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# SCS method2

## SCS method

(OP)
I am sizing a detention pond with a 260 ac watershed. I am using the SCS method and coming up with 22 ac-ft of storage. The actual developed property within the watershed is 60 acres. This seems to be a high required volume. When I say high volume the only thing I have to compare it to is the rational method with a smaller watershed. The time of concentrations and CN values are relatively close for the pre and post. Is their another method that I can use for a comparison other than the rational? I do not trust the rational.......

### RE: SCS method

You can't do volume calculations using the rational method. There are plenty of threads that explain the ins and outs of it, and also a recent one debating the Modified Rational Method which can give a volume.

### RE: SCS method

Oh, and, a 260 acre watershed almost certainly has a listed stream coursing through it, which you can't put a detention basin into.  You will have to detain the flow from the development separately before it enters the stream.  This will be much cheaper as your detention basin volume will be significantly smaller.

### RE: SCS method

Modified Rational will always give lower storage, due to both the pre and post Mod Rat inflows are higher.

Francesca is correct.  Don't analyze 260 acres, analyze your 60 acres.  Just provide a swale sized for the other 200 acres to bypass your storage volume, or provide for it to flow through the volume undetained.

22 ac-ft is more storage than I would expect for 60 acres.  Assuming post CN of 86, 50 cfs in, 10 cfs out with 8 inches of rain, you should be seeing a storage estimate of about 2/3 that.

Engineering is the practice of the art of science - Steve

### RE: SCS method

A quick volume method is to use apply the SCS loss based on your curve numbers to your total precipitation to determine the total storage.  This also helps give a rough estimate for changing land use (its original intent).

Q= (P-0.2S)^2/(P+0.8S)
where CN=1000/(10+S)

The source is NRCS Hydrology Manual at:
http://www.info.usda.gov/CED/

Are you required to look at the entire watershed?  I know in the west at least, some communities require that the first developer has to put in detention for the whole watershed and then gets reimbursed for this "regional" facility as the rest of the shed is developed.  A 260 acre watershed could possibly be this.

### RE: SCS method

(OP)
Thanks for everyone help. Now I have another question. Where can I find the rainfall distribution curves for my area. I was using the one in PondPack for Type III 24 hr rainfall distribution for this watershed.

### RE: SCS method

Most hydrology programs will include the SCS Type I, IA, II, and III rainfall distributions.  If you can use one of these tables, all you need to provide is the total rainfall depth for the n-year event.

A few locatilies have developed their own rainfall distributions.  In this case, the details should be in the stormwater regulations.  You can use this information to create a custom rainfall table for most programs.

### RE: SCS method

A nice feature of HMS is running the frequency based method.  You enter precipitation for durations up to your storm duration and then weight it.  A center or 2/3 is a typical conservative weighting for most locations depending on how you model losses.

### RE: SCS method

Careful iha,

There are at least two methods out there calling themselves the Modified Rational Method. One is really Hokey and the other is a little Hokey but even a Hokey Method can be calibrated to produce reasonable results.

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