PIERRE, S.D. (KELO) — Huge amounts of snow melt and rain runoff that rushed down from the Rocky Mountains and poured off the northern plains into the Missouri River 10 years ago this spring not only forced emergency measures but also have since exposed a troubling possible danger at one of the dams.
The U.S. Army Corps of Engineers found that the emergency spillway at the dam that forms Garrison Reservoir in North Dakota was potentially compromised. Water leaving Garrison Dam flows into Oahe Reservoir that spans North Dakota and South Dakota.
A high-water failure at Garrison would cause an immense problem downstream for people living along Oahe in both Dakotas, from Bismarck and Mandan to Pierre and Fort Pierre. Three more Missouri Rivers dams downstream, near South Dakota communities Chamberlain, Lake Andes and Yankton, could potentially be affected.
The problem found after the 2011 event at Garrison Dam was that a manhole cover came loose on the emergency spillway and allowed water to flow under the spillway’s concrete slabs.
Corps officials determined that “under extreme operating conditions, significantly larger than the 2011 flood, removal of the drainage system manhole covers would force water under the spillway which could cause the slabs to become dislodged” and cause spillway failure.
Corps officials recently announced they have started a study on how they might address the situation. Project manager Jeff Greenwald told KELOLAND News that the corps determination of risk includes two factors: The probability of failure and the consequences of failure including life safety, economic and environmental impacts.
“The risk for Garrison Dam is largely driven by consequences as there is a considerable number of residents at risk of flooding downstream of the dam, several hundred thousand, with the population closest to the dam being the most vulnerable,” Greenwald said. “In quantitative terms, a low probability of failure means that the likelihood of an uncontrolled release is estimated to be less than 1 in 100,000.”
What would it mean for South Dakota? “In the unlikely event that the Garrison spillway or embankment failed, conditions could lead to the Lake Oahe reservoir elevation nearing or even exceeding the top elevation of Oahe Dam and could potentially result in a large uncontrolled release downstream of Oahe,” Greenwald said.
The April 1 forecast for annual runoff in the upper Missouri River basin above Sioux City, Iowa, has averaged 25.8 million acre feet. The corps forecast this year is 21.3 million. The 2011 runoff exceeded 60 million, the first time the upper basin had been above 50 million since the dam system was completed in the 1960s. Runoff broke the 60 million level again two years ago.
South Dakota’s then-Governor Dennis Daugaard was only a few months into his first term of office in 2011 when flood waters in May hit Pierre, Fort Pierre and the Sioux City area of South Dakota, Iowa and Nebraska. Water lapped at the top of the Oahe emergency spillway and, depending on the direction of the wind, trickled down the back of the giant gates.
“The 2011 Missouri River flood was the largest on record for the entire basin in terms of volume of water, and record peak flows were also experienced in some reaches,” Greenwald said. He said the “unprecedented” runoff from record rainfall over portions of the upper basin combined with plains and mountain snowpack that led to record peak releases from the Missouri River dams:
65,000 cubic feet per second (cfs) at Fort Peck in Montana.
150,000 cfs at Garrison.
160,000 cfs at Oahe.
166,000 cfs at Big Bend.
160,000 cfs at Fort Randall.
160,000 cfs at Gavins Point.