Rubber dam failure

Chris,

Ok, I gave up trying the model on my computer. I have another question as to how DAMBRK handles a certain kind of spillway configuration, though.

The earth dam I'm currently working on has an inflatable rubber dam, 6 feet tall and all across the length of its spillway (basically it adds 6 feet of storage above the initial normal pool (crest elevation) of the spillway). The Fabridam (as it is called) is set to deflate, almost instantly, when the head against it is, say, 1 foot above the top of the rubber. This occurs about 7 feet below the top of the dam itself. It is normally filled with compressed air (like a giant balloon).

This poses a challenge as to how to get DAMBRK to model a complete dam failure, because once the level in the dam reaches 1 foot above the spillway, the fabridam will deflate first and essentially act as a "spillway" breach. This of course would release a he** of a lot of water. I understand how to do this in DAMBRK. But what I don't understand is if it's possible to continue on with the actual earth dam failure. Is DAMBRK exclusive in that you can either have a spillway breach (confined to the width of the spillway) OR can you have this happen first, then have a "traditional" breach happen later at another (obviously lower) trigger elevation?

I tried coding in elevations for both the dam failure, breach width, time to failure, and spillway breach (YES), and the program crashed. Before I spend time trying to massage things to get the program to run, though, I need to know if this is at all possible to mode in this fashionl.

Thanks,
Eric.

Dear Eric :

This sounds relatively tough because what is happening is you have 2 separate failures at the dam, and DAMBRK cannot really model that. You would have to model 2 separate models, modeling the rubber dam failure first (in order to get the outflow hydrograph) and then modeling the earthen dam (using the new flow values from the previous model).

Why your rubber dam model is most likely crashing is that DAMBRK does not like instantaneous dam failures (unless you have micro-sized time steps). What happens is there are too many changes that are occurring over too short a time frame, and DAMBRK becomes unstable and cannot reliably compute the water surface elevation--and therefore terminates. The trick is to model the failure over a 1 hour time period (or whatever is necessary to get the model to run), and then slowly shorten up the dam failure time until DAMBRK stops working. This is the minimum failure time you can use. Note that it helps if you use a very small time step-- in addition.

If you have any additional questions or need any further information, please contact me again.

Sincerely,

Chris E. Maeder, M.S.
Senior Technical Engineer
chris.maeder@bossintl.com