In a nutshell, releases from Gavin's Point Dam will drop to 150,000 cfs by Monday, then hold there through mid-August. They say this is needed to provide maximum evacuation of flood storage before the drawdown. Starting in mid-August, releases will drop 5,000 cfs per day, reaching a goal of 90,000 by August 27. Releases will hold there for approximately two weeks for the Corps to check for damage in dam structures and levees. The stepped drawdown is designed to allow floodwaters to drain slowly from the floodplain, causing less damage to levees as it exits. In addition, the goal is that as waters slowly drop, levees will be able to begin drying as hydrologic pressure from the river is released, causing less slumping.
Releases at Garrison and Oahe are scheduled to reach 85,000 cfs by August17 & 24 respectively, allowing the river to enter its banks again at Bismarck and Pierre.
In mid-September, releases will step down to a goal of 40,000 cfs by October 1. Releases will hold here and eventually will be drawn down to 20,000 cfs. by Dec. 1. Here's a graph of the Gavin's Plan, followed by the plan for upstream reservoirs.
This plan is designed to have the reservoirs out of the exclusive pool as soon as possible and begin getting citizens back into their homes and businesses to begin recovery as soon as possible.
The Corps plans do not involve providing for additional flood storage space next year. As Gen. McMahon explains in the second release reprinted below, the decision was made to time releases to both evacuate the exclusive flood control storage as soon as possible, then begin lowering levels to allow residents, farmers and communities the most time before winter to rebuild, clean up and assess damages.
Links on 2011 Floodwater Evacuation Plan
- Omaha District Reservoir Release Information Page
- Press Release, July 29, "Corps announces strategy for evacuating floodwaters"(also reprinted below)
- Press Release, July 29, "Corps’ drawdown plan aims to be ready for 2012 runoff" - an explanation of the plan by Brig. Gen. John McMahon, Commander of Northwest Division. (also reprinted below)
- Pdf powerpoint describing drawdown plan
- Spreadsheet detailing drawdown plan for all reservoirs
U.S. ARMY CORPS OF ENGINEERS
NEWS RELEASE
U.S. Army Corps of Engineers – Northwestern Division, 1616 Capitol Ave., Omaha, Neb. 68102
For Immediate Release: July 29, 2011
Corps announces strategy for evacuating floodwaters
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 and 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.
U.S. ARMY CORPS OF ENGINEERS
NEWS RELEASE
U.S. Army Corps of Engineers – Northwestern Division, 1616 Capitol Ave., Omaha, Neb. 68102
For Immediate Release: July 29, 2011
Corps’ drawdown plan aims to be ready for 2012 runoff
- Brig. Gen. John McMahon, Commander Northwestern Division, U.S. Army Corps of Engineers
After much deliberation with my team, I have selected a plan for responsibly evacuating flood waters from the Missouri River Mainstem System through the remainder of 2011. This risk-based decision was not made lightly. We must get the water back into the river banks and out of the floodplain so that people can return to their homes, farms and businesses as soon as possible.
The release schedule selected prepares the basin to be ready for the 2012 runoff season. Our number priority, as always, is public safety. This drawdown schedule is the safest option to evacuate floodwaters from the reservoirs in a timely manner, while simultaneously decreasing the risk to temporary and permanent levees, our six mainstem dams and other critical infrastructure.
We’ve opted to use a gradual drawdown approach. This will provide us with the best chances of minimizing the amount of additional damage we might otherwise face if we attempted to draw down too quickly. The risks associated with too slow a drawdown would leave high water on temporary levees and permanent flood risk reduction structures longer than necessary, which increases our chances of overtopping and/or breaching levees. If we make too rapid a drawdown, we run risks to include potential damage to infrastructure, extensive bank erosion, and sloughing in the levees.
2011 will be the highest runoff season in the Corps’ 113 years of record keeping in the Missouri River Basin. The Mighty “Mo” has reminded us just how unpredictable she can be. This is why it’s so important for us to be prepared for the 2012 runoff season. In light of this year’s runoff, several of the drawdown alternatives considered whether more mainstem system flood control storage is necessary for the 2012 runoff season. None of the options before us could ever eliminate all flood risk. We thoroughly evaluated options of adding an additional 1.3 million acre-feet and 3.6 million acre-feet to the existing 16.3 MAF of flood control storage in the system. These options and others have serious consequences to getting us ready for the 2012 runoff season.
First, the additional time it would take to evacuate any additional volume of water is precious time we don’t have before the onset of cold weather in the Basin. Second, there is unacceptable risk of breaching and/or overtopping additional levees, especially those protecting people and communities; this is due to the prolonged duration of increased releases to accommodate these additional volumes of water through the system. Third, neither the weather forecasts for the remainder of 2011 and for 2012, nor the probability of re-occurrence of this 2011 event in 2012 warrant such additional risk.
The mainstem system was designed based on the 1881 flood. That year, the basin experienced 40 million acre feet of runoff above Sioux City, Iowa, from March to July, the worst flood on record known to modern man. Hence, 16.3 million acre-feet of flood control storage was allocated in the system— which would have been the required amount of storage to manage the 1881 flood waters while keeping system releases at or below 100,000 cubic-feet-per-second. Since the construction of the mainstem system, that amount of storage has been sufficient to adequately handle every runoff season until this year. Runoff from March to July in 2011 is expected to total 49 million acre-feet, 20 percent higher than what the system was designed to manage.
The release schedule does not increase flood control storage prior to the 2012 runoff season. Increasing flood control storage before March 2012 would mean significantly higher releases for a longer period of time this fall. That would further increase the strain on temporary and permanent levees and other critical infrastructure. It would significantly limit our ability to inspect, assess and repair damages because water would be higher longer. Simply put, providing for more flood control storage would gravely jeopardize the basin’s ability to be ready for the 2012 runoff season.
Part of our analysis included weather forecasts through 2012. The forecast predicts a wetter than normal fall 2011. The forecast for a wet fall contributed to our drawdown release decision. Based on the gradual drawdown release schedule, our plan is to decrease releases at Gavins Point Dam to 40,000 cubic feet per second by the end of September. This would give the system the flexibility needed to store additional floodwaters if another significant rainfall event happens this year.
Our evaluation included the consideration of eight drawdown options with a thorough risk analysis of each. We took into account the impacts to homes, farms and businesses within the floodplain, temporary and permanent levees, our dams and other critical infrastructure. The review also took external factors, such as funding, weather and contractor availability, into account. Given our review and assessment of the associated risks, the release schedule we selected was the best option. The plan allows us the time we need to inspect, assess and repair damages. This drawdown schedule provides the best path for the basin to be ready for the 2012 runoff season.
There are limitations and obstacles we must consider as we prepare for the 2012 runoff season. We must quantify and obtain funding to initiate and complete the repairs. We must work closely with contractors to ensure the work is completed safely, on time and within budget. The majority of the work will have to be done during the harsh winter months. The release schedule puts us in a good position to get water levels low enough to begin those inspections and assessments and put contracts in place to begin work as early as 1 December.
We will have to prioritize our efforts based on an applicable set of criteria that puts protection of life and human safety first, followed by protection of key infrastructure and valuable cropland. Given the time constraints, we may not be able to repair everything in time for the 2012 runoff. Those decisions will not be easy.
Meanwhile, a full post-flood assessment will begin soon. The assessment will require us to answer many of the same questions you have been asking us. We will look at how we managed the dams and reservoirs from the winter of 2010 through the end of this flood fight. We will conduct a full-scale assessment to determine what, if anything, needs to change in our operating procedures. All of this will take time.
Our primary objective with this gradual drawdown schedule is to be ready for the 2012 runoff season. To do that, we must evacuate water from the reservoirs and the floodplain in a safe and responsible manner. We are 100 percent committed to this flood fight, and will remain vigilant throughout the coming months as we evacuate this water responsibly, get people back in their homes, farms and businesses, and begin the process of repairing the damage to get ready for 2012.
- Brig. Gen. John McMahon, Commander Northwestern Division, U.S. Army Corps of Engineers
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