Contractors from ten construction and engineering firms attended an industry day at the Garrison Dam in North Dakota at the end of 2023. The industry day was part of early planning efforts for a US Army Corps of Engineers’ multi-year project, considered to be the largest dam safety modification project in USACE history.
The project will modernise 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 1950s, was not originally designed for the reservoir elevations and spillway releases that would be expected under today’s conditions,” says Andrew Barry, Chief of the USACE Omaha District’s Dam Safety Production Centre. “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 pressurisation of the spillway’s subdrainage system, damaging the system and concrete within the spillway chute.
Modifications
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. They also include new seepage drains on the embankment near the dam’s powerhouse.
“Flooding can still occur, even when a dam performs as it was designed,” he says, adding that while these modifications will improve conditions at the dam, no dam is risk-free and no dam can eliminate flooding. “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,” says 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 US and the largest reservoir operated by USACE.
The Garrison Dam and Lake Sakakawea Project was authorised 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.
Tools
In other news, researchers at the US Army Engineer Corps of Engineers have unveiled tools for predicting infrastructure failure risks. With approximately 740 dams and associated structures under its purview, USACE is pushing the envelope to address the vulnerabilities of ageing.
“A lot of these structures were built 70 to 80 years ago – and in some cases 90 to 100 years ago – and the design life for most of them was 50 to 75 years,” explains Willie Brown, a research civil engineer at the US Army Engineer Research and Development Centre “Some of these structures have outlasted what they were supposed to be able to provide – and that’s a great thing – but as that continues to be the case, the risk of failing becomes greater and greater. For the communities surrounding them – from a flood risk management perspective – if we don’t keep our facilities in good shape, the consequences could be dire.”
In order to effectively operate and maintain those dams, USACE applies risk-informed decision-making to evaluate, prioritise and justify dam safety decisions in the most effective manner within a constrained budget. To streamline this process, USACE researchers have developed a systemwide approach for assessing risk in flood risk management watersheds.
The focus of Brown’s team is on the likelihood of failure, aiming to revamp the assessment process. The new methodology steps away from the subjective nature of the current system, promising a more transparent and consistent approach. This shift enables a nationwide comparison of facilities, pinpointing the highest-risk assets for targeted resource allocation.
Having already undergone successful trials in diverse case studies across four watersheds, the team is now eyeing a nationwide rollout in collaboration with USACE headquarters.
Complementing this approach is the introduction of the USACE Risk Management Centre’s (RMC) quantitative risk analysis software, RMC-TotalRisk, to enhance and expedite risk assessments within the flood risk management, planning and dam and levee safety communities of practice.
Haden Smith, a senior hydrologic engineer with the RMC, underscored the software’s versatility, saying: “It can do simple things all the way to the most complex projects. In terms of value-added, this tool stands to significantly reduce study costs,” Smith adds. “It has an intuitive user interface with very fast runtimes. It allows users to run more alternatives and really dive in. Because it’s so fast, they can quickly see cause-and-effect on inputs and outputs. It stands to not only improve the quality of studies going forward, but also reduced the overall time to develop a model.”
The RMC-TotalRisk tool offers an array of risk measures to weigh risk reduction alternatives, upping the ante on dam and levee safety activities. Used in all new dam and levee safety studies at the risk management centre, the tool is open to the public and accessible at https://www.rmc.usace.army.mil/
