Rainfall distributions, Rational Method and SCS

MickeyD30 · Jun 25, 2007

The requirement is that I use the rational method for the pre-existing conditions and compare it to the critical storm. In order to find this I have to run 2, 5, 10, 25, 50 and 100 year storms for the 1 hr, 4 hr, 8hr etc. storm for the developed conditions. I talked to the regulatory agency and normally they do not use HydroCad. The following is from a post 3-years ago which sound like what I am trying to do:

A competitor's software has an option which allows hydrographs to be generated using the Rational Method, but with the intensity from SCS 24 hour storm distribution. The Return Rate, Precipitation (in inches) and SCS Rainfall Type are specified...from there, an IDF curve is (apparently) generated.

Does anybody have any ideas how to do this with HydroCad?

psmart · Jun 25, 2007

Although it is theoretically possible to derive an IDF relationship from a rainfall distribution, I'm not sure why you would want to do this. Why not use your local IDF data and create an IDF file for HydroCAD? See

http://www.hydrocad.net/idf.htm

Of course, there have been MANY other posts on the limitations and complexities of the Rational method. Please see

http://www.hydrocad.net/rational.htm

RWF7437 · Jun 26, 2007

Psmarts answer is, of course, correct.

But why, oh why, would anyone want to compare the Rational Method with a hydrograph method ? First, you can't really do this. The Rational Method does not yield a hydrograph, only a peak flow. Second, the Rational Method is only allowed by most regulatory agencies to be used for very small drainage basins ( usually 20 acres or less in my part of the U.S.) Even if you use the so called Modified Rational Method you only get artificial triangular and trapezoidal hydrographs which don't look like anything seen in nature.

When you do such comparisons you will probably find wide disparities which can't be explained by any logical reasoning.

Finally, what is accomplished by all this ?

good luck

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Rainfall distributions, Rational Method and SCS

MickeyD30

Civil/Environmental

psmart

Active member

RWF7437

Civil/Environmental

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