SCS method
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
RE: SCS method
RE: SCS method
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
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
RE: SCS method
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
RE: SCS method
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.