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Rational Method with 3 times Tc rise hydrograph

Rational Method with 3 times Tc rise hydrograph

(OP)
All the municipalities that I work in have adopted a new common Stormwater Management Ordinance that was pushed upon them by our State's DEP. One of the requirements for using the Rational Method is to have the length of the ascending leg of the hydrograph equal to 3 times Tc and the descending leg length 7 times Tc. I'm having trouble modeling this in HydroCAD in a way that is acceptable to the reviewers. I have set my duration to 3 Tc and my rate of rise as 3 and rate of fall as 7. The reviewer is not accepting this because the Intensity used to calculate the peak of the hydrograph in not the Intensity for the Tc but the Intensity for the duration (3xTc). Setting duration to equal Tc and keeping the rise/fall at 3/7 doesn't work either. Is there a way to force HydroCAD to use the Intensity and therefore the peak Q that is associated with the Tc but have that peak occur at 3xTc and have the descending leg 7xTc? Thanks!

RE: Rational Method with 3 times Tc rise hydrograph

With respect to your agency, what they're suggesting is a misapplication of the Rational method. They're basically asking you to use the 10 minute intensity (as an example) but apply that full intensity for 30 minutes. This results in a tripling of the rainfall volume - it's neither the 10-minute depth nor the 30-minute depth.

Unfortunately, there have been many attempts to expand the applicability of the Rational method beyond it's original scope, and we're not able to support these non-standard approaches. That's one of the reasons to use a different runoff procedure, such as the SCS/NRCS method.

Peter Smart

RE: Rational Method with 3 times Tc rise hydrograph

Slight correction to my previous post: If you really want to force HydroCAD to use an intensity that's different from the IDF value, you can remove the IDF file from the project. This will let you manually enter any duration and intensity you choose. Of course, you will no longer be able to use the IDF file to easily switch events. You'll have to do this by manually changing the duration and intensity.

Peter Smart

RE: Rational Method with 3 times Tc rise hydrograph

(OP)
Thank you Peter. I know this is a misapplication of the Rational Method but no amount of explanation on my part can get the reviewer to change is opinion. His exact quote was "In 35 years of reviewing storm water designs, I've never seen it done this way." He is being irrational about the Rational Method. I appreciate the work around. Luckily his method didn't make me change my design. I won't be sealing that report though.

RE: Rational Method with 3 times Tc rise hydrograph

Sometimes I wish we could force reviewers to stamp things that they make us do and that we don't want to stamp. Might change their opinions a little.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com

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