Hec-1 Utilization of SCS Methodology

[RHOeng]

The SCS TR55 release dated January 1975 uses slope adjustment factors for computing peak discharge. It computes runoff then has a page of tables with multipliers based on basin area and slope. It defines 3 classes of slopes.

Flat Slopes = less than or equal to 2%
Moderate Slopes = 3% to 7%
Steep Slopes = 8% to 50 %

I was told in HEC-1 class years ago that only moderate slopes were written into HEC-1 and for slopes outside that range all hydrographs and flows had to be manually adjusted. This was questined recently by a young reviewer. Does anyone know if this is true and if so where is it documented?

Charleyo

 

[RHOeng]

Thanks to everyone. I had 3 people send me direct email. SCS is limited in HEC-1 and HEC-HMS because of the formula that was used. They coded in a constant of 484 which is for moderate slopes 2-7 percent. In reality this hydrograph shape factor evidently varies from 300 to 600.

 

[psmart]

484 is actually the "peak factor" of the standard SCS unit hydrograph, as used in the TR-20 runoff model, from which the TR-55 tables were derived. To use a different PF with TR-20, you actually provide a different UH, such as the Delmarva UH, which has a PF of 284.

 

[kknowle2]

There are also supposed to be standard or calibrated UH curves for PF of 256 and 323. Can anyone provide these or tell me a way (other than purchasing certain proprietary computer programs) that I can obtain them?

http://www.kcesco.com