2D Area inflow

[themoy08]

I have a question on inflows to a 2d area. So I have a 2d area along a specific reach of a main channel. My 2d area was specifically placed where I have a calculated HMS flood hydrograph. How wide should I be making my inflow line (BC Line) across the flood plain? I know that the main channel overflows given the peak flow of the hydrograph, but I don’t necessarily know how wide spread the flooding is. Do I make the inflow line the entire flood plain width? This also brings me to another question. If my inflow line for the main channel is across the entire flood plain width, but there is another smaller stream that has its own inflow hydrograph as well; how does that get handled? Would you have 2 inflow lines across the same cells representing the main channel and secondary channel inflow?

Thanks for the input

[Anonymous]

I don’t think you can have two inflow lines across the same cells. So I would probably make the secondary inflow its own boundary. If the peak flow overflows the main channel, then I would generally recommend extending the main boundary line across the flood plain. So the main boundary line would extend up to wherever you started the secondary inflow.
For the flow boundary, you have to enter a energy grade slope. The shallower you make this (smaller number closer to zero), the more the water will be spread out at the boundary. A bigger number will keep more of the flow in the channel.
The distribution of flow is not going to be perfectly accurate right at the boundary, but it should generally be ok as the flow moves downstream. You can adjust the slope up and down and see if it effects the model where you are trying to get good results.

[Lonnie A]

If you imagine the boundary line being a XS it might help you visualize where to set the limits. RAS is going to essentially cut a XS along the boundary line and when you apply a hydrograph it will fill the XS starting at the low point. So if you have a hydrograph for only one drainage area you don’t want the boundary to cut across into an adjacent drainage area. Or don’t cut across two channels if they have independent hydrographs.

[Anonymous]

Thanks for the input. I guess I’m still wondering if at the peak flow I know I have a water surface that extends all the way across my flood plain (we model a lot of PMFs) I would want to have my inflow line the whole way across the flood, but then that inflow also covers up that secondary stream that has it’s own inflow too. So I guess what you’re saying would be to have two inflow lines (main and secondary) that have bigger energy grades to contain the high flows somewhat within their channels, and know that results near the inflow would not be as realistic as results downstream after conditions have time to equalize?

[Author]

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