Wednesday, June 20, 2018

Tumble Bays and Hoover Dam Floods & Drought

I first encountered the term "Tumble Bay" in a comment by DanielG concerning the Croton Dam in Michigan. Some research has indicated two distinct uses of the term. One use is as a "splash bay" for the outfall from a Tainter gate. Another use is as the tapered receiving channel at the bottom of a long wier.

A Google search showed only one dam outside of Michigan that used this design: Elkhart Hydro Dam. And that is close to Michigan and has the same geology of a deeply buried bedrock by sand, silt and clay.
For sites without rock bottom in reach, great care is necessary in defending the channel below from erosion. For such sites the "tumble-bay" dam is best adapted, in which the force of the falling water is dissipated within the confines of the structure. In dams of this type, the sheet of water is given a sheer fall, being well ventilated behind, into the pool or bay which has a depth of 1/2 to 2/3 the height of the fall, and the water escapes from the bay over a breast wall or roll-way with little or no destructive energy. [The Americanna]
Photo
Cooke Hydroelectric Plant, Spillway, Cook Dam Road at Au Sable River, Oscoda, Iosco County, MI
Photo
Cooke Hydroelectric Plant, Spillway, Cook Dam Road at Au Sable River, Oscoda, Iosco County, MI
You can tell that this is not a common design because every source I have looked at has a different name for the downstream wall that creates the pool. For example, this diagram of the Croton Dam labels it the "tumble bay weir."
mi0529data, p6
The tumble bay weir is 20' high and 12.5' thick at the base. p3
An important aspect of this tumble-bay design is a long apron downstream of the breast wall. You can easily see the long apron in this satellite view.
Satellite
Jerry caught the middle gate open. It is open a small fraction of its total height, but it is making quite a splash over the breast wall.   Robert also caught a gate open. Note the watermarks on the side wall. This gate had been making a big splash. Gary caught the middle gate open even wider.

I looked at the photos of the dams upstream from this one, but none of them caught water flowing through the spillway. But I noticed the Alcona Dam does not have a spillway. The Mio Dam has a single gate spillway and an emergency spillway.

Screenshot from the following video
[Technically, this is probably a ski-jump spillway rather than a tumble bay. But this teaches me that part of the effectiveness of the ski-jump spillway is that it collects water at the bottom to help absorb the flow's energy.]
(new window)   The Richard B. Russel Project dam at 2:23 in this video also uses a tumble bay.





The other use of the term is for the tapered receiving channel along a spillway weir.


Reservoirs in a Changing World: 12th Conference
More terminology: some call the tumble bay the collecting trough and the downstream end of the trough empties into a chute. [ASCElibrary] Other terms are receiving and discharging channels. [core.ac.uk]

(Update: other example of dams using a tumble bay in their emergency spillway:


)

USBR-essays
I've seen images of this usage of tumble bay when I researched Croton Dam, but now I can't find them. However, I remember that the spillways for the Hoover Dam have tumble bays. Now that I'm taking a closer look at them, I see the bay below the gates is deeper than the chute leading to the tunnel. In all other examples of a receiving channel that I have seen, the depth of the receiving channel is not lower than the discharging channel.

The depth of the receiving channel is tapered. It starts shallow at the end of the weir away from the discharge and slopes down to the depth of the discharge channel as it receives more water from the weir. (I spent some more time looking for some photos of a normal side-channel spillway for a plain earthen dam, but I still can't find any. I did find some neat photos for the Croton Dam, so those notes have been extensively updated.)

USBR-essays
There are spillways on both the Arizona and Nevada sides. They were used in 1941 to test them and again in 1983 when there was a flood. The capacity of the spillways is high enough that they can cause flooding downstream.




USBR-essays
The spillways use the outer (lower) tunnels. They are the second line of defense to avoid over topping the dam. The inner (upper) tunnels are running full bore because they are the first line of defense. They, along with the powerhouse penstocks, are fed by the intake towers.
USBR-tnlfaqs, 1998

(new window)


Photo, Caption
Water enters the Arizona spillway (left) during the 1983 floods. Lake Mead water level was 1,225.6 ft (373.6 m)
The fact that the Colorado River had been allocated over 100% of its flow has caught up with the government agencies. But no one seems willing to blink and reduce their take, so Lake Mead is being drained. (The variance before 1965 shows how the Glen Canyon Dam smooths out the river flow.)
USBR-lakemead
The spillways are now high and dry. It will be a long time before they see water again, if ever.
3D Satellite
At least the low water levels allow them to save maintenance on the spillway gates. It looks like they removed them.
Screenshot
"Because water levels are always low, turbines are now being replaced with more efficient ones that have a more favorable blade pitch to take advantage of less water pressure through the tunnels used for power generation."
Kathleen's Window, Feb 2014
USlakes

Forgotten Railways, Roads, and Places shared
Dennis DeBruler http://mead.uslakes.info/level.asp
K.a. Hughes Wonder how high the silt comes now?Dennis DeBruler I read that one reason they built the Glen Canyon Dam was to collect the silt so that Lake Mead doesn't fill up with dirt.


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