1D unsteady Culvert/Bridge creating rise in WSEL.

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1D unsteady Culvert/Bridge creating rise in WSEL.

ce1319
I have a 1D unsteady HEC-RAS 5.0.3 model where the flow is contained within the channel banks. When I place a culvert in the model, I get a maximum water surface elevation rise of around 0.06 ft throughout the reach.

As a test, I placed a bridge in the same location that spans the entire channel so it doesn't restrict the flow, and I get the same result. However, if I replace the bridge with a lidded cross section using the same geometry, the rise in maximum water elevation goes away. I've tried changing the bridge modeling routine, but that doesn't seem to help.

Does anyone know why the unsteady model is showing a rise the water surface elevation if I place a bridge or culvert, even if the bridge/culvert doesn't restrict the flow? Thanks.