The Peak Discharge from Drainage Basin Runoff is found using the Rational Method: `Q=CIA`, where:
- `C` = runoff coefficient
- `I` = average rainfall intensity
- `A` = drainage area
Use the Rainfall Intensity Calculator if you need `I`.
The Math / Science
The Rational Method estimates peak discharges for drainage areas of up to about 200 acres (80 hectares) and no significant flood storage. Although the Rational Method estimates the peak discharge, it cannot estimate a flow volume or time series of flow.
Assumptions under the Rational Method: (If any one of these conditions is not true, the user should try an alternative method.)
- tc for the drainage area must be less than the duration of peak rainfall intensity
- Runoff is directly proportional to rainfall intensity
- Rainfall intensity is uniform throughout the storm
- Rainfall is distributed evenly over the drainage area
- Rational Method does not account for storage
- Available storage is assumed to be filled
- Frequency of occurrence for the peak discharge is equal to the frequency of the rainfall producing that event
- Minimum duration of rainfall intensity is 10 minutes
- If the time of concentration (tc) computed is less than 10 minutes, then 10 minutes should be adopted for rainfall intensity computations
References
Online Manuals (http://onlinemanuals.txdot.gov/txdotmanuals/hyd/rational_method.htm)
Geography AS Notes (https://geographyas.info/rivers/discharge-and-hydrographs/)