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NickV

Modeling a Weir/Dam Feature in SMS

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All,

I am setting up an SMS SRH-2D model that has a dam/weir structure in the main channel. I have assigned three weir BC's as there is three segments with different heights. The existing wall is about 20 inches wide. The *OUT.dat file says SRH-2D successfully executed, but the results vary greatly across each time step. I get successful runs at both high and low flow conditions with the same varying results. I thought I had the mesh and boundary conditions for the weir entered correctly, but I cannot figure out what is going on as this is my first attempt at using a weir structure. Does anyone know of a checklist of sorts that I can run through to confirm I set it up correctly? Any advice or tips? Thanks in advance!

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I originally had the weir BC snapped to the top edges of the wall, I moved the weir BC to snap to the bottom edges of the wall. This seems to have fixed the problems at the weir and upstream from it. I am still having problems downstream of the weir and around the box culvert further downstream. 

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If your results are varying from one output timestep to the next, your computational timestep is likely to large.  Just because SRH-2D ran to completion does not indicate that the results are correct.  For a steady flow simulation, review the case_INF.dat file to confirm model stability.  The last two columns of data are helpful to confirm stability.   If the number of wet cells is still changing or oscillating at the end of the simulation, the results are not stable.  Furthermore, if the final net flow flux is not less than about 2-5% of the initial flux into the domain, convergence has not been satisfied.  

Using a smaller timestep will usually solve most stability issues.  If not, you should review the mesh quality.  Abrupt transitions from very small to large elements can cause instability, as well as elements with small interior angles.

Good luck.

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Thanks for your response. I tried changing the timestep, changing the size of my mesh elements, etc. Only after I removed my pressure structure downstream did I get a smooth model run. I knew the problem was there. After a closer check, I had assigned the up- and down-stream low-chord elevations to the correct elevations (1500' or so) and I set the crest elevation using an elevation from another project (1300' or so). The model was trying to run water under 1500' but over 1300'. It was an operator error, but is there anyway to get a warning of this type of error in future releases? Something that will alert a user that these two elevations do not make sense? I know for future reference to check this area first but a warning would be nice.

Thanks

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