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# Length of BC

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How can the length of the inflow and outflow boundary conditions, along a model domain border, impact the rivers response to such inflow and outflows respectively?

I attached a visual representation of my question for added clarity, if needed. The Red Lines represent the "Long BC" which was part of the training exercise provided on the FHWA/Aquaveo SMS Training website and defined according to the provided instructions from the website. The yellow line represents another possible option to define the boundary condition for the river response as the "Short BC" line.

What is the best manner to determine if a "Long BC" or a "Short BC" is needed for a model domain?

Mark

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• 9 months later...

It depends on the flow characteristics, the terrain and the type of BC you choose

For the inflow BC, depending on the initial conditions,topography and BC type, SRH will bring water inside the model on the available length.

It is more noticeable on the outflow bc. If you use either BC (red or yellow), SRH will evacuate water on the available  cells. And it all depends on the type of BC you choose. For example for a top of  riverbank at 40m ; if your D/s BC is "exit-h" at 30m,  the flow will most probably stay in the riverbed and be evacuated only on the cells neigboring the BC ; if it is "exit-h" at 50m, the river flood outside its bed and will use more cells of the BC.

Hope this helps,

Pablo

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Mark,

As Pablo mentions, it does depend on various factors. You should consider the terrain at the boundary as well as other hydraulic factors (i.e. roughness, anticipated velocities, magnitude of inflows). Although these factors will assist in determining the width of the inflow/outflow boundary, it still ends up being an educated guess which could be near or far from what the flow width really is.

The suggestion I have is that you simply ensure that the Inflow/outflow boundaries are far enough away from the area you are interested in for your analysis. If you move these boundaries far enough upstream/downstream away from the area of interest, then within the model, any poor guess that was made for the width of flow will not affect the results since the width of flow will have ample space to either contract or expand into a more natural flow width before it arrives in the area of interest. This process of extending the model upstream/downstream may be an iterative process as you do some initial runs and look at the results to see what is occurring.

Hope this helps.

-Cody

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