Sequential dam failure

[Leo]

Greetings,

I am modeling a sequential dam failure event in HEC-RAS 2D. I am wondering if there are alternatives to model a domino-like dam failure (the flows from a dam failure reach a downstream reservoir and overtops another dam causing a secondary failure).
This was initially modeled with two Storage Areas and two 2D Flow Area/Conn (one for the upstream reservoir and the other one for the downstream reservoir). The problem with this strategy was that the 2D mesh was covering the entire area of the downstream pond, which created some conflict with the HEC-RAS methodologies, also the model was giving me an error of extrapolating outside of the reservoir capacity curve for the downstream reservoir.

As an effort to simulate a sequential dam failure, I came up with the following approach:
• First, I simulated the overtopping of the downstream dam (in a separate Plan). The results from this simulation were used to obtain the breach hydrograph.
• Second, I added this hydrograph as an internal boundary condition below the downstream dam and run the model for the failure of the upstream dam. This included adjustments in timing so by the time the flood flows reach the crest of the downstream dam, the hydrograph from the first step would start entering flows.

This approach seems to work okay and represents what I am trying to model, however I would like to know if anyone has modeled something similar, or what other ideas/ways would you recommend for this while taking into consideration the HEC-RAS tools and limitations.

I appreciate any help,

[Jarvus]

Doing the downstream dam by itself and then trying to add the breach hydrograph to the upstream hydrograph, seems a bit strange. You may be getting okay results but it would be hard to know.

I’ve mentioned this in other threads, but my first suggestion would be to try and model the entire system as a single 2D area with internal SA connections to represent the dams. I think this would be the approach that you could have the most confidence in the results.

With everything a single 2D area, it is a bit awkward to get the reservoirs to have the correct starting water surface elevation, but there are ways to handle that.

[Jesse]

I’ve read numerous posts from you that suggest modeling a sequential dam breach suing only a single 2D Flow Area. Using your posts as a guide, I’ve been able to to do; I’m using a SA for the upstream reservoir, but am modeling the downstream reservoir as part of the single 2D flow area (and using an internal boundary condition to fill the reservoir to max level before the incoming upstream breach flow). This is a sunny-day failure and no inflow from tributaries is being modeled.

However, I’m having a problem whereby the second (downstream) reservoir is not completely emptying even though the 2D Connection breach parameters have a bottom elevation that is the same as the reservoir bottom elevation. There still is about 10 feet of water in the reservoir when the simulation ends. It’s as if the dam breach bottom only affects the inline structure width (that is, the dam crest width of 20 feet), and isn’t cutting the terrain for the dam back to the reservoir bottom (3:1 inside slope); e.g. the dam crest might be getting to the specified breach bottom elevation, but that elevation isn’t being “cut back” into the reservoir.

This problem doesn’t exist when I use a simple SA – the SA is populated with elevation-capacity relationship, and the breach bottom elevation properly drains the entire reservoir.

Any insight you can provide would be greatly appreciated. Thanks!

[Jarvus]

If the dam is part of the terrain, that can be awkward.

During the breach, RAS will lower the cells immediately adjacent to the breach, but not any cells that are farther away. So if you have small cells, RAS will create a “slot” down the center of the dam, but you can end up with high ground that holds water back like you describe.

So there are at least two possibilities to model your situation. You can try and make the cells wider at the breach. If the upstream embankment is only one cell wide and the downstream embankment is only one cell wide, I think that would take care of the problem. But this might not work if the large cells cause other problems.

Another option: you could cut the breach into the terrain ahead of time. I’m not an expert on modifying the terrain. I would probably created a 1D channel and then use the channel to cut the terrain using the tools inside of RAS. Someone who knows what they are doing would probably do something easier.

After cutting a breach through the terrain, enter the station/elevation data of the 2D connection to represent the top of the dam. The water won’t flow through the terrain cut as long as the 2D connection hasn’t breached. When the 2D connection does breach, the terrain won’t have any high ground that’s holding the last ten feet or so of water back.

It is true that I generally push to do everything in a single 2D area. I think it is generally more accurate and it is generally more stable. But I’m not completely single minded about it. Sometimes there are other options that are easier to model that give results that are satisfactory for given problem.

[Jesse]

Hi Jarvus – thank you for your time and your reply.

After I posted, I thought maybe the 2D mesh was the culprit – I’m using 40 foot cell sizes, so there was 5 cells between the 2D Connection Dam and the bottom of the reservoir. I removed points from the mesh until the cells at the breach location stretched to the bottom of the reservoir. I’m just now reviewing the results, but my preliminary review suggests this method has worked.

I will also incorporate your solution of making the breach covered by only one cell.

Again, I appreciate your time!

[Author]

Posts