Overview - Converting Between Solvers

In general, users can switch solvers readily. However, there are special elements, behaviors and components that differ between the programs that the user needs to be aware of.

Even when the current solver does not support some element types, it may be possible to create or edit that element. However, the property grids are "filtered" such that only properties relevant to the current solver are displayed. For example, if the active solver is Implicit, it is not possible to establish controls for a GVF-Convex variable speed pumping scheme. The Alternatives Manager is not filtered in this way. Therefore, it is possible to review or edit properties that are not applicable to the active solver using the Alternatives Manager.

While essentially all element types are available to be included in models with any solver as active, their behavior can be different depending on the solver. For example, channel link elements can be specified with any solver, but only the Implicit solver can correctly transition non-prismatic channels between different cross sections at each end. Other solvers will assume the upstream cross section governs, and defines the prismatic shape of the entire channel link.

To simplify the process of moving models between solvers, users are advised to avoid elements and features that can complicate this process. Some of those special considerations are described below. They key areas where the solvers differ include flow splits, pump definitions, control statements and ponds, plus some other minor differences.

Flow Splits

Flow splits are calculated differently in the various solvers. The implicit and explicit (dynamic wave) solvers determine the flow split based on hydraulics and downstream boundary conditions. The GVF convex and rational solvers use a rating curve to split flow between the primary downstream link and diversion links. The explicit (kinematic wave and uniform) solvers use a 'splitter" node.

Treatment of Time Steps

The implicit and explicit dynamic wave solvers use a constant time step throughout the run. The GVF-convex solver has the ability to use different size time steps for hydrologic routing, gravity hydraulics and pressure hydraulics. This enables it to take larger default time steps and allows the pressure solver to insert time steps corresponding to pump switches on a subnetwork basis.

The dynamic wave solvers do not perform steady state solutions. On the other hand, GVF-convex solver can perform steady state or extended period calculations and the GVF rational solver can only perform steady state calculations.

Pumping

The solvers share the same Pump Definition Library, but not all pump types and functionality are usable in both products. In general, the multipoint pump curves is the safest to use if the user is switching between solvers.

Controls

There are three ways of controlling elements in time simulations in the storm sewer products;

1. On / off levels as a pump property. These are available in all solvers but are limited to pumps controlled by wet well levels.
2. SWMM type control statements for any element type. These are available only in the explicit solver.
3. Pressure controls for pressure elements in GVF-convex solver. These are available for pressure elements and can be more complex than the elementary on/off controls.

If a user plans to move models between solvers, it is best to keep the control statements as simple as possible or to create different scenarios with control statements in operational alternatives suited to that solver only.

Gravity Flow Control Structures

Head losses through gravity flow control nodes (e.g. manholes) and handled differently in the different solvers. The GVF solvers have the most choices while the implicit solver has many of the same except for the HEC-22 and AASHTO methods. The explicit solver handles node head losses by placing them on the outgoing link elements.

Conduits

Starting with the release of SELECTseries 3, essentially all of the conduit shape options are available in every solver. A minor exception is Pipe Arch shapes, which are specified differently depending on the solver. See help topic "Pipe Arch and Arch Behavior".

Ponds

While pond elements can be included in any model, they are supported differently depending on the solver. In the Implicit and Explicit solver, they are hydraulically calculated based on the inflow, water level, tailwater and pond outlet structure conditions.

In the GVF solver for extended period simulations, the routing does not account for backup of tailwater but uses the pond inflow, level and outlet structure to determine flow. Then, the new level will be the higher of the value from the storage routing or GVF backwater calculations. For GVF steady runs, flow continuity governs. Inflow and outflow to the pond is balanced, and the pond's water surface water surface level is determined from the control structure. If the pond does not have an outlet control, the user supplied initial elevation is utilized.

Open Channels

The Implicit solver has two types of gravity flow links, conduits and open channels. The other solvers have only conduit elements, where open channels are simply a special type of conduit as described in the table above.

In the explicit solver, the open channel properties are set at channel cross sections and vary in size between cross section nodes. In other solvers, the channels are prismatic with constant properties along the link element. When importing from the implicit solver to other solvers, the shape of the upstream cross section is used as the cross section for the entirety of prismatic channel.

Friction Loss Methods

The Implicit and Explicit solvers only use the Manning's equation for friction loss calculations. When moving scenarios that do not employ the Manning's equation between solvers, a pipe's roughness is converted into an equivalent Manning's n coefficients. The GVF solvers can use the Manning's, Darcy-Weisbach and Hazen Williams methods for pressure pipes while for gravity pipes, it can use all of the above plus the Kutter's equation.

Air Valves

Air valves at high points are unique to the pressure portion of the GVF Convex solver and are imported as pressure junctions to other solvers. However, the downhill side of a high point that may or may not flow full may be modeled in the solvers as gravity conduits that have bolted manholes.

Gutters

The flow properties of gutter elements are not calculated in the GVF-Convex solver which is intended more for sanitary sewers.

Catch Basin Inlets

The Explicit solver has a unique method for dynamically calculating depth and spread for catch basins where the capture is hydraulically calculated. If the models are to be moved between solvers, simpler inlet types such as full capture and percent capture should be used instead.

Friction Loss Methods Associated with Each Solver (Link Elements)

Each solvers offers the choice of several Friction Loss methods. However, there are some exceptions. The relationship between solvers and these loss methods are summarized in the table below. When a method is not supported by a solver an error message is generated at run time.

Manning's equation for gravity flow is unique in that Manning's n can be treated as a constant or as a function of depth or flow. This choice of implementations is only supported by the Implicit (Dynamic Wave) solver.

For channel links, the roughness coefficient is specified at the cross section element, not the channel link. When an irregular channel is specified for a channel, Manning's can be treated as a constant or varied across the cross section. For example, an irregular channel supports different roughness values for the left and right banks, as well as, the main channel. However, a trapezoidal channel cannot. Furthermore, an irregular channel could support a variety of roughness weighting schemes. (See also Irregular Open Channel.) Where this modeling customization and flexibility is needed, selection of an irregular channel shape is suggested.