Dam Safety

Corps wins award for Isabella Dam Safety Modification Project

Corps wins award for Isabella Dam Safety Modification Project

The U.S. Army Corps of Engineers Sacramento District has received the National Academy of Construction’s 2024 Recognition of Special Achievement Award for its work on the Isabella Dam Safety Modification Project at Isabella Lake.

Completed in 1953, Isabella Dam is about 40 miles northeast of Bakersfield, Calif. The reservoir is impounded by two earthen dams on the Kern River and Hot Springs Valley. Isabella Lake and its dams reduce flood risk for Bakersfield and the surrounding region and is a primary source for water users throughout Kern County. It also impounds water for the 11.95 MW Isabella hydro project.

At Lake Isabella, a 70-year-old service spillway controls outflow for the two earthen dams. The spillway can discharge about 120,000 cubic feet of water per second, far short of the estimated half-million required during a maximum precipitation event. In 2005, the Corps began an extensive dam safety study at Isabella Dam, which identified important safety modification needs. In 2017, construction began on a new emergency spillway.

“The NAC is thrilled to select the Isabella Dam Safety Modification Project for our third annual Recognition of Special Achievement Award,” announced NAC President and Chief Executive Officer Edd Gibson. “This award highlights and celebrates innovation, safety, and resourcefulness in planning and solving design challenges among international practitioners in the engineering, design, and construction industries, and the Sacramento District’s project delivery team at Isabella Dam rose to top of this year’s nominees by excelling in all areas judged.”

Prior to the project, USACE dam safety experts considered Isabella Dam to be one of the highest-risk dams in the portfolio for failure or overtopping. The $650 million modification project addressed several deficiencies by raising the main and auxiliary dams 16 feet and creating a new emergency spillway with a labyrinth weir to greatly lower flood risk for more than 400,000 people downstream of Isabella Lake.

“The impact of this remarkably complex megaproject cannot be overstated,” said Chief of Engineers and USACE Commanding General Lt. Gen. Scott Spellmon. “From conception through completion, the project delivery team displayed the USACE ethos, developing creative and innovative solutions to solve significant challenges without sacrificing project schedule, budget, and, most importantly, safety.”

The Sacramento District relied on the USACE South Pacific Division’s Dam Safety Production Center for a weir design that could safely regulate water flow through the emergency spillway, rather than just hold it back, like a dam. The result was a three-story-tall, zig-zagging labyrinth concrete weir built in a roughly 1,300-foot opening. If straightened out, the weir would have an effective length of about 3,000 feet. The creative configuration significantly increases the amount of water that can be safely discharged from the relatively narrow space.

Before weir construction began, contractors had to blast out 3 million cubic yards of rock to create the space for the new emergency spillway as well as source of materials for the dam raises. The spillway blasting was dangerous and time-consuming, but planners recognized that it offered several benefits that could help solve logistical, cost and environmental challenges. Their solution was an onsite aggregate plant that crushed the blasted stone into different gradations for use in the main and auxiliary dam raisings.

Additionally, upon substantial project completion in 2022, the project team had logged over 2.6 million exposure hours with no significant accidents or injuries — an achievement that speaks to a sustained culture of safety, the Corps said.