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.

Weighting NFF Results with Observed Data

Flows at gaging stations with short records can best be estimated by weighting the regional and station flood estimates. This procedure, which can be done using NFF, uses the number of years of gage record and the accuracy of the regional flood-frequency relations (regression equations), expressed as "equivalent years of record," as weighting factors.

NFF allows the weighting of the regression results and peak estimates from observed data at gaging stations. An exception is in situations in which 500-year event data are unavailable. The estimation of extreme events will be discussed in next month's bulletin.

Flood estimation at gaged sites is usually best accomplished by weighting the gage analysis results with the regional regression equations. NFF provides this option for most of the States' equations.

First, run the regression equations at the station of interest. For this example, we will use the station on Rock Creek near Fairmont, Georgia, No. 02383000.

Peak Streamflow Qualification Codes

  • 7 -- Discharge is an Historic Peak
  • E -- Only Annual Maximum Peak available for this year

From the latitude and longitude given, we find that the station lies in hydrologic region number 1 and has a drainage area of 6.17 square miles.

The next prompt from NFF asks if we want to calculate a weighted average. We will enter "Y" and continue the calculation. In the next window, we will be prompted to enter the 2-, 5-, 10-, 25-, 50-, 100-, and 500-year values, which can be determined either from the results of a log-Pearson III analysis of the gage data or from published results of the gage analysis.

The table of weighted event values includes the equivalent years of record for each recurrence interval event. These values indicate an improvement of the results in longer equivalent years of record.

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
  • Examples in which USGS regression equations are used for NFIP purposes
  • How to treat State Line faults (basins lying in more than one state)
  • Estimating drainage area and cross sections from USGS topo maps
  • Measures of accuracy in NFF

Upcoming Bulletin Topics

  • Estimation of Extreme Floods

View the archive page for all Flood Hazard Mapping listservs.

Last Modified: Monday, 25-Jun-2007 11:57:20 EDT