| Product(s): | SewerGEMS, CivilStorm |
| Version(s): | V8i, CONNECT Edition |
| Area: | Calculations |
Catchment runoff hydrograph results are different between using the Unit Hydrograph Runoff Method and the EPA-SWMM Runoff Method.
The Unit Hydrograph and EPA-SWMM Runoff methods are two entirely different methodologies, so the runoff results are not expected to be the same. There are different sets of coefficients and other inputs involved in computing the overall runoff hydrograph, and their different methodologies can also lead to differences in results (apples-to-oranges comparison).
See pages 72-73 for information on how the EPA-SWMM (NonLinear Reservoir) runoff method works:
https://www.epa.gov/sites/production/files/2019-02/documents/epaswmm5_1_manual_master_8-2-15.pdf
Also see the SWMM Hydrology manual here: https://www.epa.gov/water-research/storm-water-management-model-swmm#downloads
In contrast, see our Stormwater Conveyance Modeling and Design book for information on how the SCS Unit Hydrograph Method works.
Also see: http://learn.hydrologystudio.com/hydrology-studio/knowledge-base/scs-nrcs-hydrographs-2/