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RE: Total discharge boundary condition failure

Jacob A Morgan, modified 2 Months ago.

Total discharge boundary condition failure

Youngling Posts: 3 Join Date: 4/27/18 Recent Posts

I am trying to set up a morphodynamic river model with multiple upstream boundaries/tributaries specified as total discharge boundaries. The total discharge at each of the boundaries is an unsteady hydrograph. However, at the upstream main-stem boundary (and a couple tributaries) the discharge drops off to zero after the discharge increases for some time. I can delay the failure of the boundary discharge by reducing the time step but it always eventually fails. The channel gradient at the boundary is fairly steep (~1%). I can specify a water level time-series boundary condition and it seems to work fine. But this type of boundary is not desired as the bed level will not necessarily be constant at the boundary.

Is there a better way to introduce flow at a steep boundary? Is there some Courant number requirement for a total discharge boundary?

 

Thanks,

Jacob

boundary conditions total discharge river discharge boundary conditions problem river modelling river boundary conditions
Richard Measures, modified 2 Months ago.

RE: Total discharge boundary condition failure

Jedi Knight Posts: 164 Join Date: 3/23/11 Recent Posts

Hi Jacob,

I have had similar problem at total discharge boundaries before in steep rivers - no quick fix but a few things to try/check could be:

  • What is your threshold depth for drying/wetting? Sometimes increasing this can help stabilize things.
  • Check the elevation assigned at the cell edges along the boundary (you can get this from the TRIM file in quickplot I think) - sometimes (depending on grid MN orientation relative to the boundary and how bed levels are specified) I have ended up with a situation where elevations along the cell-edges at the boundary were very wrong due to interpolating null values.
  • What is happening to the morphology at the boundary when it goes unstable - could the underlying cause be morphological? Do you get the same problem if you turn morphology off?

Good luck finding a solution!

Cheers,

Richard

boundary condition total discharge upstream boundary
Jacob A Morgan, modified 10 Days ago.

RE: Total discharge boundary condition failure

Youngling Posts: 3 Join Date: 4/27/18 Recent Posts

Thanks for your suggestions, Richard!

The threshold depth for wetting/drying seemed to work for a while but just increased the discharge threshold where failure occurs. The elevations at the cell edges also seem to be interpolated correctly. Although the end goal is a full morphodynamic model, this test includes only hydrodynamics.

We *tentatively* solved the issue by adding a "flume" of several cell lengths to the upstream portion of the boundary with a milder slope. However, we have since concluded that for stability (and computational time limitations) it is best to move our upstream boundary a short distance downstream to exclude this steepest part of the domain.