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Limitations of flood-frequency estimation techniques: Computations for multiple regions

Methods for estimating flood discharges for specific recurrence intervals for sites on large streams that drain more than one flood-frequency region differ from methods used for sites on smaller streams that fall within one region. The Ocmulgee, Altamaha, Flint, Ogeechee, and Withlacoochee Rivers are the only streams that drain more than one region. Example of the method for a basin in two regions In this example, the 100-year flood discharge (Q100)is needed for an ungaged site that has a drainage basin in two regions. Essentially, a weighted average based on the percentage of the drainage area in each region is used to compute the flood discharge. An example is shown below, or you can use our multi-region calculator to compute the flood-frequency discharge. In this example, the total drainage area of the site is 2,000 mi2, with 1,200 mi2 in region 2 and 800 mi2 in region 3. The Q100 is first computed by using the regional flood-frequency relation for region 2. Assume the drainage area is totally within region 2, and then the Q100 is computed assuming the drainage area is totally within region 3. Region 2: Q100 = 794A.605  =  794(2,000).605   =  78,900 ft3/second. Region 3: Q100 = 340A.627  =  340(2,000).627   =  39,900 ft3/second. The adjusted Q100 for the site is then calculated as a weighted average based on the percentage of the drainage area in each region: Q100 adjusted = 78,900 * (1,200/2,000) + 39,900 * (800/2,000)           = 47,300 + 16,000 = 63,300 ft3/second.

Home | USGS Water Resources of Georgia Select site by: Site number | Site name | County | Area | Region Ungaged site information | Ungaged calculator | Two-region calculator | Limitations This URL: http://nts4dgaatl.er.usgs.gov:80/floodfrequency/multiregions.cfm Last updated: 11/28/2001 10:12:18 AM Note: Information and data on this Web site come from "Techniques for Estimating Magnitude and Frequency of Floods in Rural Basins of Georgia," USGS Water-Resources Investigations Report 93-4016, by Stamey, T.C., and Hess, G.W., 1993 USGS Privacy Statement || Disclaimer