Modified Rational Method

The Modified Rational Method provides a way to calculate the hydrograph from a catchment based on rational method C values and the peak intensity. There is no "loss method" associated with the modified rational method. The underlying assumption is that the peak intensity is maintained for a long enough duration to reach peak flow at the outlet of the catchment. This results in a trapezoidal hydrograph as shown below.

Qpeak is determined from the rational method (link to rational method topic)

Q = C i A

When using English units i is intensity in in/hr, A = area, acres, Q = flow, cfs and C is runoff coefficient, dimensionless.

The time to reach the peak is based on the time of concentration in the catchment which the user can manually enter or calculate using a variety of methods (Rational Method).

The length of the recession leg is based on the time of concentration times a recession multiplier which is set in the calculations options.

The intensity and duration are taken from the IDF curves (tables) based on the duration and frequency (return period) of the storm.

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