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. The NFF program 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.

Estimating Drainage Area and Cross Sections from USGS Topo Maps

The most expedient way to determine the size of a particular basin is to locate the downstream-most point of interest on the USGS 7.5' topo map(s) covering the area and follow the mapped contour lines to the top of the watershed. Following is an example using the map from last month's bulletin.

After finding the point of interest on the tributary to Cappo Run and delineating the break lines by moving upstream at right angles to the contour lines to the top of the basin, we measured the drainage area as 905 acres (1.414 mi2) using a planimeter set at a scale of 1:24,000.

Basin delineation is a simple process, but requires care in interpreting the lines and symbols on the USGS map(s). The process is best performed on an original color copy of the map(s). Using a black and white photocopy can lead to confusion and misinterpretation of basin divides.

Even though cross sections themselves are not usually part of the data needed to operate NFF, the resulting flows from NFF are often used to estimate flood depth by calculating normal flow depth in a cross section.

USGS 7.5' topo maps can also be used to estimate a floodplain cross section when other sources are lacking. Even a 20-foot contour interval map will provide sufficient information for estimating flood depths for larger flows. From the above topo map we can measure cross section coordinates on the tributary to Cappo Run at the Pennsylvania-West Virginia State line as follows:

280', 1200'; 380', 1180'; 420', stream centerline; 460', 1180'; 490', 1200'

The stream centerline elevation can be interpolated by measuring the distance between the downstream and upstream contour line crossing location, which equals 1160', and dividing it into the contour interval, 20'. This gives 20'/1160' = .01724, which is the average stream bed slope in the reach where we want to estimate the elevation. Our cross-section location is 1000' upstream of the 1160' contour crossing, or 1000'x.01724=17.2' higher than 1160' = 1177.2'.

When entered into Quick-2, our cross section looks like this:

The plotted cross section looks like this:

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)

View the archive page for all Flood Hazard Mapping listservs.

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