Bentley SewerGEMS CONNECT Edition Help

What is the Difference Between the Implicit and SWMM Engines?

The "engine" refers to the type of numerical finite difference solution used to solve the St. Venant equations, which describe unsteady one-dimensional, free surface flow. The software contains two types of engines:

  • The implicit engine uses a four-point implicit finite difference solver to find the numerical solutions for the hydrodynamic Saint-Venant equations. The implicit engine tends to be more stable than an explicit solver. The implicit engine in the software is based on the solver in the National Weather Service FLDWAV model.
  • The SWMM engine uses the solver from the EPA Stormwater Management Model version 5. This is an explicit solver which, while more prone to stability problems, exactly matches the results from SWMM 5.
  • There is an initial elevation attribute for manholes using the SWMM engine so that the calculation can simulate a filling process if the initial elevation is lower than the downstream start elevation. However in the Implicit engine the manhole initial elevation is not considred, so the initial mahole elevation is assumed to be the same as the downstream start elevation.

Inflow hydrographs are also handled differently by the two engines. The implicit engine interpolates flows between the final flow in the hydrograph and the end time. The SWMM engine assumes that all flows after the final inflow point are zero.

You select the engine you want to use in the Calculation Options manager.

Note: If a catchment is using the EPA SWMM runoff method and not using the default infiltration method specified in the SWMM calculation options then neither hydrology or network will calculate.

If the user is not using the EPA SWMM runoff method, then any combination of other runof methods can be used.