Rational Method for a Composite Catchment:  Description

The script ONLINE_RATIONAL_COMPOSITE will calculate a peak discharge for a composite catchment using the rational method, following the Example. The calculator can take either SI or U.S. Customary units. In SI units, subareas are given in km2; in U.S. Customary units, in mi2.

The time of concentration (same as rainfall duration) is given in minutes (min). The return period is given in years (yr).

The Intensity-Duration-Frequency curve (IDF curve) is defined as a function of return period T and rainfall duration tr, which is made equal to the time of concentration tc. Defaults values for the parameters of the IDF curve are as follows: k = 1000 (mm/hr); b = 20 (min); n = 0.2; and m = 0.7. These default values are intended for testing purposes.

       k T n
I  =  _______________
       (tr  +  b)m

The time of concentration tB should be greater than the time of concentration tA, as shown in the figure. The output is peak discharge Qp in m3 or cfs, depending on the choice of units, and the rainfall duration trd associated with the peak discharge, or design rainfall duration.

rational method for a composite catchment