The U.S. Army Corps of Engineers, Omaha District, is conducting a dam safety modification study at Garrison Dam, near Riverdale, North Dakota to evaluate what repairs or improvements are needed to address structural concerns at certain places at the dam. These concerns pose risks to USACE’s ability to operate the dam as designed during extreme flood events to reduce risks to people and property downstream.

As part of its Dam Safety Program, USACE conducts detailed technical and risk analyses to ensure that its dams are safe and operate as designed.
Garrison Dam is operating as designed but, like other dams, is considered a high risk because of the significant consequences of uncontrolled reservoir releases on downstream populations, including twelve states and numerous urban centers along the Missouri River. 

USACE determined the need for this study 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. The spillway passed water that could not be stored in Lake Sakakawea behind the dam during flooding in 2011. 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. 

The team discovered that the high flow in 2011 dislodged a manhole cover, one of many covers that provides access to the drainage system under the concrete slab of the spillway chute. The system, designed to drain normal water seepage under the spillway, was overwhelmed by the additional water that passed through the missing cover. Dam safety experts evaluated the covers to determine their 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 to perform as designed and would reduce USACE’s ability to control high water releases from the reservoir.

Potential solutions could include thickening the spillway slab with various types of concrete to make the drainage system redundant, redesigning and constructing a new drainage system that relocates the manholes outside of the spillway slab, retrofitting the manhole covers to make them more resilient, or a combination of these measures.

USACE’s dam safety experts also identified concerns that the very rare, extreme floods for which the capacity of the spillway was originally design would now not safely pass through the spillway. This is due to the increased size of the potential floods since the dam’s design in the 1940s.  During extreme floods (much larger than what was experienced in 2011), overtopping of the spillway gates, the walls lining the spillway chute and/or the dam’s abutment walls is possible. These rare, extreme floods also could lift up portions of concrete in the lower portion of the chute near the stilling basin.  Even though the likelihood of these events occurring is extremely remote, consequences are estimated to be high due to the population and infrastructure downstream. Therefore, as responsible dam owners, the Omaha District has initiated a dam safety modification study to evaluate solutions to modify the spillway to reduce risk. 

Through the modification study process, USACE is evaluating potential solutions—individually, or in combination— to reduce or eliminate risks to the dam from the drainage system issue. These include thickening the spillway slab with various types of concrete to eliminate the need for the drainage system, redesigning and constructing a new drainage system that relocates the manhole covers outside of the spillway slab, retrofitting the covers to make them more resilient to high flows, or a combination of these measures. 

USACE also is investigating ways to modify the spillway gates and walls and the other structural features around the stilling basin. The team will evaluate additional measures to increase the dam’s overall performance, such as improving the stability of slopes located near the powerhouse at the west end of the dam, and the benefits of constructing a new spillway on the west abutment that would be large enough to safely pass extremely large and rare flood events.

It is important for local emergency management organizations and members of the impacted public to stay informed about the modification study and to share with USACE how any potential impacts from potential risk reduction solutions. Additional opportunities for public involvement will be available at other stages in the study process, including public meetings when a draft report that presents a preferred solution is released. Up-to-date study 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.