National Flood Frequency Bulletin

Issue No. 3

The purpose of the NFF mailing list is to provide information on the U.S. Geological Survey (USGS) National Flood Frequency (NFF) program. NFF is a computer application that is used to estimate peak discharges for unregulated streams. Values (discharges) derived by the program often serve as input for other applications (such as hydraulic computer models) that are used to technically support end-products (map revisions) of the National Flood Insurance Program (NFIP).

Use of USGS Regression Equations in the NFIP

The USGS regression equations can be used in the following manner for NFIP purposes:

To compute peak discharges used in support of flood hazard mapping and map revisions
To evaluate alternate hydrologic analysis results.

Criteria for Using USGS Regression Equations in the NFIP

Before using the discharges computed using the USGS regression equations for NFIP purposes, please ensure that the following criteria are met:

Criterion 1: The discharges computed using the regression equations are more accurate than the discharges computed using any available hydrologic analysis of the subject watershed.

Typically, regression equations are acceptable for computing and revising peak discharges for streams if:

The effective FIS discharges for the subject stream are based on the USGS regression equations
The discharges for the stream are not determined in the effective FIS; or,/li>
FEMA has determined that the effective hydrologic analysis methods are not appropriate to compute discharges for the watershed and FEMA approval has been obtained to use USGS regression equations.

Criterion 2:The peak discharges are computed using the most recent regression equations published by the USGS.

Criteria 3:The subject stream is not regulated (i.e., not affected by flood-control structures). Regression equations are not appropriate to compute discharges for streams affected by flood regulation. Regulated streams are those affected by flood-control structures, such as flood-control reservoirs or flood-diversion structures. Please ensure that no flood regulation takes place on the subject stream.

Criterion 4:The resulting discharges are compatible with discharges computed for other locations of this stream and for other streams in the vicinity with similar watersheds studied for the FIS.

Upcoming Bulletin Topics

Revisions to the USGS regression equations since the NFF software was released
Locations from which to obtain the NFF program and USGS regression equation documentation
Unusual parameters of USGS regression equations and how to obtain them - Part 1
Unusual parameters of USGS regression equations and how to obtain them - Part 2
Examples in which USGS regression equations are used for NFIP purposes

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Last Modified: Friday, 22-Jun-2007 11:57:20 EDT