Unstable 2D Box Culvert on Low Gradient Channel

[Scott Miller]

I have a very low gradient channel with a bridge across it. I’m modeling the bridge as a box culvert in 2D. I’ve tried making grid cells connected to the culvert large to have sufficient volume, but am not sure I’ve made them large enough. The configuration is limited by a break line, an internal 2D connection.

The upstream and downstream water surface elevations apparently switch too easily. The grid cells iterate to the maximum count, very frequently, and I end up with large volume errors.

Any thoughts on what I need to do?

[Jarvus]

I imagine you are already aware of this, but potentially for other people reading this thread:

Trying to keep the cell size large and the time small is your best bet.

If the instability is when the downstream WSE is almost as high as the upstream WSE, happens during low flow, you could, with work, try to run that part of the model just using the terrain (adding the piers and abutments to the terrain as needed).

[Scott Miller]

Thank you, Jarvus. I did a bit of testing with smaller time steps. Had lock them in rather than allowing reasonable Courant conditions from lengthening the time step. I used a ten day period with storms, and got the following results:

So, it took 56 hours at 0.2 seconds to model 1 year. The same model without the box culvert takes 5 1/2 hours at 2 seconds, Courant number entirely below 0.5. The smaller time step was needed entirely because of the box.

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