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. 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. 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. Even though the likelihood of these events occurring is extremely remote, consequences are considered high due to the population and infrastructure downstream. Also known as hydraulic jacking, this could result in spillway failure which would reduce Garrison Dam’s ability to control reservoir releases.

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. 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”, said Cullin. “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. The drainage system collects leakage under the spillway slab and was overwhelmed by the additional water allowed to enter through the missing cover. “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,” said Ross Cullin, Omaha District Dam Safety Program Manager. 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. 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. The study addresses risks associated with the design of the concrete spillway. Army Corps of Engineers, Omaha District is conducting a dam safety modification study at Garrison Dam, near Riverdale, North Dakota.

Garrison dam