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).

Discussion on USGS Regression Equations and the NFF Program

The USGS developed a computer program titled "National Flood Frequency" or "NFF" that estimates the flood frequency and magnitude for ungaged sites through the application of the appropriate regional regression equations. NFF was released in 1993 and does not incorporate any revisions to regional regression equations that occurred after September 30, 1993. Since 1993 a significant number of the regression equations have been revised. The USGS is in the process of revising the NFF computer program to incorporate the updated regression equations. The revised version of NFF will be released soon.

The regional regression equations are currently being used for National Flood Insurance Program (NFIP) purposes. Therefore, FEMA would like to continue with this listserv and discuss issues of interest in the application of the regional regression equations for NFIP purposes. Upon the release of the revised NFF program, the focus of this listserv will shift to assist users in becoming familiar with the revised NFF program and its application for NFIP purposes.

Examples in Which USGS Regression Equations are Used for NFIP Purposes

Although most communities participating in the NFIP have been provided with Flood Insurance Studies (FISs) and issued Flood Insurance Rate Maps (FIRMs) that include Base Flood Elevations (BFEs), many areas of approximate Zone A without BFEs remain on most communities' FIRMs. Nonetheless, community floodplain administrators are required to "review all permit applications to determine whether proposed building sites will be reasonably safe from flooding." [44 CFR 60.3(a)(3)]. In approximate Zone A areas, the use of NFF to estimate the base (100-year) flood flow has been confirmed by a 1981 U.S Water Resources Council report as being the most reliable over more costly, time consuming models or calculation methods.

Care should be taken when using the equations to assure the conditions in the subject watershed are similar to those assumed in developing the equations. The discussion with each State's equations contains descriptions of the limiting assumptions and should be examined closely.

Many requests for Letters of Map Amendment and Letters of Map Revision based on Fill include NFF or State regression equation calculations as supporting data for BFE calculations. If the method is properly applied, the results are usually accepted by FEMA for property-specific map change actions.

Similarly, many Letter of Map Revision requests contain calculations of base flood discharges using NFF. These requests usually involve approximate Zone A areas, but where it can be demonstrated to be appropriate and statistically significant, detailed study areas may be revised using flows calculated from NFF. The test for statistical significance requires demonstrating that the previously established discharges do not fall within the 5- and 95-percent confidence limits of the new discharges. The methods described in Bulletin 17B, Guidelines for Determining Flood Flow Frequency, September 1981, revised March 1982, should be used to determine the confidence limits.

Regression equations provide a convenient procedure to evaluate the results of other hydrologic methods submitted in support of requests to revise the NFIP maps in approximate Zone A areas. If the submitted discharges compare favorably with the results from NFF (within the standard error), they may be accepted for use in revising the maps. If they do not, another method, or modifications to the submitted calculations, may be advised.

Many FISs have been published using NFF or State regression equations as the source of flood flows for calculating flood elevations. The Guidelines and Specifications for Study Contractors, FEMA 37, January 1995, contain descriptions of acceptable uses of regression equation estimates for FIS work. Both FEMA 37 and Bulletin 17B are available for downloading at: http://www.fema.gov//plan/prevent/fhm/en_main.shtm.

Previous Bulletin Topics

  • Introduction to the NFF Program and USGS regression equations, the applicability of the regression equations, and the advantages and limitations of the regression equations
  • Use of USGS regression equations in the NFIP and Criteria for using USGS regression equations in the NFIP
  • Revisions to the USGS regression equations since the NFF software was released
  • Part 1. Unusual Parameters of USGS Regression Equations and How to Obtain Them
  • Part 2.Unusual Parameters of USGS Regression Equations and How to Obtain Them
  • Part 3.Unusual Parameters of USGS Regression Equations and How to Obtain Them

Upcoming Bulletin Topics

  • How to Treat "State Line Faults" (Basins Lying in More Than One State)

View the archive page for all Flood Hazard Mapping listservs.

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