The Modified Rational Hydrograph also assumes that the time to peak is equal to the tc, but allows for the duration of the storm to be longer than tc, resulting in a trapezoidal shaped hydrograph as shown below. A coefficient to modify the slope of the receding limb may also be applied with this method.
where:
tc = time of concentration.
Q = Flow at time t, in cfs.
Qp = Peak flow.
r = falling limb coefficient
sd = storm duration
Related Topics:
Computing Hydrographs with Rational Method
Rational Equation