Unsteady Flow Model outlflows to sea boundary Conditions

[thod]

What is the proper downstream boundary condition to use for an unsteady flow model that outflows to the sea? In Unsteady flow there is not an option to use Known WS and if I use a steady stage hydrograph the results are not reasonable.

[Anonymous]

You can use a stage vs. time hydrograph that reflects the local tidal cycle. As the tidal variation usually depicts the WSEL for a large area, suggest that your downstream-most cross section be large enough that the velocity head is negligible. Have used that approach successfully in many applications.

[thod]

I tried to insert a steady stage hydrograph downstream boundary condition but the result was this:

while using normal depth gives this result:

It is like the flow has no way out of the system. Maybe I do something wrong. Is there any other way to “lock” the stage to a certain value at the downstream end in unsteady flow?

[Anonymous]

Hard to tell what is going on here. Looks like there is a problem at the downstream-most section or the one above it. Check the convergence messages, number of warm-up time steps, and the hydraulics at the sections at and near the downstream boundary. Perhaps run a steady flow model with the base flow that you are using for the unsteady flow model to get more info. about the cross section hydraulics. If you can put your data somewhere, I’ll give it a look.
Good Luck, Michael

[Anonymous]

Did you have any luck finding a solution? I am currently have the same issue. I have a known DS water surface, but the result is identical to yours..

[Anonymous]

Having the same issue. But actually, reading this post, I realized that model is not wrong and Known WS condition is not missing. In the 2D manual its written that when a stage hydrograph is inserted, stages above the terrain will put in flow into the system and stages below the terrain will draw it out.

That means that if you have a WS wich is higher than terrain you will always flow going inside the model. And it does make sense because water cant find an exit over there, therefore it has to got out trhough the flood plains.

Do you guys think im right with this poor reasoning?? Im having so much problems with the Output boundary conditions in unsteady models.

[Author]

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