Open Boundary Condition for 2D Flow

Hey there :)

I'm setting up a M21 model to simulate 2D overland flow. I have a closed boundary (zero normal velocity) and an open boundary which is a straight line through the topography (no river, no topolographic sink, may be an elevated point etc.). The open boundary should allow for flow coming to a edge element to flow outside directly. How can I do this?
I tried specified discharge, but the choice of the value is rather arbitrary.

Help is much appreciated!

Best, Sam

Sam,

What flow behavior is the model exhibiting?  How does this differ from what you expected?

I would infer that you are getting pooling instead of flow leaving the domain.  I neglect to use a 'Boundary' at all for my M21 overland flow simulations.  Instead, I create an artificial boundary by manually raise the elevation at all of the boundary cells to something arbitrarily large to contain any flow (or else you will get an error) and place 'Sinks' on the cells where you want an outflow boundary, setting the 'Magnitude' to something arbitrarily large to allow a free outfall.

I'm sure there is a way to do this with the boundary approach, but like many other aspects of this software, I'm not sure how.  Maybe you will find this workaround useful.

ab

Hey ab

Thank you very much for your reply.
The water is basically flooding the domain and is not leaving it (I guess that's what you meant by pooling). I would like to have a boundary where flow may flow outwards if topology or water level allows.
I also included sinks at the boundary cells but this gave me large negative total water levels, so it seems that a certain iterative calibration would be needed.
In another model I implemented an output in "line" format which measures the discharge flowing through a line near and along the boundary. I used this output file in the specification of the boundary condition with the option "specified discharge". This yielded more realistic flooding values. I don't know if this aproach makes sense, but it's already better than the first version.
What do you think about this?

Sam

Have you tried to combine this with suggestion from the other post to fix negative water levels?

In several places along the boundary the increase in flooding/drying depth solved the problem, but somehow there are still some spots that do not become positive.
I guess I'll have to go with an artificial trench around the domain and setting a constant level boundary within via a xyz polyline. At least this should allow for having outflow of the domain whenever there is something coming near the boundary.