SCS Unit Hydrograph

[godpneuma]

I have seen several reports that refer to a calculation method called the SCS Unit Hydrograph. Is this the same as the Tabular method, or the graphical method as described in the TR-55 manual, both, or something completely different?

Thanks

 

[SiPaul]

I generally associate the unit hydrograph with the TR-55 method of computing flow rates. The typical unit hydrograph is adjusted for the particular conditions (RCN Tc Area etc).

 

[psmart]

The SCS (now NRCS) Unit Hydrograph is typically used with the SCS runoff procedure, as embodied in TR-20 and other programs. The UH is used to predict the runoff from a single burst of rainfall. Complete runoff hydrographs are produced by combining a large number of UH's for an entire rainfall distribution. (Its actually a bit more complicated, but this is the general outline.) For further info see

http://www.hydrocad.net/tr-20.htm

The TR-55 Tabular Method is derived from TR-20, and is designed to approximate the runoff hydrographs that would be obtained with TR-20 itself. TR-55 does't use the UH directly, since it is already embodied in the TR-20 results that it employs. For details see

http://www.hydrocad.net/tr-55.htm

A number of hydrology applications, such as HydroCAD, employ the actual SCS runoff procedure, making it easy to use a different UH if required for special applications. For details see

http://www.hydrocad.net

 

[sapaeng]

National Engineering Handbook Section 4-Hydrology, USDA Soil Conservation Service, 1972

This covers the engineering and science behind the unit hydrograph. Type in a search engine and you can download a copy. It is sometime listed as NEH 4