Extreme jump in the upstream water elevation by minimal change of the gate opening

[isabellala]

I’m running a steady flow analysis of a river between two lakes with a weir in the middle. As boundary conditions i have Known W.S. on all profiles. By knowing the upstream water elevation i am trying to find out the needed gate opening. It works for every profile but one:

The downstream water surface elevation is 1cm higher than the upstream ground elevation. When opening the gate there is no effect (to closed gate) until 0.02cm open – then the water surface upstream goes down to the same as an completly open gate. If I take a 15cm higher water surface downstream hec-ras has no problems anymore.

Is this about the minimal difference between the upstream ground elevation and the downstream water surface? may i change something about the gate or the gate type?

I am using open air overflow of a sharp crested weir with rehbook equation.

[cameron]

did you look into changing the different calculation tolerances to be smaller?

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