Hydraulic Models Guide Design of Garrison Dam Spillway, Mega Dam Safety Project

Engineers from the U.S. Army Corps of Engineers’ Omaha and Huntington districts and Risk Management Center are advancing the Garrison Dam Spillway Modification Project using scale models to enhance safety and improve the performance of the dam’s spillway near Riverdale, North Dakota.

Curtis Miller, lead hydraulics engineer on the project from the Huntington District, said the team is using a 1-to-55 scale comprehensive model at the U.S. Army Engineer Research and Development Center, or ERDC, that replicates the spillway, along with the approach and exit channels.

“The purpose of this model is to complement our two sectional models,” Miller said. “We can evaluate any three-dimensional effects and the comprehensive nature of the spillway.”

The project team is using the models to test design features and refine solutions before implementation. Early work with deflector beams in front of the spillway gates led to unexpected results, prompting the team to explore vortex suppressors instead.

“We initially installed the deflector beams based on findings from the crest sectional model, but we had to modify that,” Miller said. “This already paid off in guiding our design decisions.”

The spillway modifications focus on addressing six potential risk-driving failure modes, Miller explained. Measures include strengthening the trunnion hubs on the radial gates , raising abutment monoliths to prevent overflow, armoring behind chute walls, and installing an overlay in the lower chute and stilling basin.

“The lower chute is where a lot of the more expensive and higher-risk work is going on,” Miller said. “We’re modeling trapezoidal labyrinth steps , a relatively novel application of this technology, to ensure durability and safety.”

Jay Ling, Garrison Dam Spillway Modification Project program manager, said the modifications are critical due to the scale and importance of Garrison Dam within the Corps’ inventory.

“Ranked in reservoir storage capacity, Garrison Dam is the third-largest manmade reservoir in the U.S. and the largest in the Army’s inventory,” Ling said. “Having a spillway that can safely handle major flood events is critical with such a large reservoir.”

Ling said leaving the spillway unmodified would pose long-term risks during extreme flood events.

“If we didn’t do anything to the spillway, another flood event would occur that would require its use, and there’s a possibility it would not perform adequately in conveying those floodwaters,” Ling said.

Two additional models, located at the Bureau of Reclamation laboratory in Denver and at Colorado State University, address sectional issues. Denver’s model evaluates deflector beam and vortex suppressor configurations at specific gates, while Colorado State’s model focuses on a 100-foot-wide section of the lower chute and stilling basin .

The project is currently in the early phases of preconstruction engineering and design, known as PED. The team completed its 15% design milestone in November and is targeting 35% design completion by the end of 2026.

“We are still early in the PED phase, where we’re conducting field investigations, understanding site conditions and beginning design,” Ling said. “Completing 15% design allows us to get early feedback and refine the project before moving forward.”

Ling said the effort is a broad collaboration across multiple organizations and disciplines.

“The Garrison Dam Spillway Modification Project is a collaboration between experts from the Dam Safety Production Centers, the Risk Management Center, the Omaha District, the Bureau of Reclamation, academia and industry,” Ling said. “It’s a highly qualified team working together to address an infrastructure challenge.”

The Garrison Dam Spillway Modification Project is considered the largest dam safety project in U.S. Army Corps of Engineers history. The modifications will improve the safety, efficiency and longevity of one of the Missouri River’s key flood control and navigation structures.