Modeling a Combined Sewer

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


How can I model a Combined Sewer system, where both stormwater runoff and sewer flows in the same pipe system?


The StormCAD GVF-Rational solver is used for modeling storm water conveyance systems, whereas the SewerCAD GVF-Convex solver is used for modeling of sewage collection system. Both of these solvers support peak flow modeling with the application of extreme flow factors (i.e. peak factors) which are applicable for steady state flow only. The SewerCAD solver also supports EPS (Extended period simulation) e.g. for 24 hours of time period.

However, the need may arise to model a Combined Sewer system, where both stormwater runoff and sewer flows in the same pipe system.


SewerGEMS and CivilStorm are well-suited for modeling a Combined Sewer, as they provide two dynamic solvers (Implicit and Explicit dynamic) along with the GVF-Rational and GVF-Convex solvers.

The Dynamic solvers (Implicit and Explicit) are suitable for:

  • Modeling challenging cross-connections, if there are loops in the network or dynamic surcharging and ponding of water.
  • It can handle some of the complexity often involved with combined sewers, such as flow splits and control structures on the pipes like orifice, and overflow situations. Modeling a flow split (diversion) in SewerGEMS or CivilStorm
  • It will use the same solver for both storm water and sewage water calculations. Please make sure that this matches with your requirements and country specific guidelines. For example you may have requirements that dictate the use of certain methodology.

The GVF-convex solver is suitable for:

  • Complicated pumping, pressure sewers, and when the use of an extended period simulation convex (EPS) routing is suitable, as opposed to fully dynamic routing.
  • If you need to perform a constraint-based constraint based design (i.e. velocity, slope, cover)
  • If you need to run a steady state simulation, such as for a peak flow analysis with Extreme Flow methods i.e. peak factor calculations.
  • If you do not need to account for overflow and do not have many flow splits.

Selection of the solver depends on your needs; if you want to use the same solver for both storm and sewer flows which includes overflow in the system, modeling of control structures, then you should use Implicit or Explicit dynamic solvers. Whereas if your system includes complicated pumping and peak flow calculations (and you don't need then you should go got GVF- Convex solver.

See Also

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