entrance or exit for tidal flow through culvert

[Anonymous]

HEC-RAS asks for entrance and exit (local energy) loss coefficients of the culvert. However, under the bi-directional tidal flow, entrance at the upstream side (the land side) during the ebb tide (water flowing from the land to the ocean) will function as an exit during the flood tide (water flowing from the ocean to the land). For the same reason, exit at the downstream side (the ocean side) during the ebb tide will function as an entrance during the flood tide.

Will HEC-RAS automatically and properly choose the given “entrance” loss coefficient or the given “exit” loss coefficient depending on the tidal flow direction? If not, what to do?

Thanks.

[Anonymous]

The short answer is: yes.

The longer answer is:

If this is a channel culvert crossing then RAS will compute internal boundary curves. For negative flow, RAS will use the same curve. So the entrance and exit loss are handled correctly (as long as the inlet and the outlet have the same shape). However, if the culvert inlet is much higher than the culvert outlet, this isn’t being modeled as accurately as it could be.

If the culvert is part of a lateral structure then it computes the culvert flow every time step (does not create curves), and I think it handles it correctly including the invert at the inlet and outlet.

If you wanted more control over it, I suppose you could compute the culvert hydraulics outside of RAS and then use either the user defined gate curves or the Advance Rule Operations to get those results into RAS.

[Author]

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