Submerged Culvert, high effluent sea level, correct calc in HECRAS?

[James]

URGENT!
A big box culvert is almost submerged because of high effluent (constant) seawater.
Downstream Boundary Conditions is therefore set to Known W.S = Seawater level.
Most of the culvert is submerged due to very high seawater (covers 90% of the inlet culvert area). However, HECRAS computes a very low culvert influent loss and it seam like it calculates with the total influent area “available”. I would expect that it will be only the culvert area “above seawater” that will be “available” for the river flow? I.e. hydraulicly the seawater should be treated as the “bottom” of the culvert. I have also tried with LID instead of culvert but still a very low influent headloss is computed.

Is it possible to calculate this situation in HECRAS? (Or are my assumptions wrong? I.e. will the riverflow be able to “push the seawater” through without high energy loss?)

(Using only the culvert area above sealevel the headloss will be around 0,6 meter. HECRAS calculates only 2 cm with the total area included the submerged part).

[Anonymous]

I don’t see any reason to think RAS is wrong. Rather, I think you are thinking about the problem incorrectly. What if the culvert was 100% submerged? Does that mean that water cannot flow through the culvert? No, of course not.
There is some difference in energy grade between the upstream side of the culvert and the downstream side of the culvert. That head can push water through using the full area of the culvert.
If the culvert flows directly into the sea, make sure that you use a large cross section just downstream of the culvert (the velocity in the cross section representing the sea should be close to zero) and that the exit loss coefficient is set to 1.0.

[Author]

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