Just when you thought it was safe to empty your sandbags, turns out it is not. For many residents of the Midwest, the US Army Corps of Engineers "Flood Threat Assessment" provides nothing more than worrisome news:
"The Kansas City District U.S. Army Corps of Engineers' reach of the Missouri River, which extends from Rulo, Nebraska to the Mississippi River, will experience flood stages similar to 2010 levels, possibly greater. We caution communities and levee districts along the river to prepare for flood fight and possible evacuation."
So what the heck is this all about? Where is all the water coming from?
The headwaters of the "Mighty Mo" is high in the Rocky Mountains, at an elevation of 4,045 feet. About 25 miles northwest of Bozeman, Montana, the Madison and Jefferson Rivers come together, creating the Missouri River. From this point, the Missouri River will flow 2,320.7 miles to its confluence with the Mississippi River at St. Louis.
About 15 miles down stream, the Missouri encounters the first of many dams, placed to control flooding hundreds of miles down stream.
Toston Dam is operated by the Montana Department of Natural Resources and Conservation (DNRC.)
- Concrete gravity dam
- 705 feet long
- 56 feet high
- Completed in 1940
- Retrofitted to include a 10 megawatt generating capability in 1989.
The numbers they generated came as no surprised, given the deeper than normal snow pack in the Rockies, and unseasonable rainfall. This is the actual forecast, and it is not a pretty sight:
The "Exceedance Forecast" is fueled by 140 percent of normal snow melt runoff into the lakes, along with record spring precipitation on top of that, essentially filling up the flood storage in the Missouri River lakes and necessitating record releases. The high releases are expected to last all summer and into the fall and winter.
And the National Weather Service says it is more worried about the duration of the coming flood than the crest level. There is a potential to remain at flood stage on the Missouri through August. That's a lot of stress on those levees.
In the Missouri River Basin, there are more than 80 flood control structures on the dozens and dozens of tributaries that feed the Missouri River. Including the State operated Toston Dam, there are a total of 15 flood control structures, from headwaters to St. Louis, to manage flood control:
Federal, US Department of Interior, Bureau of Reclamation:
- Canyon Ferry
- Hauser Dam, 14 miles northeast of Helena
- Holter Dam, 43 miles northeast of Helena
- Black Eagle Dam, 2 miles northeast of Great Falls. See video.
- Rainbow Dam, 6 miles northeast of Great Falls
- Cochrane Dam, 8 miles northeast of Great Falls
- Ryan Dam, 10 miles northeast of Great Falls. See video.
- Morony Dam, 15 miles northeast of Great Falls
- Fort Peck
- Big Bend
- Fort Randall
- Gavin's Point
Canyon Ferry Dam
- Concrete gravity dam
- 1,000 feet long
- 225 feet high
- 50,000 kilowatts output
- Impounds Lake Sewell
- Completed: 1954
- 20 miles east of Helena Montana
- Inflow to reservoir, 16,713 cf/s
- Flowing 13,430 cf/s downstream
- 76% full
Fort Peck Dam
In researching this series of dams, I discovered the Fort Peck Dam is fascinating example of extreme engineering.
With the proposed length of the dam more than six miles, it was quickly determined that dump trucks would not be practical for moving the estimated 125,626,000 cubic yards of earth required to construct the dam.
So a process called "hydraulic earth fill" was used to constructed the dam. This required a fleet of dredges and miles of pipeline. In essence, the Missouri River bottom and banks were dredged and pumped through pipe lines to construct the dam.
In addition to the dredges, a railroad trestle was built to bring in material, dumped off the trestle with side dump cars. It is an amazing story way too complex to relate here. But if you are interested, there is a website chock full of photographs, documenting the building of Fort Peck Dam.
- Hydraulic earth fill and rolled earth fill
- 21,026 feet long (6.4 miles)
- 250 feet high
- 5 generators; 185,250 kw output
- Impounds Fort Peck Lake
- Completed: 1940
- 20 miles southeast of Glasgow, Montana
On Friday, June 10 the water flow rate will increase by 5,000 cubic feet per second (cf/s) to 55,000 cf/s. The increased flow should balance the water levels between Fort Peck and Garrison Dam in North Dakota.
- Rolled earth fill
- 11,300 feet long (2.1 miles, 3.4 km)
- 210 feet high
- 5 Generators; 583.3 megawatts output
- Impounds Lake Sakakawea
- Completed: 1953
- Midway between Bismarck and Minot North Dakota
- Rolled earth fill
- 9,360 feet long (2,850 meters)
- 245 feet high (75 meters)
- 7 generators; 786,000 kw output
- Impounds Oahe Lake
- Completed: 1962
- Seven miles north of Pierre, South Dakota
Big Bend Dam
- Rolled Earth
- Length - 10,570 feet
- Height - 95 feet
- 8 generators; 493,300 kw output
- Impounds Lake Sharpe
- Completed: 1954 A
- About 150 miles northwest of Sioux Falls, South Dakota
A special viewing area on the south side of the spillway was be opened up as a vantage point for the public and media to photograph, view or shoot. The gates opened Friday to release an additional 15,000 cf/s, bringing the total release out of Big Bend Dam to 100,000 cf/s.
After Friday's opening, the gates will be raised daily to allow for additional flow through the spillway. On Tuesday, June 7 the gates reached their peak, allowing for a target spillway release of 65,000 cf/s. When combined with the 85,000 cf/s of water being released through the powerhouse, an historic 150,000 cf/s will be released into Lake Francis Case, behind Fort Randall Dam.
Fort Randall Dam
- Rolled earth
- 10,700 feet long
- 165 feet high
- 8 generators; 320,000 kw output
- Impounds Lake Francis Case
- Completed: 1954
- West of Wagner South Dakota
- Rolled earth
- 8,700 feet long
- 74 feet high
- 3 generators; 132,297 kw output
- Impounds Lewis & Clark Lake
- Completed: 1957
- Near Yankton, South Dakota
Perhaps you have noticed in the photographs, some dams feature tall structures resembling grain elevators.
The purpose of these surge tanks is to prevent water hammer from damaging the dam. "Water hammer," is a phenomenon that occurs when water, passing though a pipe or conduit, is suddenly stopped. We remember from high school physics that water cannot be compressed.
When a tunnel full of water, moving at 5 to 10 miles per hour, is suddenly stopped by a valve controlling water flow into the power house (above) a powerful shock wave - water hammer - moves back through the column of water, with enough energy to inflict serious damage if it cannot be disbursed or absorbed.
The surge tank absorbs the water hammer shock wave, thus protecting the structure from damage. Not all structures require this safe guard. They are installed when there is a significant distance between the intake and the turbine.
[Ed Note: There is a YouTube video showing a catastrophic failure of a dam turbine in Russia, in August of 2009. The video incorrectly attributes the explosion as being the result of "water hammer." It was not. It was the failure of the bolts securing the turbine into the powerhouse floor, which caused launched a thousand ton turbine 50 feet into the air like a bottle rocket, killing dozens.]
Well, that completes my armchair assessment of the flood control structures built into the Missouri River System and the steps being taken to deal with an aggressive runoff originating more than 2,000 miles away, high in the Rocky Mountains.
So the question is, "As the "Mighty Mo" joins "Old Man River" at Saint Louis, will this result in another round of high water affecting the Mississippi?"
My recommendation as an armchair river watcher?
Don't get rid of those sand bags just yet!
Follow up: 2 Lane Freeway!
"Balance of Harms: The Blast at Bird's Point"