Solving instability in downstream boundary rating curve

[gustavo]

Hello,

We are performing a dam breach model, with the upstream boundary inflow as a dam breach hydrograph.
The downstream boundary is located 200km down the river. It is another dam, run-of-river plant (no storage volume), with a gate spillway and a concrete dam designed to support overtopping.
The water level stays constant at elevation 251.5 m for flows up to 6700 m³/s, after this the level starts too rise and the concrete dam is overtopped.

We defined this boundary condition as a rating curve in Hec Ras, as shown in figure below.

Some instability appeared in the downstream boundary flow rates results.

Any ideias on how to solve this instability in our downstream boundary?
Or should we change our downstream condition of a run-of-river plant?

I am guessing our problem is with the horizontal values of the curve above. with many possybilities of Flow for the same Water Surface elevation.

[gustavo]

in this image you can see the instability I mentioned.

[cameron]

have you tried it with having a small slope at the beginning of the curve? Have you tried to change the time step?

[Anonymous]

Tried with a very little slope, 1 cm.
I will try with 10 cm slope at the begining.
When I have some results I reply here.

Thank you again, Cameron!

[Author]

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