Does dam breach flow use weir coefficient? Classic List Threaded 8 messages Open this post in threaded view
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Does dam breach flow use weir coefficient?

 Hello, I am running a dam breach model with reservoir modeled as a 2D area (dynamic routing) connected to the 2D flow area, with a 2D connection representing the dam. It is modeled as a partial breach using overtopping method. The initial water surface is below the crest, so there is no initial flow over the dam crest - flow does not start until the breach has begun progressing downward (I know this is not realistic). For this setup, is the weir coefficient used to calculate flow through the dam breach? Or are the full Shallow Water eqns used with the land cover data? (I am using Full Momentum). Is there any way to force Shallow Water eqns? Thank you!
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Re: Does dam breach flow use weir coefficient?

 If you want to model the dam breach using the shallow water eqns, then I think you will have to model it as a single 2D area with the dam inside of the 2D area.   For a connection between two, 2D areas, I believe it uses the weir equation.
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Re: Does dam breach flow use weir coefficient?

 Thanks Jarvis. I have been using an initial elevation for the 2D reservoir area, and breaching at a set time. If I use an initial elevation with only one 2D area, wouldn't it also fill the downstream area to this value? I guess I could input an artificial inflow hydrograph upstream of the reservoir and instead of breaching at set time, breach at a set elevation...Do you know of any better way to do this?
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Re: Does dam breach flow use weir coefficient?

 Unfortunately, you will have to fill the reservoir.  You can put a BC flow line inside of the 2D near the dam and use a large flow to fill it up more quickly. You can also fill the reservoir up and then write out a restart file and then use the restart file as the initial conditions.  This way, you won't have to fill the reservoir up for each run.
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Re: Does dam breach flow use weir coefficient?

 In reply to this post by DS_ras I think if you model dam breach using breach function of RAS, the only equation used to calculate flow out breach is weir equation. I have never heard using the other core equation to model flow out breach. Beacuse most breach model will simplify the process to 1D(BREACH,WINDAM.etc).Of course, after the flow  pass through the breach, the flow will be simulated used shallow water equation.
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Re: Does dam breach flow use weir coefficient?

 No, I don't think that is correct.  If you put the dam inside of the 2D area and, from the Connection Data editor, select Normal 2D equation Domain, the flow through the breach uses the 2D shallow water equation.  You can test this by flipping back and forth between 2D equation and Weir Equation and see that you get different answers.