Rational formula. The rational formula used in calculating runoff is Q = CIA. In this equation, Q is the peak runoff in cubic feet per second, C is the runoff coefficient which depends on the nature of the land cover, I is the intensity of rainfall in inches per hour for a duration equal to the time of concentration for the drainage area involved, and A is the watershed area in acres. The time of concentration is the time required for runoff from the upper reaches of the watershed to reach the point for which runoff rates are being calculated.
Values of coefficient (C). Runoff coefficients used in the rational formula shall be based on the guidelines in the following table and acceptable to the Borough Engineer:
Values of rainfall intensity (I). Rainfall intensities to be used in the rational formula are as follows:
Average velocities for overland flow. Engineers use several methods to determine the time of concentration for the drainage area in question. Seeley's Nomograph is reproduced here. The average velocities for overland flow presented in PennDOT's Design Manual are also widely used to determine the time of concentration. Any method acceptable to the Borough Engineer may be used.
Manning's equation to determine the velocity of flow in open channels and closed drains not under pressure is listed below. The second equation is used to determine the capacity after the velocity has been determined.
The coefficient of roughness used shall be as follows, unless different coefficients are approved by the Borough Engineer.
Maximum stream velocities in open channels.