FEDERAL EMERGENCY MANAGEMENT AGENCY

MITIGATION DIRECTORATE

Ensuring That Structures Built on Fill In or Near Special Flood

Hazard Areas Are Reasonably Safe From Flooding

in accordance with the

National Flood Insurance Program

FIA-TB-10

(5/01)

Technical

Bulletin

10-01
Any comments on the Technical Bulletins should be directed to:

Federal Emergency Management Agency

Mitigation Directorate

Program Policy and Assessment Branch

500 C Street, SW.

Washington, DC 20472

Wave design on cover based on the Japanese print The Great Wave Off Kanagawa, by Katsuchika Hokussai

(1760–1849), Asiatic Museum of Fine Arts, Boston.

Key Word/Subject Index

This index allows the user to locate key words and subjects in this Technical Bulletin. The

Technical Bulletin User’s Guide (printed separately) provides references to key words and

subjects throughout the Technical Bulletins. For definitions of selected terms, refer to the

Glossary at the end of this bulletin.

Key Word/Subject Index Page

Basement construction, engineered option 19

Basement construction, simplified approach 15

Basement foundation in fill, not recommended 9

Basement foundation in fill, vulnerability to subsurface flooding 1, 9

Basement foundation, in fill placed above BFE 10

Basement foundation, with lowest floor at or above BFE 10

Basement foundation, with lowest opening above BFE 11

Basement foundation, with lowest opening at BFE 12

Community permitting, administrative options for 5

Crawlspace foundation 7

Fill, placed to remove land from the SFHA 1

Fill, areas where prohibited 3

Fill, proper placement of 5

Foundation flood risk, summary table 13

Freeboard, recommendations 6

Insurance coverage for basement flooding, restrictions 9

Professional certification 4

Professional certification, sample form 4

“Reasonably safe from flooding,” defined 2

“Reasonably safe from flooding,” NFIP regulations concerning 2

Slab-on-grade foundation 8

Stem wall foundation 7

Sump pump, requirements for simplified basement construction 15
TECHNICAL BULLETIN 10-01

Ensuring That Structures Built on Fill In or Near Special Flood Hazard Areas

Are Reasonably Safe From Flooding

in accordance with the National Flood Insurance Program

Introduction

For the purpose of administering the National Flood Insurance Program (NFIP), FEMA identifies and

maps flood hazard areas nationwide by conducting flood hazard studies and publishing Flood

Insurance Rate Maps (FIRMs). These flood hazard areas, referred to as Special Flood Hazard Areas

(SFHAs), are based on a flood having a 1-percent probability of being equaled or exceeded in any

given year (also referred to as the 100-year flood or Base Flood).

Structures within the SFHA in a community participating in the NFIP are subject to floodplain

management regulations that impact building standards and are designed to minimize flood risk. For

example, Title 44, Part 60, Section 3(c)(2) of the Code of Federal Regulations—abbreviated as 44

CFR 60.3(c)(2)—requires that the lowest floor of a residential structure, including basement, built

within the SFHA be at or above the Base Flood Elevation (BFE). In addition, flood insurance must be

purchased for these structures if they are used as collateral to secure a loan provided by a federally

regulated lender. Flood insurance coverage may be purchased for all eligible structures within a

participating community. Insurance rates for structures located within the SFHA differ from the rates

for structures located outside the SFHA.

When permitted under applicable Federal, state, and local laws, ordinances, and regulations, earthen

fill is sometimes placed in an SFHA to reduce flood risk to the filled area. Under certain conditions,

when engineered earthen fill is placed within an SFHA to raise the surface of the ground to or above

the BFE, a request may be submitted to FEMA to revise the FIRM to indicate that the filled land is

outside of the SFHA. When such revisions are warranted, FEMA usually revises the FIRM by issuing

a Letter of Map Revision based on fill (LOMR-F). After FEMA has revised the FIRM to show that the

filled land is outside the SFHA, the community is no longer required to apply the minimum NFIP

floodplain management standards to any structures built on the land and the mandatory flood

insurance purchase requirements no longer apply. It is worth noting that states and local communities

may have floodplain regulations that are more restrictive than the minimum requirements of the NFIP

and may continue to enforce some or all of their floodplain management requirements in areas outside

the SFHA.

Although a structure built on a site that has been elevated by the placement of fill may be removed by

FEMA from the SFHA, the structure may still be subject to damage during the Base Flood and

higher-magnitude floods. Constructing the entire structure at or above the level of the BFE will

minimize the flood risk from the Base Flood and is therefore the most prudent approach to

constructing on fill. Conversely, a structure with a basement (subgrade area) adjacent to or near the

floodplain may well be impacted by subsurface flooding brought on by surface flooding.

1
This bulletin provides guidance on the construction of buildings on land elevated above the BFE

through the placement of fill. Several methods of construction are discussed, and the most prudent—

those that result in the entire building being above the BFE—are recommended.

In some areas of the country, basements are a standard construction feature. Individuals may wish to

construct basements on land after it has been removed from the floodplain by a FEMA revision.

Buildings with basements built in filled areas are at an added risk of flooding when compared to

buildings on other types of foundations. However, there are two major ways to minimize this

additional risk from subsurface flooding. First, the building should be located farther back from the

edge of the fill closest to the flooding source. Second, the higher the basement floor is elevated, the

less the risk. This technical bulletin provides guidance on how to determine that these buildings will

be reasonably safe from flooding during the occurrence of the Base Flood and larger floods. To be

reasonably safe from flooding during the Base Flood condition, the basement must (1) be dry, not

have any water in it, and (2) be structurally sound, not have loads that either exceed the structural

capacity of walls or floors or cause unacceptable deflections. In practice, this means that soils around

the basement must have low permeability to minimize or stop water infiltration to the basement wall

and floors. Any water that does permeate to the basement must be removed by a drainage layer on the

outside (soil side) of the basement. In addition, the foundation walls and floor slab must be designed

and constructed for any increased loads that may occur during the Base Flood condition.

NFIP Regulations

Part of a community’s application to participate in the NFIP must include “a commitment to recognize

and duly evaluate flood hazards in all official actions in the areas having special flood hazards and to

take other such official actions reasonably necessary to carry out the objectives of the program” [44

CFR 59.22 (a)(8)].

NFIP regulations at 44 CFR 60 include Subpart A: Requirements for Flood Plain Management

Regulations. Each community participating in the NFIP adopts a floodplain management ordinance

that meets or exceeds the minimum requirements listed in 44 CFR 60. Subpart A establishes specific

criteria for determining the adequacy of a community’s floodplain management regulations. The

overriding purpose of the floodplain management regulations is to ensure that participating

communities take into account flood hazards, to the extent that they are known, in all official actions

relating to land management and use.

One of the minimum requirements established by the regulations is set forth at 44 CFR 60.3 (a)(3),

which states that, for all proposed construction or other development within a participating

community, the community must “Review all permit applications to determine whether the proposed

building sites will be reasonably safe from flooding.” 44 CFR 59.1 defines “development” as

“...any manmade change to improved or unimproved real estate, including but not

limited to buildings or other structures, mining, dredging, filling, grading, paving,

excavation or drilling operation or storage of equipment or materials,”

2
3

Floodways, V Zones, and Alluvial Fan Flood Hazard Areas

This bulletin does not apply to the following:

• Construction in the floodway. The NFIP prohibits encroachments into the floodway that

would cause increases in flood stage.

• Construction in SFHAs designated Zone V, VE, or V1-V30 on FIRMs. The NFIP prohibits the use of structural fill for support of buildings in V zones. Buildings constructed in a

V zone must be constructed on an open foundation consisting of piles, piers, or posts and

must be elevated so that the bottom of the lowest horizontal structural member is at or

above the BFE. In addition, this bulletin strongly recommends that structural fill not be

used to elevate buildings constructed in A zones in coastal areas. Detailed guidance

concerning proper construction methods for buildings in coastal areas is presented in

FEMA’s Coastal Construction Manual (FEMA 55) and in NFIP Technical Bulletin 5,

Free-of-Obstruction Requirements.

• Construction in SFHAs subject to alluvial fan flooding (designated Zone A0 with depths

and velocities shown on FIRMs). The NFIP will not remove land from the floodplain

based on the placement of fill in alluvial fan flood hazard areas.

More Restrictive State and Local Requirements

NFIP Technical Bulletins provide guidance on the minimum requirements of the NFIP

regulations. State or local requirements that exceed those of the NFIP take precedence. Design

professionals should contact community officials to determine whether more restrictive state or

local regulations apply to the building or site in question. All applicable standards of the state or

local building code must be met for any building in a flood hazard area.

By issuance of this Technical Bulletin, FEMA is noting that residual flood hazards may exist in areas

elevated above the BFE by the placement of engineered earthen fill. Residual risks in these areas

include subsurface flood conditions and flooding from events that exceed the base flood. This bulletin

is intended to guide local floodplain management officials in determining whether structures placed in

filled areas are reasonably safe from flooding. FEMA will require that the jurisdiction having

authority for floodplain management determine that an area is reasonably safe from flooding before

removing it from the SFHA.

Warning

Construction of a residential building in an identified SFHA with a lowest floor below the BFE

is a violation of the floodplain management requirements set forth at 44 CFR 60.3(c)(2), unless

the community has obtained an exception to NFIP requirements from FEMA and has approved

procedures in place.
Notes for Local Officials

Professional Certification

As required by state and local floodplain management ordinances, a proposed development must be

determined to be reasonably safe from flooding. The official having the authority to make this

determination should require all appropriate information for making the determination. This may

include a certification by a qualified design professional that indicates the land or structures to be

removed from the SFHA are reasonably safe from flooding, according to the criteria described in this

technical bulletin. Such a professional certification may come from a professional engineer,

professional geologist, professional soil scientist, or other design professional qualified to make such

evaluations. A sample of such a certification is shown in Figure 1.

4

Figure 1 Sample of professional certification form.

Address and Phone

Professional Seal

Project Name and Address

Type of License

Title

Signature Date

License Number

I, certify that the design for the aforementioned

development is reasonably safe from flooding in accordance with the guidance provided within FEMA’s

Technical Bulletin 10-01 related to ensuring that structures are reasonably safe from flooding and in

accordance with accepted professional practices.

License Expiration Date
Administrative Options for Community Permitting

Communities may choose a variety of administrative procedures to assist them in gathering

information that can be used to determine whether a proposed development is reasonably safe from

flooding. Communities are encouraged to establish procedures that alert them to potential future

development of a filled area. These procedures should allow for the evaluation of future development

and a means to determine whether it will be reasonably safe from flooding. The following are

examples of such procedures:

• Require building sites to be identified on final subdivision plats and evaluate those building sites

against the standards described in this Technical Bulletin.

• Require grading plans as a condition of issuing fill permits and require that those grading plans

include building sites, and evaluate those building sites based on this Technical Bulletin.

• Require buffer zones or setback zones around the perimeter of fill pads or at the edge of the floodplain and establish construction requirements within these buffer zones to ensure that buildings are

safe from residual risk.

• Require as a condition of final subdivision plat approval that the developer agree that no basements

will be built in any flood areas.

• Adopt or have regulations that control development of areas immediately adjacent to floodplains

that would ensure that any construction is reasonably safe from flooding. For example, under the

Minnesota State Building Code, communities designate areas outside of the floodplain as “Secondary Flood Hazard Areas” where building officials evaluate plans for basements and can require

modifications to the basement if an official believes there is a residual risk.

• When issuing a permit for the placement of fill only in the SFHA, stipulate that no buildings will be

built on the site without a subsequent building permit.

Placement of Fill

Properly placing fill requires an understanding of soil mechanics, local site conditions, the specific

characteristics of the soils being placed, the methods used to place and compact the fill, and soil

testing procedures. Standard engineering and soil mechanics texts cover these subjects in detail. The

performance of these filled areas should consider, but is not limited to, the following:

• the consolidation of the fill layers and any underlying layers

• the effect of this consolidation on either excessive settlement or differential settlement

• how the permeability of the soils affects water infiltration on any structures built on the site

5
6

Building on Land Removed From the SFHA by the Placement of Fill

The safest methods of constructing a building on filled land removed from the SFHA are those that

result in the entire structure being above the BFE. Methods that place the lowest floor of the building

at, rather than above, the BFE are at greater flood risk, and methods that result in the lowest floor

(including a basement floor) below the BFE have the highest flood risk of all. Placement of the lowest

floor of these structures below the BFE, even through they are outside the SFHA, will result in an

increased threat from subsurface flooding and magnified damages from flooding that exceeds the BFE.

Loss of Storage and Conveyance

The placement of fill in the SFHA can result in an increase in the BFE by reducing the ability to

convey and store flood waters. This can result in increased flood damage to both upstream and

downstream properties. To prevent these possible results, some communities prohibit fill, require

compensatory storage for filled areas, and/or identify a more restrictive floodway.

Risk of Flood Damage in Areas Adjacent to the SFHA

Areas adjacent to the SFHA may have residual risks of flood damage similar to those in areas

removed from the SFHA through the placement of fill. Both areas are subject to residual risk

from subsurface water related to flooding and from floods greater than the Base Flood. Methods

of construction discussed in this bulletin should also be used in these areas.

The foundation types for buildings outside the SFHA described in the following sections are listed in

order of their increasing risk of flood damage.

Freeboard

Freeboard is an additional height used as a factor of safety in determining the elevation of a structure,

or floodproofing, to compensate for factors that may increase the flood height (ASCE 24-98, Flood

Resistant Design and Construction). When fill is used to protect buildings from the Base Flood, the

community should consider whether freeboard should be required. This consideration should

include whether better information exists or conditions have changed (from when the BFE was

originally established) that indicate that the BFE may be higher than originally expected. One

example of when the BFE may be higher is when a culvert or bridge is blocked by debris. Flood

modeling assumes an open channel or culvert. Even when the BFE is not expected to be higher,

freeboard may be appropriate to provide increased protection from flood events less frequent

than the Base Flood or to account for future changes that may increase the BFE.
Non-Basement Foundations

Non-basement foundations consist primarily of stem wall, crawlspace, and slab-on-grade foundations.

Stem Wall Foundation

A stem wall foundation can be used to raise the lowest floor above the surrounding grade. After the

stem walls have been constructed and extended to the desired elevation, the area enclosed by the stem

walls is filled with engineered compacted fill and a slab is poured on top (see Figure 2). Through the

placement of additional fill, the site may be elevated above the BFE. This approach provides

freeboard—an additional amount of elevation that helps protect against subsurface flooding and floods

that exceed the Base Flood. Constructing a stem wall foundation and placing this additional fill on the

site provide the highest level of flood protection.

7

Crawlspace Foundation

Constructing a crawlspace beneath the first floor will raise the lowest floor of the structure above the

surrounding grade (see Figure 3). Openings in the foundation walls are recommended. If flooding

reaches the building, the openings allow flood waters to enter the area below the lowest floor and

equalize the hydrostatic pressure on the foundation walls (see NFIP Technical Bulletin 1, Openings In

Foundation Walls).

The crawlspace alternative is less preferable than stem wall construction, which does not result in an

enclosed area under the first floor and therefore requires no flood openings. Placing additional fill to a

level above the BFE provides freeboard that helps protect against subsurface flooding and floods that

exceed the Base Flood. Constructing a crawlspace foundation and placing additional fill on the site

provide increased flood protection.

Figure 2 Structure on a stem wall foundation. The lowest floor is raised above the BFE. The

space enclosed by the stem walls is filled with engineered compacted fill.
8

Slab-On-Grade Foundation

This method normally provides less flood protection than crawlspace construction because it does not

elevate the house above the adjacent grade (see Figure 4). As a result, the lowest floor of the house can

be as low as the BFE and would be inundated by any flood greater than the BFE. Placing additional

engineered fill beneath the building to a level above the BFE would provide freeboard and therefore

increased flood protection.

Figure 3 Structure on a crawlspace foundation. The lowest floor is raised above the BFE.

Openings in the foundation walls allow water from floods higher than the fill elevation

to enter the crawlspace and equalize the pressure on foundation walls.

Figure 4 Structure on a slab-on-grade foundation. The lowest floor is typically slightly higher

than the surrounding grade.
Basement Foundations

Although basements are a desired feature in some areas of the United States, NFIP minimum

requirements generally do not allow their construction in the SFHA, because of the increased risk of

flood damages. The only instances where this is not the case are buildings for which FEMA has

granted a special exemption to allow floodproofed basements. However, once land is removed from

the SFHA through a map revision, these NFIP minimum requirements no longer apply. As a result,

builders and property owners who build on land removed from the SFHA sometimes elect to install

basements, which are at a higher risk of flood damage than the foundation types described previously.

Constructing a basement on such land is not recommended, because the basement (i.e., lowest) floor

and portions of the basement walls may well be subjected to subsurface flooding. The basement may

therefore be subject to seepage and lateral hydrostatic and uplift pressure caused by high groundwater

levels associated with flooding in surrounding areas. Additionally, when flooding exceeds the BFE,

the basement area may be totally inundated with floodwater. When builders and homeowners decide

to accept the additional risk associated with basement construction on filled land, they need to ensure

that the basement and the rest of the house are reasonably safe from flooding.

9

Flood Insurance Coverage for Basements

It is extremely important to note that the NFIP offers only limited coverage for basement

flooding. First, in order for a claim to be paid, there must be a general condition of overland

flooding where floodwaters come in contact with the structure. Secondly, the NFIP does not

provide coverage for finished nonstructural elements such as paneling and linoleum in

basement areas. Contents coverage is restricted to a limited number of items listed in the flood

insurance policy. Contact a local insurance agent for more information.

Warning

In filled areas adjacent to floodplains, floods can still greatly influence the groundwater at the

filled site. High groundwater at a site with a basement can result in water infiltrating the

basement or greatly increased hydrostatic pressures on the walls and basement slab that can

cause failure or permanent deformation. Even when floods have not reached houses with

basements, FEMA has seen numerous examples of flooded basements, bowed basement floors,

and collapsed basement walls that have resulted from the effects of high groundwater caused by

flooding. In addition, the collapse of flooded basements has also occurred when water is rapidly

pumped from basements surrounded by saturated soils whose pressure exceeds the capacity of

the basement walls.
10

Four basement construction methods are described below in increasing order of flood risk.

Basement Foundation With Lowest Floor At or Above BFE

Placing the lowest floor of the basement at or above the BFE has the effect of eliminating floodinduced damage up to the BFE (see Figure 5). In general, the higher the basement floor is above the

BFE the lower the risk of damage from seepage and hydrostatic pressure caused by flood-related

groundwater. Where possible, the basement should be built with its floor at or above the BFE. An

added benefit is that floods that exceed the BFE will cause significantly less damage to a structure

with this type of basement than to structures with basements whose floors are at greater depths.

Basement Foundation in Fill Placed Above BFE

Placing fill to a level higher than the BFE has the effect of reducing the depth of the basement floor

below the BFE (see Figure 6). It is recommended that fill be placed to a level at least 1 foot above the

BFE. In general, the higher the basement floor the lower the risk of damage from seepage and

hydrostatic pressure caused by flood-related groundwater. Where possible, enough fill should be

properly placed so that the lowest grade adjacent to the structure is raised to an elevation greater than

the BFE. An added benefit of fill placed above the BFE is that it helps protect the building from floods

greater than the Base Flood. These floods are less likely to reach the structure.

Figure 5 Basement foundation with lowest floor above the BFE. Damage from floods below

the BFE is eliminated.
Basement Foundation With Lowest Opening Above BFE

In the event that the lowest floor is not elevated to or above the BFE and fill is not placed to a level

above the BFE, the next best method of reducing flood risk is to place the lowest opening into the

basement (e.g., window well) at a level higher than the BFE (see Figure 7). This will reduce the

chances that surface flooding will enter and inundate the basement. However, the basement walls and

floor slab will still be subjected to hydrostatic pressure with the potential for damage and seepage into

the basement. In addition, the above-grade basement walls will be exposed to water from floods

greater than the Base Flood. For this reason, the lowest opening in the basement walls should be

above the BFE, as shown in Figure 7.

11

Figure 6 Basement foundation in fill placed above the BFE. The depth of the basement floor

below the BFE is less than when no fill is placed.

Figure 7 Basement foundation with lowest opening above the BFE. Surface flooding is less

likely to enter and inundate the basement.
Basement Foundation With Lowest Opening at BFE

This is the least preferable condition of all because it results in the highest flood risk and is not

recommended (see Figure 8). The lack of fill above the BFE, coupled with the lowest floor being

below BFE and lowest opening at the BFE, exposes the basement to flooding from both subsurface

flooding and any flood greater than the Base Flood.

12

Figure 8 Basement foundation with lowest opening at the BFE. The basement is exposed to

flooding from any flood greater than the Base Flood.
13

Flood Risk by Foundation Type

Table 1 summarizes the foundation construction methods described in this bulletin and ranks them in

order of increasing flood risk—the safest foundation types appear near the top; the less safe

foundation types appear near the bottom. The foundation construction methods that result in a

building that is reasonably safe from flooding are shown in the dark gray area of the table. If the

basement construction methods shown in the light gray area are used, the requirements described in

the following sections of this bulletin must be met in order for the building to be considered

reasonably safe from flooding.

Table 1 Flood Risk by Foundation Construction Method
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Basement Construction Guidance

For those who have chosen to accept the additional risk associated with basement construction below

the Base Flood on filled land that has been removed from the SFHA, this bulletin provides technical

guidance about measures that can be taken to protect basements and meet the requirement that

buildings be made reasonably safe from flooding. A simplified approach, including the requirements

that must be met for its use, is presented first. For buildings that do not meet the criteria for the

simplified approach, this bulletin provides technical guidance for the development of an engineering

design tailored to the site conditions.

Structural Design

Design of foundation elements is addressed in model building codes. This technical bulletin does

not address the structural design of basement walls or foundations. Floors and slabs should be

designed for the hydrostatic pressures that can occur from the Base Flood. For the structural

design, it is recommended that the full hydrostatic pressures be assumed unrelieved by the

drainage system. Foundation walls that have not been designed for hydrostatic pressures, such as

unreinforced masonry or pressure-treated wood wall systems, should not be used (see Figure 9).

Figure 9 Failure of this unreinforced masonry basement during flooding in East Grand

Forks, MN, in 1997 caused approximately $32,000 in damage.
Simplified Approach

Design Requirements

If, for a building and building site, all the requirements listed below are met (see Figure 10), the

building is reasonably safe from flooding. If all of these requirements are not met, the more detailed

analysis described under Engineered Basement Option, on page 19 of this bulletin, should be

performed to determine whether the building is reasonably safe from flooding.

The ground surface around the building and within a defined setback distance from the

edge of the SFHA (see next item) must be at or above the BFE.

The setback is the distance from the edge of the SFHA to the nearest wall of the basement.

The minimum allowable setback distance is 20 feet.

The ground around the building must be compacted fill; the fill material—or soil of

similar classification and degree of permeability—must extend to at least 5 feet below the

bottom of the basement floor slab.

The fill material must be compacted to at least 95 percent of Standard Laboratory

Maximum Dry Density (Standard Proctor), according to ASTM Standard D-698. Fill soils

must be fine-grained soils of low permeability, such as those classified as CH, CL, SC, or

ML according to ASTM Standard D-2487, Classification of Soils for Engineering

Purposes. See Table 1804.2 in the 2000 International Building Code (IBC) for

descriptions of these soil types.

The fill material must be homogeneous and isotropic; that is, the soil must be all of one

material, and the engineering properties must be the same in all directions.

The elevation of the basement floor should be no more than 5 feet below the BFE.

There must be a granular drainage layer beneath the floor slab, and a 1⁄4-horsepower sump

pump with a backup power supply must be provided to remove the seepage flow. The

pump must be rated at four times the estimated seepage rate and must discharge above the

BFE and away from the building. This arrangement is essential to prevent flooding of the

basement or uplift of the floor under the effect of the seepage pressure.

The drainage system must be equipped with a positive means of preventing backflow.

Model building codes (such as the 2000 International Residential Code) also address

foundation drainage (IRC Section R405) and foundation walls (IRC Section R404).

Model building codes generally allow foundation drains to discharge through either

mechanical means or gravity drains. In addition, there is often an exception to the

requirement for drainage systems in well-drained soils. However, in or near floodplains,

well-drained soils can, in fact, help convey groundwater towards the building foundation.

Therefore, this exception should not apply in or near floodplains.

15
In some cases in or near floodplains, even with standard drainage systems, hydrostatic

pressures from groundwater against the basement can result. When a standard drainage

system is unable to eliminate hydrostatic pressure on the foundation, model building

codes, including the 2000 International Residential Code (IRC Section R404.1.3), require

that the foundation be designed in accordance with accepted engineering practice. The

simplified approach contained in this Technical Bulletin assumes no hydrostatic

pressure on the foundation and should be used only when a standard drainage

system, discharged by a sump pump that is equipped with backup power and that

discharges above BFE, is employed. For other drainage systems, the designer should use

the engineered basement option presented on page 19 of this bulletin and other appropriate

building code requirements.

16

Figure 10 Requirements for use of the simplified approach to basement construction.
17

Technical Background for the Simplified Approach

The simplified approach is based on the following conditions:

1. The area of the footprint of the basement is less than or equal to 1,200 square feet.

2. The soil is saturated; therefore, there is no time lag in the development of the seepage pattern with a

change in flood water level. The groundwater table in floodplains is typically very shallow, and finegrained soils have a substantial potential for maintaining saturation above the water table by capillary

rise.

3. The tailwater level is at the elevation of the BFE. For this bulletin, “tailwater” is defined as the

groundwater level beyond the structure, on the side away from the flood water surface. This is a

reasonably conservative assumption because the flood would raise the groundwater level in the

general area. In some cases, the tailwater level can be higher than the flood level because there is

higher ground, as a valley wall, that feeds the groundwater into the floodplain soils.

4. The effective elevation of the base of the seepage flow zone can be defined (see Figure 11). This

elevation is needed to permit calculation of the quantity of seepage flow. If the base elevation is not

known, its depth below the base of the floor slab can be conservatively approximated as one-half of

the building width most nearly perpendicular to the shoreline of the flood water. This would

approximate the boundary effects of the three-dimensional seepage flow, in that it would represent the

flow coming in from all sides and meeting in the center beneath the floor slab. This approach assumes

a constant soil type and density over the flow zone. If the site has stratified soil layers, the engineered

basement option should be used (see page 19 of this bulletin).

5. The quantity of seepage flow can be calculated by a simplified method based on Dupuit’s

assumption that equipotential lines are vertical. (The Dupuit method uses Darcy’s law with specific

physical characteristics. A more detailed description can be found in the first two references listed

under “Further Information,” on page 23 of this bulletin.) The elements of the method are presented in

Figure 11. The entry surface, with hydraulic head “a,” is a vertical line extending downward from the

edge of the flood surface. The exit surface, with hydraulic head “b,” is a vertical line extending

downward from the side of the structure closest to the flood water’s edge. The length of the flow path,

“L,” is the setback distance. Flow is assumed to be horizontal, and the horizontal coefficient of

permeability is the effective permeability. For simplicity, the small inclined entry zone at the river

bank and the exit zone below the basement floor are ignored. This is a reasonably conservative

measure. The phreatic line, or the line below which the seepage flow occurs under positive pressure,

extends from the edge of the flood water to the elevation of the bottom of the basement floor slab. If

the exit zone below the basement floor were included, the hydraulic head at “b” would be higher. As

shown in Figure 11, the phreatic line is not a straight line, but within the limits of the assumed

boundary values, it is close to a straight line.
The Dupuit equation for the quantity of seepage flow is:

q = k(a2 – b2)/2L

where: q is the flow in cubic feet per second for a 1-foot width of seepage zone

k is the soil permeability in feet per second (fps) (maximum value of k is 1x10-3 fps)

a and b are hydraulic heads in feet (a < b + 5)

L is the length of the flow zone in feet (L > 20 feet)

Figure 11 Method for calculation of seepage flow.

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19

To obtain Q, the total seepage flow, in cubic feet per second, q must be multiplied by the length

around the periphery of the four sides of the structure. This is a simplifying approach that obviates the

need for a three-dimensional flow net calculation and is reasonably conservative.

It should be noted that the soil permeability does not affect the geometry of the seepage zone or the

geometry of the phreatic line. The permeability does have a significant effect on the quantity of

seepage that must be collected and discharged by the drainage layer and the sump pump. The

calculation of the quantity Q provides a basis for the selection of a sump pump of adequate capacity.

To allow for possible errors in the estimation of the soil permeability, the pump should have a capacity

of at least four times the calculated value of Q. As noted in the requirements section, a standard sump

pump of 1⁄4 horsepower or greater will generally satisfy the requirements of seepage removal for the

conditions described above.

Engineered Basement Option

If the requirements specified for the simplified approach are not met, a licensed soils engineer or

geologist should perform a detailed engineering analysis to determine whether the structure will be

reasonably safe from flooding. The analysis should consider, but is not limited to, the issues described

in the following sections.

Depth, Soil Type, and Stratification of Subsurface Soils

The depth, soil type, and stratification of the subsurface soils may be complex. Four potential

generalized scenarios are shown in Figures 12 and 13. Figure 12 shows two cases of homogeneous

soil. The depth of penetration of the basement and the depth of the flow zone are not limited to the

assumptions on which the simplified approach is based. Case I represents a foundation consisting of

clayey soils, either fill or natural deposits or a combination, which are more or less homogeneous

because they have similar engineering properties. If an adequate setback distance is provided, the

seepage quantity would be relatively low, and uplift pressure beneath the slab could be controlled by

an appropriately sized sump pump because of low permeability.

Case II represents a foundation consisting of sandy soils, either fill or natural soil deposits or a

combination, which are more or less homogeneous because they have similar engineering properties.

The seepage quantity would be fairly large, and more attention would have to be given to the setback

distance and to the provision of an adequately sized sump pump to prevent excessive uplift pressure

beneath the floor slab because of high permeability.

Figure 13 shows two simple cases of stratified soils, with impervious clays overlying pervious sands.

This is a common occurrence in natural floodplain deposits. In Case III, the contact between the two

soil strata is at some distance below the basement floor. This case would involve a moderate quantity

of seepage, depending on the thickness, d, of the impervious stratum below the basement floor. There

is also a potential for excessive uplift pressure beneath the floor, at the level of the bottom of the clay

stratum. If d is equal to h, the net hydraulic head between the flood level and the floor level, the safety

factor against uplift would be approximately 1.0. If d is less than h, there would be excessive uplift,

with a safety factor equal to less than 1.0.
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Figure 12 Case I and Case II – homogeneous soil.

Case IV shows impervious soils overlying pervious soils, with the contact between the soil strata at

some distance above the basement floor. This case would involve a large quantity of seepage and

potential for excessive uplift beneath the basement floor.

Geotechnical Investigations

Geotechnical investigations must be made for cases that do not conform with the assumptions on

which the simplified approach is based. Information that is needed to permit an adequate engineering

analysis includes the following:

• The BFE, which is to be used as the design flood water surface for calculating expected seepage.
• The elevation of the bottom of the basement floor. This can be adjusted as needed to achieve more

suitable conditions.

• The setback distance of the basement wall from the edge of the flood water. This can be adjusted to

achieve more suitable seepage control or to accommodate available space restraints.

• The elevation of the groundwater table and its seasonal variations. A high water table would cause

problems with groundwater control during construction of a basement, even without a flood event.

• The stratification of the subsurface materials, for both natural and fill soils. In general, borings

should be drilled to a depth below the bottom of the floor slab that is at least two times as great as

the depth of the bottom of the floor slab below the BFE.

Figure 13 Case III and Case IV – stratified soils.

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• The engineering classification of the soils, for both natural and fill soils. This must be done in

accordance with ASTM D2487, Classification of Soils for Engineering Purposes. This is the

Unified Soil Classification System that is universally used throughout the United States. Local or

county agricultural soil survey maps should not be used, because they do not give specific

information about location and depth of soils, and their designations are not pertinent to civil

engineering use.

• Subsurface conditions landward from the structure. This includes information about the location of

the water table, whether it is higher or lower than the flood level, and information about any

penetrations of the soil, such as ponds. Attention should be given to the possibility that higher

ground, such as valley walls, could contribute to the groundwater level in the floodplain, either

perennially or during periods of heavy rain.

• Information about any penetrations through the basement walls below the BFE, such as utility lines

and other openings.

• Analysis of seepage quantity. The analysis can be made by the conservative simplified method

described in Item 5 in the section titled Technical Background for the Simplified Approach

(illustrated in Figure 11), or by the construction of a flow net that takes into account all of the

boundary conditions more rigorously. A flow net may be required to permit analysis of uplift

pressures. Uplift pressures may be more significant in laminated or stratified soil deposits.

Buildings in Existing Filled Areas

In evaluating buildings in existing filled areas, the two approaches already described—the simplified

approach or the engineered basement option—can be used. If the simplified approach is used, all the

requirements for the use of this approach must be met. Some possible means for evaluating whether

these requirements are met include soil tests and investigations, including soil borings and hand

augers; field records from the time the fill was placed; and soil surveys. If the requirements for the

simplified approach are not met, a licensed soils engineer or geologist should perform a more detailed

engineering analysis as described under Engineered Basement Option on page 19. More extensive soil

investigations and testing may be required to complete the analysis.

The NFIP

The NFIP was created by Congress in 1968 to provide federally backed flood insurance coverage,

because flood coverage was generally unavailable from private insurance companies. The NFIP is also

intended to reduce future flood losses by identifying floodprone areas and ensuring that new development

in these areas is adequately protected from flood damage. The NFIP is based on an agreement between

the Federal government and participating communities that have been identified as floodprone. FEMA,

through the Federal Insurance Administration (FIA), makes flood insurance available to the residents

of a participating community, provided the community adopts and enforces adequate floodplain

management regulations that meet the minimum NFIP requirements. The NFIP encourages communities

to adopt floodplain management ordinances that exceed the minimum NFIP criteria set forth in Part

60 of the NFIP Floodplain Management Regulations (44 CFR 60). Included in the NFIP requirements,

found under Title 44 of the U.S. Code of Federal Regulations, are minimum building design and

construction standards for buildings located in SFHAs. Through their floodplain management

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ordinances or laws, communities adopt the NFIP performance standards for new, substantially

improved, and substantially damaged buildings in floodprone areas identified on FEMA’s FIRMs.

Technical Bulletins

This publication is one of a series of Technical Bulletins that FEMA has produced to provide guidance

concerning the building performance standards of the NFIP. These standards are contained in 44 CFR

60.3. The bulletins are intended for use primarily by state and local officials responsible for

interpreting and enforcing NFIP regulations and by members of the development community, such as

design professionals and builders. New bulletins, as well as updates of existing bulletins, are issued

periodically, as necessary. The bulletins do not create regulations; rather they provide specific

guidance for conforming with the minimum requirements of existing NFIP regulations. Users of the

Technical Bulletins who need additional guidance concerning NFIP regulatory requirements should

contact the Mitigation Division of the appropriate FEMA regional office or the local floodplain

administrator. NFIP Technical Bulletin 0, the User’s Guide to Technical Bulletins, lists the bulletins

issued to date, provides a key word/subject index for the entire series, and lists addresses and

telephone numbers for FEMA’s 10 Regional Offices.

Ordering Information

Copies of FEMA Technical Bulletins can be obtained from the FEMA Regional Office that serves

your area. In addition, Technical Bulletins and other FEMA publications can be ordered from the

FEMA Publications Distribution Facility at 1-800-480-2520. The Technical Bulletins are also

available at the FEMA web site at www.fema.gov.

Further Information

The following publications contain information related to the guidance presented in this bulletin:

American Society of Civil Engineers. 1998. SEI/ASCE 24-98, Flood Resistant Design and

Construction.

Cedergren, H. R. 1977. Seepage, Drainage and Flow Nets. Wiley. New York.

Harr, M. E. 1977. Mechanics of Particulate Media. McGraw Hill. New York.

International Code Council. 2000. International Building Code. Birmingham, AL.

International Code Council. 2000. International Residential Code. Birmingham, AL.

U.S. Department of the Army, Corps of Engineers. 1986. EM 1110-2-1901, Seepage Analysis and

Control for Dams. Washington, DC.

U.S. Department of the Army, Corps of Engineers. 1978. EM 1110-2-1913, Design and Construction

of Levees. Washington, DC.

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Glossary

Base Flood – The flood that has a 1-percent probability of being equaled or exceeded in any given

year (also referred to as the 100-year flood).

Basement – Any area of a building having its floor subgrade (below ground level) on all sides.

Community – Any state or area or political subdivision thereof, or any Indian tribe or authorized

tribal organization, or Alaska Native village or authorized native organization, which has the authority

to adopt and enforce floodplain management regulations for the areas within its jurisdiction.

Federal Emergency Management Agency (FEMA) – The independent Federal agency that, in

addition to carrying out other activities, administers the NFIP.

Federal Insurance Administration (FIA) – The component of FEMA directly responsible for

administering the flood insurance aspects of the NFIP.

Flood Insurance Rate Map (FIRM) – The insurance and floodplain management map issued by

FEMA that identifies, on the basis of detailed or approximate analysis, areas of 100-year flood hazard

in a community.

Floodprone area – Any land area susceptible to being inundated by flood water from any source.

Mitigation Directorate – The component of FEMA directly responsible for administering the flood

hazard identification and floodplain management aspects of the NFIP.

New construction/structure – For floodplain management purposes, new construction means

structures for which the start of construction commences on or after the effective date of a floodplain

management regulation adopted by a community and includes subsequent improvements to the

structure. For flood insurance purposes, these structures are often referred to as “post-FIRM”

structures.

Special Flood Hazard Area (SFHA) – Area subject to inundation by the base flood, designated Zone

A, A1-30, AE, AH, AO, V, V1-V30, or VE.

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