Transitioning from peak flow design to unsteady hydrograph routing

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


What is the workflow to use peak flow calculations for pipe design, then transition to unsteady hydrograph routing? (for example pond design)


The GVF-Rational solver used by StormCAD and the steady state solver option in SewerCAD calculate a steady state peak flow simulation and are well-suited for designing pipes, but the Implicit or Explicit (SWMM) numerical solvers available in CivilStorm or SewerGEMS must be used to route runoff hydrographs for an unsteady simulation. Here is the general workflow with supporting content:

  1. Design pipe network for peak flow - Use the GVF-Rational solver in StormCAD or the steady state solver (with extreme flow factors and unit loads configured) in SewerCAD to design pipe network elements and inlet sizes as needed.
    1. Training: StormCADSewerCAD
    2. Automated Design tips and tricks
    3. Order of automated design priorities
    4. Troubleshooting Automated Design Results

  2. Open model in CivilStorm or SewerGEMS - as long as the versions are compatible, existing StormCAD and SewerCAD models can be opened directly in CivilStorm or SewerGEMS.
    1. Model backward / forward compatibility
    2. How can I open a StormCAD model in CivilStorm?
    3. SewerCAD and SewerGEMS model file compatibility

  3. Change catchment hydrology to support runoff methods: change the runoff method in your catchments to the Unit Hydrograph, EPA-SWMM runoff method or Modified Rational Method, then select either the Implicit dynamic or Explicit (SWMM) dynamic numerical solver and specify the simulation duration and other analysis options. Note that the Modified Rational method uses Rational Method Principles but is quite different as explained here. The SCS Unit hydrograph is popular as a synthetic distribution can be constructed (see below) using standard SCS distributions along with a total rainfall depth. Consult your reviewer if needed to determine the appropriate runoff method for unsteady analysis.
    1. Differences between solvers: GVF-Convex vs. GVF-Rational vs. Implicit vs. Explicit (SWMM)
    2. How can I apply a rainfall depth or intensity to a dimensionless curve to create a storm event?
    3. Results Different between Unit Hydrograph and EPA-SWMM Runoff Methods

  4. Lay out additional elements such as Ponds: if you need to route hydrographs through detention ponds, you will need to include them in the model using the pond polygon, outfall (to connect the upstream piping to the pond) and Pond Outlet Structure node (to connect the pond to the downstream outlet structure). For sewer models, you will need to ensure any pump/wetwell stations are configured with appropriate controls to turn the pump on and off based on wetwell level.
    1. How do you direct flow from a channel or pipe into a pond?
    2. Pond and pond outlet assumptions in StormCAD, SewerGEMS, CivilStorm and SewerCAD
    3. Training: Stormwater Advanced training playlist

  5. Validate the model and clean up as needed: first ensure that the appropriate unsteady numerical solver is selected (Implicit or Explicit). For models with ponds, the Explicit (SWMM) solver is recommended, with the Routing Step calculation option is set to 1 second. Run the Validate command within the Analysis menu to check for any other issues that may need to be cleaned up or modified due to differences between the solvers.
    1. Differences between solvers: GVF-Convex vs. GVF-Rational vs. Implicit vs. Explicit (SWMM)
    2. Structure Loss methods supported by each solver

  6. Compute model and review results: computing the model with the Implicit or Explicit solver will first calculate runoff hydrographs from catchments, then route the hydrographs through the model network. Flow will be attenuated, momentum effects accounted for, pumps will turn on and off per their controls as wetwells fill and drain, and hydrographs will be attenuated in ponds based on their pond outlet structure. Note that the results you see will not be comparable to the peak flow results you saw in StormCAD or SewerCAD since the runoff method and numerical solver are different.
    1. Why don't CivilStorm results for peak flow closely match StormCAD results for peak flow?