Thursday, July 30, 2020

Tempe Town Lake Dam on Salt River in Tempe, AZ

(Satellite)

While studying UP's bridge that they damaged in Tempe, AZ, I noticed in a satellite image that Tempe had a dam to put a river next to their river parks.

Screenshot via tempe.gov
3D Satellite
I zoomed into the gates on the satellite map to verify that they can be moved to offer no resistance to the water during a flash flood. That is when I noticed that the gates have a design that I had never seen before. At first I had assumed that it was a rack and pinion drive. But then I read that this "is the country's largest hydraulically-operated steel gate dam system." [tempe.gov] In the screenshot above you can see the hydraulic rams. They are clearly long enough that they can be extended so that the gates lie flush in the splash pool. "Using hydraulic lifts, these gates can be lowered to allow storm waters to pass through and raised to capture the tail end of the flow to re-establish the lake. About two feet of water can pass over the top of the gates while they are in the fully-upright position....The cost of this project, including removal of the previous dam system, was approximately $47 million." [tempe.gov] The above video explains that the gates are also lowered when the Roosevelt Dam discharges water.

This new dam replaced a dam that used inflatable rubber bladders that was constructed in 1999. A lot of work was done to ensure that the town's water allocation stays in the river channel and doesn't seep into the surrounding land. On the west end of the lake, they used cement stabilized levees with a cement-clay cutoff wall down to bedrock. On the eastern end, where the bedrock is much deeper, they lined the Salt River channel with clay and concrete. The have also installed seepage recovery wells. They do have to consume their water allocations to compensate for evaporation losses. The gates must be able to be raised while water is still flowing over them to capture the end of a flood event to refill the lake. [water-quality] Much of the lining of the river channel had already been done before building the dams to avoid channel erosion during flood events. [original1, original2]

water-quality
The harsh desert environment was hard on the rubber dam, so it was replaced with the steel gate dam in 2016.
water-quality
Evidently they have not had a major rain event since 2016 because the only image I have found of the gates being completely lowered appear to be during a gate test.
Screenshot via tempe.gov
And there is vegetation in the downstream riverbed. When I first looked at the satellite image, it struck me how low the flow of the river can get.
Satellite
Indeed, it can get bone dry. A construction video made a point that they installed a rubber strip at the base of the gates to help seal the water in the lake. That means that the water in the lake is stagnant. I wonder if they have a problem with toxic algae growing on it. [DeBruler]
Vijay pentapati, Sep 2016
Evidently the lake is big enough that it doesn't have problems with algae, but sometimes the spillway does. But the next decent-flow event will flush that algae away.
Minette WILLIAMS, Apr 2018
The heaviest operational flow that I found.
Greg Roberts, Mar 2017
Some construction photos

(new window)  One of the bladders erupted in 2010 and they lost the lake. Also, the lake is the town's water supply. So if the intake is near the west end, that would provide some flow in the lake even if no water is flowing past the dam.


I paused the video to read the river gauge. That is when I noticed the link and bracket next to the hydraulic ram that can hold the gate in the upright position. I assume this is in anticipation of the maintenance activity of replacing a ram. You can also see the slot on this side of the river gauge where they can install a bulkhead gate.
Screenshot

(new window)  Of particular note is the number of times the old dam had to release water and they had to pump the construction zone dry again.


(new window)  Installation of the first gate. The gates were evidently fabricated in two pieces.



Unlike the two dams in Michigan, the downstream Salt River can handle the full flow of the billion gallons that are normally in the lake. So loosing just one-fourth of the dam should not have caused any downstream flooding issues.
Photo by Carter Radcliffe '11 via brophyprep
When PCL replaced the four rubber bladders, they also installed a sprinkler system to keep the rubber cool on hot days.
PCL


It appears they have had some local rains in the Summer.
USGS




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