Routing Flows in Runoff

I am in the process of learning SWMM modeling for CSO systems and was looking at an older model created in 1992 in XP-SWMM (I think version 5) and found something that I think is a problem, but I am not sure. I have attached a XP-SWMM file showing an example of my question.

The modeler used the Runoff layer to route the flow using runoff conduits to the extran layer. At the location of the extran layer connection is the CSO diversion weir and throttle pipe (in Extran). Would routing the flow in runoff cause any inaccuracies in the weir overflow estimation? My understanding is that you would want to route the flow in extran so that you could take the dynamic effects of backwater into account? Would it make a difference? I have not experimented with this yet and was wondering if I should be concerned with how this model was developed? I appreciate any opinions on how this might effect the model output.

Thank you,
Gui DeReamer
Greeley and Hansen

Gui,

The slope term dominates the Runoff layer solution. Your should have only pipes in Runoff if all of the pipes are steep or you are using the groundwater routine. You do have one pipe with a slope of 5 percent and two pipes with slopes of 1 percent in runoff. The runoff solution is a non-linear reservoir solution and does not supply the backwater, surcharge and other capabilities of Extran or even Transport. You should definitely get rid of the runoff layer pipes and use these pipes in Extran.

The weir flow is affected by whether the pipes are upstream in runoff or extran. If it just in the runoff layer the upstream flows are not connected to the weir nodal area in the extran model. If the pipes are in Extran then half of the area of the connecting pipes will be allocated to the weir node. An upstream pipe in Extran should be more stable than the same upstream pipe in Runoff.

Best Regards,
Robert Dickinson
CDM Camp Dresser & McKee Inc.