Hi, Chris: I am working on a project that a proposed roadway realignment results in the isolation of a small area of bay. The isolated area will be connected to the ocean by a culvert. The goal is to determine the size of the culvert required to provide adequate flushing time, so that water quality within the pond is maintained.
I am using HEC-RAS unsteady flow modeling, with the cross sections set up at the pond, roadway (culvert) and bay. For the downstream boundary condition, I used observed tidal elevation time series. For the upstream boundary condition, first I tried using a constant discharge (there is no significant river inflow to the pond, so i used a small Q), but there won’t be reverse flow in the culvert, and it caused some model stability problem as well. Secondly, I used a constant stage, now i could see the flood tide and ebb tide in the culvert.
Do you think this is a good assumption by holding the water surface elevation upstream as constant? Do you have any suggestions on how I could set up the upstream boundary condition? I really appreciated your help!