Santa Barbara vs Tr-55 SCS Method

[martin888888]

What is the difference between the TR-55 SCS Method and the Santa Barbara method? I have a large highway project with sub-basins in the 5ac maximum range. What would be best to use for this project? Jurisdiction doesnt specify but they want a hydrograph method used instead of the Rational Method.

 

[psmart]

Both procedures use the SCS curve numbers and runoff equation to determine the portion of the rainfall that appears as runoff (the precipitation excess.) But instead of using the SCS Unit Hydrograph procedure to generate the final runoff hydrograph, the SBUH method routes the instantaneous runoff hydrograph through an imaginary reservoir with a time delay equal to the time-of-concentration.

The method you use will generally depend on what the regulations require. If you have a choice, you may want to evaluate both methods and see what is most appropriate or advantageous for your project. Some programs offer both runoff options, making it easy to compare the results. If you don't have a program already, you can do this with the free HydroCAD Sampler at

http://www.hydrocad.net/sampler.htm

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[martin888888]

But doesnt the TR-55 use a time of concentration parameter also? I am confused by your "reservoir" comment, I have been using the SBUH for years and that doesnt make sense

 

[psmart]

Both procedures use the same Tc.

That's the way SBUH works. Rather than using a unit hydrograph to delay the runoff, it uses a reservoir routing procedure to simulate the runoff delay caused by the Tc.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[martin888888]

So whats the difference between the 2 methods?

 

[psmart]

As I stated in my original reply, SCS/TR-55/TR-20 uses a unit-hydrograph procedure (convolution) to delay the instantaneous runoff and create the final runoff hydrograph, while the SBUH procedure delays the runoff by use of a routing procedure through an imaginary reservoir. Otherwise, the two procedures are the same.

The SBUH procedure is simpler to perform, and can even be done with a spread sheet, while the unit hydrograph procedure requires procedural coding (software) and takes longer to calculate. Although both are fast enough with current computers, the SBUH procedure was developed partly to provide a faster and easier alternative to the full unit hydrograph procedure.

To determine what this means in practice you would need to run some specific examples and compare the results.

Peter Smart
HydroCAD Software

http://www.hydrocad.net