Realizing Reservoir’s Full Potential With Massive Earthwork and Construction Project

In 2018, the U.S. Army Corps of Engineers ranked Isabella Dam as its No. 1 dam safety risk in the United States. Located at the southernmost tip of the Sierra Nevada and 110 miles north of Los Angeles, the dam has served flood control, agriculture, hydroelectric and recreation needs since 1953.

On April 28, 2006, a seepage problem was found at Isabella’s auxiliary dam. The finding prompted a dam safety modification study to address the leak, seismic concerns and hydrologic issues (potential overtopping during a flood event) at both the main and auxiliary dams. Subsequently, engineers developed plans to protect the more than 300,000 people living and working below the dam, primarily in the town of Lake Isabella and the city of Bakersfield.

From the plans, USACE initiated a seismic retrofit project to modify the original earthen dam structures. The design recommended raising the main and auxiliary dams by 16 feet, modifying the service spillway and creating a new emergency spillway — featuring a 28-foot-tall, 1,300-foot-long, arced labyrinth weir, USACE’s first project with the innovative structure type — to protect nearby communities against flooding.

The new Isabella weir structure was built as a pseudo-dam — designed to regulate water flow through the emergency spillway rather than hold it back, compared to the main and auxiliary dams. The arced and zig-zag labyrinth construction maximized the volume able to flow over the weir.

Integrated USACE and HDR Staff

Our team provided on-site construction management services to the field resident office. We supported USACE staff in managing the program and overseeing earthwork for the massive 2.8 million cubic yards of rock material used on both dams, which was mined on-site,  processed, compacted and formed into earthen dam layers. Our scope also included project engineering, quality assurance for the rock aggregate material lab, scheduling support, data acquisition, and claims consultancy.

A program of this magnitude required USACE to create an entirely new resident field office, relocate USACE employees and augment field personnel with HDR employees — resulting in 40 on-site construction management personnel. Certified construction managers made up the majority of the team.

Our data acquisition team provided a comprehensive set of construction photos and developed a survey-grade reality mesh model for the site. Drone pilots captured high-resolution imagery of the site's 450 acres. From the image capture, we produced custom earthworks data for the project, assessing the site’s stockpiles and making calculations to inform the project team. Digital models showed the types and volumes of material available on-site and allowed the team to assess the amount of material hauled off-site.

Delivering Resilient Infrastructure

In early 2023, the Central Valley experienced record rainfall and snowmelt from the Sierra Nevada. To accommodate increased river flow, USACE officials returned the lake to its original design capacity of 568,000 acre-feet of water, which was made possible because of the new dam.

To celebrate the completed project, leaders and dignitaries, including the USACE chief of engineers, gathered at a ribbon-cutting ceremony in April 2023.