The U.S. Army Corps of Engineers, Omaha District, in partnership with the prime contractor, Barnard Construction Company, held a groundbreaking ceremony May 25, 2023 for a large-scale spillway project at Pipestem Dam in Jamestown, North Dakota.
Pipestem Dam was evaluated in 2007 and classified as a moderate urgency dam following detailed engineering analyses conducted by the USACE Omaha District’s Dam Safety Program. The study was part of a nation-wide effort directed by USACE to understand potential risks from flooding and to reduce these risks to public, property, and the environment.
Two years later in 2009, the Cottonwood Creek Dam, located just 50 miles southeast of Pipestem Dam, experienced severe spillway erosion and risked dam failure from flooding. This highlighted the potential for similar erosion of Pipestem Dam’s spillway due to its similar underlying geology.
Pipestem Dam’s spillway is designed to pass up to 110,000 cubic feet of water per second if water gets high in the reservoir under extreme conditions. This is known as a non-breach release that keeps pressure off the dam. But a multi-disciplinary team across USACE found that the geologic conditions under the Pipestem spillway is comprised of the same highly erodible soils as Cottonwood Creek Dam at a depth of more than 100 feet. Engineers determined that the spillway could progressively erode under extreme flood conditions, presenting the risk of a dam breach and the uncontrolled release of water from the reservoir.
While the reservoir behind Pipestem Dam has never been high enough to require spillway flow, Omaha District is now acting to improve the reliability of the spillway before it is needed, and in support of the Dam Safety Program’s highest priority: life safety. The failure of Pipestem Dam would result in catastrophic impacts to communities, including Jamestown, just a few miles downstream.
Potential property damage resulting from a failure of Pipestem Dam is estimated at more than $450 million, not including the potential loss of an additional $2 million annually in other dam and reservoir benefits such as recreation, water supply, and hydropower. These economic impacts would significantly affect local livelihoods.
The multistage spillway modification project will increase the reliability of the spillway and improve the stability of slopes located along or near the spillway. It is projected to cost $200 million and take three years to complete.
Watch a video demonstrating the pipestem spill way erosion here - (375) Dam Safety - Pipestem Spillway Erosion - YouTube