(http://www.nsgic.org/hottopics/imageryofnation.cfm). Because the committee concludes that the nation’s existing base mapping for land surface reference information is adequate, this report concentrates on the elevation data input to floodplain mapping, which has a much greater effect on the accuracy of floodplain maps and an important component of those maps, the Base Flood Elevation (BFE).

BASE FLOOD ELEVATION

Land surface elevation information defines the shape of the land surface, which is important in defining the direction, velocity, and depth of flood flows. Land surface elevation data for flood management studies of individual streams and rivers have traditionally been derived by land surveying, but the very large areal extent of FEMA floodplain mapping, which covers nearly 1 million miles of the nation’s streams and shorelines, means that land surface elevation data for Flood Map Modernization are mostly derived from mapped sources, not from land surveying. Land surface elevation information is combined with data from flood hydrology and hydraulic simulation models, to define the BFE, which is the water surface elevation that would result from a flood having a 1 percent chance of being equaled or exceeded in any year at the mapped location. A floodplain map is created by tracing the extent of inundation of the landscape by water at the BFE.

The insurance industry uses floodplain maps to determine if lenders require purchasers of new buildings to have federal flood insurance. This insurance is required if any part of the footprint (or plan view) of the building outline lies over the spatial extent of the floodplain. In other words, the flood insurance determination is made on the basis of a planimetric or horizontal criterion: Does the building outline lie within the floodplain? The current FIRMs and DFIRMs properly support this flood insurance process.

Use of the maps to regulate land development in floodplains by local communities typically requires the first floor elevation of a building to be at or above the BFE if that building is to be constructed within the floodplain. The governing criterion used is thus often stated as: Is the first floor elevation above the BFE? In some communities, a safety margin such as 1 foot of elevation is added to the BFE in order to take into account allowable encroachments into the floodplain that may raise the water surface elevation by 1 foot. This criterion, based on vertical rather than horizontal criteria, is better than that used in flood insurance determinations.

Rational floodplain management and flood damage estimation depend not only on how far the water spreads, but also on how deeply buildings are flooded and with what frequency. If the task of the nation’s flood management is observed in this larger context, accurate land surface and floodwater surface elevation information are critical. For example, in the flood damage mitigation projects undertaken by the United States Army Corps of Engineers in collaboration with local communities, flood damage estimation requires knowing the first



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