The U.S. Army Corps of Engineers, Omaha District, completed a Dam Safety Modification Study in June 2023 at Garrison Dam, near Riverdale, North Dakota. The study evaluated what repairs or improvements are needed to address structural concerns at certain places at the project, primarily within the spillway. These concerns pose risks to USACE’s ability to operate the dam during extreme flood events. The Dam Safety Modification Study also recommended a risk management plan to support the expeditious and cost-effective reduction of the risk posed by these concerns. Recommendations include modifications to the spillway that collectively address dam safety concerns with modernized designs. Key elements of these recommendations include:

• the full replacement of the spillway’s drainage system to remove manhole covers from inside the spillway to prevent the covers from dislodging during future spillway operations.

• placement of a reinforced concrete overlay in the spillway chute and stilling basin to ensure the spillway can safely pass extreme flows
• armoring the area behind the spillway chute walls to reduce potential for erosion during extreme flows
• raising the spillway abutment monoliths to prevent overtopping which can lead to erosion and failure of the structure
• constructing a deflector beam to deflect the overflow from spillway gates
• modifying the gates’ trunnion hubs to ensure they are structurally sound, and
• improving the system that deices the gates so they remain operational during winter conditions. 

Over approximately the next six years, USACE will conduct field investigations to collect data to inform the detailed design and develop plans and specifications that are necessary to award a contract to complete these recommendations. As of 2023, construction is anticipated to begin in 2029 and take several years to complete.

In the near term, USACE also plans to construct an additional line of drains on the dam’s west abutment to address a slope stability and seepage concern that was evaluated in the study. Construction could begin in 2025 and would last one season.

As part of its Dam Safety Program, USACE conducts detailed technical and risk analyses to ensure that its dams reduce risk to life and property during floods. Garrison Dam continues to operate as designed but, like other dams on the upper Missouri River, it is considered a high risk because of the anticipated significant consequences of an unlikely uncontrolled reservoir release on downstream populations, including 12 states and numerous urban centers along the Missouri River. Learn more about these risks and view inundation maps related to Garrison Dam on the National Inventory of Dams.

USACE determined the need for the DSMS following inspections and assessments of the potential future risks from conditions that developed when Garrison Dam’s spillway was used for the first time in the dam’s nearly 70-year history. During 2011 flooding, the spillway passed water that could not be stored behind the dam in Lake Sakakawea. . After dam operators closed the spillway gates and operations returned to normal, the dam’s maintenance team assessed the impacts of passing historically high floodwaters through the spillway. 

USACE’s dam safety experts discovered that the high flows in 2011 dislodged a manhole cover, one of many covers that provide access to the drainage system under the concrete slab of the spillway chute. The system, designed to drain normal water seepage, was overwhelmed by the additional water that passed through the missing cover. Dam safety experts evaluated the covers’ ability to remain in place during future spillway releases. It was concluded that under extreme operating conditions significantly larger than the 2011 flood, dislodged manhole covers would allow water under the chute and force the displacement of a portion of the slab. Known as hydraulic jacking, this condition could lead to a failure of the spillway and would reduce USACE’s ability to manage high water releases from the reservoir.

A full replacement of the drainage system will be implemented to address these concerns.

Experts also identified concerns that the very rare, extreme floods for which the capacity of the spillway was originally designed in the 1940s would not safely pass through the spillway today. The increased size of the potential floods is based on improved engineering calculations since the 1940s and changes in weather conditions in the upper Missouri River watershed. It is now possible that extreme floods, much larger than what was experienced in 2011, could overtop the spillway gates, the concrete walls lining the spillway chute, and/or the dam’s abutment walls. Such a flood also could lift up portions of concrete in the lower portion of the chute near the stilling basin. While the likelihood of these events occurring is extremely remote, life safety and property consequences are estimated to be high due to the downstream population and infrastructure.

As a responsible dam owner, USACE’s Omaha District is addressing these concerns through the spillway modification project. 

It is important for local emergency management organizations and the potentially impacted public to stay informed about the spillway modification process and to understand that completion of this risk reduction project is years away. Up-to-date modification project information can be found on this project web page. You also can send an email anytime to add your name to the project mailing list or ask a question: cenwo-planning@usace.army.mil.

Since dams do not eliminate flood risk, large and small communities downstream of Garrison Dam are encouraged to monitor National Weather Service flood forecasts during periods of heavy rain and snowmelt and learn about potential flood impacts to family, home, and community. It is important to understand community notification plans, including safe, efficient evacuation routes, and consider preparedness and emergency actions to reduce individual risks.

To learn more and get the latest information on water management in the Missouri River basin, visit: https://www.nwd.usace.army.mil/MRWM. Learn more about Garrison Dam with a visit to the National Inventory of Dams at nid.sec.usace.army.mil.