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 1

Some unusual parameters must be developed to correctly apply the equations for several States. Some of the unusual parameters include: main channel slope, Sl; main channel length, L; average basin elevation, El; basin relief, BR; forest cover, F; runoff-curve number, RCN; storage in lakes, ponds, and swamps, ST; area of limestone or Karst geology, LI; basin development factor, BDF; basin shape index, SH; and mean basin slope, Sb. Drainage area is the sole parameter for only six States.

Average basin elevation, El, appears in several of the States' revised equations discussed in last month's bulletin. El can be easily estimated using the USGS 7.5' topo map(s) covering the basin. The simplest way to make the estimate is by using a clear overlay on which is laid out some convenient equally spaced grid.

On the example above, elevations of points at the grid intersections can be estimated by eye interpolation. USGS recommends estimating elevations at 50 to 100 grid intersection points within the basin of interest and averaging the results. Within the highlighted basin, 94 points of the overlay grid are highlighted in orange. Estimating elevations at each of the 94 points and summing the elevations results in: 62' + 61' + 46' + 45' + 50'... + 61' + 61' + 58' + 55' + 59' = 4611'. Dividing by the total number of points, 94, results in an average elevation of 49.0'.

Basin relief, BR, is used in the equations for Maryland and Delaware, and is determined by subtracting the outlet elevation from El. On the example above, the outlet elevation point falls just downstream of the 10' contour crossing and is estimated to be elevation 9.9'. The value of BR would then be calculated as 49.0'-9.9' = 39.1'.

Main channel length, L, is sometimes incorrectly measured as the length from the point of interest to the end of the streamline symbol on the USGS topo map. It is actually the total distance in miles from the point of interest to the highest point on the basin boundary following the main channel route. On the example above, the main channel length is measured along the dark blue line from the point of interest to the highest basin divide point. This length is 6,100' or 1.155 mi.

Main channel slope, SL, which is another parameter that is sometimes miscalculated, appears in 27 of the 51 sets of equations. This slope, in ft./mi., should represent the average slope of the main channel of the stream upstream from the point of interest. It is calculated by determining the locations and elevations of points at 10% and 85% along the main channel from the point of interest to the basin divide, then dividing the difference in elevations by the distance in miles between the 10% and 85% points. In the example above, the points are shown at 0.1L and 0.85L, and are at 610' and 5,185' upstream of the point of interest, respectively. Estimating the streambed elevations at these points, the elevation at 0.1L = 12.1' and the elevation at 0.85L = 51.3'. The main channel slope can then be calculated as (51.3'-12.1')/ (1.155 mi.x 0.75) = 45.2 ft./mi.

SL = (El.85L-El.10L)/0.75L.

This value should not be confused with mean basin slope, which appears in 2 of the States' equations and is calculated differently. More on this in next month's bulletin.

The basin shape index, SH, is similarly easy to estimate from USGS topo maps. It is simply the ratio of the square of the main channel length in miles to the basin area in sq. mi. SH=L2/DA. In the example above, the basin area is measured as 525 acres, so the drainage area is obtained as follows: 525 ac./640 ac. per sq. mi. = 0.820 sq. mi. The basin shape index is then calculated as (1.155 mi.)2 /0.820 sq. mi. = 1.627.

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

Upcoming Bulletin

  • Part 2. Unusual parameters of USGS regression equations and how to obtain them
  • Examples in which USGS regression equations are used for NFIP purposes.

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