Connecting 2D Meshes via 2D Connection Weir Help

[Ben Skaflen]

I am modeling a watershed using multiple meshes connected via weirs, which are set to resemble the terrain. I have one weir which is not working, giving unrealistic depth answers and causing the run to crash. The full model is rain-on-mesh, however at the beginning of the simulation, I am introducing a small baseflow to help the weir calculations. The flow depth spikes from 3′ to 20,000′ just upstream of the weir, goes through the weir at a depth of 0′, then spikes again to 450′ just downstream of the weir, before continuing downstream at again 3′. I am running the weir using a coefficient of 1 and the normal 2D equation domain for the computation method. I have 5 weirs in total, and only one of them has not worked using these settings.

[Luis Partida]

Here are a couple of SA Connection tips:

1.) If a tributary is coming in along the length of the weir, it should be split into 3 separate weirs. 1 for the tributary with a low coefficient, and 2 for the adjacent sides.
2.) In some cases, you can change the equation being used to use the 2D equation instead based on submergence.
3.) In calculations options and tolerances in the General Tab, increase the lateral structure flow stability factor.
4.) Weirs should be filtered down below 100 points, which increases efficiency.

In general the weir coefficient selected should represent the underlying terrain which is why if there is an incoming tributary and or 2 meshes connecting multiple stream networks to the downstream mesh, each stream needs its own weir.

There are also initial depth errors when using spatially varied stage inputs as the DS boundary condition which can be fixed by turning off “Use Initial Stage” in the unsteady state flow editor. Hope this helps

[Luis Partida]

Oh and of course the computational time step is important as well.

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