The Corps issued the following this morning:
Omaha, Neb. – The U.S. Army Corps of Engineers announces its strategy for evacuating floodwaters from its six mainstem dams along the Missouri River today.
“This plan allows the Corps to evacuate flood water 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.
The Corps will execute a gradual drawdown , in which releases out of Gavins Point Dam, the southernmost reservoir in the system, will decrease to 150,000 cubic feet per second on Aug. 1 and will remain at that rate until approximately Aug. 16 when they 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 2 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.
For the corresponding detailed three week release forecast for the other mainstem dams, go to: http://www.nwd-mr.usace.army.mil/rcc/reports/twout.html.
“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. “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, etc.) and whether to increase the amount of flood control storage for the 2012 runoff season.
The current 2012 weather forecast predicts a 66.6 percent chance of normal or below normal precipitation, and a 33.3 percent 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.
Holding releases from Gavins Point steady at 150,000 cfs starting through mid-August will enable Fort Peck, Garrison and Oahe Dams to move out of exclusive flood control storage around Aug. 6 while Fort Randall will reach this zone around Aug. 12. This will provide operational flexibility for the Corps to respond if significant rainfall events occur.
The Gavins Point two-week release pause at 90,000 cfs will allow for preliminary inspection and assessment of infrastructure and levees before the final drawdown. Eventually, this steady drawdown from the reservoirs, and respective floodplains, will bring water levels low enough for contractors (weather and funding permitting) to begin repairs as early as Dec. 1.
“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 a nd the low probability of re-occurrence.
“The goal is to evacuate these 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 generally the start of the spring 2012 runoff season,” said 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.
“We have already seen water inflows to the system decline and empty system flood control space increase in the past three weeks” said Brig. Gen. McMahon. “We are confident that this plan will best prepare us for the 2012 runoff season.”
All dates provided above are best approximations, based on current forecast conditions and the best available information at the time. Adjustments to the release schedule may be necessary if conditions change. View daily and forecasted reservoir and river information on the Water Management section of the Northwestern Division homepage at: http://www.nwd-mr.usace.army.mil/rcc .