FEMA P646 5: Siting, Spacing, Sizing, and Elevation Considerations 51 

Chapter 5 

Siting, Spacing, Sizing, and 

Elevation Considerations

Tsunami risk is unique in that some communities may be susceptible to farsource-generated tsunamis (longer warning time), near-source-generated 

tsunamis (shorter warning time), or both. Far-source-generated tsunamis 

generally allow sufficient warning time so that emergency response plans can 

be based on evacuation out of the inundation zone. Near-source-generated 

tsunamis may not allow sufficient time for evacuation, so emergency 

response plans may need to include vertical evacuation refuge. This chapter 

provides guidance on how to locate vertical evacuation refuges within a 

community, and how to determine the size of a vertical evacuation structure. 

5.1 Siting Considerations 

Vertical evacuation structures should be located such that all persons 

designated to take refuge can reach the structure within the time available 

between tsunami warning and tsunami inundation. Travel time must also 

take into consideration vertical circulation within the structure to levels 

above the tsunami inundation elevation. Structures located at one end of a 

community may be difficult for some users to reach in a timely fashion. 

Routes to the structure should be easily accessible and well marked. 

Location of vertical evacuation structures within a community should take 

into account potential hazards in the vicinity of a site that could jeopardize 

the safety of the structure, and should consider that natural behaviors of 

persons attempting to avoid coastal flooding. 

5.1.1 Warning, Travel Time, and Spacing 

The West Coast and Alaska Tsunami Warning Center (WC/ATWC) in 

Alaska, and the Pacific Tsunami Warning Center (PTWC) in Hawaii monitor 

potential tsunamis, and warn affected populations of an impending tsunami. 

Table 5-1 summarizes approximate warning times associated with the 

distance between a tsunami-genic source and the site of interest. A farsource-generated tsunami originates from a source that is far away from the 

site, and could have 2 hours or more of advance warning time. A nearsource-generated tsunami originates from a source that is close to the site, 

and could have 30 minutes or less of advance warning time. Sites 

Vertical evacuation structures

should be located such that all 

persons designated to take refuge 

can reach the structure within the 

time available between tsunami 

warning and tsunami inundation. 
52 5: Siting, Spacing, Sizing, and Elevation Considerations FEMA P646 

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

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

these effects will likely be the first warning of the impending tsunami. A 

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

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

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

be expected to have between 30 minutes and 2 hours of advance warning 

time. 

Table 5-1 Tsunami Sources and Approximate Warning Times 

Location of Source Approximate Warning Time (t) 

Far-source-generated tsunami t > 2 hrs 

Mid-source-generated tsunami 30 min < t < 2 hrs 

Near-source-generated tsunami t < 30 min 

Consideration must be given to the time it would take for designated 

occupants to reach a refuge. To determine the maximum spacing of tsunami 

vertical evacuation structures, the critical parameters are warning time and 

ambulatory capability of the surrounding community. Once maximum 

spacing is determined, size must be considered, and population becomes an 

important parameter. Sizing considerations could necessitate an adjustment 

in the number and spacing of vertical evacuation structures if it is not feasible 

to size the resulting structures large enough to accommodate the surrounding 

population at the maximum spacing. Sizing considerations are discussed in 

Section 5.2. 

The average, healthy person can walk at approximately 4-mph. Portions of 

the population in a community, however, may have restricted ambulatory 

capability due to age, health, or disability. The average pace of a mobilityimpaired population can be assumed to be about 2-mph. 

Assuming a 2-hour warning time associated with far-source-generated 

tsunamis, vertical evacuation structures would need to be located a maximum 

of 4 miles from any given starting point. This would result in a maximum 

spacing of approximately 8 miles between structures. Similarly, assuming a 

30 minute warning time, vertical evacuation structures would need to be 

located a maximum of 1 mile from any given starting point, or 2 miles 

between structures. Shorter warning times would require even closer 

spacing. Table 5-2 summarizes maximum spacing of vertical evacuation 

structures based on travel time associated with a mobility-impaired 

population. 

Recommended Maximum 

spacing of vertical evacuation 

structures depends on warning time, 

ambulatory speed, and the 

surrounding population density.
FEMA P646 5: Siting, Spacing, Sizing, and Elevation Considerations 53 

Table 5-2 Maximum Spacing of Vertical Evacuation Structures Based 

on Travel Time 

Warning Time 

Ambulatory 

Speed Travel Distance Maximum Spacing

2 hrs 2 mph* 4 miles 8 miles 

30 min 2 mph* 1 mile 2 miles 

15 min 2 mph* 1⁄2 mile 1 mile 

* Based on the average pace for a mobility-impaired population 

5.1.2 Ingress and Vertical Circulation 

Tsunami vertical evacuation structures should be spaced such that people 

will have adequate time not only to reach the structure, but to enter and move 

within the structure to areas of refuge that are located above the anticipated 

tsunami inundation elevation. 

Increased travel times may need to be considered if obstructions exist, or 

could occur, along the travel or ingress route. Unstable or poorly secured 

structural or architectural elements that collapse in and around the entrance, 

or the presence of contents associated with the non-refuge uses of a structure, 

could potentially impede ingress. Allowance for parking at a vertical 

evacuation refuge may decrease travel time to the refuge, but could 

complicate access when the potential traffic for jams is considered. 

Stairs or elevators are traditional methods of ingress and vertical circulation 

in buildings, especially when designated users have impaired mobility. 

Ramps, such as the ones used in sporting venues, however, can be more 

effective for moving large numbers of people into and up to refuge areas in a 

structure. Estimates of travel time may need adjustment for different 

methods of vertical circulation. Disabled users may need to travel along a 

special route that accommodates wheelchairs, and those with special needs 

may require assistance from others to move within the structure. 

When locating vertical evacuation structures, natural and learned behaviors 

of evacuees should be considered. Most coastal communities have educated 

their populations to “go to high ground” in the event of a tsunami warning. 

Also, a natural tendency for evacuees will be to migrate away from the shore. 

Vertical evacuation structures should therefore be located on the inland side 

of evacuation zones and should take advantage of naturally occurring 

topography that would tend to draw evacuees towards them. Figure 5-1 
54 5: Siting, Spacing, Sizing, and Elevation Considerations FEMA P646 

illustrates an arrangement of vertical evacuation structures in a community 

based on these principles. 

Figure 5-1 Vertical evacuation refuge locations considering travel distance, 

evacuation behavior, and naturally occurring high ground. 

Arrows show anticipated vertical evacuation routes. 

5.1.3 Consideration of Site Hazards 

Special hazards in the vicinity of each site should be considered in locating 

vertical evacuation structures. Potential site hazards include breaking waves, 

sources of large waterborne debris, and sources of waterborne hazardous 

materials. When possible, vertical evacuation structures should be located 

away from potential hazards that could result in additional damage to the 

structure and reduced safety for the occupants. Due to limited availability of 

possible sites, and limitations on travel and mobility of the population in a 

community, some vertical evacuation structures may need to be located at 

sites that would be considered less than ideal. Figure 5-2 illustrates adjacent 

site hazards that could exist in a typical coastal community. 

Potential site hazards include 

breaking waves, sources of large 

waterborne debris, and sources of 

waterborne hazardous materials. 
FEMA P646 5: Siting, Spacing, Sizing, and Elevation Considerations 55 

Figure 5-2 Site hazards adjacent to vertical evacuation structures 

(numbered locations). Arrows show anticipated vertical 

evacuation routes. 

Wave breaking takes place where the water depth is sufficiently finite. In the 

design of usual coastal structures (e.g., breakwaters, seawalls, jetties), critical 

wave forces often result from breaking waves. In general, tsunamis break 

offshore. In the case of very steep terrain, however, they can break right at 

the shoreline, which is known as a collapsing breaker. 

Forces from collapsing breakers can be extremely high and very uncertain. 

Location of vertical evacuation structures within the tsunami wave-breaking 

zone poses unknown additional risk to the structure. While the possibility of 

tsunami wave breaking at an on-shore location is not zero, it is considered to 

be very rare. For these reasons, recommended sites for vertical evacuation 

structures are located inland of the wave-breaking zone, and wave breaking 

forces are not considered in this document. 

In Figure 5-2, vertical evacuation structures are located some distance inland 

from the shoreline. Structure No. 1 is located adjacent to a harbor and 

container terminal. Impact forces from ships, barges, boats, and other 
56 5: Siting, Spacing, Sizing, and Elevation Considerations FEMA P646 

waterborne debris have the potential to become very large. Locations with 

additional sources of large, possibly buoyant debris increase the chances of 

impact by one or more waterborne missiles, and increase the potential risk to 

the structure. If possible, it would be better if this structure was sited away 

from the harbor and container terminal. If there is no alternate location 

available to serve this area of the community, this structure would need to be 

designed for potential impact from the shipping containers and boats likely to 

be present during tsunami inundation. 

Structure No. 2 is located off to the side of the harbor and adjacent to a 

parking lot. This structure would need to be designed for debris consistent 

with the use of the parking lot and surrounding areas, which could include 

cars, trucks, and recreational vehicles. 

Structure No. 3 is immediately adjacent to a gas station. In past tsunamis, 

ignition of flammable chemicals or other floating debris has resulted in 

significant risk for fire in partially submerged structures. Depending on the 

potential for fuel leakage from this station in the event of a tsunami (or a 

preceding earthquake), this structure would need to be designed with fire 

resistive construction and additional fire protection. 

Structure No. 4 is adjacent to a waterfront park facility. This location can be 

ideal, as the potential for waterborne debris can be relatively low. Possible 

hazards could include debris from park structures, naturally occurring 

driftwood, or larger logs from downed trees. This area has a higher potential 

for tourists and visitors unfamiliar with the area. It would require additional 

signage to inform park users what to do and where to go in the event of a 

tsunami warning. 

Structure No. 5 is adjacent to an emergency response facility. Co-locating at 

such facilities can provide opportunities for direct supervision by lawenforcement and monitoring and support of refuge occupancies by other 

emergency response personnel. 

At two locations, Structure No. 6 is intended to aid evacuees in taking 

advantage of naturally occurring high ground. 

5.2 Sizing Considerations 

Sizing of a vertical evacuation structure depends on the intended number of 

occupants, the type of occupancy, and the duration of occupancy. The 

number of occupants will depend on the surrounding population and the 

spacing and number of vertical evacuation structures located in the area. 
FEMA P646 5: Siting, Spacing, Sizing, and Elevation Considerations 57 

Duration of occupancy will depend on the nature of the hazard and the 

intended function of the facility. 

5.2.1 Services and Occupancy Duration 

A vertical evacuation structure is typically intended to provide a temporary 

place of refuge during a tsunami event. While tsunamis are generally 

considered to be short-duration events (i.e., pre-event warning period and 

event lasting about 8 to 12 hours), tsunamis include several cycles of waves. 

The potential for abnormally high tides and coastal flooding can last as long 

as 24 hours. 

A vertical evacuation structure must provide adequate services to evacuees 

for their intended length of stay. As a short term refuge, services can be 

minimal, including only limited space per occupant and basic sanitation 

needs. Additionally, a vertical evacuation structure could be used to provide 

accommodations and services for people whose homes have been damaged 

or destroyed. As a minimum, this would require an allowance for more 

space for occupants, supplies, and services. It could also include 

consideration of different post-event rescue and recovery activities, and 

evaluation of short- and long-term medical care needs. Guidance on basic 

community sheltering needs is not included in this document, but can be 

found in FEMA 361, Design and Construction Guidance for Community 

Shelters (FEMA, 2000a). 

Choosing to design and construct a vertical evacuation structure primarily for 

short-term refuge, or to supply and manage it to house evacuees for longer 

periods of time, is an emergency management issue that must be decided by 

the state, municipality, local community, or private owner. 

5.2.2 Square Footage Recommendations from Available 

Sheltering Guidelines 

Square footage recommendations are available from a number of different 

sources, and vary depending on the type of hazard and the anticipated 

duration of occupancy. The longer the anticipated stay, the greater the 

minimum square footage recommended. 

A shelter for mostly healthy, uninjured people for a short-term event would 

require the least square footage per occupant. A shelter intended to house 

sick or injured people, or to provide ongoing medical care, would require 

more square footage to accommodate beds and supplies. For longer duration 

stays, even more square footage is needed per occupant for minimum privacy 

and comfort requirements, and for building infrastructure, systems, and 

services needed when housing people on an extended basis. 
58 5: Siting, Spacing, Sizing, and Elevation Considerations FEMA P646 

Table 5-3, Table 5-4 and Table 5-5 summarize square footage 

recommendations contained in International Code Council/National Storm 

Shelter Association, ICC-500, Standard on the Design and Construction of 

Storm Shelters (ICC/NSSA, 2007), FEMA 361 Design and Construction 

Guidance for Community Shelters (FEMA, 2000a), and American Red Cross 

Publication No. 4496, Standards for Hurricane Evacuation Shelter Selection

(ARC, 2002). 

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

on the Design and Construction of Storm Shelters

(ICC/NSSA, 2007) 

Hazard or Duration 

Minimum Required Usable Floor 

Area in Sq. Ft. per Occupant 

Tornado 

 Standing or seated 

 Wheelchair 

 Bedridden 

 5 

 10 

 30 

Hurricane 

 Standing or seated 

 Wheelchair 

 Bedridden 

 20 

 20 

 40 

Table 5-4 Square Footage Recommendations – FEMA 361 Design 

and Construction Guidance for Community Shelters 

(FEMA, 2000a) 

Hazard or Duration 

Recommended Minimum Usable 

Floor Area in Sq. Ft. per Occupant 

Tornado 5 

Hurricane 10 

Table 5-5 Square Footage Recommendations – American Red Cross 

Publication No. 4496 (ARC, 2002) 

Hazard or Duration 

Recommended Minimum Usable 

Floor Area in Sq. Ft. per Occupant 

Short-term stay (i.e., a few days) 20 

Long-term stay (i.e., days to weeks) 40 
FEMA P646 5: Siting, Spacing, Sizing, and Elevation Considerations 59 

The number of standing, seating, wheelchair, or bedridden spaces should be 

determined based on the specific occupancy needs of the facility under 

consideration. When determining usable floor area, ICC-500 includes the 

following adjustments to gross floor area: 

• Usable floor area is 50 percent of gross floor area in shelter areas with 

concentrated furnishings or fixed seating. 

• Usable floor area is 65 percent of gross floor area in shelter areas with 

un-concentrated furnishings and without fixed seating. 

• Usable floor area is 85 percent of gross floor area in shelter areas with 

open plan furnishings and without fixed seating. 

5.2.3 Recommended Minimum Square Footage for ShortTerm Refuge from Tsunamis 

For short-term refuge in a tsunami vertical evacuation structure, the duration 

of occupancy should be expected to last between 8 to 12 hours, as a 

minimum. Because tsunami events can include several cycles of waves, 

there are recommendations that suggest evacuees should remain in a tsunami 

refuge until the second high tide after the first tsunami wave, which could 

occur up to 24 hours later. 

Based on square footage recommendations employed in the design of shelters 

for other hazards, the recommended minimum square footage per occupant 

for a tsunami refuge is 10 square feet per person. It is anticipated that this 

density will allow evacuees room to sit down without feeling overly crowded 

for a relatively short period of time, but would not be considered appropriate 

for longer stays that included sleeping arrangements. This number should be 

adjusted up or down depending on the specific occupancy needs of the refuge 

under consideration. 

5.3 Elevation Considerations 

In order to serve effectively as a vertical evacuation structure, it is essential 

that the area of refuge be located well above the maximum tsunami 

inundation level anticipated at the site. Determination of a suitable elevation 

for tsunami refuge must take into account the uncertainty inherent in 

estimation of the tsunami runup elevation, possible splash-up during impact 

of tsunami waves, and the anxiety level of evacuees seeking refuge in the 

structure. 

To account for this uncertainty, the magnitude of tsunami force effects is 

determined assuming a maximum tsunami runup elevation that is 30% higher 

than values predicted by numerical simulation modeling or obtained from 

Recommended minimum square 

footage is 10 square feet 

per occupant.
60 5: Siting, Spacing, Sizing, and Elevation Considerations FEMA P646 

tsunami inundation maps. Because of the high consequence of potential 

inundation of the tsunami refuge area, it is recommended that the elevation of 

tsunami refuge areas in vertical evacuation structures include an additional 

allowance for freeboard above this elevation. 

The recommended minimum freeboard is one story height, or 10 feet (3 

meters) above the tsunami runup elevation used in tsunami force 

calculations. The recommended minimum elevation for a tsunami refuge 

area is, therefore, the maximum tsunami runup elevation anticipated at the 

site, plus 30%, plus 10 feet (3 meters). 

5.4 Size of Vertical Evacuation Structures 

Given the number and spacing of vertical evacuation structures, and the 

population in a given community, the minimum size can be determined based 

on square footage recommendations for the intended duration and type of 

occupancy. Consideration of other functional needs, such as restrooms, 

supplies, communications, and emergency power, should be added to the 

overall size of the structure. 

Given the maximum tsunami runup elevation anticipated at the site, the 

minimum elevation of the area of refuge within a vertical evacuation 

structure can be determined based on minimum freeboard recommendations. 

Recommended minimum refuge 

elevation is the maximum 

anticipated tsunami runup 

elevation, plus 30%, plus 10 feet 

(3 meters).
FEMA P646 6: Load Determination and Structural Design Criteria 61 

Chapter 6 

Load Determination and 

Structural Design Criteria

This chapter summarizes current code provisions as they relate to tsunami 

load effects, describes intended performance objectives for vertical 

evacuation structures, specifies equations for estimating tsunami forces, and 

provides guidance on how tsunami forces should be combined with other 

effects. 

6.1 Currently Available Structural Design Criteria 

Very little guidance is provided in currently available structural design 

codes, standards, and guidelines on loads induced by tsunami inundation. 

Established design information focuses primarily on loads due to rising water 

and wave action associated with riverine flooding and storm surge. While 

little specific guidance is provided, the presumption is that currently 

available flood design standards are to be used in designing for tsunami load 

effects. 

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

International Building Code. The International Code Council International 

Building Code (ICC, 2006) Appendix G provides information on flood 

design and flood-resistant construction by reference to ASCE/SEI Standard 

24-05, Flood Resistant Design and Construction (ASCE 24, 2006a). 

ASCE/SEI Standard 24-05. The American Society of Civil 

Engineers/Structural Engineering Institute (ASCE/SEI) Standard 24-05 

Flood Resistant Design and Construction (ASCE, 2006a) provides minimum 

requirements for flood-resistant design and construction of structures located 

in flood-hazard areas. Topics include basic requirements for flood-hazard 

areas, high-risk flood-hazard areas, coastal high-hazard areas, and coastal A 

zones. This standard complies with FEMA National Flood Insurance 

Program (NFIP) floodplain management requirements. 

ASCE/SEI Standard 7-05. ASCE/SEI Standard 7-05 Minimum Design 

Loads for Buildings and Other Structures (ASCE, 2006b) provides 

expressions for forces associated with flood and wave loads on specific types 

of structural components. This standard covers important definitions that 

Very little guidance is provided in 

currently available structural design 

codes, standards, and guidelines on 

loads induced by tsunami 

inundation. 

Established design information 

focuses primarily on loads due to 

rising water and wave action 

associated with riverine flooding 

and storm surge. 
62 6: Load Determination and Structural Design Criteria FEMA P646 

relate to flooding and coastal high-hazard areas related to tides, storm surges, 

riverine flooding, seiches or tsunamis. 

FEMA 55 Coastal Construction Manual. The FEMA 55 Coastal 

Construction Manual (FEMA, 2005) includes FEMA’s most recent study of 

coastal seismic and tsunami loads. This manual was developed to provide 

design and construction guidance for low-rise (less than three stories), oneand two-family residential structures built in coastal areas throughout the 

United States. The Coastal Construction Manual addresses seismic loads for 

coastal structures, and contains expressions for flood loads, wave loads, and 

load combinations for specific types of structural components. 

The Manual also provides information on tsunami hazard. Section 7.2.2 

states that: 

“Tsunamis are long-period water waves generated by undersea shallowfocus earthquakes or by undersea crustal displacements (subduction of 

tectonic plates), landslides, or volcanic activity. Tsunamis can travel 

great distances, undetected in deep water, but shoaling rapidly in coastal 

waters and producing a series of large waves capable of destroying 

harbor facilities, shore protection structures, and upland buildings... 

Coastal construction in tsunami hazard zones must consider the effects of 

tsunami runup, flooding, erosion, and debris loads. Designers should 

also be aware that the “rundown” or return of water to the sea can also 

damage the landward sides of structures that withstood the initial runup.” 

The Manual also notes that tsunami effects at a particular site will be 

determined by the following four basic factors: 

• the magnitude of the earthquake or triggering event 

• the location of the triggering event 

• the configuration of the continental shelf and shoreline 

• the upland topography 

With regard to designing to resist tsunami loads, Section 11.7 of the Manual 

states that: 

“Tsunami loads on residential buildings may be calculated in the same 

fashion as other flood loads; the physical processes are the same, but the 

scale of the flood loads is substantially different in that the wavelengths 

and runup elevations of tsunamis are much greater than those of waves 

caused by tropical or extratropical cyclones ... When the tsunami forms 

a borelike wave, the effect is a surge of water to the shore. When this 

occurs, the expected flood velocities are substantially higher...and if 
FEMA P646 6: Load Determination and Structural Design Criteria 63 

realized at the greater water depths, would cause substantial damage to 

all buildings in the path of the tsunami. Designers should collect as 

much data as possible about expected tsunami depths to more accurately 

calculate tsunami flood forces.” 

Although authors of the Coastal Construction Manual conclude that it is 

generally not feasible or practical to design normal structures to withstand 

tsunami loads, it should be noted that this study was for conventional 

residential construction, and did not take into account the possibility of 

special design and construction details that would be possible for vertical 

evacuation structures. 

City and County of Honolulu Building Code. The City and County of 

Honolulu Building Code (CCH, 2000), Chapter 16, Article 11, provides 

specific guidance for “structural design of buildings and structures subject to 

tsunamis” in Section 16-11.5(f). The loading requirements in this section are 

based on the 1980 Dames & Moore study, but with the velocity of flow in 

feet per second estimated as equal in magnitude to the depth in feet of water 

at the structure. Estimates are also given for anticipated scour around piles 

and piers based on distance from the shoreline and the soil type at the 

building site. 

6.1.2 Summary of Current Design Requirements 

Coastal areas that are subject to high-velocity wave action from storms or 

seismic sources are designated Coastal V-Zones (ASCE, 2006a). Areas 

inland of Coastal V-Zones that are subject to smaller waves caused by storm 

surges, riverine flooding, seiches or tsunamis are designated Coastal A-Zones 

(ASCE, 2006a). 

In design for coastal flooding due to storm surge or tsunamis, buildings or 

structures are proportioned to resist the effects of coastal floodwaters. Design 

and construction must be adequate to resist the anticipated flood depths, 

pressures, velocities, impact, uplift forces, and other factors associated with 

flooding, as defined by the code. Habitable space in building structures must 

be elevated above the regulatory flood elevation by such means as posts, 

piles, piers, or shear walls parallel to the expected direction of flow. Spaces 

below the design flood elevation must be free from obstruction. Walls and 

partitions in a coastal high-hazard area are required to break away so as not 

to induce excessive loads on the structural frame. 

The effects of long-term erosion, storm-induced erosion, and local scour are 

to be included in the design of foundations of buildings or other structures in 

coastal high-hazard areas. Foundation embedment must be far enough below 
64 6: Load Determination and Structural Design Criteria FEMA P646 

the depth of potential scour to provide adequate support for the structure. 

Scour of soil from around individual piles and piers must be provided for in 

the design. Shallow foundation types are not permitted in V-Zones unless 

the natural supporting soils are protected by scour protection, but are 

permitted in A-Zones subject to stability of the soil and resistance to scour. 

The main building structure must be adequately anchored and connected to 

the elevating substructure system to resist lateral, uplift, and downward 

forces. 

6.1.3 Limitations in Available Flood Design Criteria Relative 

to Tsunami Loading 

Although many of the hydrostatic and hydrodynamic loading expressions in 

currently available codes, standards and guidelines are well-established, there 

are significant differences between tsunami inundation and riverine or storm 

surge flooding. For a typical tsunami, the water surface fluctuates near the 

shore with amplitude of several meters during a period of a few minutes to 

tens of minutes. A major difference between tsunamis and other coastal 

flooding is increased flow velocity for tsunami waves, which results in 

significant increases in velocity-related loads on structural components. 

Application of existing loading expressions to tsunami loading conditions 

requires an estimate of the tsunami flood depth and velocity, neither of which 

is provided with great accuracy by currently available information on 

tsunami hazard. 

Although impact of floating debris is considered in current codes, impact 

force produced by a change in momentum is dependent on estimates of the 

debris mass, velocity, and the time taken for the mass to decelerate. No 

accommodation is made for added mass of the water behind the debris, or the 

potential for damming if debris is blocked by structural components. More 

significant forms of debris, such as barges, fishing boats, and empty storage 

tanks may need to be considered for tsunamis, depending on the location of 

the building under consideration. The size, mass, and stiffness of this type of 

debris are not considered in currently available criteria. 

No consideration is given to upward loads on the underside of structures or 

components that are submerged by the flood or tsunami flow. These vertical 

hydrodynamic loads, different from buoyancy effects, are considered by the 

offshore industry in design of platforms and structural members that may be 

submerged by large waves. 

There are two primary scour mechanisms that occur during a tsunami event. 

Shear-induced scour is similar to that observed during storm surge flooding, 

and consists of soil transport due to the flow velocity. Liquefaction-induced 

Although many of the hydrostatic 

and hydrodynamic loading 

expressions in currently available 

codes, standards and guidelines are 

well-established, there are 

significant differences between 

tsunami inundation and riverine or 

storm surge flooding. 
FEMA P646 6: Load Determination and Structural Design Criteria 65 

scour results from rapid drawdown as the water recedes. Without sufficient 

time to dissipate, pore pressure causes liquefaction of the soil resulting in 

substantially greater scour than would otherwise occur. Although current 

codes require consideration of scour, little guidance (other than rough 

estimates) is given as to the potential extent of scour. 

6.2 Performance Objectives 

While specific performance objectives for various forms of rare loading can 

vary, acceptable structural performance generally follows a trend 

corresponding to: 

• little or no damage for small, more frequently occurring events; 

• moderate damage for medium-size, less frequent events; and 

• significant damage, but no collapse for very large, very rare events. 

In the case of earthquake hazards, current model building codes, such as the 

International Building Code, assign seismic performance objectives to 

buildings based on their inherent risk to human life (e.g., very large 

occupancies) or their importance after an earthquake (e.g., emergency 

operation centers or hospitals). Buildings and other structures are classified 

into Occupancy Categories I through IV, in order of increasing risk to human 

life or importance, and code prescriptive design criteria are correspondingly 

increased, with the intention of providing improved performance. For 

Occupancy Category IV, design rules are intended to result in a high 

probability of buildings remaining functional after moderate shaking, and 

experiencing considerably less damage than normal buildings in very rare 

shaking. 

Currently available performance-based seismic design procedures are 

intended to explicitly evaluate and predict performance, instead of relying on 

the presumed performance associated with prescriptive design rules. 

However, performance-based design is an emerging technology and the 

targeted performance cannot be delivered with 100% certainty. The current 

standard-of-practice for performance-based seismic design contained in 

ASCE/SEI 41-06 Seismic Rehabilitation of Existing Buildings (ASCE, 

2006c) defines discrete performance levels with names intended to connote 

the expected condition of the building: Collapse, Collapse Prevention, Life 

Safety, Immediate Occupancy, and Operational. Seismic performance 

objectives are defined by linking one of these building performance levels to 

an earthquake hazard level that is related to the recurrence interval (return 

period) and the intensity of ground shaking, as shown in Figure 6-1. 
66 6: Load Determination and Structural Design Criteria FEMA P646 

Figure 6-1 Seismic performance objectives linking building performance 

levels to earthquake hazard levels (adapted from SEAOC, 

1995). 

When determining performance objectives for natural hazards, the most 

difficult issue is deciding how rare (or intense) the design event should be. 

For seismic design in the United States, this issue has been resolved through 

the adoption of a national earthquake hazard map defining the Maximum 

Considered Earthquake (MCE) and the intensity of shaking associated with 

such an event. 

6.2.1 Tsunami Performance Objective 

In this document, the design tsunami event is termed the Maximum 

Considered Tsunami (MCT). Unfortunately, there are no national maps 

available for defining this hazard. In addition, due to the complexity of the 

tsunami hazard, which must consider near and distant tsunami-genic sources 

and highly uncertain relationships between earthquake events and subsequent 

tsunami, no firm policy has been established defining a methodology for 

setting a Maximum Considered Tsunami at a consistent hazard level. 

Current methods for tsunami hazard assessment are described in Chapter 3. 

Vertical evacuation structures designed in accordance with the guidance 

presented in this document would be expected to provide a stable refuge 

when subjected to a design tsunami event consistent with the Maximum 

Considered Tsunami identified for the local area. 

In general, the Maximum Considered Tsunami will be a rare, but realistic 

event with large potential consequences. Consistent with the general trend of 

The Tsunami Performance 

Objective includes the potential for 

significant damage while 

maintaining a reliable and stable 

refuge when subjected to the 

Maximum Considered Tsunami. 

Most structures would be expected 

to be repairable, although the 

economic viability of repair will be 

uncertain 
FEMA P646 6: Load Determination and Structural Design Criteria 67 

acceptable performance for “Maximum Considered” loadings, the 

performance of vertical evacuation structures in this event would include the 

potential for significant damage while maintaining a reliable and stable 

refuge. Most structures would be expected to be repairable after a large 

event, although the economics of repair versus replacement will be uncertain, 

depending on the specifics of the situation including the magnitude of the 

actual event, interaction with the local bathymetry, and the design and 

construction of the facility. 

6.2.2 Seismic and Wind Performance Objectives 

The performance objective for vertical evacuation structures subjected to 

seismic and wind hazards should be consistent with that of code-defined 

essential facilities such as hospitals, police and fire stations, and emergency 

operation centers. Following the prescriptive approach in the International 

Building Code, vertical evacuation structures are assigned to Occupancy 

Category IV, triggering design requirements that provide enhanced 

performance relative to typical buildings for normal occupancies. 

In the specific case of earthquakes, design for enhanced performance is 

necessary to assure that the structure is still usable for a tsunami following a 

local seismic event. To obtain a higher level of confidence that a vertical 

evacuation structure will achieve enhanced seismic performance, the design 

developed by prescriptive code provisions can be evaluated using currently 

available performance-based seismic design techniques and verification 

analyses. Utilizing the approach in ASCE/SEI 41-06, the performance 

objective for code-defined essential facilities would be Immediate 

Occupancy performance for the Design Basis Earthquake (DBE) and Life 

Safety performance for the Maximum Considered Earthquake (MCE). 

6.3 Earthquake Loading 

The recommended basis for seismic design of vertical evacuation structures 

is the International Building Code, which references ASCE/SEI 7-05 

Minimum Design Loads for Buildings and Other Structures for its seismic 

requirements. These requirements are based on the NEHRP Recommended 

Provisions for Seismic Regulations for New Buildings and Other Structures

(FEMA, 2004a) and additional information provided in the Commentary

(FEMA, 2004b). Vertical evacuation structures should be designed using 

rules for Occupancy Category IV buildings. 

The recommended basis for seismic evaluation and rehabilitation of existing 

buildings that are being considered for use as vertical evacuation structures is 

the SEI/ASCE Standard 31-03 Seismic Evaluation of Existing Buildings

Seismic and Wind Performance 

Objectives are consistent with the 

code-defined performance of 

essential facilities such as hospitals, 

police and fire stations, and 

emergency operation centers. 
68 6: Load Determination and Structural Design Criteria FEMA P646 

(ASCE, 2003b), using the Immediate Occupancy performance objective, and 

ASCE/SEI Standard 41-06 Seismic Rehabilitation of Existing Buildings, 

using the performance objectives specified in Section 6.2.2. 

6.3.1 Near-Source-Generated Tsunamis 

A vertical evacuation structure located in a region susceptible to near-sourcegenerated tsunamis is likely to experience strong ground shaking 

immediately prior to the tsunami. As a properly designed essential facility, it 

is expected that sufficient reserve capacity will be provided in the structure to 

resist the subsequent tsunami loading effects. The reserve capacity of the 

structure, which will be some fraction of the original, needs to be evaluated. 

It is recommended that the condition of the structure after the Design Basis 

Earthquake (DBE) be used to determine the adequacy for tsunami loading. If 

inadequate, the resulting design would then need to be modified as necessary 

to address tsunami effects. For areas that are subject to near-sourcegenerated tsunamis, this sequential loading condition will clearly control the 

design of the structure. To help ensure adequate strength and ductility in the 

structure for resisting tsunami load effects, Seismic Design Category D, as 

defined in ASCE/SEI 7-05, should be assigned to the structure, as a 

minimum. 

A properly designed essential facility is also expected to have improved 

performance of non-structural components including ceilings, walls, light 

fixtures, fire sprinklers, and other building systems. For evacuees to feel 

comfortable entering a vertical evacuation structure following an earthquake, 

and remaining in the structure during potential aftershocks, it is important 

that visible damage to both structural and non-structural components be 

limited. Particular attention should be focused on non-structural components 

in the stairwells, ramps, and entrances that provide access and vertical 

circulation within the structure. 

6.3.2 Far-Source-Generated Tsunamis 

Although a vertical evacuation structure may not experience earthquake 

shaking directly associated with a far-source tsunami, seismic design must be 

included as dictated by the seismic hazard that is present at the site. Even in 

regions of low seismicity, however, it is recommended that Seismic Design 

Category D be assigned to the structure, as a minimum, to help ensure 

adequate strength and ductility for resisting tsunami load effects. 

A vertical evacuation structure 

located in a region susceptible to 

near-source-generated tsunamis is 

likely to experience strong ground 

shaking immediately prior to the 

tsunami. 
FEMA P646 6: Load Determination and Structural Design Criteria 69 

6.4 Wind Loading 

The recommended basis for wind design of a vertical evacuation structure is 

the International Building Code, which references ASCE/SEI 7-05 Minimum 

Design Loads for Buildings and Other Structures for the majority of its wind 

requirements. In many locations affected by tsunami risk, earthquake 

loading will likely govern over wind loading, but this is not necessarily true 

for all regions. 

At locations where wind loading controls the design, the use of special 

seismic detailing for structural components should be considered. It is 

recommended that Seismic Design Category D be assigned to the structure, 

as a minimum, to help ensure adequate strength and ductility for resisting 

tsunami load effects. 

6.5 Tsunami Loading 

The following tsunami load effects should be considered for the design of 

vertical evacuation structures: (1) hydrostatic forces; (2) buoyant forces; (3) 

hydrodynamic forces; (4) impulsive forces; (5) debris impact forces; (6) 

debris damming forces; (7) uplift forces; and (8) additional gravity loads 

from retained water on elevated floors. 

In this document, wave-breaking forces are not considered in the design of 

vertical evacuation structures. In general, tsunamis break offshore, and 

vertical evacuation structures should be located some distance inland from 

the shoreline. The term ‘wave-breaking’ is defined here as a plunging-type 

breaker in which the entire wave front overturns. When waves break in a 

plunging mode, the wave front becomes almost vertical, generating an 

extremely high pressure over an extremely short duration. Once a tsunami 

wave has broken, it can be considered as a bore because of its very long 

wavelength. Further justification for not considering wave-breaking forces 

can be found in Yeh (2008). 

Wave-breaking forces could be critical for vertical evacuation structures 

located in the wave-breaking zone, which is beyond the scope of this 

document. If it is determined that a structure must be located in the wavebreaking zone, ASCE/SEI 7-05 Minimum Design Loads for Buildings and 

Other Structures and the Coastal Engineering Manual, EM 1110-2-1100, 

(U.S. Army Coastal Engineering Research Center, 2002) should be consulted 

for additional guidance on wave-breaking forces. 

6.5.1 Key Assumptions for Estimating Tsunami Load Effects 

Tsunami load effects are determined using the following key assumptions: 

Tsunami Load Effects include: 

(1) hydrostatic forces; 

(2) buoyant forces; 

(3) hydrodynamic forces; 

(4) impulsive forces; 

(5) debris impact forces; 

(6) debris damming forces; 

(7) uplift forces; and 

(8) additional gravity loads from 

retained water on elevated floors. 
70 6: Load Determination and Structural Design Criteria FEMA P646 

• Tsunami flows consist of a mixture of sediment and seawater. Most 

suspended sediment transport flows do not exceed 10% sediment 

concentration. Based on an assumption of vertically averaged sedimentvolume concentration of 10% in seawater, the fluid density of tsunami 

flow should be taken as 1.2 times the density of freshwater, or ρs = 1,200 

kg/m3

 = 2.33 slugs/ft3. 

• There is significant variability in local tsunami runup heights, based on 

local bathymetry and topographic effects, and uncertainty in numerical 

simulations of tsunami inundation. Based on empirical judgment from 

past tsunami survey data, it is recommended that the design runup 

elevation, R, be taken as 1.3 times the predicted maximum runup 

elevation, R*, to envelope the potential variability. 

• Because of uncertainties in modeling tsunami inundation, design 

parameters (e.g., flow velocity, depth, and momentum flux) derived from 

numerical simulations should not be taken as less than 80% of the values 

obtained from the analytical solutions described in Appendix E, and 

provided in Equation 6-6, Equation 6-9, and Figure 6-7. 

6.5.2 Hydrostatic Forces 

Hydrostatic forces occur when standing or slowly moving water encounters a 

structure or structural component. This force always acts perpendicular to the 

surface of the component of interest. It is caused by an imbalance of 

pressure due to a differential water depth on opposite sides of a structure or 

component. Hydrostatic forces may not be relevant to a structure with a 

finite (i.e., relatively short) breadth, around which the water can quickly flow 

and fill in on all sides. Hydrostatic forces are usually important for long 

structures such as sea walls and dikes, or for evaluation of an individual wall 

panel where the water level on one side differs substantially from the water 

level on the other side. 

Hydrostatic and buoyant forces must be computed when the ground floor of a 

building is watertight, or is sufficiently insulated and airtight to prevent or 

delay the intrusion of water. In this situation, the hydrostatic force should be 

evaluated for individual wall panels. The horizontal hydrostatic force on a 

wall panel can be computed using Equation 6-1: 

2

max 2

1 ApF wch == ρ s gbh , (6-1) 

where pc is the hydrostatic pressure, Aw is the wetted area of the panel, ρs is 

the fluid density including sediment (1200 kg/m3

 = 2.33 slugs/ft3), g is the 

gravitational acceleration, b is the breadth (width) of the wall, and hmax is the 
FEMA P646 6: Load Determination and Structural Design Criteria 71 

maximum water height above the base of the wall at the structure location. If 

the wall panel with height hw is fully submerged, then the horizontal 

hydrostatic force can be written as Equation 6-2: 

Fh = pcAw = ρsg hmax − hw

2

⎛

⎝

⎜ ⎞

⎠

⎟ b hw (6-2) 

where hmax is the vertical difference between the design tsunami runup 

elevation R and the base elevation of the wall at the structure, zw, as shown in 

Equation 6-3: 

hmax = 1.3R * − zw = R − zw (6-3) 

where R* is the maximum tsunami runup elevation taken as the estimated 

maximum inundation elevation at the structure from a detailed numerical 

simulation model, or the ground elevation at maximum penetration of the 

tsunami from available tsunami inundation maps. The design runup 

elevation, R, is taken as 1.3 times the predicted maximum runup elevation, 

R*. The moment about the base of the wall can be evaluated using the line of 

action of the hydrostatic force resultant, as shown in Figure 6-2. 

Figure 6-2 Hydrostatic force distribution and location of resultant. 

6.5.3 Buoyant Forces 

Buoyant or vertical hydrostatic forces will act vertically through the centroid 

of the displaced volume on a structure or structural component subjected to 

partial or total submergence. The total buoyant force equals the weight of 

water displaced. Buoyant forces on components must be resisted by the 

weight of the component and any opposing forces resisting flotation. 

Buoyant forces are a concern for structures that have little resistance to 

upward forces (e.g. light wood frame buildings, basements, empty tanks 
72 6: Load Determination and Structural Design Criteria FEMA P646 

located above or below ground, swimming pools, components designed 

considering only gravity loads). 

For a watertight structure, the total buoyant force is given by Equation 6-4: 

Fb = ρs gV (6-4) 

where ρs is the fluid density including sediment (1200 kg/m3

 = 2.33 

slugs/ft3

), and V is the volume of water displaced by the building, i.e., the 

volume below the level of hmax as determined by Equation 6-3. Bouyant 

forces on an overall building are shown in Figure 6-3. If there is insufficient 

building weight to resist buoyant forces, tension piles may be used to 

increase the resistance to flotation, but reduction in pile side friction due to 

anticipated scour around the tops of the piles must be considered. 

R

DATUM

DESIGN RUNUP HEIGHT

Building

Weight

Fb

Pile Tension

Total Displaced

Volume, V

hmax

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

levels. 

6.5.4 Hydrodynamic Forces 

When water flows around a structure, hydrodynamic forces are applied to the 

structure as a whole and to individual structural components. These forces 

are induced by the flow of water moving at moderate to high velocity, and 

are a function of fluid density, flow velocity and structure geometry. Also 

known as drag forces, they are a combination of the lateral forces caused by 

the pressure forces from the moving mass of water and the friction forces 

generated as the water flows around the structure or component. 

Hydrodynamic forces can be computed using Equation 6-5: 

2

max

1 ( ) 2

F C B hu d sd = ρ (6-5) 
FEMA P646 6: Load Determination and Structural Design Criteria 73 

where ρs is the fluid density including sediment (1200 kg/m3 = 2.33 

slugs/ft3

), Cd is the drag coefficient, B is the breadth of the structure in the 

plane normal to the direction of flow (i.e. the breadth in the direction parallel 

to the shore), h is flow depth, and u is flow velocity at the location of the 

structure. For forces on components, B is taken as the width of the 

component. It is recommended that the drag coefficient be taken as Cd = 2.0. 

The resultant hydrodynamic force is applied approximately at the centroid of 

the wetted surface of the component, as shown in Figure 6-4. 

 

Figure 6-4 Hydrodynamic force distribution and location of resultant. 

The combination h u2 represents the momentum flux per unit mass. Note that 

(h u2

)max does not equal hmax u

2

max. The maximum flow depth, hmax , and 

maximum flow velocity, umax , at a particular site may not occur at the same 

time. The hydrodynamic forces must be based on the parameter (h u2

)max, 

which is the maximum momentum flux per unit mass occurring at the site at 

any time during the tsunami. 

The maximum value of (h u2

) can be obtained by running a detailed 

numerical simulation model or acquiring existing simulation data. The 

numerical model in the runup zone must be run with a very fine grid size to 

ensure adequate accuracy in the prediction of h u2

. 

The value (h u2

)max can be roughly estimated using Equation 6-6: 

( ) = −+

⎛ ⎞ ⎛ ⎞ ⎜ ⎟ ⎜ ⎟ ⎝ ⎠ ⎝ ⎠

2

2 2

max 0.125 0.235 0.11 z z hu gR R R

 (6-6) 

where g is the acceleration due to gravity, R is the design runup elevation, 

and z is the ground elevation at the base of the structure. The design runup 

elevation, R, is taken as 1.3 times the maximum runup elevation, R*, which is 

the maximum inundation elevation at the structure from a detailed numerical 

simulation model, or the ground elevation at maximum penetration of the 
74 6: Load Determination and Structural Design Criteria FEMA P646 

tsunami from available tsunami inundation maps. To use this formula, the 

sea level datum must be consistent with that used in the inundation maps. 

The basis of Equation 6-6 is described in Appendix E. Although the 

analytical solution is based on one-dimensional nonlinear shallow-water 

theory for a uniformly sloping beach, with no lateral topographical variation 

and no friction, the maximum value of (h u2

) obtained from Equation 6-6 can 

be used for: (1) preliminary design; (2) approximate design in the absence of 

other modeling information; and (3) to evaluate the reasonableness of 

numerical simulation results. 

R* and z can be obtained from tsunami inundation maps. Because of 

uncertainties in modeling tsunami inundation, numerically predicted values 

of (h u2

) should not be taken less than 80% of the values computed using 

Equation 6-6. 

6.5.5 Impulsive Forces 

Impulsive forces are caused by the leading edge of a surge of water 

impacting a structure. Ramsden (1993) performed comprehensive 

experiments on impulsive forces. Laboratory data show no significant initial 

impact force (impulse force) in dry-bed surges, but an overshoot in force is 

observed in bores that occur when the site is initially flooded. The maximum 

overshoot is approximately 1.5 times the subsequent hydrodynamic force, 

consistent with independent laboratory data obtained by Arnason (2005). 

Since impact momentum increases with the sudden slam of the steep front of 

a bore (Yeh, 2007), the lack of overshoot in dry-bed surge can be attributed 

to the relatively mild slope of the front profile of the water surface. If the 

runup zone is flooded by an earlier tsunami wave, subsequent waves could 

impact buildings in the form of a bore. Since the subsequent bore loading is 

greater than the initial dry-bed surge impact, dry-bed surge loading may not 

be critical. 

For conservatism, it is recommended that the impulsive forces be taken as 1.5 

times the hydrodynamic force, as shown in Equation 6-7: 

F F s d =1.5 (6-7) 

Impulsive forces will act on members at the leading edge of the tsunami bore, 

while hydrodynamic forces will act on all members that have already been 

passed by the leading edge, as shown in Figure 6-5. 
FEMA P646 6: Load Determination and Structural Design Criteria 75 

h

z

max R

DATUM

DESIGN RUNUP HEIGHT

Fs,c2

Fs,b2

Fd,c1 Fs,c1

Fd,b2

Fd,c2

F - Impulsive forces on columns and beams at leading edge of bore

d,c1

s,c1

F - Drag forces on columns and beams behind leading edge of bore

c1 and c2 - Columns at first and second levels. b2 - Beams at second level

Figure 6-5 Hydrodynamic impulsive and drag forces on components of a 

building subjected to inundation by a tsunami bore. 

6.5.6 Debris Impact Forces 

The impact force from waterborne debris (e.g., floating driftwood, lumber, 

boats, shipping containers, automobiles, buildings) can be a dominant cause 

of building damage. Unfortunately, it is difficult to estimate this force 

accurately. Background information on the development of the 

recommended impact force calculation is provided in Appendix D. 

The debris impact force can be estimated using Equation 6-8: 

F C u km i m = max (6-8) 

where Cm is the added mass coefficient, umax is the maximum flow velocity 

carrying the debris at the site, and m and k are the mass and the effective 

stiffness of the debris, respectively. It is recommended that the added mass 

coefficient be taken as Cm = 2.0. Unlike other forces, impact forces are 

assumed to act locally on a single member of the structure at the elevation of 

the water surface, as shown in Figure 6-6. 

z

R

DATUM

DESIGN RUNUP HEIGHT

W Fi

d

Figure 6-6 Waterborne debris impact force. 
76 6: Load Determination and Structural Design Criteria FEMA P646 

Debris impact forces should be evaluated considering the location of the 

vertical evacuation structure and potential debris in the surrounding area. 

For example, it is likely that floating debris would consist primarily of 

driftwood, logs and pier pilings for most coastal towns, whereas for some 

large port areas, the debris could be shipping containers. Locations near 

yacht marinas or fishing harbors should consider possible impact from boats 

that break their moorings. 

Use of Equation 6-8 requires the mass and stiffness properties of the debris. 

Approximate values of m and k for common waterborne debris are listed in 

Table 6-1. Mass and stiffness properties for other types of debris will need to 

be derived or estimated as part of the design process. 

Table 6-1 Mass and Stiffness Properties of Common Waterborne Debris 

Location of Source Mass (m) in kg 

Effective stiffness 

(k) in N/m 

Lumber or Wood Log 450 2.4 x 106

40-ft Standard Shipping Container 3800 (empty) 6.5x108

20-ft Standard Shipping Container 2200 (empty) 1.5x109

20-ft Heavy Shipping Container 2400 (empty) 1.7x109

The magnitude of the debris impact force depends on mass and velocity. 

Smaller (lighter) debris requiring little or no draft to float can travel at higher 

velocities than larger (heavier) debris requiring much larger depths to float. 

Use of maximum flow velocity without consideration of the depth required to 

float large debris would be unnecessarily conservative. The appropriate 

maximum flow velocity umax for a given flow depth can be obtained by 

running a detailed numerical simulation model or by acquiring existing 

simulation data. It is noted, however, that numerical predictions of flow 

velocities are less accurate than predictions of inundation depths, and the grid 

size for numerical simulations in the runup zone must be very fine in order to 

obtain sufficient accuracy in velocity predictions. 

When a suitable numerical simulation model is unavailable, the maximum 

flow velocity carrying lumber or a wooden log (with essentially no draft) can 

be estimated using the analytical solution for tsunami runup on a uniformly 

sloping beach with no lateral topographical variation, given by Equation 6-9: 

max 2 1 z u gR R

⎛ ⎞ = − ⎜ ⎟ ⎝ ⎠ . (6-9) 
FEMA P646 6: Load Determination and Structural Design Criteria 77 

where g is the acceleration due to gravity, R is the design runup height that is 

1.3 times the ground elevation R* at the maximum tsunami penetration, and z

is the ground elevation at the structure (the datum must be at the sea level). 

Background information on the development of this equation is provided in 

Appendix E. 

For a shipping container or other similar large debris with draft d, the ratio of 

the draft d to the maximum runup height R can be computed, and Figure 6-7 

can be used to estimate the maximum flow velocity. Draft d can be estimated 

using Equation 6-10: 

d = W

ρs g Af

 (6-10) 

where W is the weight of the debris, ρs is the fluid density including sediment 

(1200 kg/m3

 = 2.33 slugs/ft3), g is the acceleration due to gravity, and Af is 

the cross-sectional area parallel to the water surface such that the product d ×

Af represents the volume of water displaced by the debris. 

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

maximum runup elevation, R. The bottom curve represents the lower 

limit of maximum flow velocity. 

Based on the appropriate curve for d/R, and ratio between the elevation of the 

structure relative to the design runup elevation (z/R), Figure 6-7 will provide 

an estimate of the maximum flow velocity. It should be understood that 

Figure 6-7 is based on an analytical solution for tsunami runup on a 

uniformly sloping beach, with no lateral topographical variation, and no 

friction. Computed values may differ from the actual velocities, and 
78 6: Load Determination and Structural Design Criteria FEMA P646 

additional engineering evaluation and judgment should be considered. 

Background information on the development of Figure 6-7 is provided in 

Appendix E. 

When numerical models are used to determine the maximum flow velocity, 

umax, values should not be taken as less than 80% the analytical values 

predicted using Equation 6-9 or Figure 6-7. 

6.5.7 Damming of Waterborne Debris 

The damming effect caused by accumulation of waterborne debris can be 

treated as a hydrodynamic force enhanced by the breadth of the debris dam 

against the front face of the structure. Equation 6-11 is a modification of 

Equation 6-5 to include the breadth of the debris dam: 

Fdm = 1

2 ρs CdBd (hu2 )max (6-11) 

where ρs is the fluid density including sediment (1200 kg/m3

 = 2.33 

slugs/ft3

), Cd is the drag coefficient, Bd is the breadth of the debris dam, h is 

flow depth, and u is flow velocity at the location of the structure. It is 

recommended that the drag coefficient be taken as Cd = 2.0. 

The momentum flux (h u2

)max can be obtained by running a detailed 

numerical simulation model, acquiring existing simulation data, or estimated 

using Equation 6-6. Values of (h u2

) obtained from numerical simulation 

should not be taken as less than 80% of the values computed using Equation 

6-6. 

Since debris damming represents an accumulation of debris across the 

structural frame, the total debris damming force will likely be resisted by a 

number of structural components, depending on the framing dimensions and 

the size of debris dam. The debris damming force, Fdm, should be assumed to 

act as a uniformly distributed load over the extent of the debris dam. It 

should be assigned to each resisting structural component by an appropriate 

tributary width, and distributed uniformly over the submerged height of each 

resisting component. A minimum debris dam width of Bd = 40 feet (or 12 

m), representing a sideways shipping container or a mass of floating lumber, 

is recommended. The effects of debris damming should be evaluated at 

various locations on the structure to determine the most critical location. 

6.5.8 Uplift Forces on Elevated Floors 

Uplift forces will be applied to floor levels of a building that are submerged 

by tsunami inundation. In addition to standard design for gravity loads, these 
FEMA P646 6: Load Determination and Structural Design Criteria 79 

floors must also be designed to resist uplift due to buoyancy and 

hydrodynamic forces. When computing the buoyant forces on a floor slab, 

consideration must be given to the potential for increased buoyancy due to 

the additional volume of water displaced by air trapped below the floor 

framing system. In addition, exterior walls at the upper floor level will 

exclude water until their lateral resistance is exceeded by the applied 

hydrostatic pressure. This can significantly increase the displaced volume of 

water contributing to the buoyancy, as shown in Figure 6-8. 

The total upward buoyant force exerted on a floor system can be estimated 

using Equation 6-12: 

Fb = ρs g Af hb (6-12) 

where ρs is the fluid density including sediment (1200 kg/m3

 = 2.33 

slugs/ft3

), g is the acceleration due to gravity, Af is the area of the floor panel 

or floor framing component, and hb is the water height displaced by the floor 

(including potentially entrapped air). The value of hmax indicated in Figure 6-

8 should be determined using Equation 6-3. 

The upward buoyant force per unit area exerted to the floor system can be 

estimated using Equation 6-13: 

fb = ρs ghb (6-13) 

h

h

α

h

u s

max

b

f A = BxL

F

b

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

elevated floor. 

Hydrodynamic forces can also act vertically on floor slabs. During rapid 

inundation, rising water will apply uplift to the soffit of horizontal structural 

components, adding to the buoyancy uplift. The presence of structural walls 

and columns in a building will obstruct the tsunami flow passing through the 

building, and recent experiments have shown that this can result in 
80 6: Load Determination and Structural Design Criteria FEMA P646 

significant uplift forces on the floor slab immediately in front of the 

obstruction. It is recommended that the building structural layout be 

designed to minimize obstruction of tsunami flow through the lower levels of 

the building. 

Until further research results become available, the total uplift force on the 

floor system can be estimated using Equation 6-14: 

Fu = 1

2

Cu ρs Af uv

2 (6-14) 

where Cu is a coefficient (taken as 3.0), ρs is the fluid density including 

sediment (1200 kg/m3

 = 2.33 slugs/ft3), Af is the area of the floor panel or 

floor framing component, and uv is the estimated vertical velocity or water 

rise rate (adapted from American Petroleum Institute, 1993). 

The hydrodynamic uplift per unit area can be determined from Equation 

6-15: 

fu = 1

2

Cu ρs uv

2 (6-15) 

Unless a detailed hydrodynamic study is performed, the value of uv for the 

condition of sloping terrain below the building can be estimated using 

Equation 6-16: 

u u v = tanα (6-16) 

where u is the horizontal flow velocity corresponding to a water depth, hs

equal to the elevation of the soffit of the floor system, and α is the average 

slope of grade at the site, as shown in Figure 6-8. Using the maximum 

horizontal flow velocity, umax, in Equation 6-15 would be unnecessarily 

conservative since it may not correspond to a flow depth equal to the floor 

soffit elevation. The maximum horizontal velocity u in Equation 6-16 can 

also be estimated using Figure 6-7 by replacing d/R with hs/R. 

6.5.9 Additional Gravity Loads on Elevated Floors 

During drawdown, water retained on the top of elevated floors, as shown in 

Figure 6-9, will apply additional gravity loads that can exceed the loads for 

which the floor system was originally designed. The depth of water retained, 

hr, will depend on the maximum inundation depth at the site, hmax, and the 

lateral strength of the wall system at the elevated floor. It should be assumed 

that the exterior wall system will be compromised at some point so that water 

will inundate submerged floor levels. Because of the rapid rate of 
FEMA P646 6: Load Determination and Structural Design Criteria 81 

drawdown, it is likely that much of this water will be retained in the upper 

levels (at least temporarily) resulting in significant additional gravity load on 

the floor system. The maximum potential downward load per unit area, fr, 

can be estimated using Equation 6-17: 

f gh r sr = ρ (6-17) 

where ρs is the fluid density including sediment (1200 kg/m3

 = 2.33 

slugs/ft3

), g is the acceleration due to gravity, and hr is the maximum 

potential depth of water retained on the elevated floor determined using 

Equation 6-18: 

hh hh r b = −≤ max 1 w (6-18) 

where hmax is the maximum inundation level predicted at the site, h1 is the 

floor elevation above grade, and hbw is the maximum water depth that can be 

retained before failure of the wall due to internal hydrostatic pressure. 

For elevated floors without walls (such as a parking structure with open 

guardrails) water may remain on elevated floors until it has had time to drain 

off the structure. Drainage systems should be provided to ensure that the 

weight of retained water does not exceed the live load for which the floor is 

designed. 

hr

hmax

h1

u

Fr

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

by exterior walls during rapid drawdown. 

6.6 Combination of Tsunami Forces 

Not all tsunami load effects will occur simultaneously, nor will they all affect 

a particular structural component at the same time. This section describes 

combinations of tsunami forces that should be considered for the overall 

structure and for individual structural components. Other potential 
82 6: Load Determination and Structural Design Criteria FEMA P646 

combinations should be considered as needed, based on the particular siting, 

structural system, and design of the structure under consideration. 

6.6.1 Tsunami Force Combinations on the Overall Structure 

Tsunami forces are combined on the overall structure as follows: 

• Uplift due to buoyancy, Fb, and hydrodynamic uplift, Fu, have the effect 

of reducing the total dead weight of a structure, which may impact the 

overturning resistance. Buoyancy and hydrodynamic uplift appropriate 

for the design inundation level should be considered in all load 

combinations. 

• Impulsive forces, Fs, are very short duration loads caused by the leading 

edge of a surge of water impinging on a structure. As the surge passes 

through a structure, impulsive forces will be applied sequentially to all 

structural components, but not at the same time. Once the leading edge 

of the surge has passed a structural component, it will no longer 

experience the impulsive force, but rather a sustained hydrodynamic drag 

force, Fd. The total horizontal hydrodynamic force on a structure will 

therefore be a combination of impulsive forces on members at the 

leading edge of the surge, and drag forces on all previously submerged 

members behind the leading edge. Figure 6-10 shows how this 

combination would apply to a building with multiple columns and shear 

walls. The worst case lateral load will likely occur when the leading 

edge of the surge reaches the last components in the building frame. 

• Debris impact forces, Fi, are short duration loads due to impact of large 

floating objects with individual structural components. Since large 

floating objects are not carried by the leading edge of the surge, the 

effect of debris impact is combined with hydrodynamic drag forces, Fd, 

but not impulsive forces, Fs. Although many floating objects may impact 

a building during a tsunami event, the probability of two or more impacts 

occurring simultaneously is considered small. Therefore, only one 

impact should be considered to occur at any point in time. Both the 

individual structural component and the overall structure must be 

designed to resist the impact force in combination with all other loads 

(except impulsive forces). 

• Debris damming has the effect of increasing the exposed area for 

hydrodynamic loading. The debris damming force, Fdm, should be 

considered to act in the most detrimental location on a structure while 

hydrodynamic forces act on all other components of the structure. Figure 

6-11 shows typical debris dam locations that could be considered in 

conjunction with drag forces on all other submerged structural 

Not all tsunami load effects will 

occur simultaneously, nor will they 

all affect a particular structural 

component at the same time. 
FEMA P646 6: Load Determination and Structural Design Criteria 83 

components. It is conservative to ignore any shielding effect provided 

by the debris dam for components downstream of the dam. 

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

Figure 6-11 Debris dam and drag forces applied to an example building 
84 6: Load Determination and Structural Design Criteria FEMA P646 

• Breakaway walls are not part of the structural support of the building, 

and are intended, through design and construction, to fail under specific 

lateral loading. If lower level infill walls are designed as breakaway 

walls, the maximum lateral load will be the load at which the walls will 

“fail,” and the overall structure, as well as the structural components 

supporting these walls, must be designed to resist this failure load. 

Guidance on the design of break-away walls is provided in Chapter 7. 

• Design of floor systems to withstand the effects of potential retained 

water, Fr, can be performed independently of the lateral loading on the 

structure. 

6.6.2 Tsunami Force Combinations on Individual 

Components 

Tsunami forces are combined on individual structural components (e.g., 

columns, walls, and beams), as follows: 

• Impulsive force, Fs, due to the leading edge of the tsunami bore, for 

maximum h u2

. 

• Hydrodynamic drag force, Fd, plus debris impact, Fi, at the most critical 

location on the member, for maximum h u2

. 

• Debris damming, Fdm, due to a minimum 40-foot wide debris dam 

causing the worst possible loading on the member, for maximum h u2

. 

• Hydrostatic pressure, Fh, on walls enclosing watertight areas of a 

structure, for maximum h. 

For uplift on floor framing components, the following combinations of 

tsunami forces should be considered: 

• Buoyancy, Fb, of submerged floor framing components including the 

effects of entrapped air and upturned beams or walls, for maximum h. 

• Hydrodynamic uplift, Fu, due to rapidly rising flood waters, for flow 

velocity at a depth equal to the soffit of the floor system, hs. 

• Maximum uplift case: The larger of the above uplift loads combined with 

90% dead load and zero live load on the floor system, for design against 

uplift failure of floor slabs, beams, and connections. 

For downward load on floor framing components due to retained water, the 

following force combination should be considered: 

• Downward load due to water retained by exterior walls, fr, combined 

with 100% dead load. 
FEMA P646 6: Load Determination and Structural Design Criteria 85 

6.7 Load Combinations 

The load combinations presented herein are based on the guidance given in 

the Commentary of ASCE/SEI 7-05 Minimum Design Loads for Buildings 

and Other Structures (ASCE, 2006b), but are different from those used in 

model building codes or ASCE/SEI Standard 7-05. They have been 

reviewed in the development of this document, but have not been extensively 

studied. They should be considered in addition to all other load 

combinations provided by the current building code in effect, or Section 2 of 

ASCE/SEI 7-05. 

Tsunami forces that will act on the entire structure and on individual 

structural components should be calculated in accordance with Section 6.5 

and Section 6.6. The resulting member forces (Ts) should then be combined 

with gravity load effects using the following Strength Design Load 

Combinations: 

Load Combination 1: 1.2D + 1.0Ts + 1.0LREF + 0.25L

Load Combination 2: 0.9D + 1.0Ts 

where D is the dead load effect, Ts is the tsunami load effect, LREF is the live 

load effect in refuge area (assembly loading), and L is the live load effect 

outside of the refuge area. 

A load factor of 1.0 is used in conjunction with tsunami forces calculated in 

accordance with this document for the following reasons: (1) it is anticipated 

that the tsunami hazard level corresponding to the Maximum Considered 

Tsunami will be consistent with the 2500-year return period associated with 

the Maximum Considered Earthquake used in seismic design; (2) potential 

variability in tsunami runup elevations is explicitly considered by applying a 

30% increase to runup elevations used in tsunami force calculations; and (3) 

design for tsunami forces considers only the elastic response of components, 

without consideration of inelastic response and corresponding forcereduction factors (as is used in seismic design). 

Load Combination 1 considers the refuge area in the vertical evacuation 

structure to be fully loaded with assembly live load (e.g., 100 psf). The 

assembly live load represents a practical upper limit for the maximum 

density of evacuees standing in the refuge area. In combination with tsunami 

inundation, it is expected that all other floor areas will experience a reduced 

live load equal to 25% of the design live load. This reduced live load is 

consistent with live load reductions used in combination with earthquake 

Tsunami Load Combinations 

should be considered in addition to 

all other load combinations 

provided by the current building 

code in effect, or ASCE/SEI 7-05. 
86 6: Load Determination and Structural Design Criteria FEMA P646 

forces. When gravity load effects oppose tsunami load effects, Load 

Combination 2 applies. 

No additional importance factor, I, is applied to tsunami loads in this 

document. These design guidelines have been developed specifically for 

tsunami evacuation structures, and the critical nature of these structures has 

been considered throughout. 

Seismic loads are not considered to act in combination with tsunami loads. 

While aftershocks are likely to occur, the probability that an aftershock will 

be equivalent in size to the design level earthquake, and will occur at the 

same time as the maximum tsunami inundation, is considered to be low. 

6.8 Member Capacities and Strength Design 

Considerations 

Model building code provisions and engineering standards for Strength 

Design, also known as Load and Resistance Factored Design (LRFD), 

provide material-specific member capacity calculations and strength 

reduction factors for various force actions and different structural 

components. Until further research shows otherwise, it is recommended that 

capacity calculations and strength reduction factors be applied to design for 

tsunami loading in the same way they are currently applied to design for 

earthquake and wind loading. 

6.9 Progressive Collapse Considerations 

Reducing the potential for disproportionate (i.e., progressive) collapse due to 

the loss of one or more structural components will increase the likelihood 

that a vertical evacuation structure will remain standing if a column is 

severely damaged due to waterborne debris. The decision to include 

progressive collapse considerations in the design for a particular structure 

will depend on the site and the nature of the debris that could potentially 

impact the structure. Because the potential exists for localized severe 

damage due to debris impact, design for progressive collapse prevention is 

strongly encouraged. In the United States, primary design approaches for 

progressive collapse include the “tie force” strategy and the “missing 

column” strategy. 

6.9.1 Tie Force Strategy 

The Department of Defense has adopted an indirect tie force strategy to 

address the potential for progressive collapse in the design of facilities using 

UFC 4-023-03, Design of Buildings to Resist Progressive Collapse (2005). 

The tie force strategy is illustrated in Figure 6-12. 

Member Capacities and 

Strength Reduction Factors

should be applied to design for 

tsunami loading in the same way 

they are currently applied to design 

for earthquake and wind loading. 
FEMA P646 6: Load Determination and Structural Design Criteria 87 

Tension ties in reinforced concrete structures typically consist of continuous 

reinforcing steel in beams, columns, slabs, and walls, as shown in Figure 

6-13. Reinforcement required for tension ties can be provided in whole, or in 

part, by steel already sized to resist other actions, such as shear or flexure. 

In many cases, the quantity of steel provided to resist gravity and lateral 

forces for typical reinforced concrete structures is also sufficient to develop 

the necessary tie forces. 

Figure 6-12 Tie force strategy 

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

column 

Pos. Moment

Region

Neg. Moment 

Region 

Cont. Bottom Steel 

Provides Positive 

Moment Capacity 

Closely Spaced 

Stirrups Enhance 

Ductility 

Loss of Column
88 6: Load Determination and Structural Design Criteria FEMA P646 

It is reasonable to check tie force compliance after a structure is initially 

designed for gravity and lateral loading. Ties must be properly spliced and 

adequately anchored at each end in order to develop their full capacity and 

perform as anticipated. Reinforcing steel used as tension ties must have 

lapped, welded, or mechanically joined (Type 1 or Type 2) splices per ACI 

318, Building Code Requirements for Structural Concrete (ACI, 2005). 

Splices should be staggered and located away from joints and regions of high 

stress. 

Anchorage is critical to the performance of ties and must be carefully 

assessed, particularly in cases where building layout may be non-typical. 

Seismic detailing should be used to anchor ties to other ties, or at points of 

termination (such as at the perimeter of a building). This includes providing 

seismic hooks and seismic development lengths, as defined in ACI 318. 

6.9.2 Missing Column Strategy 

The General Services Administration (GSA) missing column strategy is an 

independent check performed without consideration of other loads. This 

approach is based on the concept that loss of a single column, in this case due 

to impact from waterborne debris, should not result in progressive collapse of 

the surrounding structural components. 

Current progressive collapse criteria are found in Progressive Collapse 

Analysis and Design Guidelines for New Federal Office Buildings and Major 

Modernization Projects (GSA, 2003). As illustrated in Figure 6-14, this 

strategy requires evaluation of surrounding structural components to continue 

to support anticipated gravity loads in a series of missing column scenarios. 

Live loads on the building are reduced to simulate those in place at the time 

the column is damaged. In the case of vertical evacuation structures, full live 

loads should be considered in the refuge area while reduced live loads can be 

considered elsewhere in the building. 

The missing column approach utilizes plastic design concepts in evaluating 

the capability of surrounding structural components to continue to support 

gravity loads, so some damage in these components is permitted as a result of 

a missing column scenario. Given that waterborne debris is most likely to 

impact an exterior or corner column, missing column scenarios should 

consider the potential loss of any single exterior column. Loss of interior 

columns need not be considered. 
FEMA P646 6: Load Determination and Structural Design Criteria 89 

Figure 6-14 Missing column strategy 

FEMA P646 7: Structural Design Concepts and Additional Considerations 91 

Chapter 7 

Structural Design Concepts 

and Additional Considerations

This chapter summarizes structural design concepts and other considerations 

relevant to the design of vertical evacuation structures, including retrofit of 

existing structures, permitting, peer review, quality control, planning issues, 

and potential cost impacts. 

7.1 Attributes of Tsunami-Resistant Structures 

Structural system selection and configuration, from foundation to roof 

framing, can have a significant effect on the ability of a vertical evacuation 

structure to withstand anticipated tsunami, earthquake, and wind loading. 

Many common structural systems can be engineered to resist tsunami load 

effects. 

Structural attributes that have demonstrated good behavior in past tsunamis 

include: (1) strong systems with reserve capacity to resist extreme forces; (2) 

open systems that allow water to flow through with minimal resistance; (3) 

ductile systems that resist extreme forces without failure; and (4) redundant 

systems that can experience partial failure without progressive collapse. 

Systems exhibiting these attributes include reinforced concrete and steel 

moment frame systems, and reinforced concrete shear wall systems. 

7.2 Structural Considerations for Tsunami Load 

Effects 

Foundation design must consider the local effects of scour and liquefaction. 

In many cases foundation support will consist of deep foundations (piles). 

Pile design must consider increased demands due to downdrag and additional 

lateral forces, and increased unbraced pile length due to scour. Potential 

uplift from the overall buoyancy of the structure needs to be accounted for in 

the foundation design. 

Design of individual columns for tsunami lateral loads should be performed 

assuming the appropriate degree of fixity at the column base and at each 

floor level. For example, a reinforced concrete column in a multi-story 

building supported by pile foundations can be assumed fixed at the base and 

at each floor level. A steel column forming part of a moment-resisting frame 

can be assumed pinned or fixed at the base and at each floor level. 

Tsunami-Resistant Structures 

have: 

(1) strong systems with reserve 

capacity to resist extreme forces; 

(2) open systems that allow water 

to flow through with minimal 

resistance; 

(3) ductile systems that resist 

extreme forces without failure; and 

(4) redundant systems that can 

experience partial failure without 

progressive collapse. 
92 7: Structural Design Concepts and Additional Considerations FEMA P646 

Column shape is also important. Round columns will result in lower drag 

forces than square or rectangular shapes. In addition, waterborne debris will 

be less likely to fully impact round columns. 

If shear walls are used, the plan orientation of the walls is important. It is 

recommended that the shear walls be oriented parallel to the anticipated 

direction of tsunami flow to reduce associated hydrodynamic forces and 

impact forces from waterborne debris. 

Design of reinforced concrete walls for tsunami forces should consider the 

full load on the wall, including hydrodynamic and debris impact forces, 

spanning vertically between floor levels. Reinforced concrete beams poured 

integral with the floor will be braced by the slab. Design of beams for 

horizontal tsunami forces should take into account the lateral bracing 

provided by the floor slab. Isolated beams must be designed for horizontal 

shear and bending induced by tsunami loads. 

Floor systems must be designed for the effects of buoyancy and 

hydrodynamic uplift, which will induce shear and bending effects that are 

opposite to those resulting from gravity loads. Even though lower levels of a 

vertical evacuation structure are not intended for use during a tsunami, 

failure could result in damage or collapse of columns supporting upper 

levels, including the tsunami refuge area. 

In structural steel floor systems, lateral torsional buckling of beam bottom 

flanges must be considered when subjected to uplift loading. In reinforced 

concrete floor systems, continuity of reinforcement should be provided in 

beams and slabs for at least 50% of both the top and bottom reinforcement. 

Prestressed concrete floor systems must be carefully checked for buoyancy 

and hydrodynamic uplift effects when submerged. Internal prestressing 

forces used to oppose dead loads add to these effects. Web elements of 

typical prestressed joist systems are susceptible to compression failure under 

uplift conditions, and many typical bearing connections are not anchored for 

potential net uplift forces. Localized damage to the concrete in a prestressed 

floor system can result in loss of concrete compressive capacity, and release 

of the internal prestressing forces. 

7.2.1 Foundation / Scour Design Concepts 

Scour around shallow foundations can lead to failure of the supported 

structural element. Foundations consisting of drilled shafts or driven piles 

can be designed to avoid this failure; however, they must be able to resist all 
FEMA P646 7: Structural Design Concepts and Additional Considerations 93 

applied loads after scouring has exposed the pile cap and top of the shafts or 

piles. 

Dames and Moore (1980) suggest that scour depth is related to distance from 

the shoreline and soil type. As indicated in Table 7-1, scour depth is 

estimated as a percentage of the maximum tsunami flow depth, d. 

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

(Dames and Moore, 1980) 

Soil Type 

Scour depth (% of d) 

(Shoreline Distance < 300 feet) 

Scour depth (% of d) 

(Shoreline Distance > 300 feet) 

Loose sand 80 60 

Dense sand 50 35 

Soft silt 50 25 

Stiff silt 25 15 

Soft clay 25 15 

Stiff clay 10 5 

Observations after the Indian Ocean Tsunami indicate that scour can occur 

significantly farther inland than 300 feet from the shoreline. Conservative 

engineering judgment should be exercised in categorizing the soil type at the 

site into the broad categories listed above. 

7.2.2 Breakaway Wall Concepts 

Solid enclosure walls below the tsunami inundation level will result in large 

tsunami loads on the overall building. These walls will also increase the 

potential for wave scour at grade beams and piles. Non-structural walls 

below the anticipated tsunami flow depth can be designed as breakaway 

walls to limit the hydrostatic, buoyancy, hydrodynamic, and impulsive forces 

on the overall building and individual structural members. Breakaway wall 

requirements are described in detail in the FEMA 55 Coastal Construction 

Manual (FEMA, 2005), which complies with National Flood Insurance 

Program (NFIP) requirements for construction in the mapped V-Zone. 

Breakaway walls can create wave reflection and runup prior to failure as 

indicated in Figure 7-1. 

In accordance with ASCE/SEI Standard 24-05 Flood Resistant Design and 

Construction (ASCE, 2006a), walls, partitions, and connections to the 

structure that are intended to break away are designed for the largest of the 

following loads acting perpendicular to the plane of the wall: 

• The wind load specified in ASCE/SEI Standard 7-05 Minimum Design 

Loads for Buildings and Other Structures (ASCE, 2006b). 
94 7: Structural Design Concepts and Additional Considerations FEMA P646 

Figure 7-1 Effect of breakaway walls on waves (FEMA, 2005). 

• The earthquake load specified in ASCE/SEI Standard 7-05. 

• 10 psf (0.48kN/m2). 

• Not more than 20 psf (0.6 kN/m2) unless the design meets the following 

conditions: (1) breakaway wall collapse is designed to result from a flood 

load less than that which occurs during the base flood; and (2) the 

supporting foundation and the elevated portion of the building is 

designed to resist collapse, permanent lateral displacement, and other 

structural damage due to the effects of flood loads in combination with 

other loads. 

Standard engineering practice can often result in considerable design 

overstrength, which would be detrimental to a breakaway wall system and 

the supporting structure. Care should be taken to avoid introducing 

unnecessary conservatism into the design. All components, including 

sheathing, siding, and window frame supports, must be considered in 

determining the actual strength of the breakaway wall system, and the 

resulting maximum load on the supporting structure. The most desirable 

fusing mechanism includes failure of the top and side connections while the 

bottom connection remains intact, allowing the wall panel to lay down under 

the tsunami flow without becoming detached and part of the debris flow. 

Metal Stud Walls. Metal stud infill walls are commonly used as part of the 

building envelope. Unless properly galvanized, metal studs will corrode 

rapidly in the coastal environment. Recent lateral load testing of typical 

metal stud wall configurations shows that ultimate failure occurs when the 

studs separate from either the top or bottom tracks. However, the load 

required to produce this failure is as much as four times the wind load for 

which the studs were initially designed. It is therefore necessary to introduce 
FEMA P646 7: Structural Design Concepts and Additional Considerations 95 

some sort of a “fuse” at the top track connection to ensure that the wall fails 

at a predictable load. Such a fuse might include a reduced stud section at the 

top of the studs. Testing of fuse mechanisms would be required to verify that 

they have the capacity needed to resist design loads, but will fail at 

predictably higher load levels. 

Masonry Walls. Masonry walls are commonly used as enclosures in lower 

levels of larger buildings. They can be restrained with the use of a dowel pin 

fuse system around the top and sides of the wall, without bonded contact to 

the structure. Such a system should be tested to verify that it will fail at 

predictable load levels that exceed design loads. If properly fused, the 

masonry wall will cantilever from the foundation and load will no longer be 

applied to the surrounding structural frame, upon failure of the dowel pins. 

To allow wall failure due to foundation rotation without damage to the 

remaining structure, separation of the wall foundation from the building 

foundation should be considered. 

7.3 Concepts for Modifying and Retrofitting Existing 

Structures 

It may not always be feasible to construct new buildings in an area that 

requires vertical evacuation refuge. Although retrofitting existing buildings 

to perform as a vertical evacuation structure could be expensive and 

disruptive to current users of the building, it may be the most viable option 

available. Existing buildings considered for use as vertical evacuation 

structures should possess the structural attributes listed in Section 7.1 that are 

associated with tsunami-resistant structures, and should be evaluated for 

tsunami load effects in accordance with Chapter 6. In the case of nearsource-generated tsunamis, existing buildings should also be evaluated for 

seismic effects. Because of the importance of vertical evacuation structures, 

and the need for these facilities to function as a refuge when exposed to 

extreme tsunami and seismic loading, reduced loading criteria for existing 

buildings, as is the current state-of-practice for seismic evaluation of existing 

buildings, is not recommended for evaluation of potential tsunami vertical 

evacuation structures. 

The following concepts can be considered in the modification and retrofit of 

existing buildings for use as vertical evacuation structures: 

• Roof system. Upgrade roof systems to support additional live loads 

associated with refuge occupancy. Protect or relocate existing building 

functions at the roof level (e.g., mechanical equipment) that would be at 

risk or unsafe in the immediate vicinity of high occupancy areas. Modify 

existing roof parapets for fall protection of refuge occupants. 

Existing buildings considered for 

use as vertical evacuation structures 

should possess the attributes of 

tsunami-resistant structures listed in 

Section 7.1 
96 7: Structural Design Concepts and Additional Considerations FEMA P646 

• Wall system. Consider modifying walls and wall connections in the 

lower levels of the building to perform as breakaway walls to minimize 

tsunami hydrostatic, hydrodynamic, and surge forces on the building. 

• Access. Modify ingress into the building and improve vertical 

circulation through the use of new entrances, ramps, and stairs. Consider 

placing access points on the outside of the building for ease of 

construction and high visibility. 

• Potential Debris. Remove or relocate building ground level functions 

that may become potential water-borne debris. 

• Existing hazards at the site. Consider and protect against other hazards 

that might exist at the building site, including other adjacent buildings 

that could collapse, and the presence of hazardous or flammable 

materials near the site. 

7.4 Permitting and Quality Assurance for Vertical 

Evacuation Structures 

7.4.1 Permitting and Code Compliance 

Before construction begins, all necessary state, local, building, and other 

permits should be obtained. Because model building codes and engineering 

standards do not address the design of a tsunami refuge specifically, design 

professionals should meet with building officials to discuss possible design 

requirements. 

In general, mechanical, electrical, and plumbing systems should be designed 

for the normal daily use of the facility, unless otherwise directed by the 

authority having jurisdiction. Designing these systems for the high 

occupancy load that would occur only when the structure is serving as a 

vertical evacuation refuge may not be necessary. 

7.4.2 Peer Review 

A vertical evacuation structure is a unique structure that must withstand 

special loads and load combinations. While earthquake, wind, and flood 

loading effects are well understood in the design and permitting process, 

consideration of tsunami load effects includes some new concepts and 

approaches. Considering the importance of vertical evacuation structures 

and the extreme nature of tsunami loading, peer review by a qualified 

individual or team is recommended. 

The unique nature of vertical 

evacuation structures may 

require special allowances for: 

(1) permitting and code compliance; 

(2) peer review; and 

(3) quality assurance. 
FEMA P646 7: Structural Design Concepts and Additional Considerations 97 

7.4.3 Quality Assurance / Quality Control 

Because a vertical evacuation structure must perform well during extreme 

loading conditions, quality assurance and quality control for the design and 

construction of the structure should be at a level above that for normal 

building construction. Design calculations and drawings should be 

thoroughly scrutinized for accuracy. 

The quality of both construction materials and methods should be ensured 

through the development and application of a quality control program. A 

quality assurance plan should be based on the Special Inspection 

Requirements listed in Chapter 17 of the International Building Code (ICC, 

2006). Special inspections and quality assurance provisions for primary 

seismic- and wind-resisting systems should be applied to tsunami-resisting 

elements of vertical evacuation structures. Exceptions that waive the need 

for quality assurance when elements are prefabricated should not be allowed. 

In addition to the building elements that are normally included special 

inspection programs, the following items require special attention: 

• Breakaway walls and their connections to structural components to avoid 

unintended conservatism in construction. 

• Other special components or details that are used to minimize tsunamiloading effects. 

• Piles, pilecaps and grade beams that will potentially experience the 

effects of scour. 

7.5 Planning Considerations for Vertical Evacuation 

Structures 

In addition to structural design, planning for vertical evacuation facilities 

should consider a number of issues, including access, parking, pets, 

occupancy limitations, and protection of critical functions. 

• Access and Entry. Confusion and panic will occur if evacuees arrive at 

a refuge facility, but cannot enter. Provisions should be made to ensure 

access in the event of a tsunami, while providing adequate security 

during times when the facility is unoccupied. Ideally, a vertical 

evacuation refuge should be configured so that it is always accessible, or 

can be entered without emergency personnel. 

• Americans with Disabilities Act (ADA). Vertical evacuation 

structures, when not operating as a refuge, must comply with Federal, 

state, and local ADA requirements and ordinances for the normal daily 

use of the facility. Design of ingress and vertical circulation within a 

Planning for vertical 

evacuation facilities should 

allow for: 

(1) access and entry; 

(2) Americans with Disabilities Act; 

(3) parking; 

(4) pets; 

(5) occupancy limitations; and 

(6) protection of critical functions. 
98 7: Structural Design Concepts and Additional Considerations FEMA P646 

vertical evacuation structure should consider the needs of disabled 

occupants to the extent possible, and the extent required by law, in the 

case of emergency evacuation. Given potential limitations on 

functionality of power sources and vertical conveyance systems (e.g., 

elevators and escalators) in the event of a near-source earthquake, 

disabled occupants may need assistance accessing refuge areas in vertical 

evacuation structures. 

• Parking. Parking at evacuation facilities can be a problem. Traffic 

congestion can adversely affect access to the facility, and parked vehicles 

can become waterborne debris that can damage the structure. Planning 

for vertical evacuation facilities should consider parking limitations. 

• Pets. Refuge facilities are typically not prepared to accommodate pets. 

Many people, however, do not want to leave their pets behind during a 

disaster. Planning should carefully consider the policy regarding pets. 

• Occupancy Limitations. Population density can be non-uniform, and 

can vary by time of day, week, or year. In the event of a tsunami, 

evacuation behavior of the surrounding population may result in an 

unequal distribution of evacuees among available refuge facilities. In 

determining the maximum occupancy for a refuge facility, the time of 

day, day of the week, or season of the year that will result in the largest 

number of possible evacuees should be considered. The maximum 

occupancy might need to be increased in order to accommodate 

unexpected additional occupants or visitors in the area. 

• Protection of Critical Functions. A vertical evacuation facility must be 

operational to serve its intended function in the event of a tsunami. 

Functions that are critical for operation as a short-term refuge, 

emergency response, medical care, or long-term sheltering facility must 

be protected from tsunami inundation, or located within the area of 

refuge. These might include emergency power, electrical equipment, 

communications equipment, basic sanitation needs, medical and 

pharmaceutical supplies, and emergency provisions (e.g., food, water, 

and supplies). 

7.6 Cost Considerations for Vertical Evacuation 

Structures 

Design of vertical evacuation structures for tsunami load effects will require 

more strength, ductility, and robustness than is necessary for normal-use 

structures. As recommended in this document, this can include the use of 

seismic detailing provisions, progressive collapse preventative measures, 

customized breakaway wall details, and deeper foundation systems. As such, 
FEMA P646 7: Structural Design Concepts and Additional Considerations 99 

it is expected that structural construction costs will be higher for vertical 

evacuation structures than for other structures. While there are no direct 

comparisons between the cost of a conventional structure versus the cost of a 

tsunami-resistant structure, order-of-magnitude information on potential 

structural construction cost increases can be obtained from currently 

available information. 

Structural costs, however, are only a fraction of total construction costs for a 

building. Depending on the nature of building occupancy and use, structural 

construction costs can range between 5% and 40% of total construction costs. 

Structural costs are a lower percentage of the total for occupancies with 

special uses (e.g., hospitals) requiring more expensive nonstructural systems 

and contents, and are higher percentage of the total for occupancies with 

standard uses (e.g., offices). 

Anecdotal evidence from design and construction of essential facilities (e.g., 

hospitals) in California, Oregon, and Washington indicate that the cost 

premium for seismic design requirements associated with essential facilities 

versus ordinary occupancy facilities is on the order of 10% to 20% of 

structural construction costs. This would represent an increase on the order 

of 1% to 8% in terms of total construction costs. 

In a recent study funded by the National Institute of Standards and 

Technology, Engineering Design and Cost Data for Reinforced Concrete 

Buildings for Next Generation Design and Economic Standards for 

Structural Integrity (NIST, 2007), the cost premium for progressive collapseresistant design was on the order of 10% to 20% of structural construction 

costs. Similar to seismic design, this would represent an increase on the 

order of 1% to 8% in terms of total construction costs. 

Considering additional allowances for added strength to resist tsunami load 

effects, it is reasonable to expect that a tsunami-resistant structure, including 

seismic-resistant and progressive collapse-resistant design features, would 

experience about a 10% to 20% order-of-magnitude increase in total 

construction costs over that required for normal-use buildings. While each 

project will be unique, and relative costs will depend on the specific tsunami 

hazard and site conditions, it should not be assumed that incorporation of 

tsunami-resistant design features in a vertical evacuation structure will be 

cost prohibitive. 

Structural construction costs are only 

a fraction of total construction costs 

for a building. 

Tsunami-resistant structures could 

experience about a 10% to 20% 

order-of-magnitude increase in total 

construction costs over that required 

for normal-use buildings. 

FEMA P646 A: Vertical Evacuation Structure Examples from Japan 101 

Appendix A 

Vertical Evacuation Structure 

Examples from Japan 

In Japan there are examples of structures that were designed and constructed 

specifically for the purpose of tsunami refuge. It appears that no formal 

guidance for design of these structures is available. 

Life-Saving Tower: The Life-Saving Tower (Tasukaru Tower) developed 

by Fujiwara Industries Company, Limited, Japan, is shown in Figure A-1. 

This is a simple and economical structure that enables a temporary high 

refuge for evacuees. The structure has a 5.4-meter span between the 

supporting posts, a refuge elevation of 5.8 meters from ground level, and a 

capacity of 50 people. 

Figure A-1 Life-Saving Tower 
102 A: Vertical Evacuation Structure Examples from Japan FEMA P646 

Nishiki Tower: The Nishiki Tower, shown in Figure A-2, was constructed 

in the town of Kise, Mie Prefecture, Japan. The five-story, 22-meter tall 

reinforced concrete structure resembles a lighthouse, and has a spiral 

staircase winding up the outside of the building. It was specifically designed 

to serve as a tsunami refuge, but is used for other (non-refuge) purposes on 

normal days. The first floor is used for public toilet and storage space for 

fire equipment; the second floor for a meeting room; and the third floor for 

an archival library for natural disasters. The fourth and fifth floors have 73 

square meters of refuge space for evacuees. 

Figure A-2 Nishiki Tower. 

Nishiki Tower is a well-engineered structure that is designed to withstand a 

seismic event commensurate to JMA VII on the Japanese earthquake 

intensity scale that is comparable to a MMI XII (modified Mercalli scale). 

The building is founded on a 4-meter deep sand-and-gravel layer, and is 

supported on concrete piles extending 6 meters below grade. The possibility 

of liquefaction is remote, considering the large particle size of the sand-andgravel layer. Elastic design was employed for consideration of tsunami 

forces. Based on historical data from the 1944 Tou-Nankaido Earthquake, a 

design tsunami of 6 meters in height was used for design. It is designed to 

withstand the impact of a 10-ton ship at a velocity of 10 m/sec. This 
FEMA P646 A: Vertical Evacuation Structure Examples from Japan 103 

criterion was based on size of ships moored in the neighboring port. The 

intended performance level allows for partial damage of the building without 

incurring loss of life. 

Elevated Shelter at Shirahama Beach Resort: A rather aesthetic tsunami 

refuge was constructed at a beach resort in the town of Shirahama, 

Tokushima Prefecture, shown in Figure A-3. It is designed to accommodate 

700 refugees in the area of 700 square meters. The design inundation 

elevation is 7.5 meters, based on historical data from the 1854 Ansei-Tokai 

Earthquake (M 8.4) and resulting tsunami. With a planned freeboard of 4 

meters, the evacuation platform is located at elevation of 11.5 meters. This 

reinforced concrete structure is designed to withstand a maximum base 

acceleration of 780 gal. Because of a potential for soil liquefaction, pipe 

piles were driven approximately 20 meters deep into bedrock. The facility is 

also equipped with a solar-powered lighting system. 

Figure A-3 Refuge at Shirahama Beach Resort (Photo courtesy of N. Shuto). 

Other Tsunami Refuge Structures: There are other structures in Japan 

specifically designed as tsunami refuges. A reinforced concrete structure in 

the town of Kaifu, Tokushima Prefecture, Japan is shown in Figure A-4. An 

artificial high ground (berm), shown in Figure A-5, was constructed in 

Aonae, Okushiri-Island, Japan, where the 1993 tsunami struck the hardest. 

After the 1993 Okushiri Tsunami, Aonae elementary school, shown in Figure 

A-6, was reconstructed as a tsunami resistant structure. The upper floors can 
104 A: Vertical Evacuation Structure Examples from Japan FEMA P646 

be used as tsunami refuge spaces. The ground floor of the school is 

constructed with breakaway walls to relieve tsunami forces. 

Figure A-4 Tsunami refuge in Kaifu, Japan. 

Figure A-5 Berm constructed for tsunami refuge in Aonae, Japan. 
FEMA P646 A: Vertical Evacuation Structure Examples from Japan 105 

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

tsunami refuge space. 

FEMA P646 B: Community Design Example 107 

Appendix B 

Community Design 

Example 

A hypothetical community is indicated in Figure B-1 below. In this 

appendix, the initial design and configuration of a series of vertical 

evacuation structures is illustrated. 

The community has evaluated public and private sites that might be 

appropriate for construction of new vertical evacuation structures and 

identified existing facilities for possible renovation for use as vertical 

evacuation structures. This evaluation includes consideration of the number 

of sites required based on travel time and population, as discussed in Chapter 

5. 

Figure B-1 Hypothetical sketch of example community showing potential 

vertical evacuation structure sites and evacuation routes. 
108 B: Community Design Example FEMA P646 

An assessment of the tsunami inundation depths and flow velocities is 

necessary for assessing tsunami effects within the community and 

determining tsunami design parameters. Predicted tsunami inundation depths 

for this example community are shown in Figure B-2. 

Figure B-2 Example community inundation map. Shaded areas show 

various predicted tsunami inundation depth, d. 

In this example community, the area of refuge at each site would need to be 

elevated as indicated in Table B-1. 

Table B-1 Design Elevations for Areas of Refuge 

Site 

Predicted 

Inundation 

Depth 

Freeboard 

(3 meters plus 30%) 

Design 

Elevation 

Site 1 3 m 3 m + 0.9 m 6.9 m 

Site 2 4 m 3 m + 1.2 m 8.2 m 

Site 3 3 m 3 m + 0.9 m 6.9 m 

Site 4 4 m 3 m + 1.2 m 8.2 m 

Site 5 3 m 3 m + 0.9 m 6.9 m 
FEMA P646 B: Community Design Example 109 

Tsunami inundation depths indicated in Figure B-2 are increased by 30% to 

account for local variability in numerical simulations. An additional 

minimum freeboard of 3 meters (or one-story height) is recommended to 

ensure that the area of refuge is not inundated from splash or wave action. 

The velocity at a particular site is affected by the surrounding topography as 

well as natural and man made obstructions to flow. Predicted flow velocities 

for this example community are shown in Figure B-3 and summarized in 

Table B-2. 

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

areas show various predicted tsunami flow velocities, u. 

Table B-2 Tsunami Flow Velocity at Each Site 

Site Tsunami Flow Velocity 

Site 1 9 m/s 

Site 2 12 m/s 

Site 3 9 m/s 

Site 4 12 m/s 

Site 5 9 m/s 
110 B: Community Design Example FEMA P646 

B.1 Site 1 Example: Escape Berm 

Site 1 has several unique conditions to consider. The waterfront in this area 

is somewhat industrial in nature and includes a container terminal facility at 

the harbor. Areas adjacent to the site contain some residential development. 

The evacuation population at this site would include both employees of the 

harbor industrial area and adjacent residences. 

The community has been struggling with finding ways to address other social 

issues in this area, which have included a lack of recreational facilities for the 

residents, some neglected and deteriorating properties, and a need to 

revitalize and enhance the area. At this site a man-made berm, as shown in 

Figure B-4, provides an opportunity to add new public open space in addition 

to vertical evacuation refuge. This solution creates a unique elevated park 

setting for the community, which addresses recreational needs, and provides 

a scenic overlook for the waterfront. 

With a location adjacent to a container terminal facility, there is a potential 

for shipping containers to become waterborne debris. Construction of the 

berm utilizing a sheet piles to contain the fill addresses this issue. 

Figure B-4 Example escape berm design 

The features of this escape berm, illustrated in Figure B-5, include the 

following: 

• Location 1 (Figure B-5). The semi circular configuration was selected to 

help divert tsunami flood waters and potential waterborne debris around 

the facility and away from the access stairs and ramp. The elevated area 

is over 31,000 square feet, and can handle over 3,000 evacuees at 10 
FEMA P646 B: Community Design Example 111 

square feet per person. There is sufficient space in the elevated area to 

accommodate a comfort station that could be used for both day to day 

recreational purposes and emergency use. 

Figure B-5 Example escape berm plan layout 

• Location 2 (Figure B-5). The ocean facing side of the berm is essentially 

vertical to prevent tsunami flood waters and potential floating debris 

from moving upslope into the area of refuge. Trees and other 

landscaping can be used to hide the vertical face and create an 

aesthetically appealing feature. 

• Location 3 (Figure B-5). The sides of the berm can be sloped to provide 

additional access to the area of vertical refuge. Care should be taken to 

orientate the slope so that water and debris are not inadvertently 

channeled upslope. 

• Locations 4 and 5 (Figure B-5). Stairs and ramps provide primary 

access for both recreational and emergency purposes. 

Additional considerations are illustrated in Figures B-6 and B-7 and 

described below. 

Figure B-6 Example escape berm section 
112 B: Community Design Example FEMA P646 

Figure B-7 Example escape berm rear elevation 

• Location 1 (Figure B-6). Where the elevated area is adjacent to a steep 

drop off, guard rails or walls of appropriate size and height should be 

provided for fall protection. Using a solid wall for the guardrail will 

have the added benefit of providing additional protection from tsunami 

runup or splash onto the area of refuge. Walls can be configured to 

divert splash away from the wall. 

• Location 2(Figure B-6). Materials used to help create the berm will 

need to be constructed deep enough below existing grade to ensure that 

retaining system is not undermined by scour around the perimeter of the 

berm. 

• Location 3(Figure B-7). With sufficient length, both ADA compliant 

ramps and stairs can be provided. This will address both the day to day 

recreational use of the facility as well as emergency evacuation needs. 

Sloped surfaces on the sides of the berm can be used to provide 

additional access, and can also help channel floating debris away from 

the base of the ramps and stairs to minimize the risk of blockage. 

B.2 Site 2 Example: Multi-Use Structure 

Site 2 is situated on property managed by the school district. The site is 

located adjacent to an existing school and the surrounding area contains a 

combination of residential and business use. The existing school is located 

well within the inundation zone. The waterfront in this area includes an ongrade parking lot that services businesses in the area, and a nearby oceanfront 

park. The evacuation population at this site would include children attending 

the school, neighbors in the adjacent residences, employees of nearby 

businesses, and nearby users of the oceanfront park. 

The school district has had an ongoing need for a covered gymnasium. At 

this site, the community has decided to incorporate the roof of the proposed 

gymnasium into its emergency planning. It is decided that this new structure 

will be designed to meet the requirements for a vertical evacuation structure 
FEMA P646 B: Community Design Example 113 

to serve two important community needs. The structure is illustrated in 

Figure B-8. 

Located adjacent to an on-grade parking lot, the structure will need to be 

designed for potential impacts from floating vehicles. If the community is 

located in a climate that requires the gymnasium to be enclosed, special 

attention should be paid to the design of the exterior wall system. Walls 

should be detailed as breakaway walls to minimize tsunami loading on the 

overall structure. Otherwise the structure will need to be designed to for the 

corresponding increased hydrostatic, hydrodynamic, and impulse loads. 

As a school facility, the building must also be designed to address typical 

health and safety requirements for school facilities in normal use (when not 

serving as a vertical evacuation refuge). 

Figure B-8 Example gymnasium 

Features of this multi use structure, illustrated in Figure B-9 and Figure 

B-10, include the following: 

• Location 1 (Figure B-9). The rectangular layout is selected based on the 

gymnasium requirements for the school. The elevated area is over 

10,000 square feet in size, and can handle over 1,000 evacuees at 10 

square feet per person. Using available census information, it has been 

determined that this should be sufficient for the surrounding area this 

facility is intended to serve. 
114 B: Community Design Example FEMA P646 

• Location 2 (Figure B-9). Stair access is designed using a concrete 

encased stair structure that will have its own inherent strength. The 

shape is intended to channel tsunami flow and potential debris away 

from both the structure and the stair system. 

• Location 3 (Figure B-9). An additional ADA accessible ramp system is 

considered for a future phase of the project. This could utilize sheet piles 

and fill to further channel tsunami flow and waterborne debris away from 

the structure. 

Figure B-9 Example gymnasium plan 

Figure B-10 Example gymnasium elevation 
FEMA P646 B: Community Design Example 115 

• Location 4 (Figure B-10). The structural system utilizes a concrete 

moment frame to create an open lower level that will keep hydrodynamic 

loads on the structure to a minimum. This includes using circular shaped 

columns. 

• Location 5 (Figure B-10). Additional strength can be provided in the 

system by using walls that parallel the anticipated direction of the 

tsunami inundation flow. 

• Location 6 (Figure B-9). The stairs structures can be integrated with the 

primary structure to provide additional strength, or they can be made 

structurally independent. 

FEMA P646 C: Example Calculations 117 

Appendix C 

Example Calculations 

A rectangular-shaped tsunami evacuation structure, 10 m wide, is constructed 

at a site 200 m from the shoreline, where the elevation is 4 m from the sea 

level. The local beach slope is 1/50 and there is no significant alongshore 

variation in the topography; hence, it is reasonable to assume a plane beach 

with a 1/50 slope. The tsunami inundation map indicates the elevation R* = 

10 m at the maximum inundation point (runup height of 10 m at the location 

500 m from the shoreline). A log (8.53 m long, 0.35 m in diameter, and 450 

kg mass) is considered as the design waterborne missile for the impact 

loading. In addition, the impact loading of a 40-ft shipping container (40 ft L 

x 8 ft W x 8-1/2 ft H: or 12.2 m x 2.44 m x 2.59 m) is estimated to be 30,000 

kg (30 tons). A definition sketch for these example calculations is provided 

in Figure C-1. 

R* = 10 m

z = 4 m

datum

Figure C-1 Definition sketch for example calculations: R* is the maximum 

runup elevation (the maximum inundation distance is 500 m) and 

z is the elevation at the location of the tsunami evacuation 

structure (located 200 m from the shoreline). Two horizontal lines 

represent the initial water level and the maximum inundation 

level, respectively. 

C.1 Inundation Depth 

The recommended design runup height, R, is 30% greater than the predicted 

maximum runup elevation, R*, to account for local amplification and 

uncertainty in the predicted value, i.e., R = 1.3 R* = 13 m. Therefore, the 

design inundation depth at the structure is 13 – 4 = 9 m. A freeboard of 3 m 

(10 ft) is recommended; therefore, the refuge area must be located higher 

than 9 + 3 = 12 m above the ground level. If the typical floor height is 4 m, 

then the refuge area should be located on the 4th floor or higher. 
118 C: Example Calculations FEMA P646 

C.2 Hydrostatic and Buoyant Forces 

It is recommended that all nonstructural walls at the lower levels of the 

building be designed as breakaway walls. In that case, the hydrostatic forces 

and potential uplift of the overall building are not important. However, if the 

structure, or any portion of the structure, is constructed watertight at the 

lower levels, then the wall panels must be designed for the anticipated 

hydrostatic pressure. The maximum force acting on a wall panel of 4-m wide 

and 3-m tall on the ground floor can be computed using Equation 6-2. Since 

the wall panel on the ground floor is fully submerged: 

Fh = ρs g R − (z + Δz) − hW

2

⎛

⎝

⎜ ⎞

⎠

⎟ hW b

= 1200 kg m3 ( ) 9.81m sec2 ( ) 1.3 × 10m − (4 m + 0.5m) − 3m

2

⎛

⎝

⎜ ⎞

⎠

⎟( ) 3m ( ) 4 m

= 989 kN

where Δz is the height at the toe of the wall panel from the ground level, 

assumed to be 0.5 m. Note that the fluid density ρ = 1.2 ρwater is used 

assuming a mixture of seawater and sediment. 

With the water level at 9 m at the building location, the first and second 

floors will be submerged. Assuming the nonstructural walls have broken 

away at these two levels, but not yet at the third level, then the uplift due to 

buoyancy acting on the third floor should be evaluated. Assuming plan 

dimensions of 5 m by 5 m for a typical floor panel on the third floor, and a 

floor elevation of 7 m above the ground level, as shown in Figure C-2, then 

the upward buoyant force can be computed using Equation 6-4: 

kN

mmmmkg mmm

hgAF bfsb

589 

)7)4103.1)((55)(sec/81.9)(/1200( 3 2

=

= −−××

= ρ

where hb is the water height displaced by the floor including the effect of air 

trapped below the floor, as shown in Figure C-2. 

C.3 Hydrodynamic and Impulsive Forces 

Hydrodynamic drag and impulse forces are exerted on the building as a 

whole, assuming no breakaway walls at the lower levels. The maximum 
FEMA P646 C: Example Calculations 119 

7m

9m

2m

5m

Figure C-2 Condition resulting in buoyant forces 

value of h u2 at the site can be computed using Equation 6-6, with z = 4 m, R 

= 13 m and g = 9.81 m/sec2

:

h u2 ( )max = g R2 0.125 − 0.235 z

R

+ 0.11 z

R

⎛

⎝

⎜ ⎞

⎠

⎟

2 ⎛

⎝

⎜

⎞

⎠

⎟ = 105 m

3 sec2

Hence, from Equation 6-5 the fluid force is: 

kN

mkg mm

d ds huCF

1260 

)sec/105)(10)(0.2)(/1200( 2

1 

)( 2

1

3 23

max

2

=

=

= ρ

where B = 10 m (shelter width), and Cd = 2.0. If the worst-case tsunami 

arrives at a previously flooded site, then the tsunami front may form a bore. 

The impulsive force for this condition would be 1.5 times the hydrodynamic 

force, as in Equation 6-7: 

Fs = 1.5Fd = 1890 kN

If the nonstructural walls at the lower level are designed to break away 

during a tsunami, then the hydrodynamic drag and impulse forces would be 

computed for all individual structural members (e.g., columns, shear walls) 

and combined as shown in Figure 6-10. 
120 C: Example Calculations FEMA P646 

C.4 Impact Force 

The maximum flow velocity at the site can be estimated using R = 13 m in 

Equation 6-9:

umax = 2 g R 1 − z

R

⎛

⎝

⎜ ⎞

⎠

⎟

= 2 g(13m) 1 − 4 m

13m

⎛

⎝

⎜

⎞

⎠

⎟ = 13.3m sec.

Note that this flow velocity is at the leading tongue of the flow where the 

flow depth is nil. Hence, this value of approximately 48 km/hr (30 mph) will 

be conservative. Using this conservative velocity estimate, the impact force 

due to a floating log can be computed by Equation 6-8, with Cm = 2.0, k = 2.4 

x 106

 N /m, and m = 450 kg: 

Fi = Cm umax k m

= 2.0 13.3 ( ) m sec 2.4 ×106 ( ) N m ( ) 450 kg

= 874 kN

This force would be applied locally at the assumed point of impact. 

If the assumed draft, d, of the log is 0.25m, then the velocity is evaluated 

using Figure 6-7. Using the ratios ζ = z/R = 0.31, and the flow depth, d/R = 

0.019, at the location of the site: 

umax

2 g R = 0.53

umax = 0.53 2 9.81 ( )(13) = 8.5 m sec

The impact force is then: 

Fi = Cm umax k m

= 2.0 8.5 ( ) m sec 2.4 ×106 ( ) N m ( ) 450 kg

= 560 kN

which is more realistic than the previous estimate (874 kN). The total force 

on the structure at the time of the impact can be determined conservatively 

by combining this impact force with the hydrodynamic drag force 

determined earlier: 
FEMA P646 C: Example Calculations 121 

Fi + Fd = 560 + 1260 = 1820 kN

To compute the impact force due to a floating shipping container, the draft, 

d, must be estimated: 

d = W

ρ g Abox

= ( ) 30000 kg g

1200 kg m3 ( )g ( ) 12.2 m × 2.44 m = 0.84 m

where W is the weight and Abax is the cross sectional area of the box in the 

horizontal plane, and the constant g cancels out. The maximum flow velocity 

that supports draft, d = 0.84 m, can be found from Figure 6-7. At the location 

of the site, ζ = z/R = 0.31, and the flow depth, d/R = 0.065. Figure 6-7 shows 

umax along the limit curve at 

ζ = 0.31. Hence, the maximum velocity is: 

umax = 0.15 2 g R = 2.4 m sec.

The impact force due to the shipping container is computed by Equation 6-8 

with Cm = 2.0, k = 2.4 x 106

 N /m, and m = 30000 kg: 

Fi = Cm umax k m

= 2.0 2.4 ( ) m sec 2.4 ×106 ( ) N m ( ) 30000 kg

= 1290 kN

The total force on the structure at the time of the impact can be determined 

conservatively by combining this impact force with the hydrodynamic drag 

force determined earlier: 

Fi + Fd = 1290 + 1260 = 2550 kN

C.5 Damming Effect of Waterborne Debris 

The damming effect of debris can be computed using Equation 6-11, which 

is readily obtained from the hydrodynamic force computed earlier, 

substituting the recommended debris dam width of 12 m (40 ft): 

Fdm = ( ) 1260 kN ×

12 m

10 m

⎛

⎝

⎜

⎞

⎠

⎟ = 1510 kN
122 C: Example Calculations FEMA P646 

If the building were wider than 12 m, then the damming effect should be 

considered at various locations as shown in Figure 6-11 to determine the 

worst condition for loading on the structure as a whole, and on individual 

structural elements. 

C.6 Hydrodynamic Uplift Forces 

The hydrodynamic uplift force can be computed using Equation 6-14. 

Assuming that the water depth at the soffit of the second floor is hs = 3 m, 

and at the location of the shelter site, ζ = z/R = 0.31, and the flow depth, d/R

= hs/R = 0.23, Figure 6-7 shows u along the limit curve at ζ = 0.31. Hence, 

the maximum velocity is: 

u = 0.15 2 g R = 2.4 m sec.

The vertical velocity can be computed using Equation 6-16, assuming the 

slope at the site is 1/50: 

uv = u tanα = (2.4)(1 50)= 0.048 m sec

Hence, the hydrodynamic uplift force given by Equation (6-14) is: 

Fu = 1

2

Cu ρs Af uv

2

= 1

2

( )3 1200 kg m3 ( )( ) 5 m × 5 m ( ) 0.048 m sec

2

= 103N

which is insignificant for the beach slope assumed in this example. If a 

beach slope of 1/5 is assumed, the hydrodynamic uplift force increases to 

10.3 kN. 
FEMA P646 D: Background Information on Impact Load Calculations 123 

Appendix D 

Background Information on 

Impact Load Calculations 

D.1 Available Models for Impact Loads 

The impact force from waterborne missiles (e.g., floating driftwood, lumber, 

boats, shipping box containers, automobiles, buildings) can be a dominant 

cause of building destruction. Unfortunately, it is difficult to estimate this 

force accurately. Unlike the other forces, the impact force occurs locally at 

the point of contact when the debris is smaller than the building. Impact 

forces can be assumed to act at or near the water surface level when the 

debris strikes the building. Most available models are based on the impulsemomentum concept, in which the impulse of the resultant force acting for an 

infinitesimal time is equal to the change in linear momentum: 

( ) 0

I F dt d mu ; 0 τ== →τ ∫ (D-1) 

where: 

I = impulse 

F = resultant force 

m = mass of water-borne missile 

u = velocity of the missile 

t = time 

For actual computations, a small but finite time, Δt (not infinitesimal), and 

the average change in momentum are used as an approximation. There is 

significant uncertainty in evaluating the duration of impact, Δt. The 

following are available formulae for missile-impact force estimation. 

Matsutomi (1999). Matsutomi experimentally investigated the impulse 

forces of driftwood. He performed two sets of experiments: one in a small 

water tank and the other for full-scale impact in air. In his small water tank, a 

bore and a surge were generated (a bore is a moving hydraulic jump onto a 

quiescent shallower water in front of it, while a surge is a moving water body 

onto a dry bed). A scaled-down driftwood model was placed 2.5 m upstream 

from the receiving wall. The model driftwood was picked up by the 
124 D: Background Information on Impact Load Calculations FEMA P646 

generated bore (or surge) and impacted onto the receiving vertical wall. His 

full-scale impact experiments were conducted to compensate for potential 

scale effects in his small-scale experiments. A full-scale log was tied at the 

end of a pendulum and was swung against the stationary stop equipped with 

the load cell. It is noted that this impact condition in the air may significantly 

differ from an actual waterborne case because of the absence of the added 

mass effect of water: prior to the impact, the waterborne missile is carried by 

the surrounding water flow and the momentum of the water must contribute 

to the impact force. Matsutomi compensated for the added mass effect with 

the data obtained from the small-scale water tank experiments. Based on a 

regression analysis of the large amount of data, Matsutomi proposed 

Equation D-2 for the impact force, F: 

1.2 0.4

2 1.6 f

M

w w

F u C

D L g D L

σ

γ γ

⎛ ⎞ ⎛ ⎞ = ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎝ ⎠ ⎝ ⎠ (D-2) 

where: 

γw = the specific weight of the log, 

D and L = the diameter and the length of the log, 

respectively, 

CM = the added-mass coefficient, 

u = the velocity of the log at impact, and 

σf = the yield stress of the wood. 

Matsutomi recommended σf = 20 × 106

 Pa for a wet log. From small-scale 

experimental data, he recommended a value of CM ≈ 1.7 for a bore or surging 

condition, and CM ≈ 1.9 for a steady flow. Note that the recommended values 

of CM are the upper limit when more than 60% of the receiving wall is open 

and permeable. The value of CM is smaller when the receiving wall does not 

allow the flow to pass through the receiving wall. For a solid (impermeable) 

receiving wall, Matsutomi found that CM = 0.5 for a bore and CM = 1.1 for a 

surging flow. Note that in the case of a bore striking an impermeable wall 

(i.e., no flow-through), CM is less than unity (= 0.5). This is because the flow 

reflection at the wall actually reduces the impact force. 

In spite of a thorough study with a large amount of laboratory data, the 

derived form of Equation D-2 is inconvenient due to the particular choice of 

the scaling parameters, and it is only applicable to driftwood or logs. 

Ikeno et al. (2001; 2003). Laboratory experiments similar to Matsutomi 

(1999) were performed to examine the impact forces of the objects other than 

driftwood or logs. They used cylindrical, square column, and spherically-
FEMA P646 D: Background Information on Impact Load Calculations 125 

shaped drift bodies. Note that unlike Matsutomi’s experiments, Ikeno et al. 

only examined the impact onto an impermeable vertical wall. The following 

empirical formula was derived based on small-scale experiments 

(approximately 1/100 model):

2.5

M

F u S C

g m g DL

⎛ ⎞

= ⎜ ⎟

⎜ ⎟ ⎝ ⎠

 (D-3) 

where: 

S = a constant (equal to 20 for a bore case), 

CM = the added mass coefficient, 

m = the mass of the drift body. 

CM = 0.5 regardless the shape of the objects for a bore impact onto an 

impermeable wall, which was adopted from Matsutomi’s results. For a drybed surge, Ikeno and Tanaka (2003) suggested S = 5 and CM = 0.8 for 

spherical-shaped objects and CM = 1.5 ~ 2.0 for cylinders and square-shaped 

columns. The results by Ikeno et al. are valid only for the condition of an 

impermeable wall (i.e., the entire incident flow reflects back to the offshore 

direction). This is why the added mass coefficient has a value less than unity.

Haehnel and Daly (2002). At the U.S. Army Cold Regions Research and 

Engineering Laboratory (CRREL), Haehnel and Daly performed experiments 

similar to Matsutomi (1999). They considered reduced-scale logs in steady 

flow in a small flume, and prototype logs in a large towing basin. It must be 

noted that, just as potential errors were introduced in Matsutomi’s full-scale 

pendulum impact experiments conducted in the air, the condition in the 

towing basin also differs from the actual impact condition of a waterborne 

missile. In the towing basin the water is stationary while in the actual 

condition moving water carries the missile. Instead of the impulsemomentum approach, Haehnel and Daly analyzed the data based on the 

linear dynamic model with one degree of freedom. Since the collision occurs 

over a short duration, damping effects are neglected. Assuming a rigid 

structure, the model can be formulated by Equation D-4: 

mx kx  + = 0 (D-4) 

where: 

m = the mass of the log, 

x = the summation of the compression of the building and 

the log during impact and rebound 

 the dot denotes the time derivative, and
126 D: Background Information on Impact Load Calculations FEMA P646 

 k = the effective constant stiffness associated with both the 

log and the building. 

Solving Equation D-4 yields the maximum force by Equation D-5:

. F Max k x u k m max = = (D-5) 

where: 

 u = the impact velocity. 

Based on their laboratory experiments, the effective constant stiffness k

between a log and a rigid building was estimated to be 2.4 × 106

 N/m. 

Haehnel and Daly demonstrated that the impulse-momentum approach could 

be reduced to the constant-stiffness approach shown in Equation D-5 by 

setting Δt = π

2

m

k (note that, to be consistent to Equation D-4, the force is 

considered a sinusoidal function in time). The work-energy approach can 

also be made equivalent to Equation D-5 by setting the stopping distance as 

S = u

m

k . The work-energy approach is an impact force estimation that 

equates the work done on the building with available kinetic energy of the 

floating missile. Based on their laboratory data, the following formulae were 

suggested by Haehnel and Daly: 

Constant-stiffness approach: 

F Max k x u k m u m max = =≈ . 1550 (D-6) 

Impulse-momentum approach: 

90.9

2 max

u m F um

t

π

= ≈ Δ (D-7) 

Work-energy approach: 

2

2 125 8000 max

u m F mu

x

=≈ +

Δ (D-8) 

Note that in Equations D-6, D-7, and D-8, the velocity, u, is in m/sec and the 

mass, m, is in kg. It is emphasized that errors associated with the use of a 

towing tank (instead of the realistic condition of a log being carried with 

flow) may be significant in the results by Haehnel and Daly (2002). 

SEI/ASCE Standard 7-02 (ASCE, 2003a). ASCE gives the following 

design formula based on Equation D-1: 
FEMA P646 D: Background Information on Impact Load Calculations 127 

2

muC C C C R I O D B max F

t

π

= Δ (D-9) 

where: 

m = the water-borne-missile mass, 

u = the impact velocity of the missile, 

CI = the importance coefficient, 

CO = the orientation coefficient, 

CD = the depth coefficient, 

CB = the blockage coefficient, 

Rmax = the maximum response ratio for impulsive load, and 

Δt = the impact duration. 

All of the C coefficients are based on non-peer-reviewed results of laboratory 

testing and on engineering judgment. Rmax is a coefficient to compensate for 

the effect of the degree of compliance of the building. A single value of the 

impact duration, Δt = 0.03 sec, is recommended, although there is wide 

variation in the impact duration owing to, for example, the object material, 

the flow blockage condition, and the compliance of the building. It is worth 

noting that the City and County of Honolulu Building Code (CCH, 2000) 

recommends Δt values for wood construction as 1.0 sec, steel construction as 

0.5 sec, and reinforced concrete as 0.1 sec. Furthermore, the FEMA 55 

Coastal Construction Manual (FEMA, 2005) provides the Δt values shown 

in Figure D-1. Such an excessive variation in Δt makes Equation D-9 

unreliable. 

Figure D-1 Ranges of duration of impact (FEMA, 2005). 

D.2 Summary and Discussion

Review of previous work clearly demonstrates the immaturity and 

uncertainty of the present understanding of missile-impact forces. The form 

of Equation D-9 exhibits a struggle to obtain an engineering estimate of the 

forces by adjusting five coefficients based on engineering judgment, together 
128 D: Background Information on Impact Load Calculations FEMA P646 

with the unreliable estimate for Δt. All of the prediction formulae are based 

on small-scale laboratory data by compensating with the full-scale 

measurements in the compromised conditions. For example, Matsutomi’s 

full-scale data were obtained by the impact study in air, and Haehnel and 

Daly’s data were obtained in a towing tank. Since the added mass effect 

appears important at the impact (the impact halts not only the waterborne 

missile itself but also the water flowing around it), the results derived from 

the compromised experimental conditions may contain significant errors. For 

this reason information available from the auto industry related to automobile 

crash tests were not considered in this review. 

Even if the impact velocity, u, and the missile mass, m, were given, each 

formula yields a different functional relation to predict the forces, which 

indicates complexity and uncertainty inherent in the problem: 

Constant-stiffness approach ⇒ F ∝ u m , 

Impulse-momentum approach ⇒ F ∝ u m , 

Work-energy approach ⇒ 2 F um ∝ , (D-10)

Ikeno and Tanaka (2003) ⇒ F ∝ u

2.5

mn , n ≈ 0.58, and 

Matsutomi (1999) ⇒ F ∝ u

1.2

mn , n ≈ 0.66.

Although Equation D-2 by Matsutomi is based on his substantial analyses of 

a large set of the laboratory data, the form of Equation D-2 is physically 

ambiguous in terms of the choice of the scaling parameters, is limited only to 

cylindrical shaped missiles, and is inconvenient for use in actual practice. 

The empirical Equation D-3 by Ikeno et al. is based on their small-scale 

laboratory experiments with an impermeable wall; hence, its extrapolation is 

unreliable for real-world applications. Proper estimates of Δt and Δx are 

formidable for the impulse-momentum and work-energy approaches, 

respectively. The value of the effective constant stiffness, k, is difficult to 

evaluate for Haehnel and Daly’s Equation D-5. In reality, k is not constant; it 

is likely a function of x during the impact. Hence, the linearized equation D-4 

may be inadequate. 

Until more comprehensive studies can be made, the constant stiffness 

approach of Equation D-5, suggested by Haehnel and Daly, is recommended 

because of its simple but rational formulation. In addition, as shown in the 

foregoing comparisons in Equations D-10, the functional relation of m and u

to the force F is similar to Matsutomi’s empirical Equation D-2, which was 

derived based on a very large amount of experimental data. Considering that 

Matsutomi’s empirical treatment was based on the impulse-momentum 

approach, the coincidental similarity with the constant-stiffness approach 
FEMA P646 D: Background Information on Impact Load Calculations 129 

provides additional confidence in the formulation. Since the added-mass 

effect must be included, it is recommended that Equation D-5 be modified as 

shown in Equation D-11: 

Fmax = CM u km (D-11) 

with CM ≈ 2 for conservatism (note that Matsutomi (1999) found that CM ≈

1.7 ~ 1.9 and Ikeno et al. (2001, 2003) used CM ≈ 1.5 ~ 2.0) and k must be 

determined based on the model missile (as mentioned earlier, k = 2.4 × 106 

N/m was recommended for a log by Haehnel and Daly). Note that a proper 

estimate of k is the key for this method. An added advantage for the use of 

Equation D-11 is that k is not as sensitive as Δt and Δx in the impulsemomentum and work-energy approaches, which can be shown from the fact 

that Δt and Δx are proportional to 1

k , as discussed earlier. 

FEMA P646 E: Maximum Flow Velocity and Momentum Flux in the 131 

Tsunami Runup Zone 

Appendix E 

Maximum Flow Velocity and 

Momentum Flux in the 

Tsunami Runup Zone 

E.1 Flow Velocity 

For prediction of flow velocities and depths at a site of interest for a given 

design tsunami, the best practice available is to run a detailed numerical 

simulation model with a very fine grid size (less than 10 meters) in the 

tsunami runup zone. Such a numerical model is usually run with a nested 

grid system with a grid size of several kilometers in the abyssal plain, a few 

hundreds of meters on the continental shelf, a few tens of meters near the 

shore, and less than 10 meters in the runup zone. A numerical simulation can 

provide the complete time history of flow velocity and depth at the site of 

interest. 

Alternatively, the use of analytical solutions can be considered. Although 

some simplifications and assumptions must be imposed, the results are useful 

as a guideline for checking the reasonableness of results, or as estimate of 

approximate values in the absence of other information. Available analytical 

solutions are based on one-dimensional, fully nonlinear shallow-water-wave 

theory for the condition with a uniformly sloping beach. With those 

assumptions, the exact solution for the runup of an incident bore was given 

by Shen and Meyer (1963), based on Ho and Meyer (1962). The maximum 

fluid velocity occurs at the leading runup tip as calculated by Equation E-1: 

u gx = 2 tanα , (E-1) 

where: 

α = the beach slope, 

g = the gravitational acceleration, and 

x = the distance from the maximum runup location to the location of 

interest; the location of interest must be above the initial 

shoreline. 

Results indicate that the flow close to the leading runup tip moves up the 

beach under gravity, just like a particle with simple energy transfer between 
132 E: Maximum Flow Velocity and Momentum Flux in the FEMA P646 

Tsunami Runup Zone 

its kinetic and potential energies. According to Yeh (2006), Equation E-1 

provides the upper-limit envelope of the flow velocity for all incident 

tsunami forms. Because a real beach is not uniformly sloped, it is more 

convenient to present Equation E-1 as a function of the ground elevation, 

instead of distance as follows: 

max 2 1 z u gR R

⎛ ⎞ = − ⎜ ⎟ ⎝ ⎠ (E-2) 

where: 

R = the ground elevation at the maximum penetration of tsunami 

runup, measured from the initial shoreline, and 

z = the ground elevation of the location of interest, measured from 

the initial shoreline level. 

It is emphasized that the model does not include the effects of friction and 

the maximum flow velocity occurs at the leading runup tip, where the flow 

depth is zero. Since debris requires some finite flow depth in order to float 

(draft), use of Equations E-1 and E-2 to estimate velocity for impact load 

calculations is somewhat overconservative. 

Based on Shen and Meyer’s (1963) results, Peregrine and Williams (2001) 

provided the formulae for the temporal and spatial variations in fluid velocity 

and flow depth of the incident bore runup. With slightly different scaling, 

Yeh (2007) expressed Peregrine and Williams’ formulae for the flow depth 

and velocity, respectively as follows: 

η = 1

36τ

2 2 2 τ − τ

2

( − 2ζ )

2

 (E-3) 

and 

( ) 1 2 2 2

3

υ ττ ζ

τ

=−+ (E-4) 

where, in the above equations: 

d

R η = ; 2

u

g R

υ = ; tan g t

R

τ α = ; 

z

R ζ =

d = the water depth, 

R = the ground elevation at the maximum penetration of tsunami 

runup, measured from the initial shoreline, 

u = the flow velocity, 

g = the gravitational acceleration, 

α = the beach slope, 
FEMA P646 E: Maximum Flow Velocity and Momentum Flux in the 133 

Tsunami Runup Zone 

t = the time: 0 when the bore passes at the initial shoreline, and 

z = the ground elevation of the location of interest, measured from 

the initial shoreline: this identifies the location of interest along a 

uniformly sloping beach. 

For a given maximum runup penetration, an incident bore should yield the 

maximum flow velocity. Gradual flooding of non-breaking tsunamis should 

result in slower flow velocity than that caused by the bore runup. Therefore, 

Equations E-3 and E-4 can be used to estimate the maximum flow velocity at 

a given location for a given flow depth. Combining Equations E-3 and E-4 

and eliminating τ, Figure E-1 can be derived. Each curve in the figure 

represents the dimensionless flow velocity υ versus the location ζ (in terms 

of ground elevation, z) for a given local flow depth, d. This figure can be 

used to evaluate the maximum flow velocity that can carry floating debris 

with finite draft depth, since draft of the debris must be greater than the flow 

depth to make the debris float. 

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

and maximum runup elevation, R. The bottom curve represents 

the lower limit of maximum flow velocity. 

The bottom curve in Figure E-1 is the lower limit of the maximum flow 

velocity for a given depth, d. Note that the results in Figure E-1 are based on 

the runup condition of uniform incident bore. Local inundation depth of 

other tsunami forms usually exceeds that of a bore runup, and the maximum 

flow velocity is lower than the limit curve in Figure E-1. Hence when a 

floating-debris has a draft that exceeds the flow depth of the bore runup, the 
134 E: Maximum Flow Velocity and Momentum Flux in the FEMA P646 

Tsunami Runup Zone 

design velocity umax can be estimated conservatively with the lower limit 

curve. 

E.2 Momentum Flux 

Using the exact solution algorithm, Yeh (2006) developed an envelope curve 

of the maximum momentum flux per unit water mass per unit width, hu2

, 

expressed in Equation E-5: 

( ) ( ) 2 2

2 2 0.11 0.015 hu x x

gα

= + A A A (E-5) 

where: 

hu2

 = the momentum flux per unit mass per unit width, 

α = the beach slope, 

g = the gravitational acceleration, 

x = the distance from the maximum runup location to the location of 

interest (the location of interest must be above the initial 

shoreline), and 

A = the maximum runup distance. 

Once the maximum runup distance, A, is determined (e.g., from an available 

inundation map), the momentum flux, ρ hu2

 per unit breadth at a given 

location x, can be computed by Equation E-5. It is emphasized that Equation 

E-5 is for a uniform beach slope; therefore, some adjustments need to be 

made to evaluate realistic conditions. Because a real beach is not uniformly 

sloped, it is more convenient to express Equation E-5 as a function of ground 

elevation instead of distance, as follows: 

2 2

2 0.125 0.235 0.11 h u z z

g R R R

⎛ ⎞ =− + ⎜ ⎟ ⎝ ⎠ (E-6) 

where: 

hu2

 = the momentum flux per unit mass per unit width, 

g = the gravitational acceleration, 

R = the ground elevation at the maximum penetration of tsunami 

runup, measured from the initial shoreline, and 

z = the ground elevation of the location of interest, measured from 

the initial shoreline: this identifies the location of interest along a 

uniformly sloping beach. 

Although a real beach is not uniformly sloped and tsunami runup is not a 

one-dimensional motion, Figure E-1 and Equations E-2 and E-6 provide an 

analytical basis for runup conditions. 
FEMA P646 Glossary 135 

Glossary 

The following definitions are provided to explain the terms and acronyms 

used throughout this document. Many have been taken directly from the 

FEMA 55, Coastal Construction Manual (FEMA, 2005). 

A 

ADA – Americans with Disabilities Act. Law requiring that design 

accommodations be made for persons with certain disabilities. 

A-Zone – Under the National Flood Insurance Program, the area subject to 

inundation by a 100-year flood where waves are less than 3 feet high 

[designated Zone A, AE, A1-A30, A99, AR, AO, or AH on a Flood 

Insurance Rate Map (FIRM)]. 

Armor – Material used to protect slopes from erosion and scour by 

floodwaters, such as riprap, gabions, or concrete. 

ASCE – American Society of Civil Engineers. 

ATC – Applied Technology Council. 

B 

Base flood – Flood that has a 1% probability of being equaled or exceeded in 

any given year, also known as the 100-year flood. 

Base Flood Elevation (BFE) – Elevation of the base flood in relation to a 

specified datum, such as the National Geodetic Vertical Datum or the North 

American Vertical Datum. The Base Flood Elevation is the basis of the 

insurance and floodplain management requirements of the National Flood 

Insurance Program. 

Bathymetry – Underwater configuration of a bottom surface of an ocean, 

estuary, or lake. 

Berm – A mound of soil or other earthen material. 

Bore – A long, broken wave propagating into a quiescent body of water, with 

an abrupt increase in water depth at its front face covered with turbulent, 

tumbling water. 
136 Glossary FEMA P646 

Breakaway wall – Under the National Flood Insurance Program, a wall that 

is not part of the structural support of the building and is intended, through its 

design and construction, to collapse under specific lateral loading forces 

without causing damage to the elevated portion of the building or supporting 

foundation system. Breakaway walls are required by the National Flood 

Insurance Program regulations for any enclosures constructed below the Base 

Flood Elevation beneath elevated buildings in coastal high-hazard areas (also 

referred to as V-Zones). In addition, breakaway walls are recommended in 

areas where floodwaters flow at high velocities or contain ice or other debris. 

Building codes – Regulations adopted by local governments that establish 

standards for construction, modification, and repair of buildings and other 

structures. 

Building official – An officer or other designated authority charged with the 

administration and enforcement of the code, or a duly authorized 

representative such as a building, zoning, planning, or floodplain 

management official. 

Bulkhead – A wall or other structure, often of wood, steel, stone, or 

concrete, designed to retain or prevent sliding or erosion, and occasionally 

used to protect against wave action. 

C 

CAEE – Canadian Association for Earthquake Engineering. 

Cast-in-place concrete – Concrete that is formed, placed, and cured in its 

final location in the structure. 

Cladding – Exterior surface of the building envelope. 

Coastal A-Zone – The portion of the Special Flood Hazard Area landward 

of a V-Zone or landward of an open coast without mapped V-Zone in which 

the principal sources of flooding are astronomical tides, storm surge, seiches, 

or tsunamis (not riverine sources). The flood forces in coastal A-Zones are 

highly correlated with coastal winds or coastal seismic activity. Coastal 

A-Zones may therefore be subject to wave effects, velocity flows, erosion, 

scour, or combinations of these forces. (Note: National Flood Insurance 

Program regulations do not differentiate between coastal A-Zones and noncoastal A-Zones.) 

Coastal barrier – Depositional geologic features such as a bay barrier, 

tombolo, barrier spit, or barrier island that consists of unconsolidated 
FEMA P646 Glossary 137 

sedimentary materials; is subject to wave, tidal, and wind energies; and 

protects landward aquatic habitats from direct wave attack. 

Coastal High-Hazard Area – Under the National Flood Insurance Program, 

an area of special flood hazard extending from offshore to the inland limit of 

a primary frontal dune along an open coast, and any other area subject to 

high-velocity wave action from storms or seismic sources. On a Flood 

Insurance Rate Map, the coastal high-hazard area is designated Zone V, VE, 

or V1–V30. These zones designate areas subject to inundation by the base 

flood where wave heights or wave runup depths are greater than or equal to 3 

feet. In Hawaii, the VE-Zones are generally determined where the depth of 

water from a 100-year event (as determined from tsunami and/or hurricane 

data) is greater than 4 feet. 

Collapsing breaker – A type of breaking wave associated with a steep beach 

slope and flat incident wave, which occurs right at the instantaneous 

shoreline. 

D 

Dead load – Weight of all materials of construction incorporated into the 

building, including but not limited to walls, floors, roofs, ceilings, stairways, 

built-in partitions, finishes, cladding, and other similarly incorporated 

architectural and structural items and fixed service equipment. See Loads. 

Debris – Solid objects or masses carried by or floating on the surface of 

moving water. 

Debris impact loads – Loads imposed on a structure by the impact of 

waterborne debris. 

Debris line – Markings on a structure or the ground caused by the deposition 

of debris, indicating the height or inland extent of floodwaters. 

Design Basis Earthquake (DBE) – The earthquake hazard level that 

structures are specifically proportioned to resist, taken as two-thirds of the 

Maximum Considered Earthquake (MCE) hazard level.

DoD – Department of Defense. 

Draft – The depth of water that a body needs in order to float. 

F 

FEMA – Federal Emergency Management Agency. 

FEMA MAT Report – FEMA Mitigation Assessment Team Report. 
138 Glossary FEMA P646 

Fill – Material such as soil, gravel, or crushed stone placed in an area to 

increase ground elevations or change soil properties. See Structural Fill.

FIRM – Flood Insurance Rate Map. 

Far-source-generated tsunami – Tsunami resulting from a source located 

far from the site such that it arrives in excess of a 2-hour timeframe.

500-year flood – Flood that has a 0.2% probability of being equaled or 

exceeded in any given year. 

Flood elevation – Height of the water surface above an established elevation 

datum such as the National Geodetic Vertical Datum, the North America 

Vertical Datum, or mean sea level. 

Flood Insurance Rate Map – Under the National Flood Insurance Program, 

an official map of a community upon which the Federal Emergency 

Management Agency has delineated both the special hazard areas and the 

risk premium zones applicable to the community. (Note: The latest FIRM 

issued for a community is referred to as the effective FIRM for that 

community.) 

Flood-hazard area – The greater of the following: (1) the area of special 

flood hazard, as defined under the National Flood Insurance Program, or (2) 

the area designated as a flood-hazard area on a community's legally adopted 

flood-hazard map, or otherwise legally designated. 

Footing – The enlarged base of a foundation wall, pier, post, or column 

designed to spread the load of the structure so that it does not exceed the soil 

bearing capacity. 

G 

GSA – General Services Administration. 

Grade beam – Section of a concrete slab that is thicker than the slab and acts 

as a footing to provide stability, often under load-bearing or critical structural 

walls. 

H 

Hydrodynamic loads – Loads imposed on an object, such as a building, by 

water flowing against and around it. Among these loads are positive frontal 

pressure against the structure, drag effect along the sides, and negative 

pressure on the downstream side. 
FEMA P646 Glossary 139 

Hydrostatic loads – Loads imposed on a surface, such as a wall or floor 

slab, by a standing mass of water. The water pressure increases linearly with 

the water depth; hence, the hydrostatic loads increase with the square of the 

water depth. 

I 

Impact forces – Loads that result from waterborne debris transported by 

tsunami waves striking against buildings and structures or parts thereof. 

Impulsive forces – Force induced against a vertical obstruction subjected to 

the leading edge of a tsunami during runup, also termed “surge” forces.

Ingress – The act of entering a building.

Inland zone – For the purposes of this report, the area that is inland of the Aand X-Zones (the limit of the 500-year flood). 

L 

Liquefaction – A phenomenon that occurs in saturated soils when the net 

pore pressure exceeds the gravity force holding soil particles together. Soil 

strength and stiffness decrease dramatically as the soil behaves similar to a 

fluid. 

Loads – Forces or other actions that result from the weight of all building 

materials, occupants and their possessions, environmental effects, differential 

movement, and restrained dimensional changes. 

M 

Masonry – Built-up construction of combination of building units or 

materials of clay, shale, concrete, glass, gypsum, stone, or other approved 

units bonded together with or without mortar, grout, or other accepted 

methods of joining. 

Maximum Considered Earthquake (MCE) – The most severe earthquake 

effects considered by seismic design codes and standards. The MCE is based 

on the United States Geological Survey seismic hazard maps, which are 

based on a combination of: (1) 2500-year probabilistic earthquake ground 

motion hazards; and (2) deterministic ground motion hazards in regions of 

high seismicity, with the appropriate ground motion attenuation relationships 

defined for each region. 

Maximum Considered Tsunami (MCT) – A design tsunami event based on 

a probabilistic assessment considering all possible tsunami sources, or a 
140 Glossary FEMA P646 

deterministic assessment considering the maximum tsunami that can 

reasonably be expected to affect a site. 

Mid-source-generated tsunami – Tsunami generated by a source that is 

near the site of interest, but not close enough so that the effects of the 

triggering event is felt at the site.

Mitigation – Any action taken to reduce or permanently eliminate the longterm risk to life and property from natural hazards. 

N 

National Geodetic Vertical Datum (NGVD) – Datum established in 1929 

and used as a basis for measuring flood, ground, and structural elevations; 

was previously referred to as Sea Level Datum or Mean Sea Level. The Base 

Flood Elevations shown on most of the Flood Insurance Rate Maps issued by 

the Federal Emergency Management Agency are referenced to NGVD or, 

more recently, to the North American Vertical Datum. 

Near-source-generated tsunami – Tsunami generated by a source located 

near the site such that it arrives within a 30-minute timeframe, and the effects 

of the triggering event are felt at the site. 

National Flood Insurance Program (NFIP) – The federal program created 

by Congress in 1968 that makes flood insurance available in communities 

that enact and enforce satisfactory floodplain management regulations. 

Nonstructural wall – A wall that does not support vertical loads other than 

its own weight. 

North American Vertical Datum (NAVD) – Datum used as a basis for 

measuring flood, ground, and structural elevations. NAVD, rather than the 

National Geodetic Vertical Datum, has been used in many recent flood 

insurance studies. 

P 

Pier foundation – Foundation consisting of isolated masonry or cast-inplace concrete structural elements extending into firm materials. Piers are 

relatively wide in comparison to their length, and derive their load-carrying 

capacity through skin friction, end bearing, or a combination of both. 

Pile foundation – Foundation consisting of concrete, wood, or steel 

structural elements driven or jetted into the ground, or cast in place. Piles are 

relatively slender in comparison to their length, and derive their load-
FEMA P646 Glossary 141 

carrying capacity through skin friction, end bearing, or a combination of 

both. 

Plain concrete – Structural concrete with no reinforcement or with less 

reinforcement than the minimum amount specified for reinforced concrete. 

Plunging Breaker – A type of breaking wave when the wave front curls 

over, forming a tube; it usually happens on beaches where the slope is 

moderately steep.

Post foundation – Foundation consisting of vertical support members, 

usually made of wood, set in holes and backfilled with compacted material. 

Precast concrete – Concrete, usually a discrete structural member, that is 

formed, placed, and cured at one location, and subsequently moved and 

assembled into a final location in a structure. 

Probabilistic maps – Maps of predicted tsunami effects including for 

inundation zone, flood depths, and flow velocities, based on a method 

involving probability and uncertainty. 

Progressive collapse – ASCE/SEI Standard 7-02 defines progressive 

collapse as “the spread of an initial local failure from element to element 

resulting eventually, in the collapse of an entire structure or a 

disproportionately large part of it.” 

R 

Rapid drawdown – A sudden reduction in water level immediately prior to 

the first tsunami wave, or between tsunami waves. 

Reinforced concrete – Structural concrete reinforced with steel. 

Retrofit – Any change made to an existing structure to reduce or eliminate 

potential damage to that structure from flooding, erosion, high winds, 

earthquakes, or other hazards. 

S 

Scour – Removal of soil or fill material by the flow of floodwaters, 

frequently used to describe storm-induced, localized conical erosion around 

pilings and other foundation supports where the obstruction of flow increases 

turbulence. 

Sea wall – Solid barricade built at the water’s edge to protect the shore and 

to prevent inland flooding. 
142 Glossary FEMA P646 

SEI – Structural Engineering Institute of ASCE. 

Shearwall – Load-bearing or non-load-bearing wall that transfers in-plane 

forces from lateral loads acting on a structure to its foundation. 

Special Flood Hazard Area (SFHA) – Under the National Flood Insurance 

Program, an area having special flood, mudslide (i.e., mudflow), and/or 

flood-related erosion hazards, and shown on a Flood Hazard Boundary Map 

or Flood Insurance Rate Map as Zone A, AO, A1-A30, AE, A99, AH, V, V1-

V30, VE, M, or E. 

Storm surge – Rise in the water surface above normal water level on an 

open coast due to the action of wind stress and atmospheric pressure on the 

water surface. 

Stillwater elevation – Projected elevation that floodwaters would assume, 

referenced to the National Geodetic Vertical Datum, the North American 

Vertical Datum, or some other datum, in the absence of waves resulting from 

wind or seismic effects. 

Structural fill – Fill compacted to a specified density to provide structural 

support or protection to a structure. 

T 

Topography – Configuration of a terrain, including its relief and the position 

of its natural and man-made features. 

Tsunami – A naturally occurring series of ocean waves resulting from a 

rapid, large-scale disturbance in a body of water, caused by earthquakes,

landslides, volcanic eruptions, and meteorite impacts. 

Tsunami inundation zone – The region flooded by tsunami penetration 

inland. 

Tsunami inundation elevation – The elevation, measured from sea level, at 

the location of the maximum tsunami penetration 

Tsunami runup – Rush of tsunami waves up a slope, terrain, or structure. 

Tsunami runup height – The difference between the elevation of maximum 

tsunami penetration and the elevation of the shoreline at the time of tsunami 

attack. 

Tsunami water level – The difference between the elevation of the highest 

local water level and the elevation of the shoreline at the time of tsunami 

attack. 
FEMA P646 Glossary 143 

U 

Undermining – Process whereby erosion or scour exceeds the depth of the 

base of a building foundation, or the level below which the bearing strength 

of the foundation is compromised. 

Uplift – Vertical hydrostatic pressure caused by the volume of displaced 

water under a building. 

V 

V-Zone – See Coastal High-Hazard Area. 

VE-Zone – Coastal High-Hazard Areas where the Base Flood Elevations 

have been determined through a detailed study. 

Vertical Evacuation Refuge from Tsunamis – A building or earthen 

mound that has sufficient height to elevate evacuees above the tsunami 

inundation depth, and is designed and constructed with the strength required 

to resist the forces generated by tsunami waves. 

W 

Waterborne debris – Any object transported by tsunami waves (e.g., 

driftwood, small boats, shipping containers, automobiles). 

Wave crest – The point of highest elevation in a wave profile. 

Wave height – Vertical distance between the successive local maximum and 

minimum elevations in a wave profile. 

Wave zone – Area that coincides with V, VE, or V1–V30 Zones or Coastal 

High-Hazard Areas. 

FEMA P646 References 145 

References 

Abe, S., Sugaya, C., Tanaka et al., 2005, “Guideline for Tsunami Evacuation 

Buildings,” Tsunami Evacuation Building Guideline Committee, 

http://www.bousai.go.jp/oshirase/h17/tsunami_hinan.html (translated 

from Japanese). 

Abednego, L.G., 2005, “The Contribution of Indonesian Engineers 

Association to Aceh Province After Earthquake and Tsunami,” 

Proceedings of the Scientific Forum on Tsunami, Its Impact and 

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FEMA P646 Project Participants 157 

Project Participants 

ATC Management and Oversight 

Christopher Rojahn (Project Executive) 

Applied Technology Council 

201 Redwood Shores Parkway, Suite 240 

Redwood City, CA 94065 

Jon A Heintz (Project Manager) 

Applied Technology Council 

201 Redwood Shores Parkway, Suite 240 

Redwood City, CA 94065 

William T. Holmes (Project Tech. Monitor) 

Rutherford & Chekene 

55 Second Street, Suite 600 

San Francisco, CA 94105 

FEMA Project Officer 

Michael Mahoney (Project Officer) 

Federal Emergency Management Agency 

500 C Street, SW, Room 416 

Washington, DC 20472 

FEMA Technical Monitor 

Robert D. Hanson (Technical Consultant) 

(Federal Emergency Management Agency) 

2926 Saklan Indian Drive 

Walnut Creek, CA 94595 

Project Management Committee 

Steven Baldridge (Project Technical Director) 

BASE Research & Development, LLC 

1164 Bishop Street, Suite 605 

Honolulu, HI 96813 

Frank Gonzalez 

National Ocean & Atmospheric Administration 

Pacific Marine Environmental Laboratory 

7600 Sand Point Way NE, Building 3 

Seattle, WA 98115-0070 

John Hooper 

Magnusson Klemencic Associates 

1301 Fifth Avenue, Suite 3200 

Seattle, WA 98101 

Ian N. Robertson 

University of Hawaii at Manoa 

Dept. of Civil and Environmental Engineering 

2540 Dole Street, Holmes Hall 383 

Honolulu, HI 96822 

Timothy J. Walsh 

Dept. of Natural Resources, Geology & Earth 

Resources 

1111 Washington Street SE, P.O. Box 47007 

Olympia, WA 98504-7007 

Harry Yeh 

Oregon State University 

School of Civil & Construction Engineering 

220 Owen Hall, 

Corvallis, OR 97331-3212 
158 Project Participants FEMA P646 

Project Review Panel 

Christopher P. Jones* (Chair) 

5525 Jomali Drive 

Durham, NC 27705 

John Aho 

CH2M Hill 

301 West Northern Lights Blvd., Suite 601 

Anchorage, AK 99503-2662 

George Crawford 

Washington State Military Dept. 

Emergency Management Division 

Camp Murray, WA 98430-5122 

Richard Eisner 

Governor's Office of Emergency Services 

1300 Clay Street, Suite 400 

Oakland, California 94612 

Lesley Ewing 

California Coastal Commission 

45 Fremont Street, Suite 2000 

San Francisco, CA 94105 

Michael Hornick 

DHS/FEMA, Region IX 

1111 Broadway, Suite 1200 

Oakland, CA 94607 

*ATC Board Representative 

Chris Jonientz-Trisler 

Federal Emergency Management Agency Region X 

130 228th Street SW 

Bothell, WA 98021-9796 

Marc L. Levitan 

LSU Hurricane Center 

Suite 3221 CEBA Building 

Louisiana State University 

Baton Rouge, LA 70803 

George R. Priest 

Oregon Dept. of Geology and Mineral Industries 

Newport Coastal Field Office 

P.O. Box 1033 

Newport, OR 97365 

Charles W. Roeder 

University of Washington 

Structural Eng.& Mechanics 

233B More Hall, Box 352700 

Seattle, WA 98195-2700 

Jay Wilson 

Clackamas County Department of Emergency 

Management 

2200 Kaen Road 

Oregon City OR 97045