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I've been given an assignment to model 2- and 100-year storms for a 70-acre site using EPASWMM. Previously, the local jurisdiction wasn't particular about what software was used on a project, but for some reason they are requiring SWMMM for this one. We have the appropriate IDF curves and coefficients needed to model runoff for those storms, except that EPASWMM doesn't use that data, and the users manual, etc., give no information on how to translate the IDF based data into something that can be input into the EPASWMM model. Can someone explain or point me toward a source that will describe how to develop storm runoff data using IDF coefficients, that can be input into the EPASWMM5.1 model?


You need to figure out how to model hydrology in SWMM
1. use SWMM hydrology to general hydrograph (It is basically a non linear reservoir routing method);
2. develop hydrograph outside of SWMM and introduce them at nodes as external flows;

Also, you need to understand that SWMM is a dynamic model which is different from rational method (a single peak flow), although there are some methods to convert rational method peak flow to a hydrograph - talk to you local agency get their approval first.


Your stormwater agency should be able to provide guidance on the appropriate rainfall distribution. Although it is possible to generate a temporal rainfall distribution from IDF data, predefined rainfall distributions are still used for most studies. Check your regulations for details.

Peter Smart
HydroCAD Software


Thanks guys. I see my next step is to sit down with Public Works and see if the actually do have criteria regarding the rainfall distribution. Since they have not been using SWMM in the past it might be something they haven't considered.


There's an even chance they'll want the 24 hour SCS storm distribution for your area (Type I, II, III, IA) in which case you find out your 100 year 24 hour intensity (in inches per hour) from your IDF curve, and multiply that by 24 to generate a total volume of rainfall (in inches). Then you'd multiply the hyetograph by that number on an ordinate by ordinate basis. That's if you're doing it by hand. I'm sure EPA-SWMM has functions that will do this for you, you just have to figure out what they are. My experience with EPA-SWMM is limited.

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


Hi Bank,


If NOAA (http://hdsc.nws.noaa.gov/hdsc/pfds/) has the depth duration data for your area, you can follow this hyetograph generator. It basically allows converting depth duration data to IDF coefficients--and then the IDF coefficients to rainfall hyetograph using alternating block method (Chow, Applied Hydrology).

If you need help, pls email me. Thanks


Thanks for all the comments folks. We ran the model using SCS hydrograph data and submitted it. We were then told that we needed to use a Clark's Unit Hydrograph to generate the input data. At least now I have some direction. Now I get to figure out how to do that.

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