drop in flow in front of breach wave

[MikeL]

I am running a dam breach on HECRAS 4.0. the model runs fine and i can get the breach to work. however i notice a drop in flow in front of the breach wave as it comes down. I have a tributary coming in below the dam ( modeled as another reach w/ junction) with a constant flow of 200 cfs. The flow through the dam is about 50 cfs. Right before the breach wave the flow drops down to 100 cfs. This wouldn't be that big of a deal however getting down to 100 cfs crashes the model. I have bumped the flow up on the trib, but it still drops and in some cases crashes the model.

Has anyone noticed the drop in flow before? Or is there something else going on in my model?

Thanks for any insight

[Paul M]

If you can get the dambreak unsteady flow to run with tributaries, good luck! maybe what your seeing is reverse flow up the tributary when the dam breaches? the breach flow should be significantly higher than the trib flow, so a reverse flow would occur. is it necessary to have the tributary in the model? is it large enough to take a significant amount of the dambreach flow as storage? if it is a small channel and the dam is on the main channel, you can probably lop off the tributary. if the dam comes from a small channel tributary and flows into the main river channel, then the dam breach flow would go both directions...up and down river. i had a model that had a situation like this, and i couldn't get it to run without crashing, so i removed the upstream tributary and modeled that as an off-channel storage with a side-channel weir. the weir was the overbank profile of the side valley to my model's "main channel" and the storage area was the planimetered contours of the up river valley, so as the floodwave progress water was able to flow into the storage area then out of the storage area as the floodwave receded. of course, you may not need this kind of detail to you model, and you might want to just eliminate the tributary. without actually seeing your model, that is about all i have to tell you. hope this helps.