Corps of Engineers Releases New Flood Plan

Brigadier General John McMahon

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 slumping 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 knownto 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.