sources of flooding are astronomical tides, storm surge, seiches, 

or tsunamis (not riverine sources). The flood forces in coastal A-Zones are 

highly correlated with coastal winds or coastal seismic activity. Coastal 

A-Zones may therefore be subject to wave effects, velocity flows, erosion, 

scour, or combinations of these forces. (Note: National Flood Insurance 

Program regulations do not differentiate between coastal A-Zones and noncoastal A-Zones.)

Coastal barrier – Depositional geologic features such as a bay barrier, 

tombolo, barrier spit, or barrier island that consists of unconsolidated 

152 Glossary FEMA P-646
sedimentary materials; is subject to wave, tidal, and wind energies; and 

protects landward aquatic habitats from direct wave attack.

Coastal High-Hazard Area – Under the National Flood Insurance Program, 

an area of special flood hazard extending from offshore to the inland limit of 

a primary frontal dune along an open coast, and any other area subject to 

high-velocity wave action from storms or seismic sources. On a Flood 

Insurance Rate Map, the coastal high-hazard area is designated Zone V, VE, 

or V1–V30. These zones designate areas subject to inundation by the base 

flood where wave heights or wave runup depths are greater than or equal to 3 

feet. In Hawaii, the VE-Zones are generally determined where the depth of 

water from a 100-year event (as determined from tsunami and/or hurricane 

data) is greater than 4 feet.

Collapsing breaker – A type of breaking wave associated with a steep beach 

slope and flat incident wave, which occurs right at the instantaneous 

shoreline.

D

Dead load – Weight of all materials of construction incorporated into the 

building, including but not limited to walls, floors, roofs, ceilings, stairways, 

built-in partitions, finishes, cladding, and other similarly incorporated 

architectural and structural items and fixed service equipment. See Loads.

Debris – Solid objects or masses carried by or floating on the surface of 

moving water.

Debris impact loads – Loads imposed on a structure by the impact of 

waterborne debris. 

Debris line – Markings on a structure or the ground caused by the deposition 

of debris, indicating the height or inland extent of floodwaters.

Design Basis Earthquake (DBE) – The earthquake hazard level that 

structures are specifically proportioned to resist, taken as two-thirds of the 

Maximum Considered Earthquake (MCE) hazard level.

DoD – Department of Defense.

Draft – The depth of water that a body needs in order to float.

F

Far-source-generated tsunami – Tsunami resulting from a source located 

far from the site such that it arrives in excess of a 2-hour timeframe.

FEMA P-646 Glossary 153
FEMA – Federal Emergency Management Agency. 

FEMA MAT Report – FEMA Mitigation Assessment Team Report.

Fill – Material such as soil, gravel, or crushed stone placed in an area to 

increase ground elevations or change soil properties. See Structural Fill.

FIRM – Flood Insurance Rate Map.

500-year flood – Flood that has a 0.2% probability of being equaled or 

exceeded in any given year.

Flood elevation – Height of the water surface above an established elevation 

datum such as the National Geodetic Vertical Datum, the North America 

Vertical Datum, or mean sea level.

Flood-hazard area – The greater of the following: (1) the area of special 

flood hazard, as defined under the National Flood Insurance Program, or (2) 

the area designated as a flood-hazard area on a community's legally adopted 

flood-hazard map, or otherwise legally designated.

Flood Insurance Rate Map – Under the National Flood Insurance Program, 

an official map of a community upon which the Federal Emergency 

Management Agency has delineated both the special hazard areas and the 

risk premium zones applicable to the community. (Note: The latest FIRM 

issued for a community is referred to as the effective FIRM for that 

community.)

Footing – The enlarged base of a foundation wall, pier, post, or column 

designed to spread the load of the structure so that it does not exceed the soil 

bearing capacity.

G

Grade beam – Section of a concrete slab that is thicker than the slab and acts 

as a footing to provide stability, often under load-bearing or critical structural 

walls. 

GSA – General Services Administration.

H

Hydrodynamic loads – Loads imposed on an object, such as a building, by 

water flowing against and around it. Among these loads are positive frontal 

pressure against the structure, drag effect along the sides, and negative 

pressure on the downstream side.

154 Glossary FEMA P-646
Hydrostatic loads – Loads imposed on a surface, such as a wall or floor 

slab, by a standing mass of water. The water pressure increases linearly with 

the water depth; hence, the hydrostatic loads increase with the square of the 

water depth.

I

Impact forces – Loads that result from waterborne debris transported by 

tsunami waves striking against buildings and structures or parts thereof. 

Impulsive forces – Force induced against a vertical obstruction subjected to 

the leading edge of a tsunami during runup, also termed “surge” forces.

Ingress – The act of entering a building.

Inland zone – For the purposes of this report, the area that is inland of the Aand X-Zones (the limit of the 500-year flood). 

L

Liquefaction – A phenomenon that occurs in saturated soils when the net 

pore pressure exceeds the gravity force holding soil particles together. Soil 

strength and stiffness decrease dramatically as the soil behaves similar to a 

fluid.

Loads – Forces or other actions that result from the weight of all building 

materials, occupants and their possessions, environmental effects, differential 

movement, and restrained dimensional changes. 

M

Masonry – Built-up construction of combination of building units or 

materials of clay, shale, concrete, glass, gypsum, stone, or other approved 

units bonded together with or without mortar, grout, or other accepted 

methods of joining.

Maximum Considered Earthquake (MCE) – The most severe earthquake 

effects considered by seismic design codes and standards. The MCE is based 

on the United States Geological Survey seismic hazard maps, which are 

based on a combination of: (1) 2500-year probabilistic earthquake ground 

motion hazards; and (2) deterministic ground motion hazards in regions of 

high seismicity, with the appropriate ground motion attenuation relationships 

defined for each region.

Maximum Considered Tsunami (MCT) – A design tsunami event based on 

a probabilistic assessment considering all possible tsunami sources, or a 

FEMA P-646 Glossary 155
deterministic assessment considering the maximum tsunami that can 

reasonably be expected to affect a site. 

Mid-source-generated tsunami – Tsunami generated by a source that is 

near the site of interest, but not close enough so that the effects of the 

triggering event is felt at the site.

Mitigation – Any action taken to reduce or permanently eliminate the longterm risk to life and property from natural hazards.

N

National Flood Insurance Program (NFIP) – The federal program created 

by Congress in 1968 that makes flood insurance available in communities 

that enact and enforce satisfactory floodplain management regulations.

National Geodetic Vertical Datum (NGVD) – Datum established in 1929 

and used as a basis for measuring flood, ground, and structural elevations; 

was previously referred to as Sea Level Datum or Mean Sea Level. The Base 

Flood Elevations shown on most of the Flood Insurance Rate Maps issued by 

the Federal Emergency Management Agency are referenced to NGVD or, 

more recently, to the North American Vertical Datum.

Near-source-generated tsunami – Tsunami generated by a source located 

near the site such that it arrives within a 30-minute timeframe, and the effects 

of the triggering event are felt at the site. 

Nonstructural wall – A wall that does not support vertical loads other than 

its own weight. 

North American Vertical Datum (NAVD) – Datum used as a basis for 

measuring flood, ground, and structural elevations. NAVD, rather than the 

National Geodetic Vertical Datum, has been used in many recent flood 

insurance studies.

P

Pier foundation – Foundation consisting of isolated masonry or cast-inplace concrete structural elements extending into firm materials. Piers are 

relatively wide in comparison to their length, and derive their load-carrying 

capacity through skin friction, end bearing, or a combination of both.

Pile foundation – Foundation consisting of concrete, wood, or steel 

structural elements driven or jetted into the ground, or cast in place. Piles are 

relatively slender in comparison to their length, and derive their load156 Glossary FEMA P-646
carrying capacity through skin friction, end bearing, or a combination of 

both.

Plain concrete – Structural concrete with no reinforcement or with less 

reinforcement than the minimum amount specified for reinforced concrete.

Plunging Breaker – A type of breaking wave when the wave front curls 

over, forming a tube; it usually happens on beaches where the slope is 

moderately steep.

Post foundation – Foundation consisting of vertical support members,

usually made of wood, set in holes and backfilled with compacted material. 

Precast concrete – Concrete, usually a discrete structural member, that is 

formed, placed, and cured at one location, and subsequently moved and 

assembled into a final location in a structure.

Probabilistic maps – Maps of predicted tsunami effects including for 

inundation zone, flood depths, and flow velocities, based on a method 

involving probability and uncertainty.

Progressive collapse – ASCE/SEI Standard 7-02 defines progressive 

collapse as “the spread of an initial local failure from element to element 

resulting eventually, in the collapse of an entire structure or a 

disproportionately large part of it.”

R 

Rapid drawdown – A sudden reduction in water level immediately prior to 

the first tsunami wave, or between tsunami waves.

Reinforced concrete – Structural concrete reinforced with steel.

Retrofit – Any change made to an existing structure to reduce or eliminate 

potential damage to that structure from flooding, erosion, high winds, 

earthquakes, or other hazards.

S

Scour – Removal of soil or fill material by the flow of floodwaters, 

frequently used to describe storm-induced, localized conical erosion around 

pilings and other foundation supports where the obstruction of flow increases 

turbulence. 

Sea wall – Solid barricade built at the water’s edge to protect the shore and 

to prevent inland flooding.

FEMA P-646 Glossary 157
SEI – Structural Engineering Institute of ASCE.

Shearwall – Load-bearing or non-load-bearing wall that transfers in-plane 

forces from lateral loads acting on a structure to its foundation.

Special Flood Hazard Area (SFHA) – Under the National Flood Insurance 

Program, an area having special flood, mudslide (i.e., mudflow), and/or 

flood-related erosion hazards, and shown on a Flood Hazard Boundary Map 

or Flood Insurance Rate Map as Zone A, AO, A1-A30, AE, A99, AH, V, V1-

V30, VE, M, or E.

Stillwater elevation – Projected elevation that floodwaters would assume, 

referenced to the National Geodetic Vertical Datum, the North American 

Vertical Datum, or some other datum, in the absence of waves resulting from 

wind or seismic effects.

Storm surge – Rise in the water surface above normal water level on an 

open coast due to the action of wind stress and atmospheric pressure on the 

water surface.

Structural fill – Fill compacted to a specified density to provide structural 

support or protection to a structure. 

T

Topography – Configuration of a terrain, including its relief and the position 

of its natural and man-made features. 

Tsunami – A naturally occurring series of ocean waves resulting from a 

rapid, large-scale disturbance in a body of water, caused by earthquakes,

landslides, volcanic eruptions, and meteorite impacts.

Tsunami inundation elevation – The elevation, measured from sea level, at 

the location of the maximum tsunami penetration

Tsunami inundation zone – The region flooded by tsunami penetration 

inland.

Tsunami runup – Rush of tsunami waves up a slope, terrain, or structure.

Tsunami runup height – The difference between the elevation of maximum 

tsunami penetration and the elevation of the shoreline at the time of tsunami 

attack.

Tsunami water level – The difference between the elevation of the highest 

local water level and the elevation of the shoreline at the time of tsunami 

attack.

158 Glossary FEMA P-646
U

Undermining – Process whereby erosion or scour exceeds the depth of the 

base of a building foundation, or the level below which the bearing strength 

of the foundation is compromised.

Uplift – Vertical hydrostatic pressure caused by the volume of displaced 

water under a building. 

V

V-Zone – See Coastal High-Hazard Area.

VE-Zone – Coastal High-Hazard Areas where the Base Flood Elevations

have been determined through a detailed study.

Vertical Evacuation Refuge from Tsunamis – A building or earthen 

mound that has sufficient height to elevate evacuees above the tsunami 

inundation depth, and is designed and constructed with the strength required 

to resist the forces generated by tsunami waves.

W

Waterborne debris – Any object transported by tsunami waves (e.g.,

driftwood, small boats, shipping containers, automobiles).

Wave crest – The point of highest elevation in a wave profile.

Wave height – Vertical distance between the successive local maximum and 

minimum elevations in a wave profile.

Wave zone – Area that coincides with V, VE, or V1–V30 Zones or Coastal 

High-Hazard Areas.

FEMA P-646 Glossary 159

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Project Participants

ATC Management and Oversight

Christopher Rojahn (Project Executive)

Applied Technology Council

201 Redwood Shores Parkway, Suite 240

Redwood City, California 94065

William L. Coulbourne (Project Manager)

Applied Technology Council

19266 Coastal Highway

Rehoboth Beach, Delaware 19971

Jon A Heintz (Project Quality Control Monitor)

Applied Technology Council

201 Redwood Shores Parkway, Suite 240

Redwood City, California 94065

William T. Holmes (Project Technical Monitor)

Rutherford & Chekene

55 Second Street, Suite 600

San Francisco, California 94105

FEMA Project Officer

Michael Mahoney (Project Officer)

Federal Emergency Management Agency

500 C Street, SW, Room 416

Washington, D.C. 20472

FEMA Technical Monitor

Robert D. Hanson (Technical Monitor)

(Federal Emergency Management Agency)

2926 Saklan Indian Drive

Walnut Creek, California 94595

Project Management Committee (Second Edition)

Ian N. Robertson (Project Technical Director)

University of Hawaii at Manoa 

Dept. of Civil and Environmental Engineering

2540 Dole Street, Holmes Hall 383 

Honolulu, Hawaii 96822

Gary Chock

Martin and Chock

1132 Bishop Street, Suite 1550

Honolulu, Hawaii 96813

John Hooper

Magnusson Klemencic Associates

1301 Fifth Avenue, Suite 3200

Seattle, Washington 98101

Timothy J. Walsh

Dept. of Natural Resources, Geology & Earth 

Resources

1111 Washington Street SE, P.O. Box 47007

Olympia, Washington 98504

Harry Yeh

Oregon State University 

School of Civil & Construction Engineering

220 Owen Hall, 

Corvallis, Oregon 97331

FEMA P-646 Project Participants 173
Project Management Committee

Steven Baldridge (Project Technical Director)

BASE Research & Development, LLC

1164 Bishop Street, Suite 605

Honolulu, Hawaii 96813

Frank Gonzalez

National Ocean & Atmospheric Administration 

Pacific Marine Environmental Laboratory

7600 Sand Point Way NE, Building 3

Seattle, Washington 98115-0070

John Hooper

Magnusson Klemencic Associates

1301 Fifth Avenue, Suite 3200

Seattle, Washington 98101

Ian N. Robertson

University of Hawaii at Manoa 

Dept. of Civil and Environmental Engineering

2540 Dole Street, Holmes Hall 383 

Honolulu, Hawaii 96822

Timothy J. Walsh

Dept. of Natural Resources, Geology & Earth 

Resources

1111 Washington Street SE, P.O. Box 47007

Olympia, Washington 98504

Harry Yeh

Oregon State University 

School of Civil & Construction Engineering

220 Owen Hall

Corvallis, Oregon 97331

Project Review Panel

Christopher P. Jones (Chair)

5525 Jomali Drive

Durham, North Carolina 27705

John Aho

CH2M Hill

301 West Northern Lights Blvd., Suite 601

Anchorage, Alaska 99503-2662

George Crawford

Washington State Military Dept.

Emergency Management Division

Camp Murray, Washington 98430

Richard Eisner

Governor's Office of Emergency Services

1300 Clay Street, Suite 400

Oakland, California 94612

Lesley Ewing

California Coastal Commission

45 Fremont Street, Suite 2000

San Francisco, California 94105

Michael Hornick

DHS/FEMA, Region IX

1111 Broadway, Suite 1200

Oakland, California 94607

Chris Jonientz-Trisler

DHS/FEMA, Region X

130 228th Street SW

Bothell, Washington 98021

Marc L. Levitan

LSU Hurricane Center

Suite 3221 CEBA Building

Louisiana State University

Baton Rouge, Louisiana 70803

George R. Priest

Oregon Dept. of Geology and Mineral Industries

Newport Coastal Field Office

P.O. Box 1033

Newport, Oregon 97365

Charles W. Roeder

University of Washington

Dept. of Civil and Environmental Engineering 

233-B More Hall, Box 2700 

Seattle, Washington 98195

Jay Wilson

Clackamas County Department of Emergency 

Management

2200 Kaen Road

Oregon City, Oregon 97045

174 Project Participants FEMA P-646