Thursday, June 2, 2016

Tainter or Radial Gate

(2018: I did a total rewrite.)

The US Army Corps of Engineers (USACE) uses Tainter gates in all but its oldest river dams.
Newburgh Dam on the Ohio River
During a flood, these gates will be lifted so that the support arm is almost vertical. That is, the skin plate will be lifted completely out of the flow of water.

A Tainter gate is a curved skin plate held by arms that are fastened to  trunnions on the dam's foundation. The skin plate is a semi-circle. It is opened by lifting the plate and pivots on the trunnion. Note the arms favor the bottom of the skin plate where the water pressure is the highest.
RodneyHunt, p2
RodneyHunt, p3
Jeremiah Tainter was 26 when he came to Menomonie, WI, to work for a lumber mill in 1862. He had a talent for improving the water control gates for their many mill pond dams. "There are 321 Tainter gates on the dams and locks of the upper Mississippi River Basin from Minneapolis to St. Louis....There are 195 Tainter Gates in the Columbia River Basin on 26 dams, including the Bonneville and Grand Coulee dams." [DunnHistory] He developed the current design in 1886. Tainter gates close automatically under their own weight if there is a mechanical failure. So it is a fail-safe design. [EngrColoState]

I made a special trip down the Illinois River during a flood take photos of the Dresden and Marseilles Dams during a flood. Below is a photo of the Marseilles Dam. Note that the first four gates are completely out of the water, and the river has an unobstructed flow. Also note that gates 6 and 7 are raised but no water is flowing. That is because bulkheads have been installed in those openings. They are still repairing the dam after the April, 2013, accident. There is a crane on a barge parked behind the dam on the left side of the river.
20150714 2760, DeBruler
The other reason for selecting Marseilles as an example of a river dam is that it has sheet metal over the top side of the gates. During a heavy flow, since the gates are lifted completely out of the water, the debris on the water such as tree limbs and ice flows is not a big issue because it passes through the dam with the flow of the water. But when the gates are partially raised because of a moderate flow, then debris gets caught by the gates and can build up, and it is dangerous to go near the dam to pick up the debris. For these gates that have the top covered, there are depressions under the gates so that they can be lowered as well as raised. When they are lowered, the water flows over them instead of under them and debris can be carried over the gates by the water. An example of how much debris can be flowing in a river.

A satellite image of the USACE using their Hercules 60' ringer crane to repair a gate gives us the unique view of a gate laying on a couple of barges that have been lashed together.

20140627 0062
Control works for the Marseilles North Mill Headrace
I have several photos of gates in dams along the Illinois, Ohio and Mississippi rivers (see labels wwIll, wwOhio and wwMiss). The problem is that those dams are so big that you can't get a good view of the gates. The two gates in this picture controlling the flow through a mill headrace allow me to get a closer shot. They are also unusual because they have counter weights.
(new window) Skipping to when they start opening the second gate. Note that water is flowing over the top of the gate. Tainter gates are supposed to be raised long before the water would go over the top. They must have raised the reservoir level to an excessive height to test the gates with a maximum head of water. I was impressed by the amount of churn at the end of the spillway since the gates were not very wide open. In fact, after reading the text associated with the video, I learned that for this "little" (1 of the possible 11 feet) test, the downstream channel rose six feet.

Photo referenced by EngrColoState
The following spillway test videos taught me that a Tainter gate design for the crest of reservoir dams is significantly different than those for a river dam. Specifically, the gates are tall, but most of them are designed to open just a fraction of their height. That is, unlike river dams, these gates won't be lifted out of the flow of water. The tests are for the gates machinery and alignment. It is not for the maximum capacity of the spillway because the maximum capacity is designed for floods. If they used the full capacity, the downstream river would flood. That is why the Winter reservoir level for a reservoir dam is lower than the reservoir's capacity. They want room to hold the Spring snow melt and/or rains so that excessive amounts of water don't have to be released. (Gates in river dams don't need explicit tests because they get tested often by floods since they have no storage capacity. There job is to create a pool of water that is at least 9 feet deep year round for barge traffic. They have no flood prevention capability.)

Just one gate is opened at a time to reduce how quickly the downstream river level rises. They want to avoid creating a flash flood that would not allow fishermen, kayakers, etc., time to get to higher ground.

I've noticed that active spillways for large dams are a good source of "white noise."

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(new window) The spillway volumes for the Hartwell Dam looked impressive until I realized how tall the gates are. The pivot point is at the top of the concrete pillars. In some scenes, you can see the three beams that go from the trunnion to the gate, and the beams for the open gates are not much higher than for the closed gates. When you look at the height of the training wall on the side, the gates cannot be raised much higher or the water would go over the top of the wall.

Another test of Hartwell Dam test five years after the above test.
(new window at 2:41) At 3:44 the gates jump into focus. But you still can't see much difference between the top of the opened gates and the still closed gates. You can tell by the turbulence at the outflow of the hydro plant that they did not have to shut it down during this (partial capacity) test.

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(new window)   Logan Martin Dam

So why are reservoir gates so tall that only a fraction of their height is used at the bottom? If you know, please comment.

Here are some of my speculations:
  • The gates are cheaper than the concrete needed for a higher spillway.
  • They want to release the cooler water that is lower in the reservoir.
  • The spillway has to deal with a lower head of water. But this is offset by the higher pressure of the released water.
  • If it is the spillway for an earthen dam, they are willing to open the gates really wide if it proves to be necessary to keep the water from topping the earthen part. A check zoom out of the Harwell Dam reveals that it is an earthen dam.
Fort Gibson Dam is an example of a reservoir dam that can releases a lot more water than the downstream river can handle.

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