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.

## How to Treat "State Line Faults" (Basins Lying in More Than One State)

The main shortcoming of NFF becomes apparent when a basin delineation crosses State boundaries. This raises the question of which set of equations to use. Unfortunately, there is no standard way to resolve this situation, but there are several techniques that can be applied.

Users can treat the watershed just like one crossing two hydrologic regions in the same State; the only difference in this case is that the hydrologic regions are located in different States. Compute discharges from each of the regions using the appropriate equations and weight the discharges from each region using the drainage area as the weight factor. NFF may not be able to do the weighting automatically, but it is easily done manually. Following is an example.

For the delineated basin lying in West Virginia (0.828 mi2) and Pennsylvania (0.586 mi2), applying the West Virginia equations (WRIR 00-4080) for the entire drainage area (1.414 mi2) gives the following result:

Applying the Pennsylvania equations (WRIR 00-4189) for the entire drainage area (1.414 mi2) gives the following result (the minimum drainage basin size used in developing the Pennsylvania equations was 1.5 mi2, but the example basin size is considered close enough to the limit to use in this case):

For the 100-year discharge, we can see that there is a difference of (887-699) = 188 cfs, or 21.2%, between the two estimates. The Standard error for the 100-year West Virginia flow is 31.3%, so we could accept either value. Weighting the results for the 100-year value by drainage area gives [699(0.586) + 887 (0.828)] ÷ 1.414 = 809 cfs.

Calculating the runoff for each State using only the portion of the basin within each State gives the following results (the portion of the basin lying in Pennsylvania brings the drainage area below the minimum value of 1.5 mi2 for the equations, but the calculation is being done to illustrate the application of the method):

Summing the results for the 100-year flow gives 645 + 368 = 1,013 cfs. If a combination method is decided to be the best approach, the weighted method result of 809 cfs for the 100-year flow should be used because the basin area used for Pennsylvania is well below the minimum used in developing the equations.

Data from gage stations located in different States but in hydrologically similar basins are often used in developing regression equations, as seen in the West Virginia regression equations (WRIR 00-4080). Stations from Virginia, Maryland, Ohio, and Pennsylvania and Kentucky were all used with stations within West Virginia. If the user has a drainage basin crossing State boundaries, they should review the WRIR publication to see if nearby stations in the neighboring State were actually used in developing the regression equations. There is a good chance they were used. Then the regression in one State, in this case a region in West Virginia, can be used for the entire watershed.

In the example, the values calculated using the West Virginia equations should be used for the final result. It is appropriate to do so because the basin's flow features are well represented in the equations.

## 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

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