Modeling 4' vertical drop in HEC-RAS 2D

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Modeling 4' vertical drop in HEC-RAS 2D

Greg G
I am modeling a culvert with a 4-ft drop (see photo) at the outlet that flows as a free outfall under most conditions. I am getting dry cells and most of the cells in the area reach maximum iterations without converging (see result snapshot), which slows the model down so much the runs I need to do are not feasible. In the geometry the drop is represented as a very steep short slope. I'm using the culvert function of the model and discharging into the 2D geometry onto a sill then over the drop. Is it possible to use RAS 2D to simulate flow over such a drop? What is the best approach? Smaller cells didn't seem to help, although I could try again more carefully. Model the drop as a broad weir or other structure? Lower the culvert and sill to eliminate the drop but retain the velocity jet? I'm also considering eliminating the culvert and representing it as a rectangular channel. Or, is all this more trouble than it is worth if I can get the information I need without it?

Thanks,