The Bentley storm and sanitary products have a distinct advantage over other sewer models in that it is possible for a user to choose different numerical hydraulic solvers for each scenario. This is important because there is not a single "correct" way to solve the complex hydraulic equations for a mixed gravity and pressure hydraulic network. Each problem has its own characteristics that can make one solver (sometimes referred to as an "engine") more advantageous than another.

Each of the solvers is briefly described in the section below.

• Implicit (SewerGEMS Dynamic Wave): Solves the full St. Venant equations using an implicit numerical method developed by Bentley based on the US National Weather Service FLDWAV model. It simultaneously solves for both flow and hydraulic grade and uses the same equations for gravity and pressure portions of the system. It only solves dynamic flows (no steady state). It can be applied to storm, sanitary and combined sewers.
• Explicit (SWMM solver): Solves the full St. Venant equations using an explicit numerical method based on the EPA-SWMM model. In addition to solving the St. Venant equations using a dynamic solution, it can also route flows using a kinematic wave solution and a uniform flow solution, which does not account for any backwater effects. The Explicit dynamic solver simultaneously solves for both flow and hydraulic grade and uses the same equations for gravity and pressure portions of the system. It can be applied to storm, sanitary and combined sewer, especially systems without a great deal of force mains or pressure sewers.
• GVF - Convex (SewerCAD): Separates the hydraulic problem first into a set of gravity subnetworks and pressure subnetworks. For the gravity subnetworks, it divides the solution into a flow routing problem using convex routing (a hydrologic routing method) and then solves for hydraulic grade (including depth and velocity) using gradually varied flow (GVF) (backwater) equations. For the pressure subnetworks, it uses a true pressure solver as found in Bentley's WaterGEMS. It can perform extended period simulations and steady state simulations and can base steady simulations on an extreme flow factor method that reduces peaking factors as the flow increase moving downstream. The solution method was developed for the SewerCAD model. It can be applied to sanitary and combined sewers, especially those with a great deal of pumping or pressure sewers, and those which only require a steady state analysis.
• GVF - Rational (StormCAD): Uses flows developed by the rational method (plus other additional flows). The solver outes values for CA downstream while decreasing peaking factors due to longer time of concentration moving downstream. Once flows are determined, hydraulic grade (including depth and velocity) are determined using gradually varied flow (GVF) methods for both gravity and pressure pipes. This solution method was originally developed for the StormCAD model. It can be applied to stormwater system.

While the solvers were developed with an orientation to storm and sanitary sewer design, to the extent to which they capture the behavior of the systems, the solvers can also be applied to flood control systems, irrigation systems, natural streams, treatment plants and any other hydraulic systems comprised of a mixture of gravity and/or pressure flow elements.

SewerGEMS contains all of the numerical hydraulic solvers. The association of other products and solvers is shown below.

Available Solvers by Product