National Flood Frequency Bulletin

Issue No. 17

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.

NFF Versions 3.0 and 3.1

After many adjustments, the Windows®-compatible version of NFF has been released. Version 3.0, released in November 2002, was updated by Version 3.1 in February 2003, to eliminate an error that sometimes occurred with urban flood estimates when one or more of the rural estimates used in the urban equations was missing. The database was updated to eliminate a problem of compatibility with Windows® XP. Version 3.0 users will need to update both the software and the database for the program to work.

To download the new software, visit the USGS web page at: http://water.usgs.gov/software/nff.html, and follow the download instructions there. When you have completed the installation, open the program by clicking on the desktop shortcut. When you do, the reduced window shown in Figure 1 will appear on your desktop. The default units of operation are English, but you can click the Metric radio button to change to metric units. Your local system user name will be displayed in the “User” text box, and the path to the installed NFF program on your computer will be displayed in the “Project” text box. You can change the username text by highlighting the displayed text and retyping the name you want.


Figure 1. Startup Window

You can work with the window reduced or open it to the full screen view by clicking the “full screen” button between the “minimize” and “ close window” buttons in the upper right-hand corner of the window. Click the “Run” button to begin your computing session. When you do, the calculation window shown in Figure 2 will appear on your desktop. Again, it will be reduced and you can either work in the reduced window or open it to full screen.


Figure 2. Calculation Window

Choose the State or area in which you wish to do calculations by clicking on the down arrow in the “State” text box. A drop-down list will appear from which you can select the State or area desired by scrolling down to view the entire list. Click on the State or area desired and the window shown in Figure 3 will appear. The revised software includes equations for the 50 U.S. States, Southwestern U.S., American Samoa, and Puerto Rico.


Figure 3. State Calculation Window

For this example, you will use Tutuila Island in American Samoa. In the “Site Name” text box you can enter the stream name, community name, or any identifier you want to use. In this case, enter “Aualii Stream.” Note that this text box does not clear from screen to screen or from State to State. It must be cleared or reentered for each site. To input the needed data, you will click on the “New” button on the Rural (left) side of the window. When you do, the window shown in Figure 4 will appear.


Figure 4. Data Entry Window

When you select the region with the cursor in the “Regions” text box, the window shown in Figure 5 will appear, which highlights the necessary data window(s) for entry. Since drainage area is the only parameter in the American Samoa equations, it appears and the range of values is shown.


Figure 5. Secondary Data Entry Window

Enter a drainage area of 0.33 square mile in the “Drainage Area” text box. Since American Samoa lies outside the 50 States, there is no Crippen & Bue region to enter, so the default “None” is correct. When you click the “Ok” button, the data entry window closes and the resulting window shown in Figure 6 appears.



Figure 6. Computation Results

To save the results, click on “File” on the menu bar in the upper left corner and select “Report.” The window shown in Figure 7 will appear. Enter a file name under which you want to save the results in the “File name” text box. You could enter “samoaAualii.” You can save the file in “text” format or “all files” format.

The text option is preferable; this allows you to open, read, and print the report using any word processing software, such as WordPad®, Word®, WordPerfect®,etc. When you click the “Save” button, the text file is saved in the NFF folder created when you installed the software.


Figure 7. Save Report Window

When you open the saved file with MSWord® or WordPad®, it looks like the following:

National Flood Frequency Program
Version 3.1
Based on Water-Resources Investigations Report 94-4002
Equations from NFF data base version 1.0
Equations for American_Samoa developed using English units

Site: Aualii Stream, American_Samoa
User: username
Date: Wednesday, July 03, 2002 09:00 AM

Rural Estimate: Rural 1
   Basin Drainage Area: 0.330 mi2
   1 Region
   Region: Island_of_Tutuila
   Drainage_Area = 0.330 mi2
Flood Peak Discharges, in cubic feet per second

Estimate Recurrence
Interval, yrs		 Peak, cfs Standard Error, Equivalent
Years
Rural 1
2
297
47
  
 
5
452
46
  
 
10
553
45
  
 
25
681
44
  
 
50
775
44
  
 
100
870
44
  
 
500
1080
   

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