Tuesday, May 5, 2020

Oahe Dam on the Missouri River upstream of Pierre, SD

(no Bridge Hunter; John A Weeks IIISatellite)

View of Oahe Dam, located six miles north of Pierre, S.D. On August 17, 1962, President John F. Kennedy spoke at Oahe’s ‘Power on the Line’ dedication ceremony. The first of Oahe’s seven 85,000 Kilowatt generators was put into operation in March 1962 and the final generator went into operation in June 1963. The largest electricity producer on the river, Oahe produces 2.8 billion Kilowatts annually.
"The Oahe Dam is the second largest dam on the Missouri River, and the 15th largest earth fill dam on the planet. In comparison, it contains more than twice the fill material as the Aswan High Dam on the Nile River....Construction of the dam was authorized in 1944 by an act of Congress. The US Army Corps of Engineers began work on the project in 1948. Early work included the relocation of towns, bridges, and roads that would eventually be flooded out. The dam was closed in 1958 and completed in 1961, and the reservoir was full for the first time in 1962. The power plant was completed in 1963." [JohnWeeks]

An overview of the construction   The new dam flooded the farms, homes and businesses of a lot of people, many of them were Native Americans.

A close-up view of the power plant water intake control towers. These towers contain valves that regulate the water flow into the tunnels that carry water under the dam to the power plant. Each steel-lined tunnel is 24-feet in diameter and is about 3,650 feet long. The water is approximately 200 feet deep in this area.
It is interesting to contrast the water level in John's view above with a street view below.
Street View, Aug 2009

USACE-image, Photo by Cheryl Moore
Oahe Dam Surge Tanks
Located near Pierre, South Dakota, the surge tanks at Oahe Dam are each 70 feet in diameter. There are two tanks per penstock. The surge tanks help regulate water flow to the power house turbines.
This is the first time I have seen a hydropower plant with surge tanks.

"Oahe’s generators were upgraded in the 1980s and are now capable of producing 112,290 Kilowatts each, for a total potential output of 786,000 Kilowatts. Oahe is the largest power producer on the Missouri River, producing 2.8 billion Kilowatts annually" [USACE-hydro]
Are the surge tanks needed because of the length (3,280' to 4,005 long [USACE-facts]) and/or the curve of the penstocks? I include the map scale in this screenshot. Were they part of the original design or were they added later when problems were discovered?
The archivist notes imply the surge tanks were part of the original design.
USACE via SDdigitialarchives
This Power Plant at the Oahe Project near Pierre, S.D., is nearing completion by the U.S. Army Corps of Engineers. The surge tanks, each 145' high and 70' in diameter, help to control flows of water from the reservoir above the turbines below.
"The spillway and outflow tunnels are buried under the dam, so only this small concrete pad sticks up above the dam. The crane is used to operate each of the 8 Tainter gates. The gates are 50 feet wide by 24 feet tall, and are located 240 feet below the crane. Pairs of gates feed 4 outflow conduits which are nearly 20 feet in diameter and are approximately 3,500 feet long." [JohnWeeks]  I see six outflow conduits.
Street View
And it appears the Tainter gates are submersible. I've never seen that before, and I can't figure out how they work. A construction photo doesn't help me figure out how the Tainter gates work at the bottom of the reservoir.

Street View, Sep 2008
I was confused as to why there were two different discharge capacities. Then I realized that normally the height of the pool should not go above 1,620'. I assume the height of 1,644' is at the top of the earth embankment. That capacity is about twice the record discharge of 160,000 cfs during the 2011 flood.
The "Top of Spillway Gates (closed)" figure of 1620' seems to contradict John's statement that the gates are 240' below the crane.

I wonder what discharge rate causes flooding downsream.
Evidently before 2011, any discharge from the spillway would cause the flooding of some homes! [RapidCityJournal] I wonder what the "other release gates" are.

In 2011, the USACE ignored advice about a big snow pack melting late in the year because of a long Winter, saturated soils and heavy rains; and they did not open spillways early. So later in the year they had to open the spillways even further creating an even bigger flood.
ArgusLeader (paycount 5)
The release volumes on the six dams also were moving up aggressively. By May 26, officials forecast releases of 100,000 cfs by mid-June and 110,000 cfs by early July - amounts well above previous records. And amounts that would mean major flooding for Pierre, Fort Pierre and Bismarck....
Concerns also grew for Oahe Dam. The last time the dam experienced high water, the earthen embankment moved south. The corps dispatched geotechnical staff to begin taking instrument readings. It was an area of concern that needed to be watched.
Officials also were working on models of potential release volumes from the dams. Farhat noted May 28 that, "Just adding one rainfall event like the one two weekends ago pushed the releases up to 150,000 (cfs)."
And that's what happened. More rain came.
Later that day, the corps issued a news release stating that five of six dams would ramp up to 150,000 cfs by mid-June.
[So if 110,000 cfs would cause major flooding, what did 15,000 cfs do?]
So many people went to watch the record water release that they created a traffic jam. The USACE built a temporary parking area.
In a June 8, 2011 photo, sightseers look at water pouring from the outlet tunnels at Oahe Dam near Fort Pierre, S.D. The U.S. Army Corps of Engineers has opened the tunnels to help get rid of heavy rains and an impending snowmelt upstream in the Missouri River basin. The result is a pectacular show. (AP Photo/Chet Brokaw)
[So in June the snowmelt is still impending.]
From what I've read, they repeated the mistake in 2019 of not drawing down the reservoirs for flood control. They gave priority to recreational use in the Summer instead of flood mitigation. The USACE has high discharges during the Winter of 2020 because they are still dumping the water from the 2019 floods. "The Corps estimates 2020 runoff will reach 36.3 million acre-feet, ninth highest out of 122 years. The highest year was 2011 with 61 million acre-feet, followed by 2019, with 60.9 million acre-feet." [TheGazette]

USACE posted five images with the comment:
Yesterday [posted Sep 11, 2018], crews at Oahe Dam on the Missouri River opened one regulating tunnel. Releases are flowing from the power plant and regulating tunnel during scheduled maintenance to two units at the Oahe power plant.
Today's project orders at Oahe are for releases at 45,000 cubic feet per second. The current river stage on the Missouri River at Pierre is 10.87 feet (Sept. 11, 10 am) down from 12.26 on Sunday night. (Sept. 9)
[That is an impressive hydraulic jump.]





construction video  That page also has a video about the 2011 floods. (A YouTube version of the 2011 flood video)

(new window)    June 18, 2011, at 155,000 cfs     June 19, 20011, at 160,000 cfs

(new window)    spillway outlet footage    Big Bend spillway 

(new window) A lot of 85,000 cfs action

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