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).
A diversion can be modeled as a pond with multiple pond outlet structures. The pond's elevation-area or elevation-volume table would be configured to represent the volume of the chamber that the flow diverts away from. If the chamber can become pressurized, consider adding a skinny "chimney" to the elevation-volume curve (tiny increase in volume for elevations above the physical top of the chamber, to allow surcharging without additional storage effects). This option may not work as well if the diversion chamber size is very small, as you will be more at risk for stability issues. You might need to use the Explicit (SWMM) solver with a one second Routing Timestep.
Is it possible to model a weir within a catch basin or manhole in the the storm-sewer OpenFlows products?
Modeling a flow split (diversion) with the SewerCAD or StormCAD numerical solver
Troubleshooting unstable SewerGEMS and CivilStorm results using the implicit solver