I'm trying to model a stream with a parallel culvert. Think: an open trapezoidal channel with a box culvert alongside it.
The problem is that the culvert will have to span several stream cross sections, and I was thinking to just model a bunch of culverts between each cross section. However, it looks like HEC-RAS won't let me use a zero distance to the upstream cross section for the culvert "deck", which would seem to preclude me from doing that. Unless I somehow use a nominal gap between culvert segments.
How about setting the culvert up as a completely separate, but parallel reach? Then simulate the culvert with cross sections with "lids". Lids will be better, since the empirical culvert routines included in RAS are meant for realtively short culverts (i.e. under highway bridges), not long tunnels. Suggest reading up on "Lids" and Priessman Slot in the manuals.
The culvert invert will basically mimic the channel invert; can I set up the separate reach to do that? I believe the intent is to allow the culvert to have periodic openings in the side so that water from the channel can equalize the level of water in the two conveyances. Kind of like a very long (wide?) bridge.