Flood conditions and TVA data


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Posted by Kerry Grissett on May 11, 2003 at 16:04:11:

A lot has been said about how the readings at lakeinfo.tva.gov are wrong during these flooded conditions. But, I would like to point out a few things that may have not been considered. No, I am not saying TVA's data is always correct, but I think many folks don't understand the dynamics involved.

First, let us say that the river is at rest, no spillways open and nothing going through the generators at the dams. In this scenario, water finds its own level. That is to say that the actual surface water level at any given point along any of the lakes on the Tennessee river will be roughly equal as far as "mean sea level" goes.

OK, I think you will all agree with that, right?

Now, consider that the river flows because it is lower in elevation downstream, no matter where you are on the river. The river bottom slopes (in the case of the Tennessee river) roughly from east to west downward, whether or not the river is flowing. However, when you open up the flood gates or even the turbines at a high rate, you have a lower level near the dam than you will have upstream. Think about it. The level downstream MUST be lower or there is NO flow even if the flood gates are open. Makes sense, right?

Also, where you have rivers or creeks/streams that are pouring into the Tennessee river after heavy rains, those areas will be higher. Just a matter of concentration of water flow.

OK, now with that under our belts, you also have to consider that the water levels below the dams will be considerably higher, relative to "normal" conditions, during the times when massive amounts of water are being released either from the turbines or especially during times when it is necessary for TVA to open the spillgates. Look at McFarland in Florence, Ditto landing, etc.

Someone on the NAFF mentioned that "bottle-necks" might cause higher water levels as well. I think the specific site mentioned was the train bridge at Decatur. This is probably correct, but I don't have enough data to confirm that hypothesis. (big words...grin) although I am sure there is something to consider there.

Along with all this, remember that there was a huge amount of rain that fell up in Tennessee that has to make its way down here eventually. Hopefully, this will help some of you understand why the river can be at different flood stages at the same time and also why the information at lakeinfo.tva.gov may not be wrong at all. For a better idea of water levels at different locations on the Tennessee river, take a look at the USGS water pages. Unfortunately, there are not many (if any) reporting stations for Guntersville, but there is one at Decatur and several at Huntsville. Many of these stations report real-time water level conditions or near real-time.

Over many years I have realized that the current river level may not make any difference or little difference to the flood levels on many creeks and streams. I used to live next to West Flint creek south of Decatur and it could flood even when the Tennessee river was very low.

In any case, I am having withdrawal symptoms! Not catching anything worth talking about and I will not get my boat out in this high water.

Good luck and good fishin'

Kerry

USGS Real-time water levels at specific point in Alabama
http://al.water.usgs.gov/rt-cgi/gen_tbl_pg



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