Product(s):	SewerGEMS, StormCAD, CivilStorm, SewerCAD
Version(s):	10.XX.XX.XX and higher
Area:	Layout and Data Input

 

Overview

OpenFlows Storm and OpenFlows Sewer applications support several runoff calculation methods. This article explains which methods are supported by each solver, and which rainfall (storm data) types are supported for each runoff method. Note that this article was written based on Connect Edition Update 4, 10.04.00.XX. Compatibility for earlier versions may be different.

Runoff Methods supported by each solver

Solver	Rational Method	Modified Rational (UK)	Modified Rational	ILSAX	EPA-SWMM Runoff	Unit Hydrograph	Time - Area	User Defined Hydrograph	None
Implicit	N*	N	Y	Y	N	Y	Y	Y	Y
Explicit	N*	N	Y	Y	Y	Y	Y	Y	Y
Convex	N*	N	Y	Y	N	Y	Y	Y	Y
Rational	Y	Y	N**	N	N	N	N	N	Y

* Modified Rational Method will be assumed in this case (and a global storm duration will need to be entered)

** Rational Method will be assumed in this case (and ignore the global storm duration)

Rational Runoff Method - The Rational method solves for peak discharge based on watershed area, Rational coefficient, and rainfall intensity for the watershed. The equation Q=CIA is used to compute flow using the Rational method: You will need to enter C = Weighted runoff coefficient for drainage area, I = intensity of storm and A = area of catchment.

Modified Rational (United Kingdom) Runoff Method - In case of Modified Rational (United Kingdom) Method, you can specify soil Index, Antecedent wetness, Surface intended to drain % for a catchment, which will help you in computing runoff coefficient, this is specific to UK region and generates a peak flow similar to the Rational Method.

Modified Rational Runoff Method - The Modified Rational Method provides a way to calculate the hydrograph (instead of just a peak flow) from a catchment based on Rational Method C values and the peak intensity. See more here: Understanding the Modified Rational Method

ILSAX Runoff Method-  The version of the ILSAX method used in SewerGEMS is the same version used in the DRAINS program. ILSAX is also the name of a freeware application that was superseded by DRAINS, but SewerGEMS implements only the hydrological method. The ILSAX Runoff Method combines a triangular unit hydrograph approach with Hortonian Infiltration to compute the precipitation losses for the pervious portion of the catchment. Therefore, the method requires the input parameters corresponding to the physical characteristics of soil at the site, and its antecedent moisture condition.

EPA-SWMM Runoff Method – You need to choose the SWMM engine to use EPA-SWMM runoff method, all catchments must use that runoff method and the loss method specified on the calculation options manager (only Green -Amt, Horton, or SCS are supported).

Unit Hydrograph Runoff Method – Here you can choose from RTK Unit Hydrograph, SCS Unit Hydrograph or Generic Unit Hydrograph runoff methods.  With each method, input data will change.

Time-Area Runoff Method - The time-area method of hydrologic catchment routing transforms an effective storm hyetograph into a runoff hydrograph. The method accounts for translation only and does not include storage. Time-area method is based on the concept of time-area histogram, i.e. a histogram of contributing catchment subareas.

User Defined unit Hydrograph Runoff Method – Here you can enter values into the table of flows (for a unit of rainfall) vs. time in a catchment property directly.

A note on Low Impact Development Controls

While LID elements are not a runoff method, it is worth noting their solver compatibility here. LIDs are supported by the Explicit (SWMM) solver, but only with the Time-Area and EPA-SWMM runoff method. See more here: Modeling Low Impact Development (LID) Controls

Rainfall Methods supported by each Runoff Method

All of the hydrologic methods can be used with any of the non-steady state hydraulic numerical solvers. For the steady runs using the GVF-Rational (StormCAD) solver, the rational method must be used while for steady runs with the steady state GVF-convex solver, the user cannot use catchment hydrology.

In general, for a long term continuous simulation, the SWMM hydrology methods should be selected because they work more effectively with long term rainfall data.

In general, all of the catchments should use either Bentley hydrology methods or SWMM hydrology methods and not mix them in a given run. When the SWMM based hydrology solvers are used, the same loss method must be used for all catchments and the loss method is specified in the Calculation options rather than as a catchment property.

The two RTK methods use different internal solvers so a user should decide beforehand which one to use and not try to switch back and forth. Unlike the other hydrology methods, the SWMM RTK method is applied to nodes such as manholes. It is generally more appropriate for I/I studies.

See Also

Structure Loss methods supported by each solver