• This method is a very useful method for evaluating the peak rate of run-off.
  • This method is based on the fact that if a rainfall is appli to an impervious surface at a constant rate,
  • The resulting run-off from the surface would finally reach a rate equal to the rate of rainfall.
  • In the beginning only a certain amount of water will reach the outlet, but after sometime, the water
  • Will start reaching the out let from the entire area and in this case the run-off rate would become equal to the rainfall rate.
  • The time require to reach this equilibrium condition is know time of concentration and the peak rate of run-off would be equal to the rate of rainfall.
  • This is the basis of the rational method. The peak rate of run-off can be estimated using the following formula:

\[_{Rp}= \frac{1}{36}kpA\]


Rp = Peak rate of run-off in cumecs.

k = Run-off coefficient

p = Rainfall intensity, during the critical rainfall duration is equal to the time of concentration in cm/hr.

A = Drainage area hectares of the catchment in hectares.