Difference between the boundary condition (Upstream WS) and what is shown in the water profile

[carlomonte]

Hi,

I have just discovered this forum. I am trying to model a very simple rectangular channel (spillway sluice), with a length of 14 m, a depth of 10 m and a difference in elevation of 1 m. The flow is 120 m3/s, and the upstream boundary condition is a water surface elevation, 7 m over the bottom of the channel at the upstream position.

The analysis works well (no error message, supercritical flow), however when I verified the upstream boundary in the profile, it was at 4 m over the bottom of the channel at the upstream position.

I have tried to increase the upstream boundary condition of the water surface elevation up to 10 m, and the upstream boundary condition in the water profile (the graph produced by HEC-RAS) remains at 4 m.

It is as if the software was capping at 4 m. It would be appreciated if someone could help me out.

Thank you in advance

[carlomonte]

Here are some screenshots:

[PaoloP]

In the upstream cross section you have subcritical flow the known WS is disregarded

Paolo P

[carlomonte]

Thank you very much for your answer PaoloP. However, I don’t undestrand why the known WS upstream boundary condition would be disregarded if the flow is subcritical. In real life, I have seen and filmed this spillway sluice with approximately this upstream level and this flow. It was subcritical upstream and supercritical downstream. I was trying to reproduce what I had seen in real life.

[Anonymous]

If the flow is subcritical, the water surface is controlled by what is happening downstream. If the flow is correct, then it would appear there is some other problem with the model. Cross section shape is wrong, manning’s n is too low, etc.

[Author]

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