Guidelines for Design 

of Structures for Vertical 

Evacuation from Tsunamis

FEMA P646 / June 2008

FEMA

FEMA P646 / June 2008 

Guidelines for Design of Structures for 

Vertical Evacuation from Tsunamis 

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) 

Jon A. Heintz (Project Manager) 

William T. Holmes (Project Tech. Monitor) 

PROJECT MANAGEMENT COMMITTEE 

Steven Baldridge (Project Technical Director) 

Frank Gonzalez 

John Hooper 

Ian N. Robertson 

Timothy J. Walsh 

Harry Yeh 

PROJECT REVIEW PANEL 

Christopher P. Jones* (Chair) 

John Aho 

George Crawford 

Richard Eisner 

Lesley Ewing 

Michael Hornick 

Chris Jonientz-Trisler 

Marc L. Levitan 

George Priest 

Charles W. Roeder 

Jay Wilson 

*ATC Board Representative 
Notice 

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

reflect the views of the Department of Homeland Security’s Federal Emergency Management Agency 

(FEMA), the National Oceanic & Atmospheric Administration (NOAA), or the Applied Technology 

Council (ATC). Additionally, neither ATC, DHS, 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 from this publication assume all liability arising from such use. 

Cover photographs provided courtesy of Magnusson Klemencic Associates, Seattle, Washington. 
FEMA P646 Foreword iii 

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 and Indian Ocean tsunami. While this event 

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. Without realizing it, these survivors were among the first to 

demonstrate 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; 

• Determining tsunami and earthquake loads and structural design criteria 

necessary to address them; and, 

• Structural design concepts and other considerations.
iv Foreword FEMA P646 

This publication is the first of two documents on this issue. The second, 

currently under development, will present 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 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. 

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 
FEMA P646 Preface v 

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 the development of 

design guidance for special facilities for vertical evacuation from tsunamis, 

which ATC designated the ATC-64 Project. The effort was co-funded by 

FEMA and the National Oceanic and Atmospheric Administration (NOAA). 

The developmental process involved a variety of activities including review 

of relevant research and state-of-the-practice documentation and literature, 

preparation of 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 

desired architectural and structural system attributes for vertical evacuation 

facilities. 

The resulting guidance for design of special facilities for vertical evacuation 

from tsunami, as presented herein, addresses a range of relevant issues. 

Chapter 1 defines the scope and limitations of the guidance. 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, and design concepts and other considerations. The document 

concludes with a series of appendices that provide supplemental information, 

including examples of vertical evacuation structures from Japan, example 

tsunami load calculations, a community design example, development of 

impact load equations, and background on maximum flow velocity and 

momentum flux in the tsunami runup zone. 

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

participated in the development of this document. The Project Management 

Committee, consisting of Steven Baldridge (Project Technical Director), 

Frank Gonzalez (who participated in early portions of the project), 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 
vi Preface FEMA P646 

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. Peter N. 

Mork and Bernadette Hadnagy provided ATC report production services. 

The affiliations of these individuals 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), and William Holmes (ATC Project Technical Monitor). 

Jon A. Heintz Christopher Rojahn 

ATC Director of Projects ATC Executive Director 
FEMA P646 Table of Contents vii

Table of Contents 

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

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

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

List of Tables ................................................................................................. xv 

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 Hurricane Katrina .............................. 23 

2.2.4 Implications for Tsunami-Resistant Design ..................... 28 

3. TSUNAMI HAZARD ASSESSMENT ................................................... 31 

3.1 Current Tsunami Modeling and Inundation Mapping ................... 31 

3.2 The NOAA Tsunami Program: Forecast Modeling and 

Mapping ......................................................................................... 32 

3.3 The National Tsunami Hazard Mitigation Program: Credible 

Worst-Case Scenarios .................................................................... 34 

3.4 The FEMA Map Modernization Program: Probabilistic 

Tsunami Hazard Assessments ....................................................... 37 

3.5 Limitations in Available Modeling and Mapping Products ........... 40 

3.6 Hazard Quantification for Design of Tsunami Vertical 

Evacuation Structures .................................................................... 40 

3.7 Recommendations to Improve Tsunami Hazard Assessment ........ 42 

4. VERTICAL EVACUATION OPTIONS ................................................ 43 

4.1 Vertical Evacuation Considerations .............................................. 43 

4.1.1 Single-Purpose Facilities .................................................. 43 

4.1.2 Multi-Purpose Facilities ................................................... 44 

4.1.3 Multi-Hazard Considerations ........................................... 44 

4.2 Vertical Evacuation Concepts ........................................................ 45 

4.2.1 Existing High Ground ....................................................... 45 

4.2.2 Soil Berms ........................................................................ 46 
viii Table of Contents FEMA P646 

4.2.3 Parking Garages ............................................................... 46 

4.2.4 Community Facilities ....................................................... 47 

4.2.5 Commercial Facilities ...................................................... 47 

4.2.6 School Facilities ............................................................... 48 

4.2.7 Existing Buildings ............................................................ 49 

5. SITING, SPACING , SIZING AND ELEVATION 

CONSIDERATIONS .............................................................................. 51 

5.1 Siting Considerations .................................................................... 51 

5.1.1 Warning, Travel Time, and Spacing ................................ 51 

5.1.2 Ingress and Vertical Circulation ...................................... 53 

5.1.3 Consideration of Site Hazards.......................................... 54 

5.2 Sizing Considerations ................................................................... 56 

5.2.1 Services and Occupancy Duration ................................... 57 

5.2.2 Square Footage Recommendations from Available 

Sheltering Guidelines ....................................................... 57 

5.2.3 Recommended Minimum Square Footage for 

Short-Term Refuge from Tsunamis ................................. 59 

5.3 Elevation Considerations .............................................................. 59 

5.4 Size of Vertical Evacuation Structures ......................................... 60 

6. LOAD DETERMINATION AND STRUCTURAL DESIGN 

CRITERIA .............................................................................................. 61 

6.1 Currently Available Structural Design Criteria ............................. 61 

6.1.1 Current U.S. Codes, Standards, and Guidelines .............. 61 

6.1.2 Summary of Current Design Requirements ..................... 63 

6.1.3 Limitations in Available Flood Design Criteria 

Relative to Tsunami Loading ........................................... 64 

6.2 Performance Objectives ................................................................ 65 

6.2.1 Tsunami Performance Objective ...................................... 66 

6.2.2 Seismic and Wind Performance Objectives ..................... 67 

6.3 Earthquake Loading ...................................................................... 67 

6.3.1 Near-Source-Generated Tsunamis ................................... 68 

6.3.2 Far-Source-Generated Tsunamis...................................... 68 

6.4 Wind Loading ............................................................................... 69 

6.5 Tsunami Loading .......................................................................... 69 

6.5.1 Key Assumptions for Estimating Tsunami Load 

Effects .............................................................................. 69 

6.5.2 Hydrostatic Forces ........................................................... 70 

6.5.3 Buoyant Forces ................................................................ 71 

6.5.4 Hydrodynamic Forces ..................................................... 72 

6.5.5 Impulsive Forces .............................................................. 74 

6.5.6 Debris Impact Forces ....................................................... 75 

6.5.7 Damming of Waterborne Debris ...................................... 78 

6.5.8 Uplift Forces on Elevated Floors ..................................... 78 

6.5.9 Additional Gravity Loads on Elevated Floors ................. 80 

6.6 Combination of Tsunami Forces ................................................... 81 

6.6.1 Tsunami Force Combinations on the Overall 

Structure ........................................................................... 82 

6.6.2 Tsunami Force Combinations on Individual 

Components .................................................................... 84 
FEMA P646 Table of Contents ix

6.7 Load Combinations ........................................................................ 85 

6.8 Member Capacities and Strength Design Considerations .............. 86 

6.9 Progressive Collapse Considerations ............................................. 86 

6.9.1 Tie Force Strategy ............................................................ 86 

6.9.2 Missing Column Strategy ................................................. 88 

7. STRUCTURAL DESIGN CONCEPTS AND ADDITIONAL 

CONSIDERATIONS .............................................................................. 91 

7.1 Attributes of Tsunami-Resistant Structures ................................... 91 

7.2 Structural Considerations for Tsunami Load Effects .................... 91 

7.2.1 Foundation / Scour Design Concepts ............................... 92 

7.2.2 Breakaway Wall Concepts ................................................ 93 

7.3 Concepts for Modifying and Retrofitting 

Existing Structures ......................................................................... 95 

7.4 Permitting and Quality Assurance for Vertical 

Evacuation Structures .................................................................... 96 

7.4.1 Permitting and Code Compliance ........................................ 96 

7.4.2 Peer Review ......................................................................... 96 

7.4.3 Quality Assurance / Quality Control .................................... 97 

7.5 Planning Considerations for Vertical Evacuation Structures ........ 97 

7.6 Cost Considerations for Vertical Evacuation Structures ............... 98 

APPENDIX A – VERTICAL EVACUATION STRUCTURE 

EXAMPLES FROM JAPAN ................................................................ 101 

APPENDIX B – COMMUNITY DESIGN EXAMPLE ............................ 107 

B.1 Site 1 Example: Escape Berm...................................................... 110 

B.2 Site 2 Example: Multi-Use Structure ........................................... 112 

APPENDIX C – EXAMPLE CALCULATIONS ...................................... 117 

C.1 Inundation Depth ......................................................................... 117 

C.2 Hydrostatic and Buoyant Forces .................................................. 118 

C.3 Hydrodynamic and Impulsive Forces .......................................... 118 

C.4 Impact Force ................................................................................ 120 

C.5 Damming Effect of Waterborne Debris ....................................... 121 

C.6 Hydrodynamic Uplift Forces ....................................................... 122 

APPENDIX D – BACKGROUND INFORMATION ON IMPACT 

LOAD CALCULATIONS .................................................................... 123 

D.1 Available Models for Impact Loads ............................................ 123 

D.2 Summary and Discussion ............................................................ 127 

APPENDIX E – MAXIMUM FLOW VELOCITY AND MOMENTUM 

FLUX IN THE TSUNAMI RUNUP ZONE ......................................... 131 

E.1 Flow Velocity .............................................................................. 131 

E.2 Momentum Flux .......................................................................... 134 

GLOSSARY ................................................................................................ 135 

REFERENCES ............................................................................................ 145 

PROJECT PARTICIPANTS ....................................................................... 157 

FEMA P646 List of Figures xi

List of Figures 

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

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

Figure 2-1 Maximum computed tsunami amplitudes 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 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 NihonkaiChubu 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 water-borne 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 
FEMA P646 List of Figures xii

Figure 2-15 Example of surviving reinforced concrete mosque in 

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

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: 

(a) Beam-column connection failures; (b) Soft story 

failure .................................................................................. 24 

Figure 2-22 Gulf Tower Apartment building suffered substantial 

non-structural damage at the ground floor level, but 

remained structurally sound ................................................ 25 

Figure 2-23 Pass Christian office building with cast-in-place pan 

joist floor system that suffered non-structural damage 

at first two floors but no structural damage ......................... 26 

Figure 2-24 Condominium building in Gulfport, Mississippi, with 

wave and storm surge damage to non-structural 

elements at the ground floor, but no reported structural 

damage ................................................................................ 26 

Figure 2-25 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 ..................................................................................... 27 

Figure 2-26 Failure of prestressed piles due to damming effect of 

shipping container ............................................................... 28 

Figure 2-27 Negative bending failure of a prestressed double-tee 

floor system due to uplift forces .......................................... 29 

Figure 2-28 Concrete frame of three-story apartment building that 

partially collapsed due to failure of the post-tensioned 

flat slab in the bay closest to the Gulf of Mexico ................ 30 

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

for the Tsunami Forecasting System ................................... 33 

Figure 3-2 Tsunami inundation modeling products for Seattle, 

Washington ......................................................................... 35 

Figure 3-3 Tsunami inundation map for Seattle, Washington 

produced and published by the state of Washington, 

using modeling products as guidance .................................. 35 
FEMA P646 List of Figures xiii

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

three inundation lines ........................................................... 37 

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

exceeded in 50 years ............................................................ 38 

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

depicting maximum have heights that are met or 

exceeded at an annual probability of 0.2% .......................... 39 

Figure 4-1 Soil berm combined with a community open space. ........... 46 

Figure 4-2 Parking garage. Open structural systems allow water 

to pass through with minimal resistance, and interior 

ramps allow for easy ingress and vertical circulation .......... 47 

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

type of structure can accommodate circulation and 

service needs for large numbers of people ........................... 48 

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. .................................................................................. 48 

Figure 4-5 Evacuation map for Waikiki, Hawaii, indicating use of 

existing buildings for vertical evacuation ............................ 49 

Figure 5-1 Vertical evacuation refuge locations considering travel 

distance, evacuation behavior, and naturally occurring 

high ground .......................................................................... 54 

Figure 5-2 Site hazards adjacent to vertical evacuation structures ........ 55 

Figure 6-1 Seismic performance objectives linking building 

performance levels to earthquake hazard levels .................. 66 

Figure 6-2 Hydrostatic force distribution and location of 

resultant ................................................................................ 71 

Figure 6-3 Buoyant forces on an overall building with watertight 

lower levels .......................................................................... 72 

Figure 6-4 Hydrodynamic force distribution and location of 

resultant ................................................................................ 73 

Figure 6-5 Hydrodynamic impulsive and drag forces on 

components of a building subjected to inundation by 

a tsunami bore ...................................................................... 75 

Figure 6-6 Waterborne debris impact force ........................................... 75 

Figure 6-7 Maximum flow velocity of depth, d, at the ground 

elevation, z, and maximum runup elevation, R. ................. 77 

Figure 6-8 A definition sketch for upward buoyant force exerted 

on an elevated floor .............................................................. 79 

Figure 6-9 Gravity loads exerted on an elevated floor with water 

retained by exterior walls during rapid drawdown .............. 81 
FEMA P646 List of Figures xiv

Figure 6-10 Impulsive and drag forces applied to an example 

building ............................................................................... 83 

Figure 6-11 Debris dam and drag forces applied to an example 

building ............................................................................... 83 

Figure 6-12 Tie force strategy ................................................................. 87 

Figure 6-13 Detailing of reinforcing steel for potential loss of a 

supporting column ............................................................... 87 

Figure 6-14 Missing column strategy ..................................................... 89 

Figure 7-1 Effect of breakaway walls on waves ................................... 94 

Figure A-1 Life-Saving Tower ............................................................ 101 

Figure A-2 Nishiki Tower .................................................................... 102 

Figure A-3 Refuge at Shirahama Beach Resort ................................... 103 

Figure A-4 Tsunami refuge in Kaifu, Japan ........................................ 104 

Figure A-5 Berm constructed for tsunami refuge in Aonae, Japan ..... 104 

Figure A-6 Aonae Elementary School. Upper floors are intended 

for use as tsunami refuge space ......................................... 105 

Figure B-1 Hypothetical sketch of example community showing 

potential vertical evacuation structure sites and 

evacuation routes ............................................................... 107 

Figure B-2 Example community inundation map ............................... 108 

Figure B-3 Example community inundation flow velocity map ......... 109 

Figure B-4 Example escape berm design ............................................ 110 

Figure B-5 Example escape berm plan layout ..................................... 111 

Figure B-6 Example escape berm section ........................................... 111 

Figure B-7 Example escape berm rear elevation ................................. 112 

Figure B-8 Example gymnasium ......................................................... 113 

Figure B-9 Example gymnasium plan ................................................. 114 

Figure B-10 Example gymnasium elevation ......................................... 114 

Figure C-1 Definition sketch for example calculations ....................... 117 

Figure C-2 Condition resulting in buoyant forces ............................... 119 

Figure D-1 Ranges of duration of impact ............................................ 127 

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

elevation, z, and maximum runup elevation, R ................. 133 
FEMA P646 List of Tables xv

List of Tables 

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

Locations (Dunbar, et. al., 2008). .................................................8 

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 .................. 52 

Table 5-2 Maximum Spacing of Vertical Evacuation Structures 

Based on Travel Time ................................................................ 53 

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

the Design and Construction of Storm Shelters ......................... 58 

Table 5-4 Square Footage Recommendations – FEMA 361 Design 

and Construction Guidance for Community Shelters ................. 58 

Table 5-5 Square Footage Recommendations – American Red Cross 

Publication No. 4496 .................................................................. 58 

Table 6-1 Mass and Stiffness Properties of Common Waterborne 

Debris ......................................................................................... 76 

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

Depth, d ...................................................................................... 93 

Table B-1 Design Elevations for Areas of Refuge .................................... 108 

Table B-2 Tsunami Flow Velocity at Each Site ........................................ 109 

FEMA P646 1: Introduction 1 

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. 
2 1: Introduction FEMA P646 

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, 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 

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. 
FEMA P646 1: Introduction 3 

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

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 

various evacuation alternatives, vulnerability of the existing building stock, 
4 1: Introduction FEMA P646 

and locations of existing of 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 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 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. 

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 

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 
FEMA P646 1: Introduction 5 

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. 

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 

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. 

FEMA P646 2: Background 7 

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. The tsunami created by the 

magnitude-9.3 underwater earthquake devastated coastal areas around the 

northern Indian Ocean. The tsunami took anywhere from 15 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. 

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.
8 2: Background FEMA P646 

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. 

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 

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 
FEMA P646 2: Background 9 

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

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

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 most recent damaging tsunami 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 (PararasCarayannis, 1976). 

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 
10 2: Background FEMA P646 

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 

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 

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. 
FEMA P646 2: Background 11 

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 

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. 
12 2: Background FEMA P646 

 (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). 

• 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. 
FEMA P646 2: Background 13 

-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

time (hours)

 (a)

-0.5

0.0

0.5

1.0

1.5

2.0

25 26 27 28 29 30 31 32 33 34 35

time (hours)

 (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). 

• 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 

can focus or disperse tsunami wave energy along certain shoreline 

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

Tsunami runup heights vary 

significantly in neighboring areas 

due to variations in offshore 

bathymetry that can increase or 

decrease local tsunami impacts. 
14 2: Background FEMA P646 

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 

photos) 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 wind-generated 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. 

 

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). 

A 

B 

C 

D 
FEMA P646 2: Background 15 

• 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). 
16 2: Background FEMA P646 

2.2 Tsunami Effects on Buildings 

Damage studies from historic tsunami events, the 2004 Indian Ocean 

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 highrise 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. 
FEMA P646 2: Background 17 

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 Fig. 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 (Weigel, 

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 
18 2: Background FEMA P646 

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. 
FEMA P646 2: Background 19 

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. 
20 2: Background FEMA P646 

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 waters 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. 
FEMA P646 2: Background 21 

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. 
22 2: Background FEMA P646 

 

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). 
FEMA P646 2: Background 23 

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. 

2.2.3 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. 
24 2: Background FEMA P646 

 

(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). 
FEMA P646 2: Background 25 

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 past tsunamis, most multi-story engineered buildings 

along the coastline survived the surge with damage to nonstructural elements 

at the lower levels (Figures 2-22 to 2-24). 

Figure 2-22 Gulf Tower Apartment building suffered substantial nonstructural damage at the ground floor level, but remained 

structurally sound (Hurricane Katrina, 2005). 

Estimated Inundation Level

Observations from Hurricane 

Katrina appear to support many of 

the effects documented with 

tsunami inundation and the 

conclusions drawn from historic 

tsunami data. 
26 2: Background FEMA P646 

Figure 2-23 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). 

Figure 2-24 Condominium building in Gulfport, Mississippi, with wave and storm surge damage to nonstructural elements at the ground floor, but no reported structural damage (FEMA 549, 2006). 

Estimated Inundation Level 
FEMA P646 2: Background 27 

Also consistent with past tsunami observations, Hurricane Katrina illustrated 

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

shown in Figure 2-25, impact from a barge-mounted casino failed a lower 

level column resulting in progressive collapse of the surrounding portions of 

the structure. In Figure 2-26, 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 2-27, 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 as shown in Figure 2-28. Cast-in-place concrete parking 

structures did not experience this type of failure. 

Figure 2-25 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). 
28 2: Background FEMA P646 

Figure 2-26 Failure of prestressed piles due to damming effect of shipping container 

(Hurricane Katrina, 2005). 

2.2.4 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, 

and supporting evidence from extreme storm surge flooding associated with 

Hurricane Katrina result in the following implications for tsunami-resistant 

design: 

• Vertical evacuation structures 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. 

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. 
FEMA P646 2: Background 29 

Figure 2-27 Negative bending failure of a prestressed double-tee floor system due to uplift forces (Hurricane 

Katrina, 2005). 

• Vertical evacuation structures should be located away from the wave 

breaking zone. 

• Impact forces and damming effects from water-borne 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. 
30 2: Background FEMA P646 

Figure 2-28 Concrete frame of three-story apartment building that partially collapsed due to failure of the posttensioned flat slab in the bay closest to the Gulf of Mexico (Hurricane Katrina, 2005). 
FEMA P646 3: Tsunami Hazard Assessment 31 

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 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 

Worst-Case Scenarios 

The FEMA Map Modernization 

Program: Probabilistic Tsunami 

Hazard Assessments 
32 3: Tsunami Hazard Assessment FEMA P646 

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 V.V. Titov (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) 

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. 
FEMA P646 3: Tsunami Hazard Assessment 33 

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

Tsunami Forecasting System. 
34 3: Tsunami Hazard Assessment FEMA P646 

• 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). 

• 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 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. 

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 
FEMA P646 3: Tsunami Hazard Assessment 35 

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). 

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). 
36 3: Tsunami Hazard Assessment FEMA P646 

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. 

• In addition to worst-case scenarios, maps in Alaska also depict 

inundation from a number of locally generated scenario tsunamis.

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 

reports are available for viewing, download, or purchase from the following 

state web sites: 

• Alaska: http://www.dggs.dnr.state.ak.us/pubs/pubs

• Oregon: http://www.oregongeology.com/sub/earthquakes/

Coastal/Tsumaps.htm

• Washington: http://www.dnr.wa.gov/AboutDNR/Divisions/GER/

Pages/home.aspx
FEMA P646 3: Tsunami Hazard Assessment 37 

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

inundation lines (Priest et al., 1997a; Priest et al.,1997b). 

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 occurrence, respectively). 
38 3: Tsunami Hazard Assessment FEMA P646 

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). 

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

exceeded in 50 years (FEMA, 1997). 
FEMA P646 3: Tsunami Hazard Assessment 39 

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. 

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 

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). 
40 3: Tsunami Hazard Assessment FEMA P646 

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. 

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. 

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 

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. 
FEMA P646 3: Tsunami Hazard Assessment 41 

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 the 2500-year return period associated with 

the Maximum Considered Earthquake used in seismic design. 

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 

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 Deterministic Maximum Considered 

Earthquake (Deterministic MCE) as the source (initial condition) of the 

tsunami model. At a minimum, the Deterministic MCE for a near-sourcegenerated tsunami in the United States should be the largest potentially 

tsunami-genic earthquake reported in the “Quaternary Fault and Fold 

Database of the United States” http://earthquake.usgs.gov/regional/qfaults/. 

Otherwise, selection of the Deterministic MCE should conform to ASCE/SEI 

Standard 7-05 Minimum Design Loads for Buildings and Other Structures

(ASCE, 2006b), and the following additional guidance proposed for 

inclusion in the next update of the standard: 

• Ground motions for the Deterministic MCE shall be based on 

characteristic earthquakes on all known active faults in a region. 

• The magnitude of a characteristic earthquake on a given fault should be a 

best estimate of the maximum magnitude capable for that fault but not 

less than the largest magnitude that has occurred historically on the fault. 

• The maximum magnitude should be estimated considering all seismic 

geologic evidence for the fault, including fault length and paleoseismic 

observations. 

• For faults characterized as having more than a single segment, the 

potential for rupture of multiple segments in a single earthquake should 

be considered in assessing the characteristic maximum magnitude for the 

fault. 

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.
42 3: Tsunami Hazard Assessment FEMA P646 

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. 

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, 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). 

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 possible goal for tsunami-resistant 

design of vertical evacuation structures is to achieve a uniform level of safety 

across all communities subjected to tsunami risk. In seismic and wind 

design, the starting point is probabilistic mapping of earthquake and wind 

risk. 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 difficult to develop, probabilistic maps for tsunami hazard can be 

made and are needed for reliable design of tsunami-resistant structures for 

uniform risk. 
FEMA P646 4: Vertical Evacuation Options 43 

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. 

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: 

• 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. 

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. 
44 4: Vertical Evacuation Options FEMA P646 

• 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 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. 

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 
FEMA P646 4: Vertical Evacuation Options 45 

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 accommodations for these and 

other potential conflicting requirements. 

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 

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. 

Vertical evacuation structures can be 

soil berms, parking garages, 

community facilities, commercial 

facilities, school facilities, or existing 

buildings. 
46 4: Vertical Evacuation Options FEMA P646 

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 ramp slope in the range of one foot vertical 

rise to four feet horizontal run (1 in 4) is recommended. 

Figure 4-1 Soil berm combined with a community open space. Ocean 

facing walls can deflect incoming waves while sloped sides 

provide for quick access. 

4.2.3 Parking Garages 

Parking garages are excellent 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. 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 revenue generating 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 
FEMA P646 4: Vertical Evacuation Options 47 

and construction inspection by local jurisdictions, parking garages could be 

effective vertical evacuation structures. 

Figure 4-2 Parking garage. 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 

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. 

Priority should be given to uses with complementary functions, such as 

accommodations for large numbers of people. 

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. 
48 4: Vertical Evacuation Options FEMA P646 

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

structure can accommodate circulation and service needs for 

large numbers of people. 

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. 

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 
FEMA P646 4: Vertical Evacuation Options 49 

structures. This has the added benefit of possible additional public support 

for projects that increase the safety of school-age children. 

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 4-5. In a tsunami evacuation 

map for Waikiki, concrete and steel framed buildings six or more stories in 

height are considered to provide increased protection on or above the third 

floor, and are identified as potential areas of refuge. 

Figure 4-5 Evacuation map for Waikiki, Hawaii, indicating use of existing 

buildings for vertical evacuation.