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How can I model a flow split / diversion, where a control structure such as a weir or orifice plate controls the split?
Using the Implicit or Explicit dynamic solver in SewerGEMS or CivilStorm, you can model a diversion as follows:
1) Multiple conduits leaving a junction/manhole
In this case, the hydraulics of the flow split are controlled by the implicit solver based on the size and invert of the downstream conduits
2) Conduit control structures
In this case, a weir or orifice can be modeled with a start control structure on the adjacent conduit(s). When doing this, the results may be unstable or very sensitive due to the relative large change in flow that tends to occur for a relatively small change in head with such control structures. So, you may need to use a very small calculation timestep, or use the Explicit (SWMM) solver with a small Routing Timestep (such as 1-5 seconds).
3) Pond
A diversion can be modeled as a pond with multiple pond outlet structures.
Is it possible to model a weir within a catch basin or manhole in the the storm-sewer Haestad products?
Modeling a flow split (diversion) with the SewerCAD or StormCAD numerical solver
Troubleshooting unstable SewerGEMS and CivilStorm results using the implicit solver