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.

Urban Flood Hydrograph Estimation Using NFF

As discussed in last month's bulletin, the NFF program contains a procedure for computing the average hydrograph for a basin. Last month, we looked at a rural basin in West Virginia as an example. Following is an example hydrograph calculation for a small, urban watershed using NFF.

This month we will use a small urbanized basin in Bloomington, IL, lying on the Bloomington East, IL, USGS quad. Figure 1 shows the delineation of the drainage basin. For some developed urban areas, the basin area served by storm sewer may not always coincide with the natural drainage defined by the USGS contours. For these conditions, storm sewer drainage information should be used in delineating the basin divides.

Figure 1. Bloomington, IL, Urban Basin Delineation

First, we must use NFF to calculate the rural peak flows. The needed parameters are basin area, average main channel slope, and the 2-year, 24-hour precipitation. We measured the basin size as 260 acres or 0.406 mi2, the slope as 76.3 feet per mile, and the 2-year, 24-hour precipitation as 3.1 inches. Plotting the basin location on Figure 1 in the Illinois section of the NFF publication, we determined that the basin lies in hydrologic region III. Inputting the parameters to NFF provides the results shown in Figure 2.

Figure 2. Rural NFF Results

To get to the urban calculation window, enter "N" in answer to the intervening questions. When we enter "Y" to the prompt for urban calculations, the window in Figure 3 requires that we select "1" for the National Urban equations.

Figure 3. National Urban Equations Window

When we enter "1," the ensuing windows prompt us for the urban parameters needed. The equations should not be used if any of the seven variables (except slope, which is limited to 70 feet per mile) are outside the range of those used in the original regression study. We measured the basin length, L, as 6,000 feet, or 1.136 miles; estimated the basin development factor, BDF, as 8; estimated the impervious area, IA, as 50%; entered the main channel slope, SL, as 70 feet per mile because it is the maximum value (ours measured 76.3 feet per mile); determined the 2-year, 2-hour rainfall as 1.7 inches; and estimated the storage factor as 0%. After entering these parameters and answering "Y" in the window asking if we want to calculate a hydrograph, the window shown in Figure 5 asks if we want NFF to estimate the basin lag time.

Figure 5. Basin Lag Time Prompt Window

Entering "Y" results in the window in Figure 6, which shows the calculated lag time of 0.38 hour.

Figure 6. Basin Lag Time Results

The next prompt asks us for which peak value we want to calculate the hydrograph. We will choose the 100-year peak, which results in the window shown in Figure 7.

Figure 7. 100-year Urban Hydrograph

When we enter "100," NFF displays the hydrograph ordinates in Figure 7. The prompt asks if we want to plot the resulting hydrograph. Unfortunately, this version of NFF can only display the resulting graph to the screen. In some cases, depending on the computer operating system, NFF may black out and/or lock up the work station. If it does display a plot of the hydrograph, the plot cannot be printed or saved because of the incompatibility between MS DOS programs and the MS Windows environment. Beyond the above window in NFF, if you wish to plot, save, and/or print the resulting hydrograph, you must save the ordinates to an NFF file and import them to Excel. (Remember, when saving files in MS DOS format, the file names can only be up to 8 characters long.) Then, using the charting option in Excel as in last month's example, you can plot, save, and print the hydrograph.

The revised version of NFF will eliminate display, plotting, and printing problems currently encountered in the current DOS version.

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
  • Weighting NFF results with observed data
  • Estimation of extreme floods
  • Flood hydrograph estimation using NFF

Upcoming Bulletin Topics

  • Revised NFF Software

View the archive page for all Flood Hazard Mapping listservs.

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