Problem

When attempting to compute a model using the GVF-Convex or GVF-Rational solver, the following user notification appears and the model fails to compute:

Gravity sub-network has more than one outfall or wet-well

Solution

This message indicates that the model contains flow splits or loops, where one or more gravity subnetwork is connected to more than one outfall. This includes ponds with multiple outlets (pond outlets, channels, headwalls, etc). This is because these particular solvers do not support flow split calculations or loop calculations.

The model as a whole can have multiple wetwells and outfalls, as long as they are in different subnetworks. 

To resolve this issue, you must consolidate the outfalls and/or wet wells in the model so that each gravity subnetwork drains to a single outfall or a single wet well, or handle the flow split using the diversion rating table method. Meaning, one of the conduits downstream of the split would be set as a diversion rating table by selecting "true" for "is diversion link?", with the rating table configured. 

You can visually search for where these situations are occurring in the model. Another option is to use Network Navigator queries. If you are using SewerCAD, to use Network Navigator, go to View > Network Navigator. Select the ">" button and choose Network Review > Find Gravity Sub-network with more than one Outfall or Wet well.

Note: If you are using SewerGEMS or CivilStorm, this query is located under the Network Trace menu in Network Navigator. If you're using SewerGEMS and do not see this query, you will need to upgrade to a later version in order to access (08.11.05.58 or greater) . Previous versions of SewerGEMS do not have this query. This was largely because the Implicit (SewerGEMS) solvers can have multiple outfalls or wet wells, as it uses a dynamic wave solver to solve a network. The other solvers do not allow for multiple outfalls for a single gravity sub-network.

Network Navigator will populate with the elements that have more than one outfall or wet well associated with it. To view these in the drawing, select either the "Highlight" or "Select in Drawing" button. This will help locate the areas where the issue is occurring and allow you to edit the drawing.

If this query does not help (for example for a large model where the query highlights a very large section), you can try visualizing this by making the symbol size larger for wetwells and outfalls (in Element Symbology). Visually inspect areas between wetwells and outfalls for cases where more than one of them are connected together. For example a series of conduits connected between two wetwells, where a high point is in the middle would cause a flow split.

Another option would be to use one of our dynamic solvers, which can handle flow splits and loops. The Implicit and Explicit (SWMM) solvers are available in SewerGEMS and CivilStorm. Existing SewerCAD models can be opened in SewerGEMS without any conversion needed (as long as the SewerGEMS version number is greater than or equal to the version of SewerCAD used to save the model)