Technical data

Chapter 3 Performing Hydrologic Studies
44
To calculate the peak discharge using the Rational Method (continued)
Steps
Use
to look up
8 Specify the runoff coefficient.
You can type in a value for the runoff coefficient
that represents the ratio of runoff to rainfall, or
click Coef to select a single value from a list of
standard runoff coefficients. You can also click
CmpCoef to calculate a composite runoff
coefficient value, if applicable, for your site.
Specify the Rational
Runoff Coefficient
9 Select an adjustment factor.
The adjustment factor edit field is not directly
editable, but you can click Factor from the
Rational Method dialog box to display the
Frequency Factor Editor dialog box. Select the
Use Frequency Factor check box, and then select
the appropriate storm event from the list of
events. Click OK to return to the Rational Method
dialog box, and add the appropriate adjustment
factor for the specified storm event to the
adjustment edit field.
Specify a Frequency
Adjustment Factor
10 Specify the time of concentration value.
You can type a value for the time of
concentration, or click Tc to display the Time of
Concentration Calculator.
You can use this calculator to specify the sheet
flow, shallow flow, and channel flow parameters
and compile the time of concentration data.
Calculate the Watershed
Time of Concentration
11 Click Save to display the Save Rational Method
Data dialog box. Enter the file name and click
Save to return to the Rational Method dialog box.
12 Click OK when you are finished to close the
Rational Method dialog box.
To determine the runoff peak discharge for other storm events, select
the new storm frequency from the popup list in the Rational Method
dialog box. The software automatically re-calculates the appropriate
rainfall intensity and the runoff peak discharge.
For example, if you select 100 from the Rainfall Frequency popup list,
the runoff peak discharge for the 100-year storm event is calculated.