Modeling a flow split (diversion) with the SewerCAD or StormCAD numerical solver

Product(s): SewerGEMS, SewerCAD, StormCAD, CivilStorm
Version(s): CONNECT Edition, V8i
Area:  Layout and Data Input


In some situations, there is a need to model a flow split, where the flow approaching a node splits and goes two different directions.

With the Implicit or Explicit (SWMM) solver in SewerGEMS and CivilStorm, the downstream pipes can be defined and the flow split is calculated automatically based on the dynamic equations.

With the GVF-Convex solver (available in SewerCAD and SewerGEMS) and the GVF-Rational solver (available in StormCAD, CivilStorm and SewerGEMS), the numerical solver cannot calculate the flow split automatically, and you will need to use a diversion rating table to define how much flow is diverted down one of the pipes.

The Solution below applies to the latter, diversion rating table approach with the GVF-Convex and GVF-Rational solvers. 

You can find more information as to which solver is best to use for your model in this article**  


First, you will need to identify one of the outgoing conduits as a diversion link. If you double-click the conduit to view the properties you will find a property field titled "Is Diversion Link?". Set this to "True" and new property fields will be available to enable the addition of a diversion rating curve.

To configure the diversion rating curve, click the ellipsis (...) button in the field "Diversion rating curve". This will open a new dialog where you can enter a table for incoming flow versus diverted flow. In other words, for a given upstream incoming flow approaching the split, you are defining how much of that flow will travel down the pipe marked as the diversion. The remaining flow will go down the other downstream pipe. What you are doing here is manually defining how the flow split functions. For example, you could assume a 50/50 split. You can also have no diverted flow for a part of the curve and only allow diverted flow once it reaches a certain flow (which may be correlated to depth.)

In the above example, when you have an incoming flow of 10 cfs none of it will be going down the diversion pipe; all of the 10cfs will be going down the other pipe.  This might happen if the diversion pipe has a higher invert and the water surface has not reached that elevation yet.  Once you have this set up you can then compute the model. The flow should split based on the diversion rating curve that you entered.

Caveats / Limitations

  1. When you are using diversions, you can only have one diversion link coming out of a given node. In other words, two pipes can be coming out of a node, with one of them set up as a diversion. If three or more pipes are coming out of a given node, the following user notification will be generated: "Multiple diversion links coming out of the node. Only one diversion link is allowed downstream of a node." If you have a case like this, the best option is to use the Implicit or Explicit solver

  2. If you encounter a message stating that a conduit control structure cannot be used on a diversion and you are using SewerGEMS or CivilStorm, change the active numerical solver in the calculation options to "Implicit", check the conduit and set "has start control structure" and "has stop control structure" to "false". Then, revert back to the desired solver and compute again.

  3. Currently (as of version  the flow reported at outfall node elements does not include diverted flow. In earlier versions, a workaround was to insert a transition element (click the transition layout tool, click in the middle of the conduit and choose Yes when prompted if you want to split the pipe, then unselect the diversion option in the new conduit), but as of, this is no longer possible - related defect # 172041. Update: this issue is scheduled to be resolved starting with version 10.03.00.XX, due to be available February, 2020.

  4. Velocity and other results in the conduit marked as the diversion are not calculated. This includes the "subnetwork outfall" field and the calculated bend angle. See: Velocity and other results fields in a Diversion pipe

  5. A tap node cannot be connected to a diversion link; a user notification will be presented in this case. If you have a property connection that ties into the conduit downstream of a diversion, consider splitting the diversion conduit with a manhole or transition just upstream of the lateral connection point, then remove the diversion setting on the new, downstream conduit and reattach the tap. 

Using The Implicit or Explicit solver in SewerGEMS or CivilStorm to develop the diversion rating table

Sometimes you may not always know exactly how the flow is split. If you have access to SewerGEMS or CivilStorm, you could use these to help.  SewerGEMS and CivilStorm use the Dynamic Wave solver (Implicit solver or SWMM solver) and can model split flows in a storm or sanitary system automatically. As a result, you can use these programs to help create the diversion rating curve, which you can then use in StormCAD or SewerCAD. To check which solver you have selected go to Analysis > Calculation Options and double click the active calculation option with the red check mark on it. Check to see what the active numerical solver is set to.

To do this, set up a small model with the manhole in question and incoming/outgoing pipes, with a load on the upstream node covering a range of expected flows. Compute the model and look at the flow through one of the conduits, then build your diversion rating SewerCAD or StormCAD based on these results.

Using CivilStorm as an example, below is a simplified model:

At manhole "X", an Inflow (Wet) Collection has been set up to represent flow into the system.

You can see the steps by the graph for the different times and flows.

When the flow goes through Manhole "Y" it is split between Pipe "B" and pipe "C".  How much flow goes into each pipe will depend on the diameter of the pipe and the invert elevations.  As noted above CivilStorm and SewerGEMS implicit and explicit solvers can calculate the split flow automatically.  After computing, the results are graphed below.  You will see the flow for pipes A, B, and C.

You can enter the data from points on the graph into the diversion rating curve in StormCAD or SewerCAD, or you can copy/paste from the data tab.  If you are going to copy/paste, you would copy the data from the column of the upstream pipe to Upstream Flow (cfs).  You will copy the data from the pipe that will be the diversion pipe to Diverted Flow (cfs).

For a given "step" of the flow in the upstream pipe, that flow amount would be the "Upstream flow" on the diversion rating curve for StormCAD or SewerCAD. At the same time you observe that upstream flow, you would look at the flow in one of the downstream pipes and use that as the "diverted flow" on the diversion rating table. The pipe you pick is the one that you will be using as the diversion link in StormCAD or SewerCAD (or with the GVF-Convex or GVF-Rational solver). For example when using the implicit or explicit solver in SewerGEMS or CivilStorm, if you have a flow of 10 L/s in the upstream pipe in the first "step" of the pattern, and if you select pipe "C" as the diversion link, if pipe C has a flow of 3 L/s at the time you observe the 10/Ls in the upstream pipe, then 3 L/s would be used in the diversion rating table as the "diverted flow".

NOTE: you may need to extend the duration of the first set of inflows since the initially dry pipes will take time to fill

In summary, if you have a model with several complicated diversions you may want to consider using CivilStorm (Implicit solver) or SewerGEMS (Implicit solver), as they are best suited for more complicated flow split situations.

See Also

Modeling a flow split (diversion) in SewerGEMS or CivilStorm

Is it possible to model a weir within a catch basin or manhole in the the storm-sewer OpenFlows products?

Velocity and other results fields in a Diversion pipe