Thanks for your reply! I think using the culvert option in GSSHA will work for areas where the open channel flows in a closed conduit and doesn't receive any additional from from any lateral storm sewer connections. However, there are still a couple of spots in my watershed where I have an open channel that flows into a pipe but then several pipes converge before discharging back into an open channel. So, I think in these situations the culvert option won't work because the flow volume is changing. I tried out a few ideas of how I could simulate this:
I found a report titled "Unsteady Storm Drainage Modeling Within the U.S. Army Corps of Engineers GSSHA Model" and it says that inflow to the SUPERLINK is possible via culverts which capture natural stream channels. I found another paper called "Modeling Subsurface Storm and Tile Drain Systems in GSSHA with SUPERLINK" and it writes about the different inlet codes:
0-10: # of grate inlets with 10 capturing all flow and 0 capturing no flow
888: junctions that discharge to an overland flow cell
999: junctions that discharge into a stream node
777: node connects to a junction
In WMS I'm able to specify the inlet type for the SUPERLINK storm drain nodes as 0-9 grate inlets, empty to grid, or empty to channel, but there doesn't appear to be an option for some sort of culvert or node/junction connection. Do you know if there's a way to do this? I tried to add a culvert to the terminal node of my upstream set of stream arcs (before they enter the closed conduits), but then GSSHA crashes because the model can't determine the flow through the culvert if I have channel routing enabled. I don't think this would capture the stream flow and connect it to the storm drain coverage though.
My next idea was to try to get the stream channel to discharge onto the overland grid where the storm drain could capture the flow, so I turned on the overbank flow option and set the end of my stream channel upstream of the storm sewer to have a tiny depth and width so that the channel flow would overflow onto the grid. However, this didn't seem to work as I intended and the stream flow didn't discharge onto the overland grid.
Since I couldn't get that to work, I played around with some boundary conditions. I thought that maybe I could run a GSSHA model on the stream arcs/overland flow before the storm sewer section to get an upstream hydrograph that I could then set as a point source/boundary condition for the storm drain. However, when I set the "Stream BC Type" to "Variable flow (discharge)" and specify the hydrograph boundary condition for the upstream most storm drain node it does not apply that boundary condition to the pipe flow, but instead applies it to all of the stream arc nodes. When I tried adding the same boundary condition to just one stream arc node, then it only applies the boundary condition as a point source to that specific node, so I'm not sure why adding the boundary condition to the storm drain node ends up applying it to all of the stream nodes. Is there some other way that I can separate my GSSHA model into two models 1) upstream of the storm sewer and 2) the storm sewer and the downstream stream arcs and be able to specify a flow rate time series into the upstream end of the storm drain?
Finally, I saw that there is an option to link a GSSHA model to other GSSHA models using the "GSSHA Model Linkage" dialog. Would this allow me to specify the inflow to the storm drain? If so, are there any tutorials or guidance on how to link models?
Thanks for your help! I guess if there isn't a way to get the stream flow routed through the storm drain coverage then I'll look into setting up a SWMM model instead.