Bowstring method/Modified Rational Method

[ajs75]

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.

 

[RWF7437]

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

 

[RWF7437]

http://www.forester.net/sw_0107_stormwater.html