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In order to control the inflows of water on a building land, I try to size a wall with a calibrated opening to allow the correct flow of water to pass for a given return period.
According to the HEC-RAS manual, the flow over a weir is given by the formula Q = CLH^(3/2), with C the weir coefficient, L the length of the weir, and H the difference between the upstream energy grade line and the top of the overflow. The value of C is automatically reduced when the weir is submerged more than 75%.
To better understand how this works, I created a simplified model containing a channel and a rectangular overflow on the left bank. The overflow rates follow exactly the relationship described above.
Regarding my main model (wall with opening), the discharged flow does not follow this equation. Although part of the water coming from the upstream passes to the right of the wall, this does not prevent that for a given WSE upstream of the wall, the flow passing over the latter should follow the relation Q = CLH^(3/2)? I calculated the submergence rate of around 80% which is not enough to reduce the weir coefficient (discharge reduction factor is more or less 1).
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