Does dam breach flow use weir coefficient?

[Geo_Wick]

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!

[Anonymous]

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.

[Geo_Wick]

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?

[Anonymous]

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.

[lilith894]

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.

[Anonymous]

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.

[cameron]

First off, you should not use 2D equation for a dam (see below).

To answer the breach question, it uses the weir coefficient you specify in the breach input window.

If you do have overtopping of a Dam, the weir equation would be the better option to use than 2D equation anyway as that type of flow fails to meet the shallow water equations assumptions. It is more like a waterfall. This would be similar to any large levee that over tops.

The 2D Users Manual states this specifically on page 3-67.

[Geo_Wick]

Thanks for the reference, Cameron, I somehow missed that section in the manual. However, the dam I am modeling is only 9ft high and unlikely to cause freefall, so it sounds like the shallow water equations may still apply. I will be sure to compare my results with the weir option results.

[Author]

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