Updating Existing Flood Maps Efficiently

The first pilot study was conducted in the lower reach of the Nisqually River Basin of western Washington, to develop a method to update flood maps efficiently. The method reduces the time and expense of updating flood inundation maps for the National Flood Insurance Program (see Map Disclaimer) by using locally produced high-accuracy elevation data, existing numerical models, and Geographic Information Systems (GISs) to update existing Flood Insurance Rate Maps (FIRMS).

• High accuracy elevation data produced from low level aerial photography and photogrammetry were provided by Thurston County Geodata Center.
• Flood discharges associated with the 50- and 100-year recurrence intervals were calculated using updated peak flow records.
• Flood elevations were estimated from stage-discharge relations developed from the results of a previous hydraulic model used for the 1982 Flood Insurance Study (FIS).
• A Geographic Information System (GIS) was used to create inundation, depth, and velocity vector maps.

Flood maps created for this effort cover a limited area near the mouth of the Nisqually River. These maps also are used in a subsequent pilot to display forecast flood maps in near real time over the Internet.

An overview of the method is presented in U.S. Geological Survey Open-File Report 98-200 — Updating Flood Inundation Maps Efficiently.

A detailed description of the method is presented in U.S. Geological Survey Water-Resources Investigations Report 01-4051 — Updating Flood Maps Efficiently Using Existing Hydraulic Models, Very-High-Accuracy Elevation Data, and a Geographic Information System: A Pilot Study on the Nisqually River, Washington. This report is in PDF format (PDF: 2.3 MB/30 pages)

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U.S. Department of the Interior
U.S. Geological Survey, Tacoma, WA, USA
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Last modification: Tuesday, 22-Jan-2008 17:06:13 EST