2D flow areas connection (1D/2D flow fluctuation and errors)

[Kinagoto]

Dear Modelers,

I have a dam breach and floodplain delineation model where the dam (2D Area connection) is located between two2D flow areas.

To be able to setup the initial water level upstream the dam t the spillway level, a separate 2D flow area was created. The lateral inflow hydrograph into the reservoir of the dam was generated from HEC-HMS.

Under the No Breach plan, HEC-RAS 5.0.7, determined the peak discharge and pick water surface elevation at the dam that match HEC-HMS output results.
Despite selecting, I think, the appropriate grid size and time step (50 feet and 0.5 seconds), I am experiencing a lot of fluctuations and 1D/2D flow errors (see screen captures). The fluctuations begin right after the breach occurs with release of a lot of flow. The time slicing did not help.

I may have to try to have a warmup run to establish a baseflow equal the 68 cfs, the outflow discharge at the spillway at top of dam elevation. The overtopping breach occurs when at the top of dam.
Every model is specific, but I searched on HEC-RAS forum using “Combined 1D/2D Flow errors” key words and found discussions related to my issues but they do not help solving it.

Any guidance on how to solve the issue?

Thanks,

Kinagoto

[Anonymous]

Putting the dam between two separate 2D areas should work.

However if you are having stability problems I would recommend a single 2D area with the dam inside. This configuration is more stable because the breach flow is updated more frequently. (It is updated for each iteration of the 2D solution. In your configuration it is only updated at the end of time step.)

The down side is you can’t use an initial water surface for the reservoir. You have to fill the reservoir with an inflow. There are some tricks for this. You may be able to use a really large inflow for filling. You might want to put this inflow near the dam. Also look into writing out and using a restart file to save time.

.5 seconds sounds short enough. You might experiment with smaller cells.

[Anonymous]

For filling the reservoir, you can put the boundary condition line inside the reservoir in version 507. This may allow larger flows and quicker filling.

[Author]

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