Results Different between Unit Hydrograph and EPA-SWMM Runoff Methods

Product(s): SewerGEMS, CivilStorm
Version(s): V8i, CONNECT Edition
Area: Calculations

Problem

Catchment runoff hydrograph results are different between using the Unit Hydrograph Runoff Method and the EPA-SWMM Runoff Method.

Solution

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 boom for information on how the SCS Unit Hydrograph Method works. Also see: http://learn.hydrologystudio.com/hydrology-studio/knowledge-base/scs-nrcs-hydrographs-2/ 

Recommended
Related