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Flood Hazards
Mapping NWS Flood Forecasts

The second pilot study was conducted in the Nisqually and Snoqualmie River Basins to investigate the feasibility of mapping forecast flood discharges.

A method was developed to provide the capability to generate and deliver inundation maps for forecasted flood discharges via the Internet (see Map Disclaimer) The intent was to use state-of-the art data, numerical models, and mapping software to provide flood forecast maps previously impossible to create within a useful time period.

These forecast flood discharge maps are needed to fill a critical gap in flood response. Existing 50-, 100-, or 500-year recurrence interval flood flows are determined statistically for a specific site on a river, and serve as a planning tool for emergency response officials — they do not reflect actual forecast flood discharges. The National Oceanic and Atmospheric Administration's (NOAA) National Weather Service (NWS) operates several River Forecast Centers (RFCs) across the country that forecast and model precipitation to estimate flood discharges associated with specific storms. These forecasts take the form of peak flows, peak flood elevations, or plots of one or both of these over time. The forecasts, however, are restricted to the immediate proximity of the streamflow stations used in the predictions; no flood elevations are predicted for areas between the stations. This gap between the availability of flood maps for planning purposes, and the availability of forecast flood flows is the target of the this second pilot study.

For the Nisqually River Basin, flood inundation maps produced using the method for updating flood maps using a GIS were used to generate forecast flood inundation maps for this basin. Maps generated for ½-foot increments of flood stage at the NWS forecast point are provided for the viewer to select and compare.

For the Snoqualmie River Basin, the methodology uses locally produced high-accuracy elevation data, a two-dimensional flow model, and Geographic Information Systems (GISs).

  • High accuracy elevation data were collected using airborne laser range-finding.
  • Flood elevations were determined with a semi-explicit, semi-lagrangian, two-dimensional depth averaged flow model that is unique in its ability to simulate wide ranges of flow over long stream reaches.
  • A Geographic Information System (GIS) was used to create inundation and depth maps.

Internet map serving software is used to deliver forecast flood maps in a flexible, user-friendly format for both the Nisqually and Snoqualmie River Basins.

For a general summary of the method, see U.S. Geological Survey Fact Sheet 2004-3060-- Mapping a Flood...Before It Happens. The fact sheet is available from this Web page in PDF format, 420 KB. A detailed description of the method is presented in U.S. Geological Survey Water-Resources Investigations Report 02-4251 — Near-Real-Time Simulation and Internet-Based Delivery of Forecast-Flood Inundation Maps Using Two-Dimensional Hydraulic Modeling--A Pilot Study for the Snoqualmie River, Washington. This report is in PDF format (PDF: 4.0 MB/40 pages)

Part of a map of the Snoqualmie River Basin, showing predicted flooding near Carnation, 1986

Forecast-Flood Map for the Snoqualmie River Basin,
January 2009 Flood



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U.S. Geological Survey, Tacoma, WA, USA
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Last modification: Tuesday, 18-Jun-2013 10:34:02 EDT