2.0

Time of Concentration and Time of Travel
567
17 In the Time of Concentration column, enter a Tc. You can also click Tc at the
bottom of the dialog box to calculate the areas Tc.
NOTE
If you enter a value in this field, the Rainfall Intensity value is reported from
the IDF file. Enter zero (0) if you want to enter a value manually in the
Rainfall Intensity field.
18 In the Rainfall Intensity column, enter the average rainfall rate.
If you loaded an intensity duration frequency file, and entered a time of
concentration in the previous step, then this value is calculated automatically
for you. For more information, see Specifying the Rainfall Intensity in this
chapter.
After you have entered all of the required data for the rational equation, the
Rational Method calculator displays the computed runoff flowrate for the area
in the Flowrate column.
19 Click Save to save the information.
20 Click OK.
Rational Method Formula
The following formula is used to determine the peak discharge when you work
with the Rational Method:
QCCIA
f
where:
Q = Peak discharge (cfs)
C = Runoff coefficient, a ratio of runoff to rainfall
C
f
= Frequency factor for use with major storms
I = Average rainfall intensity for a duration equal to the time of concentration,
for a selected return period (in./hr)
A = Watershed drainage area (ac)