USGS Washington Water Science Center
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WA270 - Determination of 100-year Flood Levels for Federal Property - Completed FY1986
Problem - By law, buildings or public facilities constructed on Federal lands or with Federal funds must be located or protected from damage by a 100-year flood. Not all agencies have the expertise needed to evaluate inundation areas for a 100-year flood. The Water Resources Division (WRD) has that expertise.
Objectives - To determine elevations and inundation areas of a 100-year flood from streams for small areas of Federal lands where development is proposed and study is requested by another Federal agency or bureau.
Approach - One of two approaches may be used depending on the requirements of the requesting agency. The least expensive but longest-term approach is that of documenting a range of selected flow events with measurements of elevation and discharge at selected sites in the study area. The fastest, most accurate, and most detailed approach involves methods similar to those used nationally by WRD for Federal Emergency Management Agency (FEMA) Flood Insurance Studies. It requires the collection of cross-section and topographic data, the collection of historic flood-elevation data if available, the application of step-backwater analyses to determine the elevation profile for a 100-year flood discharge estimated from an existing regional regression equation, and the transfer of the flood elevations to maps that define boundaries of inundation.