King County Rational Method

This program supports a narrow subset of the traditional Rational Method implementation. The following procedure is only applicable in King County, Washington .

King County uses the following Rational Method equation:

where

• Qr=peak flow (cfs) for a storm of return frequency R
• C=estimated runoff coefficient
• Ir=peak rainfall intensity (inches/hr) for a storm return frequency R
• A=drainage sub-basin area (acres).

The formulation of ‘C’ is the same as the traditional method. It is the Ir value that is interesting. Ir is the peak rainfall intensity computed by:

where

Pr= the total precipitation at the project site for a 24-hour duration storm event, for a given return frequency.

• Tc is the time of concentration in minutes, (must be between 6.3 and 100).
• “ar”and “br” are coefficients used to adjust the equation for he design storm

Using the King County Rational Method also requires that the travel times used for computation be computed using the SCS TC equation.

Ron Mayo P.E. formulated the above equation is based on the original Renton/Seattle Intensity/Frequency/Duration curves. The procedure is specific only to the King County, Washington.