The Rational Method Equation and Parameters
The primary equation for Rational Method calculations is: Q = CiA, where:
A is the area that drains to the design point of interest (the drainage area) in acres (for SI units: ha)
C is the runoff coefficient for that drainage area, that is, the fraction of rainfall on the drainage area that becomes storm water runoff. It is dimensionless.
i is the design rainfall intensity in inches/hr (for SI units: mm/hr)
Q is the peak storm water runoff rate from the drainage area due to the design storm in cfs (for SI units: m3/s)
A note on units: The equation, Q = CiA is for the U.S. units given above. This equation actually gives Q in acre-in/hr. The unit conversion comes out to be 1.008 cfs/(acre-in/hr), so the equation, Q = 1.008CiA could be used, but in view of the accuracy with which the runoff coefficient can be determined, the conversion constant is typically taken to be simply 1. With the S.I. units given above for Q, i, and A, the Rational Method equation is Q = 0.0028 Cia.
Calculation of the peak storm water runoff rate, Q, is quite straightforward if values are known for C, i, and A. Some information on typical ranges for C values is given in the table at the right. The next section presents and discusses an Excel spreadsheet for calculating the design rainfall intensity, i, based on rainfall intensity duration frequency data for the design location, and then calculating the design storm water runoff rate, Q.
For more background information about the Rational Method and the parameters, Q, C, i, and A, see the article, "The Rational Method for Calculation of Peak Storm Water Runoff Rate." For background information about calculating design rainfall intensity, i, see the article, "Calculating Design Rainfall Intensity for Use in the Rational Method."