The U.S. Army Corps of Engineers announced last week its strategy for evacuating floodwaters from its six mainstem dams along the Missouri River.
"This plan allows the corps to evacuate floodwater from the reservoir system in a responsible way to prepare for the 2012 runoff season, while reducing the risk of further damages and gets affected homeowners, farmers and businesses back on their properties to begin repair and recovery as quickly as possible," said Brig. Gen. John McMahon, Northwestern Division commander, Omaha, Neb.
The corps is executing a gradual drawdown, in which releases out of Gavins Point Dam, the southernmost reservoir in the system, began decreasing to 150,000 cubic feet per second on Aug. 1, and will remain at that rate until Aug. 16, when it will be stepped down 5,000 cfs daily until reaching 90,000 cfs around Aug. 27. The Gavins Point Dam releases will stay at 90,000 cfs for approximately two weeks and then will drop 5,000 cfs every two days, until reaching 40,000 cfs, which is slightly above the typical fall release rate, on or about Sept. 30.
Releases from Garrison and Oahe dams are scheduled to reach 85,000 cfs on Aug. 17 and 24, respectively. This is the estimated release to get the water back within the river channel and to begin floodplain drainage along the river at Bismarck, N.D. and Pierre, S.D.
This plan provides the opportunity for the corps to begin inspection and repair of levees and other critical infrastructure and ensures adequate storage for the 2012 runoff season.
"We meticulously reviewed each of eight drawdown options with technical experts and leadership within the Northwestern Division, Omaha and Kansas City Districts," said Jody Farhat, chief of Missouri River Basin Water Management Division, Omaha, Neb. "This release schedule puts us in the best position to drawdown the water as quickly and as responsibly as possible, while allowing us time to inspect, assess and repair damages."
In making the decision, the corps considered criteria such as the potential impacts to homes, farms and businesses within the floodplain, weather forecasts through 2012, acceptable release rate reductions from the dams, water levels on the temporary and downstream levees, getting the reservoirs out of the exclusive flood control zones, impacts to other critical infrastructure (tributary reservoirs, roads, facilities) and whether to increase the amount of flood control storage for the 2012 runoff season.
The current 2012 weather forecast predicts a 66.6% chance of normal or below normal precipitation, and a 33.3% chance of wetter than normal conditions. However, fall 2011 is forecasted to be wetter than normal; both of these predications contributed to the drawdown decision. Further consideration was given to the low probability of the re-occurrence of this 2011 500-year event again in 2012.
"It's important that we drawdown these releases with full consideration of the many risks that remain," said Brig. Gen. McMahon. "A rapid drawdown with high flows could cause extensive bank erosion and slumping in the levees, while too slow of a drawdown could leave high water on temporary and permanent levees, dams and other critical infrastructure, further increasing risks for overtoppings and breaches." We assess these risks to be unacceptable in the context of the weather forecast and the low probability of re-occurrence."
The goal of the corps is to evacuate the historic and unprecedented floodwaters responsibly and bring the entire system back to its full annual flood control capacity of 16.3 million acre feet by March 1, which is the start of the spring 2012 runoff season, according to Farhat. This will put the flood control pool to
a system storage level of 56.8 million acre feet. Prior to the Flood of 2011, and since 1881, this amount has been adequate to capture spring runoff and manage water flow through the system.
View daily and forecasted reservoir and river information on the Water Management section of the Northwestern Division homepage at www.nwd-mr.usace.army.mil/rcc.
Source: U.S. Army Corps of Engineers