In 2011 the mainstem Missouri River Reservoir System experienced the largest volume of flood waters since the initiation of record-keeping in the nineteenth century. In Montana flooding affected the Fort Peck Dam, the Missouri River and the Lower Yellowstone River, considered a tributary of the Missouri.
The high levels of runoff from both snowpack and rainfall stressed the system’s capacity to control flood waters and caused massive damage and disruption along the river. As a result of its experiences during the flood, the U.S. Army Corps of Engineers (Corps) appointed a panel to conduct an independent technical review of its operations of the reservoir system during the flood event. The panel was appointed in September of 2011 and held its first meeting in early October in Omaha, Nebraska. Members of the panel represent Federal agencies with missions in water data and studies and Colorado State University. The panel’s work was conducted independently from the Corps and is similar to a technical performance audit. The ninety-nine page report reflected the system as a whole, with some parts particular to the Mondak region.
In conducting the review of the Missouri River mainstem reservoir system during the 2011 flood, the review panel divided its findings into two areas:
• Conclusions regarding regulation of the reservoir system by the Corps during 2011 for the authorized purpose of flood control. The conclusions are summarized as answers to a set of questions that served as the starting point of the review process. The original questions posed by the Corps were supplemented by additional questions that arose during the panel investigation.
• Recommendations for actions to prevent or reduce damages from similar, or larger floods in the future.
One of the questions the panel was asked to address was whether the water-management decisions made during the flood of 2011 appropriate and in line with the approved water control manuals? The report said “Rather than speculate after-the-fact about differences in the required and actual operating decisions, the panel identified ways that future flood operation might be improved through lessons learned.”
Another question the panel addressed was if the Corps could have prevented or reduced the impact of the flood by taking other management actions leading up to the flood. The report said “In summary, the Corps could have reduced the impact of the flood with more storage and higher releases before the flood, but these actions carried risks and consequences that did not seem appropriate to the Corps at the time they were required.”
The panel addressed whether operations for environmental or other purposes influence flood risk management operations, and if so how did they influence the operations and determined that operations for system purposes other than flood control were suspended or assigned secondary priority once significant flooding started. During the flood the Corps did not operate for environmental or other purposes in a way to influence flood risk. In noting the complexity of the communication systems required to manage the mainstem reservoirs, while considering the status of weather, downstream flooding, inflows, and storage in tributary reservoirs, the panel observed that a program of modernization is needed to create an effective decision support system linked to a modern interactive graphic forecast system.
One of the questions the panel was asked to address was about the role the weather forecasts played in the flood management. The report says “The Corps’ long-term regulation forecasts did not accurately account for the runoff volume, however, no forecasting agency accurately predicted the volume of the extreme runoff.”
The report suggested that the events of the spring of 2011were extreme and rare with an annual .2% probability and characterized as a 500-year flood event. It also noted “The recurrence interval is not known exactly, but the 500-year designation might be a reasonable approximation of the extreme nature of the flooding.” As the snowpack increased and as the rain fell, the forecasts rose but did not predict the record precipitation and runoff.
Specific to Montana portion of the flood area, the report addressed agricultural damages. Farmland flooding was particularly devastating. In the upper reaches, land uses are mostly agricultural and undeveloped lands and a number of small communities are located between Fort Peck and Garrison Dams. As anticipated by the Corps’ Master Manual, damages in the upper reaches focus on agricultural losses and flooding in smaller urban areas, along with disruption of transportation routes.
The panel made six recommendations for future management of the Missouri River Mainstem Reservoir System including:
1. Support for a program of infrastructure enhancement to ensure all flood release spillways and tunnels are ready for service and that all levees are in good condition.
2. Hydrologic studies to update the design flood with new probabilities. The panel recommends re-examining the Missouri River System planning that is based on the entire historical record and adjusting to the recent decades of varying climatic extremes.
3. A review of the System storage allocations, based upon the 2011 flood event.
4. The panel recommends improved future cooperation and collaboration with the National Weather Service (NWS), and its already-established forecast systems as well as with USGS. They said those coordination meetings should be held with the other agencies that produce water supply forecasts, specifically the NWS and the Natural Resources Conservation Service (NRCS), to help alert the Corps to potential trouble spots.
5. Studies to enhance data collection, forecasting, and resulting runoff from plains snow.
6. A decision support system to include real-time status information on tributary reservoirs
The report concluded with conclusions on flood infrastructure effects from the 2011 Flood.
Overall, the panel said “The flood control infrastructure performed well during the 2011 event, but a good bit of damage occurred and will require timely repair to prepare for future events.” Most facilities operated near their maximum capabilities and water levels at Fort Peck and Garrison entered the surcharge zone. They agreed that infrastructure condition was a factor in operation of the reservoirs, and with such massive and high-risk structures it will always be a factor in such extreme events. Additionally the report noted that 2011 damage to infrastructure included the erosion at Fort Peck of a spillway plunge pool.
Finally, the panel said “It is imperative that the facilities be maintained at high levels of readiness and equally imperative that they be monitored and protected during the heat of operation while extreme events are in progress. The panel believes that the Corps did an impressive job of using the infrastructure and that the absence of major failures is evidence of that.”