The U.S. Army Corps of Engineers, Omaha District is conducting a dam safety modification study at Garrison Dam, near Riverdale, North Dakota. The study addresses risks associated with the design of the concrete spillway. These risks were identified during the 2011 Missouri River flood, when the Garrison Dam’s spillway was used for the first time in the dam’s nearly 70 year history to help control the flow of the Missouri River and lessen the impacts of severe flooding on downstream communities like Bismarck.
Once the spillway gates were closed and operations returned to normal, the dam’s maintenance team began assessing the impacts of passing historic floodwaters over the spillway. One of the issues encountered was the removal of a manhole cover which provides access to the drainage system under the concrete spillway slab, bringing into question their reliability during future releases through the spillway. The drainage system collects leakage under the spillway slab and was overwhelmed by the additional water allowed to enter through the missing cover.
As part of its Dam Safety Program, the Corps conducts detailed technical and risk analyses to ensure dams that are owned and operated by the agency are safe and that risks to the public, property and the environment are minimized to the extent possible. Some dams, like Garrison Dam, are considered higher risk, not because the dam is unsafe, but because the consequences of uncontrolled reservoir releases would significantly affect downstream populations, including twelve states and numerous urban centers along the Missouri River.
Through the course of this technical and risk analysis process, USACE dam safety experts determined that under extreme operating conditions, significantly larger than the 2011 flood, removal of the drainage system manhole covers would force water under the spillway which could cause the slabs to become dislodged. Also known as hydraulic jacking, this could result in spillway failure which would reduce Garrison Dam’s ability to control reservoir releases. Even though the likelihood of these events occurring is extremely remote, consequences are considered 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.
Potential solutions could 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 manholes outside of the spillway slab, retrofitting the manhole covers to make them more resilient, or a combination of these measures.
In addition to studying the issues with the spillway manhole covers, the team will be looking at a few additional measures to increase overall performance at the dam including ways to improve the stability of slopes located near the powerhouse at the west end of the dam. If these solutions are determined to be viable, the public will be encouraged to comment on these alternatives along with the potential solutions to address the spillway.
It is important for local emergency management organizations, community groups and members of the impacted public to be involved in the dam safety modification study for Garrison Dam. It is important to think about how any potential solutions might impact you, your community, your property, or the environment.
Additional opportunities for public involvement will be available throughout this study, including public meetings when a draft report is released. Up-to-date study information can be found on this project web page (https://www.nwo.usace.army.mil/Missions/Civil-Works/Planning/Planning-Projects/Garrison-Dam-Safety-Modification-Study/).
To submit study comments:
Study comments can be mailed or emailed to the following:
Mr. Jeff Greenwald (Project Manager)
U.S. Army Corps of Engineers Omaha District
1616 Capitol Avenue
Omaha, NE 68102-4901
To have your name added to the list for study updates, please email firstname.lastname@example.org.