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Table Of Contents
Calculating the Runoff from Watershed Areas
547
Selecting the Rainfall Frequency for a County
When you use the TR-55 Graphical, Tabular, or the TR-20 method of calculating
runoff for a site, you are required to input a rainfall amount. The rainfall
amount is based on a 24-hour storm and the frequency you want to design for.
Rainfall frequencies can be entered for the county in which the site is located.
Autodesk Civil Design provides a Rainfall Frequency Editor which you can use
to enter and look up these frequencies.
The rainfall frequency data that Autodesk Civil Design uses is stored in the
county.rf file. This file is stored in the c:\Program Files\Land Desktop
R2\land\prot\hd folder. By default, a sample county.rf file for the state of
Maryland is provided with Autodesk Civil Design.
To obtain data for your state, you can use a county.rf file provided by the local
SCS (now NRCS) office. Or, you can obtain the written data, and then use the
Rainfall Frequency Editor to input the data into Autodesk Civil Design for easy
look-up and retrieval.
For more information about adding the rainfall frequency values for your state
into the Rainfall Frequency Editor, see Viewing, Editing, and Defining Rainfall
Frequency Values for Counties in this chapter.
To select the rainfall frequency for a county
1 From the Hydrology menu, choose Runoff ä Rainfall Frequency to display the
Define Rainfall-Frequency dialog box.
NOTE
The Define Rainfall Frequency dialog box is available from within the TR-55
Tabular Hydrograph Method dialog box and the TR-55 Graphical Peak
Discharge Method so you can easily look up and enter the values. One of
the options in the dialog box, Apply Rainfall, is only available when you
access the Define Rainfall-Frequency dialog box from within the TR-55
Tabular Hydrograph dialog box.