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Old 12-15-2009, 10:51 PM   #1
mriceman
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Potential Branson Table Rock Dam Water Release Discussion

Hi all! New to the forum, but not Branson... lived here 28 years. Thought I would *bump* this thread up a bit, as I highly doubt any of us have forgotten the twenty08 flood, nor the wetter than average year, this year. So, I think what could happen is valid for discussion, even now.

Quote:
Originally Posted by Grizzly View Post
Gary,

Do you know what our Max output can be. Not that we ever want to see it. Somehow, I had the idea we maxed at 50K + generators, or was that per gate?
From the materials I've read, the spillway on the main dam was designed for a probable maximum flood event, with a discharge of 558,000 cfs w/ the lake at its design pool of 942.0 ft. and all 10 gates opened 37 feet. That's like if the gates weren't even there.



As you can see, the gates are capable of opening much farther than they were in April '08. The top of the dam is 947 ft. When the lake reaches 931 ft, the dam goes into what is called "surcharge operation," which means, whatever comes in has to be released, to prevent going over the design pool of 942.

The auxiliary spillway was built because of the study that was mentioned in the thread earlier, which estimated that the lake could reach 952 ft. if a Probable Maximum flood event occured. One statistic I'm having trouble finding is just how big is a probable maximum flood for Table Rock Lake. Well, I'm guessing that it's a lot bigger than the event of 2008. In a survey report from 1972, for a proposed but never built lake on the James River east of Springfield, quoting memorandum no. 33 from the US Weather Bureau, it mentions a probable maximum flood would need about 27 inches of rain over the studied basin during 72 hours, or a near 10 inch rain during 6 hours. That's an unimaginable amount of rain!

One common misconception I've heard folks mention is that the aux. spillway was built in case Beaver Dam were to fail. Although the spillway would help in that case, it's not the primary reason. Beaver Dam was rehabilitated in the early 1990s, when a cutoff wall was built to stop seepage through the foundation of the earth portion and since completion, the problem has been controlled to a great degree, so the possibility of Beaver failing is much less now, to the point that it is not a consideration. I have a copy of the completion report if anyone wants a copy.

But basically, Table Rock is capable of releasing over 10 times the volume as during last April. The auxiliary spillway can release another 450,000 cfs, through 8 gates, meaning the whole project can release upwards of 1,008,000 cfs, which it can probably release a little more if the 4 sluices at the base of the dam were opened as well.

I have a copy of the tailwater rating curve for Table Rock, from 1954. At a discharge of 550,000 cfs., the water below the dam would be at about 775 ft. To give some perspective, when all 4 units are running at overload, the water below the dam rises to about 712 ft. The powerhouse would already be almost totally underwater and that's without opening the aux. spillway.

I've been studying the White River basin since last Spring, when all of the flooding occurred. Honestly, it was the catalyst for my interest in doing so. If anyone has any other questions, of course, the COE is the best place to ask, but after the year of studying the projects, I can probably be of some help.
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Old 12-16-2009, 04:44 AM   #2
TheOleSeagull
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Thank you for posting this. Your work should serve as an excellent starting point for further discussion on this matter.

Interestingly, even at its height in 2008, less than the CFS (Cubic Feet Per Second) for just one of the ten flood gates was being released. The Ole Seagull has been told by sources he believes reliable that although the ten gates on the original dam can open in increments that is not the case with the auxiliary gates. At lake level 937 they open full bore. Has anyone else heard the same thing? Man, that would be a lot of water.

This thread was started with a post to http://www.1branson.com/forum/t34374.html the previous thread on this topic which had not had a post until this post in over a year an a half. Because of the potntial importance of the topic a new thread was started.
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Old 12-16-2009, 05:39 PM   #3
mriceman
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Thanks for starting a new thread, Seagull.

As far as the operating range of the new spillway, I'm not very familiar with it, other than that the floodgates are hydraulically operated. On the main dam, 2 step motors are installed at each gate opening, one on each side, which use a cable and drum harness to lift each gate.

The original method of hoisting the floodgates on the main dam was by a chain and sprocket arrangement. The control panels along the catwalk were operated by hand and the sequence of operation allowed for gate travel of one foot each time the "raise" or "lower" push buttons were pressed momentarily and released, the gate travel being stopped by a preset limit switch. I'm not sure in what increments the new control scheme allows for... 6 inch increments, maybe.

With all of the development that has taken place along Taneycomo, I shudder to think how a flood on the scale of even half of the spillway design flood would do. My business location is downtown at an elevation of about 735 ft. mean sea level (MSL). The tailwater curve at a release of 250,000 CFS puts the water level at about 755, which is about 3-4 feet below the powerhouse lobby level. Just a shot in the dark, I'd say that downstream at Branson, Taneycomo would be around 730-735 MSL, which it is normally around 700-702 with modest generation (say... 3 operating units) at Table Rock.

If the rain event we had on the 24th of April 2008 had been as bad as the event on the 10th... or even the March event, which by volume was much worse, I would not be surprised if Taneycomo had risen that much, because of the resultant releases at Table Rock. It would have been on par with the Flood of 1945, which saw floodwater in the railroad depot (now Branson Scenic Rail), the old hotel, and 6+ feet of it in the Ye English Inn in Hollister. The reason is simply because when Beaver Lake is full, as it was at the time, COE is forced into operating Beaver Dam in "surcharge" mode, just as they must at Table Rock, making it as though there are no dams.

There are 4020 mi.^2 of drainage area above Table Rock. In April of 1945 a peak streamflow of 200,000 CFS was measured at the Table Rock Dam damsite. Makes you think how worse it could be.

Honestly, what worries me about the system as a whole (even though an event of this magnitude would be most unlikely) is that there is a dam downstream of Table Rock that can only release about half as much... Bull Shoals. It's Spillway design flood release is 550,000 CFS, with a design pool of 702 MSL, I believe. Again, this means that Bull Shoals must release the incoming water, when it gets to 695 MSL.

What happens if Bull Shoals is at 695 MSL (full flood pool, as it was in April 2008) and Table Rock has to release 1,000,000 CFS (which it wasn't in April 2008, thank our luck stars) because of a massive rain event? Not likely, but where does that extra 500,000 CFS go, over the top of Bull Shoals? It's like having a drain in a shower that can only drain 1/2 of the water coming through the shower head. It's a question that I would like an answer to, for the sake of those downstream from Bull Shoals Dam, along with the rest of us.
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