Introduction
The purpose of this analysis was to provide a supplement to the 2006 Evaluation of the
National Flood Insurance Program’s Building Standards
1
and, further, to determine the
cost-effectiveness of including freeboard
2
within the foundation height of new residential
buildings constructed in floodplains. The incorporation of freeboard will effectively
reduce the potential flood risk to the building. This analysis was conducted to determine
the benefits associated with constructing coastal houses higher than the current National
Flood Insurance Program (NFIP) requirements mandate. In addition, the analysis was
conducted to establish which factors should be considered when determining how many
feet above the minimum required elevation a house should be constructed at in order to
maximize the cost-effectiveness of the additional height.
The report will demonstrate the cost-effectiveness of utilizing additional freeboard in the
construction of a house and point out the factors that impact the cost-effectiveness of
freeboard. Foundation types, house size (i.e., layout) variations, flood hazard conditions,
and costs of construction were all evaluated in order to determine which factors should be
specified as the primary considerations in determining how much freeboard is
economically justified. In almost all situations studied, freeboard proved cost-effective
for both 1 and 2 feet above the minimum requirements. In some situations, 3 and 4 feet of
freeboard were still deemed cost-effective. In addition to cost-effectiveness, a general
discussion of risk reduction (i.e., damages avoided) is provided, as well as some of the
engineering considerations that should be evaluated when selecting the appropriate
foundation type.
Background
The NFIP was instituted by congressional order in 1968 to assist in regulating
floodplains. The NFIP divides the Special Flood Hazard Area (SFHA – 1-percent annual
chance floodplain) into V Zones (i.e., areas of high-velocity wave action) and A Zones
(i.e., some wave action, but primarily inundation and low-velocity floodwaters). Due to
the differences in risks associated with these zones, this report will use these zones as a
method of differentiating flood risk levels and as a proxy for different methods of
construction. It is also important to further differentiate the risk level within different
portions of the A Zone. The A Zone has been subdivided into the Coastal A Zone and the
A Zone (see Figure 1). Each of these zones is defined by the depth of the stillwater and
wave height during a design event flood.
The design event used for designation of the zones is a flood event with the annual
probability of exceedance of 1 percent. The 1-percent annual chance flood event is
1
Evaluation of the National Flood Insurance Program’s Building Standards, American Institutes for
Research, Christopher P. Jones, William Coulbourne, Jamie Marshall, and Spencer M. Rogers, Jr.,
December 2006.
2
Freeboard is defined by the American Society of Civil Engineers (ASCE) in ASCE 24-05, Flood
Resistant Design and Construction, as “additional height used as a factor of safety in setting the minimum
elevation of a structure, or floodproofing measures applied to a structure, to compensate for factors that
may increase flood heights and for uncertainties inherent in determining flood frequencies and flood
elevations.”