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.

Unusual Parameters of USGS Regression Equations and How to Obtain Them - Part 3

Some unusual parameters must be developed to correctly apply the equations for several States. In the last two months we discussed main channel slope, Sl; main channel length, L; average basin elevation, El; basin relief, BR; forest cover, F; runoff-curve number, RCN; and storage in lakes, ponds, and swamps, ST. This month we will discuss basin development factor, BDF; areas of limestone or Karst geology, LI; and mean basin slope, Sb.

The basin development factor, BDF, is used to account for the effects of urbanization on the rural regression equation results. The BDF can be determined from topographic maps and field inspection. After the drainage basin has been delineated, divide the area into approximate thirds by upper, middle, and lower areas. The division should be made such that each section contains approximately one-third of the total basin, and travel distances along the streams are approximately equal. Each third will be evaluated using four categories: curb-and-gutter streets; storm sewers; channel improvements; and channel linings. Below are some examples of subdivision for different shaped basins.

Each category is assigned a code of "0" or "1," depending on the degree affected by the condition. If the area is more than 50% affected by the condition, assign a "1," if the area is less than 50% affected, assign a "0." Curb-and-gutter streets: if more than 50% of the subarea has residential, commercial, and industrial development and more than 50% of the streets are curb-and-gutter, assign a "1," if not, assign a "0." Storm sewers: if more than 50% of the secondary tributaries are enclosed in storm sewers, assign a "1," if not, assign a "0." Channel improvements: if more than 50% of the combined lengths of the main channel and the principle tributaries have been straightened, enlarged, deepened, cleared, or otherwise hydraulically improved, assign a "1," if not, assign a "0." Channel linings: if more than 50% of the combined lengths of the main channel and principle tributaries have been lined with impervious material, assign a "1," if not, assign a"0." After evaluation of all thirds, sum the assigned codes to determine the BDF, which can range from 0 to 12.

Mean basin slope, Sb, in feet per mile, is different than main channel slope and can be calculated from data determined from USGS topographic maps. The calculation is done by measuring the lengths in miles of the contour lines within the delineated drainage basin. In basins that are less than 50 square miles, reasonable accuracy can be obtained by measuring only the 100-foot contour lines on 1:24,000 scale (7.5 minute) maps. For basins between 50 and 300 square miles, the 1:250,000 scale (1°x2°) maps can be used. For basins above 300 square miles, Sb generally approaches an average of 500 feet/mile For comparison purposes, we have used the basin (525 acres = 0.820 square miles) from the September bulletin and measured the 20' contour lines (60', 40', 20').

The lengths measured for the 60', 40', and 20' contour lines were 2.576 miles, 2.746 miles, and 1.439 miles, respectively. The next step is to multiply the sum of contour line lengths by the contour interval, 20', which gives 6.761 miles x 20 feet = 135.228 ft.-mi. The final step in the calculation is to divide the result by the drainage basin area in square miles, which gives 135.228/0.820 = 164.8 feet./mile. When compared with the main channel slope of 45.2 ft./mi. calculated in the September bulletin, the difference in parameters is obvious.

The final parameter we will discuss is areas of limestone or karst geology, LI. Karst hydrology is an issue that merits more investigation and study. In areas of known karst geology, it is appropriate to include the variable in the regression equations. The value is used as the percentage of the basin of interest where known karst conditions exist. Areas can be measured from available geologic maps and divided by the basin size, then multiplied by 100 to get the percentage. Because of the many uncertainties in karst areas, results from regression equations in karst areas should be evaluated against available gage data in similar watersheds. Gage data can be found in the Water Resources Investigation Reports (WRIRs) for the State of interest, or at the USGS web site at http://water.usgs.gov/nwis/peak.

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

Upcoming Bulletin Topics

  • Examples in which USGS regression equations are used for NFIP purposes.

View the archive page for all Flood Hazard Mapping listservs.

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