Rational Method

The Rational method solves for peak discharge based on watershed area, Rational coefficient, and rainfall intensity for the watershed. The following equation is used to compute flow using the Rational method:


Q	=	Flow (cfs) for drainage area A
C	=	Weighted runoff coefficient for drainage area A
i	=	Intensity (in/hr.) for the given design frequency and storm duration (this value is taken from the I-D-F curves for your design area)
A	=	Drainage area (acres)

Note: A conversion factor of 1.008 acre inches/hour per cfs makes the Rational equation unit-consistent, and is used by PondPack.

Some localities have C adjustment factors for different storm frequencies.

C, the Rational coefficient, is the parameter that is most open to engineering judgement. In many cases, an area weighted average of C coefficients is used as the C for the entire drainage area. Bentley SewerCAD calculates the weighted C for drainage areas.