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 km^{2};
in U.S. Customary units, in mi^{2}.
The time of concentration (same as rainfall duration)
is given in minutes (min).
The return period is given in years (yr).
The IntensityDurationFrequency curve (IDF curve) is defined as a function of return period T and rainfall duration t_{r}, which is made equal to
the time of concentration t_{c}. 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 = ^{_______________}
(t_{r} + b)^{m}

The time of concentration t_{B} should be greater than the time of concentration t_{A}, as shown in the figure. The output is
peak discharge Q_{p} in m^{3} or cfs, depending on the choice of units, and the rainfall duration t_{rd}
associated with the peak discharge, or design rainfall duration.
