Stage hydrograph as 2D bc line

[Anonymous]

Hello everybody.
I wish to use a stage hydrograph as 2D boundary condition of a 2D flow area.
I was reading the documentation available to understand which equations HEC-RAS uses to calculate the flow.

I report the only thing I found: “A Stage Hydrograph can be used to bring flow into or take flow out of a 2D flow area. If the water surface elevation in the Stage hydrograph is higher than the cell water surface elevation (or dry elevation), flow will go into the 2D Cells. When the water surface elevation of the Stage Hydrograph is lower than the water surface in the 2D flow area, flow will go out of the 2D area. If a cell is dry and the stage boundary condition is lower than the 2D flow area cell minimum elevation, then no flow will transfer. The flow is computed on a per cell basis. For instance, if the Stage Hydrograph water surface is higher than the water surface of some of the 2D boundary cells and lower than that of other 2D boundary cells, water will simultaneously enter and exit the Stage Hydrograph boundary.”

How is the flow calculated? What “per cell basis” means?

Thanks

[Jarvus]

If you are thinking that RAS computes a flow at the boundary before the time step, I don’t think that is correct. I think it comes out of the 2D solution.

For example, if you are using the diffusion method and two adjacent cells have different water surfaces, over the period of the time step, there will be a flow between the two cells based on the finite volume diffusion solution.

I think a boundary condition works the same way. The boundary condition forces a water surface and the 2D solution computes a flow like it would between any two cells.

“Per cell”: The stage is applied to each cell and each cell computes its own flow. So, for instance, some cells could have water entering the 2D area while other cells could have water leaving.

It is not taking an average water surface along all of the cells.

So the ‘equation’ that RAS uses to compute flow depends on which methodology you choose (diffusion, full momentum, etc.)

[Anonymous]

Thank you for your reply Jarvus.

My question was linked to the following trouble I am working on.
In a work I’m working on it is required to me to apply to a 2D flow area a stage hydrograph derived from a previous 1D model (made by Cross section+SA). Basically I am using as stage hydrograph of my BC the TW stage of the LS connected to the SA in the previous model. In this way I am able to obtain into my 2D flow area the same WSE obtained in the SA of the previous model. On the other hand if I compare the flow leaving of LS of the previous model with the flow computed at the boundary condition of my model the maximum flow and the volume are totally different.

I expected that it were different, but I hoped less than results I obtained. In my opinion this difference is due to the following elements (please let me know if I am rigth):
In the previous model the flow over the LS is computed by the standard weir equation, i.e. the flow is linked to the HW stage. In my case I am using the TW stage, which is that less than HW stage.
The computational methods of flow is different: in the previous model was based on the weir equation, in my model on Full momentum. Isn’t it?

[Anonymous]

Yes. I would agree with that. I would not expect the weir equation to produce the same flow.

[Author]

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