RIVERDALE, N.D. - Contractors from 10 construction and engineering firms attended an industry day Oct. 11 at the Garrison Dam located between Riverdale and Pick City, North Dakota. The industry day was part of early planning efforts for a U.S. Army Corps of Engineers multi-year project considered to be the largest dam safety modification project in USACE history.
The project will modernize the dam and reduce risks associated with the spillway’s performance when used to reduce flooding and balance river flows in the Upper Missouri River basin during significant storms or snowmelt.
“The dam, built in the 1950’s, was not originally designed for the reservoir elevations and spillway releases that would be expected under today’s conditions,” said Andrew Barry, chief of the USACE Omaha District’s Dam Safety Production Center. “Modifying the spillway and other structural work will help ensure the dam and spillway can be operated at the full range of potential flows and continue to reduce the impact of flooding on people and property downstream of the dam.”
Concerns with the spillway were identified during dam safety inspections following record runoff into the Upper Missouri River Basin and Lake Sakakawea in 2011. High water levels in Lake Sakakawea required the opening of the dam’s spillway gates for flood risk management purposes for the first time since the dam became operational in 1955. The peak flow of about 60,000 cubic feet per second of water released through the spillway created a high pressurization of the spillway’s subdrainage system that caused damage to the system and concrete within the spillway chute.
Barry explains that the modifications primarily focus on critical spillway components, including the spillway’s Tainter gates, abutments, areas behind the spillway chute walls, drainage pipes, chute slab and stilling basin. Modifications also include new seepage drains on the embankment near the dam’s powerhouse.
Barry added that while these modifications will improve conditions at the dam, no dam is risk-free and no dam can eliminate flooding.
“Flooding can still occur, even when a dam performs as it was designed,” he said. “It’s important for people to understand the risks associated with living downstream of a dam.”
The industry day provided an opportunity for contractors interested in bidding on the modification project and working with USACE to tour the spillway and learn more about the scope of construction and its unique challenges.
“Two of the biggest challenges are the shorter construction seasons in North Dakota due to severe winters, and that much of the construction will occur in the emergency spillway,” said Jeff Greenwald, a planner and project manager with the USACE Omaha District. “We need to plan for the possibility of using the spillway in a high runoff season which could coincide with the construction season. The contractor would have a short period to close down their work before the impending spillway operation. This coordination with construction contractors during the design phase benefits the project.”
The modification project is currently in the pre-construction engineering and design phase. Construction is expected to begin around 2029 and take approximately six years.
Garrison Dam is one of six USACE mainstem dams on the Upper Missouri River. Lake Sakakawea behind the dam is the third largest reservoir in the United States and the largest reservoir operated by USACE with a current capacity of 23.5 million acre-feet.
The Garrison Dam and Lake Sakakawea Project was authorized by the Flood Control Act in 1944 as part of the general comprehensive plan for flood risk management, hydroelectric power, water supply, water quality, irrigation, recreation, navigation, and fish and wildlife conservation in the Missouri River Basin.
To learn about the USACE Dam Safety Program, visit https://www.nwo.usace.army.mil/Missions/Civil-Works/Dam-and-Levee-Safety/.