Differences between solvers: GVF-Convex vs. GVF-Rational vs. Implicit vs. Explicit (SWMM)

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


What are the differences between the numerical solvers? -

  • GVF-Convex (SewerCAD)
  • GVF-Rational (StormCAD)
  • Implicit dynamic (SewerGEMS and CivilStorm)
  • Explicit (SWMM) (SewerGEMS and CivilStorm)

When should I use StormCAD instead of CivilStorm?


The SewerCAD GVF-Convex solver uses convex routing and a gradually varied flow profile for design and analysis of sewer networks including mixed gravity and pressure flow.

SewerGEMS is a superset of SewerCAD, including all its functionality, plus two fully dynamic solvers (Implicit and Explicit) and ArcGIS integration support.

NOTE : Each solver supports different calculation methods for various individual network components. See more in articles such as these:

How do I know which solver is best for me to use?

SewerCAD (GVF Convex) vs. SewerGEMS (Implicit and Explicit)

The SewerCAD application (and the GVF Convex numerical solver in SewerGEMS) is best used in systems that have complicated pumping, pressure sewers, and only need to use extended period simulation convex (EPS) routing as opposed to fully dynamic routing. SewerCAD should also be used if you need to perform a constraint-based design or if you need to run a steady state simulation, such as for a peak flow analysis with Extreme Flow methods.

SewerCAD can be thought of as a bread-and-butter package that delivers conventional design and capacity analysis. Municipal-scale master planning is certainly part of it but serves very well in site/civil arena as well. Routing is hydrologic with conventional back-water dominant hydraulics. Gravity analysis is complete with well-accepted state-of-the-practice hydraulic grade analysis with form losses. Diversions or splits are handled in explicit ways. I&I, similarly, is modeled using an array of fundamental and appropriate simplifying models.

On the other hand, the SewerGEMS/Civlstorm applications layer into the mix solvers for dynamic wave simulation (implicit and explicit (SWMM), with ArcGIS integration support. So, if you have challenging cross-connections, loops or dynamic surcharging and ponding, this gives you the capabilities of EPA SWMM along with Bentley's own implicit solver. SewerGEMS (Implicit or Explicit Dynamic numerical solvers) is best for analyzing existing problematic systems, where catchment rainfall-runoff calculations are required, or dynamic wave solutions are needed (if required by the reviewer or by way of the complicated nature of the particular network) or if you must work inside the ArcGIS platform.

SewerGEMS can handle complex things like control structures, diversions (without having to enter a diversion rating curve required in SewerCAD/GVF Convex solver) or ponds. Long term continuous simulations would be done using the Explicit solver in SewerGEMS. The "solver" 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 different solvers::

Implicit solver - Uses a four-point implicit finite difference solver to find the numerical solutions for the hydrodynamic Saint-Venant equations. This solver is based on the National Weather Service FLDWAV model. The implicit solver tends to be more stable with pumping situations (for example using a timestep of 0.01 hr - 0.005 hr)

Explicit (SWMM) solver - Uses the solver from the EPA Stormwater Management Model version 5 (SWMM). The results from this solver should exactly match the results from SWMM 5. The explicit solver tends to be more stable with fast changing areas such as ponds or control structures where the flow or elevation changes quickly over multiple time steps.

  • Although the SWMM documentation mentions it uses an “implicit” Euler method, this is referring to how the SWMM solver progresses the Explicit iterations, so it is still an Explicit-based scheme. On the other hand, the “Implicit" solver in SewerGEMS and CivilStorm refers to how the underlying scheme is implicit.
  • 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 considered, so the initial manhole 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.

    *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 you are not using the EPA SWMM runoff method, then any combination of other runoff methods can be used.

Note: Starting with the CONNECT Edition Update 2 release of SewerGEMS and CivilStorm, you now have the option to select and use either the Bentley-enhanced version of the SWMM solver or the straight SWMM solver also found in EPA-SWMM. You can select this from the properties of the Calculation Option.

GVF-Convex (SewerCAD) solver is not intended to handle overflow situations such as a case where you want to analyze a problematic existing system.  When an overflow condition arises with the GVF Convex solver, the HGL is reset to the rim for an overflow condition.  However, the dynamic solvers in SewerGEMS (Implicit and Explicit SWMM solvers) do handle overflow, as they are intended for situations like this (problematic existing systems and/or complex situations).  SewerGEMS Implicit and Explicit solvers automatically calculate the overflow using the weir equation.

So, SewerGEMS differentiates in the market as being a singular, "top of the line" tool that will carry the engineer though all stages of design and analysis from conventional capacity and constraint based design of pipe networks into complex hydraulics of combined-sewer systems. SewerGEMS will handle both storm and sanitary models. Importantly, if you have any old StormCAD, SewerCAD or CivilStorm files they can all be loaded into SewerGEMS and brought cleanly ahead.

The GVF Rational Solver

Note: Currently the StormCAD numerical solver is included with CivilStorm, so CivilStorm has all of the functionality of StormCAD included, by way of selection of GVF Rational as the active numerical solver.

The StormCAD product (and the GVF Rational Solver in SewerGEMS and CivilStorm) uses the rational method to analyze or design a system under peak flow conditions based on peak rainfall intensity, while the other solvers in CivilStorm and SewerGEMS such as the Implicit or Explicit, takes rainfall hyetographs (rain vs. time) and develops hydrographs (flow vs. time) for each pipe and routes the flows dynamically. If you are studying a small area where only peak flow is of interest, or if you need to design a system based on the standard rational method, then StormCAD or the StormCAD solver (GVF Rational) should be adequate. If you are working on a large area where hydrograph routing and storage are significant, where you need to use a dynamic solver, or if you need to otherwise analyze more complex effects such as flooding and controls structures, then the Implicit and Explicit solvers in CivilStorm (or SewerGEMS) is what you need.

If you get involved with combined sewers where rain and sanitary sewage is carried in the same pipe, we recommend you use one of the dynamic solvers in SewerGEMS.

See Also

OpenFlows SewerGEMS Solvers Part 1: Choosing the Right Solver for Your Stormwater Network

Conduit start/stop control structures