2.0

Calculating the Runoff from Watershed Areas
549
Editing and Defining Rainfall Frequency Values
for Counties
When you use the TR-55 Graphical and Tabular methods of calculating runoff
for a site, you are required to input the rainfall frequency 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.
The storm frequency you are designing for and the location of the site
determine the rainfall amount that you use to calculate runoff. Typically, the
municipality in which you are working provides requirements that your design
must meet. 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 enter the data into the Rainfall Frequency Editor by using the following
steps.
To edit and define rainfall frequency values for counties
1 From the Hydrology menu, choose Runoff ä Rainfall Frequency Editor to
display the Rainfall-Frequency Editor dialog box. The dialog box contains
rainfall depths by county for various storm frequencies. These depths are in
inches, and are for 24-hour time frames.
2 Edit the values as needed, using the following guidelines:
If you want to} Then}
change a county name or existing
frequency data
double-click within the cell and type the
new value.
add rainfall data for a new county place your cursor in the row you want to
insert the new row above, and click Insert.
delete the record for a county place your cursor in the row you want to
delete and click Delete.
locate a value use the navigation buttons.
The following describes the navigation buttons available in the Rainfall-
Frequency Editor dialog box:
 and : To move up or down one row at a time.
PgUp and PgDn: To move up or down one screen at a time.
Home and End: To move to the beginning or end of the list.
NOTE
When you click in a row in this editor, the row is not highlighted. The
position of the blinking cursor indicates the selected row.
3 Click OK.