| Product(s): |
SewerGEMS, StormCAD, CivilStorm, SewerCAD |
| Version(s): |
CONNECT Edition, V8i |
| Area: |
Layout and Data Input |
Overview
OpenFlows Storm and Sewer products have been supporting several runoff calculation methods. This article explains which methods are supported by each solver. However the runoff methods supported can vary based which version you are using. This article is written based on Connect Edition Update 4, 10.04.00.XX.
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 ILSAX is also the name of a freeware application that was superseded by DRAINS, but SewerGEMS implements only the hydrological method.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.
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)
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
See Also
Structure Loss methods supported by each solver