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.