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Bowstring method/Modified Rational Method

Bowstring method/Modified Rational Method

Bowstring method/Modified Rational Method

(OP)
What is the difference between using the Rational "bowstring" method versus just the Rational Method for calculating required storage volumes? I assume the bowstring method uses a curve factor. I cannot find any lit on the subject? Thanks for any insight.

RE: Bowstring method/Modified Rational Method

Don't know what the "bowstring" method is; at least not by that name.

You can't simply use the Rational Method for volumes because it does not produce a hydrograph.  There are several modified Rational Methods which can be used to produce a hydrograph, however.  The simplest uses triangular and trapezoidal hydrographs to arrive at a supposed "critical storm duration" and volume of storage.  It is best explained in the Washington State Highway Dept. Hydrology Manual.  Frankly I don't find it convincing and use it only when required to by some local regulatory agency and only for small drainage areas ( <25 acres).

A "better" modified method is used in Riverside County California.  It is better in the sense that it uses actual historic storm patterns and can be calibrated the produce realistic results.  I don't know if it has been calibrated but the results do appear reasonable and can be easily calculated using a spreadsheet.

good luck

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