SCS Unit Hydrograph method - TR-20 or TR-55?

  Applies To 
  Product(s): PondPack, SewerGEMS, CivilStorm
  Version(s): V8i, CONNECT Edition
  Area:  Modeling
  Original Author: Dan Iannicelli, Bentley Technical Support Group


Is the unit hydrograph method available in SewerGEMS, CivilStorm, and PondPack based on TR-20 or TR-55? How can I get runoff hydrograph results such as peak flow to match TR-20 or TR-55?


If you are using SCS Unit hydrograph method with SCS loss method, the results should match closely with TR-20 hydrology. TR-20 represents the full bore implementation of these methods, whereas TR-55 is a more simplified version that lent itself more to hand calculations. TR-55 was a simplified procedure that was used when engineers didn't have computers that could properly crunch the numbers for them. It was simplified from TR-20. TR-20 is typically preferred in modeling software.

So, SewerGEMS, CivilStorm and PondPack match TR-20 well when looking specifically at hydrology, but when you are routing flow through channels, and ponds, the dynamic numerical solver behind SewerGEMS and CivilStorm is much more sophisticated. The results should be close if there are few tailwater effects, but once backwater comes into play, the dynamic solvers in SewerGEMS and CivilStorm will be much more accurate.

PondPack may be a better choice for the user who wants to match up more closely with overall TR-20 results when dealing with Pond routing and channel routing.

In PondPack, SewerGEMS and CivilStorm, the SCS unit hydrograph method's default approach uses a hybrid of the strengths of HEC-1 and TR-20. To choose to strictly use HEC-1 or TR-20 in PondPack, change the method under Tools > Options. Also in the advanced option dialog for each catchment, set the internal time increment to match the internal calculation increment used for that subarea within the TR-20 or HEC-1 program respectively.

The curve number tables PondPack uses were taken from TR-55.

Further references in the PondPack help documentation:

1) The section titled "Overview of the SCS Unit Hydrograph Method" (location may differ depending on your PondPack version: using search is recommended).

2) The section titled "CN Weighting", which discusses how the CN information is based on TR-55.