Breach Parameters for an Earth Embankment Dam 2

[1467]

When simulating a dam breach for the design storm, are you supposed to breach it at the highest water surface elevation, or should you take into account the erodibility of the soil and calculate velocities over the dam to determine when the dam will breach?

 

[dgillette]

Would it make a big difference for your case (more than just the "noise" in the flood hydrograph, the erodibility of the embankment, the dimensions of the breach, etc.)? Off hand, I wouldn't expect a huge difference in peak Q or the inundation area. It would seem more realistic to breach it with the reservoir level at the time of the breach, but it's awfully hard to make a good estimate of the timing of the breach.

On that topic, see

http://customwire.ap.org/dynamic/st...US&TEMPLATE=DEFAULT&CTIME=2006-03-15-08-25-11

Click on the picture for a bigger view. The picture is not the dam that failed, but another one in the area. This one must have been overtopped by a pretty good depth to have that big tree on the crest. I think the scallops in the crest are scarps from head-cutting.

Reading these news stories always seem a bit voyeuristic, but I suppose it's OK if they remind us of how careful we need to be with dams and public safety. It's OK to stare at car wrecks if it reminds you to drive more carefully (and you don't cause another wreck while staring).

 

[1467]

The dam overtops by almost 4 feet from the design storm. If I breach the dam before the peak water surface elevation, then it would probably make a big difference. There would not be as much storage if I breach the dam prior to the peak water surface elevation.

 

[1467]

I have used a 30 minute breach scenario, The top width is twice the height of the breach and the side slopes of the breach are 1 to 1. FYI

 

[1467]

The other Breach paramters that I have used are; The top width of the breah is twice that of the height, the side slopes of the breach are 1 to 1, and the total time to reach full breach is 30 minutes.

 

[dgillette]

I pulled out a plot I made a few years ago showing breach width at mid height vs height of water in breach. (Data mostly borrowed from Froelich, D.C. "Embankment Dam Breach Parameters Revisited" in a 1995 conference. See link below for full citation.) The ratios ran from about 2 for small reservoir (<1000 ac-ft) to 10 for large ones (>10,000). I don't know how much the smaller ratios were constrained by the abutments (so they couldn't get any wider like could happen with a very long dam. Some of the higher ratios came from flow slides (e.g., Buffalo Creek WV). For small reservoirs, I drew a line at 2.5 H + 180 feet, which almost forms an upper bound. The best fit to the data for small reservoirs would be more like 2 H + 100 feet. (This is out the window, of course, if the abutments constrain the size of the breach.

The best source on breach parameters is:

http://www.usbr.gov/pmts/hydraulics_lab/pubs/PAP/PAP-0735.DSO-98-004.pdf

That's a pretty thorough review of the subject.

Please do not use the prediction of breach occurrence in the Dewey & Oaks and Dewey & Gillette papers cited therein. It's been shown to be questionable, or worse.

As to the depth of overtopping to initiate breaching, you might pick 1 to 2 feet or even more, depending on the erosion resistance of the material and the duration. It is not uncommon for dams to be overtopped by 2 feet and survive, if they are constructed of clayey material, have good turf cover, etc. It's going to be a SWAG anyway.

I'm curious about how much difference there is in the peak Q between 1 foot and 4 feet of overtopping. Keep us posted please.

 

[1467]

The Dam is 1000 feet long, so if you look at it as a giant weir with a coefficient of 2.8.

Q=CLH^(1.5)

1'=2800 cfs
4'=22,400 cfs

 

[dgillette]

Sorry, I should have specified that I was curious about the peak BREACH discharge (Q and inundation area).

 

[cvg]

1 foot of overtopping is generally accepted conservative estimate here before assuming the dam breaches

 

[blueoak]

I think it is important to look at what you are really modeling. THis idea that you are actually modeling the true shape of the hydrograph is a little presumptive. For a PMP event, I normally use the USBR late 2/3 method, but that is for a conservative flow not for a conservative breach.
If you are doing an incremental assesment it looks like you already have a problem if your design storm is overwhelming your dam to such a degree that you do this timing thing.
I would run the breach for a 1 foot overtopping storm that lasts longer than the material will handle(the NRCS SITES is a good guessing tool) and then just stick the breach on the top of the "design storm" for a breach.
As for the time for breach, be really careful for those long breach formation times. The breach formation time is typically is from the initiation of removal of crest materail, so most of that downstream section is gone from an embankment dam.(Think Tom Sauk) I typically like to look at a sensitivity analysis and run a few methods( the Tony Wahl USBR papers are fabulous). I like the NWS BREACH model but rely on the empirical stuff.

Good Luck, because evacuation plans will be based on your work. I think engineers are really under bidding what flood inudation mapping requires.

 

[BigH]

Might want to look at references below: The first comes from the second.

http://www.geiconsultants.com/images/library/sub29.pdf

http://www.geiconsultants.com/papers.php?id=36&category=Water+&+Dam+Engineering