2D Dam Breach, flow into additonal storage areas downstream.

[gwheyduke]

Background
I’m trying to model a 2d dam breach of a reservoir. Let’s call this reservoir Barr Lake. Once the dam at Barr lake breaches due to a piping failure, the outflow will flow downstream and into a second reservoir (Milton Lake). I’d like to model the second reservoir (Milton Lake) as being full (i.e a water surface at or just below the spillway elevation) when the wave from Barr Lake hits it. I’d like to model this second reservoir’s spillway discharge curve, and pass some flow downstream via the spillway. Once/if the spillway at the second reservoir is full, I’d like the second dam to overtop and fail.

Finally, all of the outflow from the second reservoir (either via the spillway, or a dam overtopping) flows flows downstream and enters a third reservoir (Lower Latham). As with the second reservoir in the chain, I’d like to model this third reservoir as having a water level elevation just below it’s spillway. Any additional rise in water levels would result in flow out of the spillway. If and when the water level at the third reservoir rises above the dam crest of the third reservoir, I’d like to simulate the breach of this reservoir as well.

The Question
I’ve read the documentation and it indicates I can set an inline weir within a 2d area, and that I can have this weir breach. However, I’m unclear on how to simulate a full reservoir, and a spillway flow (based on an elevation flow table) using this method. Is it possible to set a starting water level and model a spillway with an inline weir in the middle of a 2d area?

Follow up questions
Assuming its not possible to use an inline weir, and instead rely on a storage area for the 2nd and 3rd reservoir, I assume I will need a 2D to SA connection, which will need a weir that matches the ground surface between the upstream 2d area along the boundary of the reservoir and the storage area?

There is also the potential for the flow to backup and flow down a separate drainage, in which case I’ll need to connect two 2D areas. Again I assume a weir between them as a 2D connection?

Below is an image of my current setup, using storage areas rather than inline weirs. I’m open to any and all suggestions on how you might set this up.

[Jarvus]

“However, I’m unclear on how to simulate a full reservoir, and a spillway flow (based on an elevation flow table) using this method.”

For a dam inside of a 2D area, use a 2D-SA connection. You can enter a spillway curve and the location of the spillway curve. If you want to spread the spillway flow out horizontally, you can enter several gates. And then each gate can flow can be based on a user entered curve, with all of the gates adding up to the correct total.

“Is it possible to set a starting water level and model a spillway with an inline weir in the middle of a 2d area?”

Short answer: No, there is not a simple way to do this. And it is unfortunate RAS hasn’t added this yet.

Long answer: You can fill a reservoir up using a flow hydrograph. A flow hydrograph can be placed inside of a 2D area. So for each reservoir, create a flow hydrograph in front of the dam. And then create a flow hydrograph that fills the reservoir to the desired level. You can just create a big flow hydrograph, run it, and then sees when it gets to the correct level and go back and chop the flow hydrograph off at that point. If you snake the location of the flow hydrograph around the lowest part of the reservoir, you can usually use very large flows while filling. Basically, as long as it doesn’t go crazy unstable. If you take this approach, you might want to write out a restart file so you can skip this step in the future.

I generally recommend trying to do everything in a single 2D area, especially for dam breaks. The other approaches tend to have various drawbacks including more stability problems.

[Anonymous]

Thanks for the reply. Right now I’ve got the model working ok with multiple 2D areas, but I also have the computation timestep set really low.

Perhaps I’ll re-build a geometry file with just one 2D area and use hydrographs like you suggested to fill my reservoirs.

Finally, I’ll have to read about restart files. That’s something new to me.

Thanks again for your help.

Hayden

[Anonymous]

If you already have multiple 2D areas and it is working, you may be good to go. Multiple 2D areas don’t automatically have problems. But if you find yourself having stability issues, definitely think about it.

[Author]

Posts