Using SCS or other hydrograph method in StormCAD (GVF Rational solver)

  Applies To 
  Product(s): StormCAD, CivilStorm, SewerGEMS
  Version(s): V8i, CONNECT Edition
  Area:  Modeling
  Original Author: Jesse Dringoli, Bentley Technical Support Group


Can I use another method such as the SCS Unit Hydrograph method to calculate peak flows for pipe design? (with StormCAD or the GVF Rational solver in SewerGEMS or CivilStorm)


Traditionally, the Rational method is used in storm sewer pipe sizing. This is a standard practice in many locations for pipe sizing. So, this is the only runoff method available in StormCAD (or the GVF rational solver in SewerGEMS and CivilStorm). 

If you use one of the other available solvers in CivilStorm or SewerGEMS (SWMM and Implicit dynamic) you can model other runoff methods such as the SCS method. If you open the Sample1 model included with CivilStorm for example (located in the Samples folder within the CivilStorm installation folder) you will see that the catchments are configured with the SCS Unit HydroGraph Method. However, if you need to use the automated design feature to size pipes according to peak flow you'll need to use StormCAD or select the GVF Rational solver in CivilStorm, but this only supports the Rational runoff method.

If you would like to override the rational method and enter your own peak flows (for example peak from a SCS method runoff hydrograph), for use with automated design, you can use the "Flow (Additional Carryover)" field in the properties of your catchbasins.

This basically replaces the rational method with user defined flows. Meaning, you would not have catchments in the model (so no runoff calculations) but rather enter the desired peak flows directly into the catchbasins (and those peak flows could be based on an external calculation from the SCS method for example). You can then set the calculation model to Design in the calculation options and it will size the pipes and inlets as it normally would, except it would be using the user-defined flows that you enter instead of flows calculated from the catchments using the rational method.

Obtaining hydrograph method peak flow results

When you have the active numerical solver set to GVF Rational, you'll see the additional carryover flow field in the catchbasin properties.

If you need to calculate the SCS method-based peak flows first, you can do this in SewerGEMS or CivilStorm. First, set up and compute your scenario using a unit hydrograph method to compute the flows, with the active numerical sover set to Implicit or Explicit. Note the catchment peak flow results. You can copy/paste from a FlexTable to Excel, or you might like to set up ModelBuilder to sync the data out to an Excel file.

Next, create a new scenario that uses the 'GVF-Rational (StormCAD)' solver. Now, copy or import the flows copied to Excel into the "Flow (Additional Carryover)" field as described above. The easiest way to do this is through ModelBuilder, but you can also copy and paste the data into the element FlexTables. Make sure all other flows/catchments are set to zero or inactive in this scenario.

Now, set the calculation type to 'Design' and compute. The program will then design the pipe diameters and inverts based on the peak flows from your unit hydrograph calculations.