Boundary condition


I’m simulating a river that has 4 inflows. I’m running my model for different scenarios with different inflows. I’m wondering how should I specify the boundary condition at outlet?

1- Flow: I know the cumulative outflow (mass balance) but I don’t know the time series of flow because I don’t know the travel time from each BC to the outlet.

2- Open BC: In order to specify open BC, I need the pressure (water depth) at outlet which I don’t have it (I’m running the model with hypothetical inflows).

I would really appreciate any help.



If you are looking into the impacts caused by river inflows into the overall system, I would suggest using open BC / water surface elevation boundary on the downstream. The reason you want to use the water surface elevation boundary is because your model would be more stable and you can compute the impact by only changing the flows. If you have outflows as your downstream boundary, you need to make sure you have mass balance in the system. If there is not water balance, then either the flow will keep on increasing or decreasing and eventually will crash.

Are you looking at a hypothetical river with hypothetical river inflows ? Then you can create your first run by specifying constant downstream elevation. Then you can increase your upstream flows and see what impacts would it having when we have same boundary condition.

Though constant boundary condition on downstream is not ideal, but it will give more insight into your system. If you are dealing with some hydraulic structures / real systems, then discharge depth relationship or weir equation would be reasonable approximation.



Hey Amir, what did you use finally ?