Stormwater Drainage

Stormwater Runoff Calculator

Modified Rational Method

Calculate peak flow rates and total runoff volume based on local rainfall data, drainage area, and surface conditions.

1
Input Parameters

Select the design storm length (often matched to time of concentration); NOAA intensities below are shown as in/hr for this duration, and shorter durations raise the intensity.

2
Results

Peak Flow Rate

0cfs

Total Volume

0cf

About the Modified Rational Method

The Modified Rational Method estimates peak discharge and runoff volume from small drainage areas, extending the traditional Rational Method by incorporating storm duration for volume calculations.

Q = CIA

  • Q = Peak flow rate (cfs)
  • C = Runoff coefficient
  • I = Rainfall intensity (in/hr)
  • A = Drainage area (acres)