Monday, February 27, 2017

Chickamauga Dam in Tennessee River in Chattanooga, TN

(3D Satellite)
While studying "Tenbridge," I discovered this dam.
The 60-by-360-foot lock at Chickamauga lifts and lowers river craft about 50 feet between Nickajack and Chickamauga reservoirs. [TVAdam]

GeoCaching, construction started in 1936 and it began operations in 1940
Deep Zoom has some more construction pictures.

The dam was designed in a way that allowed a larger 110 feet (34 m) x 600 feet (180 m) lock to be installed if increases in river traffic ever required it. [GeoCaching] I noticed when I looked at the satellite image that the lock seemed rather small. I confirmed that it can pass only one barge at a time instead of the 9 that a 600x110 lock can pass or the 15-barge tow that a 1200x110 lock can handle. I also noticed the cofferdam big enough for a new lock, but no other signs of construction. In fact, construction stopped in 2012 and the batch concrete plant was removed because of a lack of funding. [TimesFreePress2016] But in this case a new lock is also needed because the old one is falling apart faster than normal. It suffers from "concrete growth:"
Chickamauga Lock has growing concrete in it's structure, which is a reaction between the alkali in the cement and the minerals in the stone. This growing concrete has brought many problems – in some places large chunks of concrete have broken loose from the lock walls – and because the massive blocks that make up the lock have expanded at different rates, the top of the structure is uneven. Lengthwise, the lock has actually grown five inches inside the lock chamber. The approach walls have grown even more. Corps of Engineers and TVA working together continues making temporary repairs to the project spending large maintenance dollars. Corps and TVA have determined that Chickamauga Lock does have a finite life. [usace]
This has caused problems that have shutdown the lock completely such as "a crack in a steel support beam on the upper gate" in 2014. The government is having to waste a lot of money doing serious maintenance work on this like every year the new lock is delayed. [TimesFreePress2014] Fortunately for the eastern Tennessee economy, congress has chosen to fund the Olmsted Dam cost overruns out of the general fund so that money from the Waterways Trust Fund can be used to replace other locks in the nation such as this one and the Kentucky Dam Lock. This PDF file describes both the Kentucky and Chickamauga Lock projects. Of note is the "cofferdam stabilization." Does it need to be stabilized because it has set for four years or because that would have been the next phase of construction anyhow? The USACE also has a page concerning the new lock construction.

Note above that GeoCaching indicated the dam was designed to allow a bigger lock to be built. I have to wonder if the current design was the one that was envisioned in 1936. It seems dangerous to remove about a third of the spillway capacity. Or maybe they don't need as big a spillway now that more dams with flood control reservoirs have been built upstream.

I'm reminded that this is a TVA dam because, like the Kentucky Dam, I'm having a hard time figuring out how the spillway gates work. This is my current best guess.

Looking at Phil Thach's photo and the ones below, it appears there are two sluice gates in each bay, one on top of the other. They normally raise the upper gate and let water squirt out between the two gates.

Note that there are no gates in the side rail. This must be a construction photo taken before they installed the gates, and we are looking at the normal river flow passing through the spillway structure.
Alamy has 102 pictures of the dam including several with the gates at flood stage. When shut, they do completely stop the flow.

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