Procedure for using the Rational Method

The rational formula estimates the peak rate of runoff at a specific location in a watershed as a function of the drainage area, runoff coefficient, and mean rainfall intensity for a duration equal to the time of concentration. The rational formula is:
EquationObject3144
Equation 4-20.
Where:
  • Q
     = maximum rate of runoff (cfs or m
    3
    /sec.)
  • C
     = runoff coefficient
  • I
     = average rainfall intensity (in./hr. or mm/hr.)
  • A
     = drainage area (ac or ha)
  • Z
     = conversion factor, 1 for English, 360 for metric