The U.S. Army Corps of Engineers Sacramento District awarded a $204-million contract on September 18 to Flatiron/Dragados/Sukut (FDS) joint venture of Benicia, CA, to construct modifications to the Isabella Lake Dam in Lake Isabella, CA. 

The Lake Isabella Dam Safety Modification Project (DSMP) consists of major improvements which the Corps says will enhance the structural capability of the existing dam and increase public safety, as well as restore water storage capacity in Lake Isabella. 

Modifications to the dam have been in the works for many years. Originally constructed in 1953, the structure has been the focus of seismic stability, overtopping, and seepage concerns for the past decade. These safety concerns have caused the dam to operate at limited capacity in recent years.  

New construction at the main dam, led by the FDS JV team, will include excavation of a new emergency spillway, modifications to the existing service spillway, a 16-ft raise of the main dam, modifications to California State Route 155 at the main dam’s right abutment, and modifications at several recreation facilities north of the auxiliary dam.

Sukut Construction's website says the basic features of the contract work includes excavation of 2.95 million cu-yards of earth and rock for a new emergency spillway; construction of a 28-ft-tall Labyrinth Weir using 56,000 cu-yards of structural concrete; buttressing and raising the existing dam structures by 16 ft; improvement and enhancements of the dam foundation; processing and placement of more than 2.3 million cu-yards of zoned fill embankment; modifications to CA State Route 155 to accommodate the increased dam height; and site work and improvements to several adjacent recreational facilities and roadways.

The contract contains an option to also perform work on the auxiliary dam. If the Corps decides to exercise the option at a future date that would bring the total contract award to $241.75 million. Work on the auxiliary dam would encompass abandonment of the Borel outlet works and modifications to the auxiliary dam, including a 16-ft raise similar to the main dam.

Construction is scheduled to begin this winter and is expected to be complete in 2022.

Isabella Reservoir is located forty miles northeast of Bakersfield, and consists of an earthfill main dam and auxiliary dam across Kern River and Hot Springs Valley, respectively. The dam was authorized under the Flood Control Act of 1944 and construction completed in 1953. The reservoir provides flood-risk management, irrigation and recreational benefits.

With more than 300,000 people living and working below the dams, primarily in the town of Lake Isabella and the city of Bakersfield, the U.S. Army Corps of Engineers began a dam safety modification study in 2006 to address seismic, hydrologic (potential over-topping during an extreme flood event) and seepage issues at the dams.

The Isabella Dam construction contract comes as Oroville Dam, which failed last February, is currently getting repaired, and as the California Department of Water Resources continues its push to inspect 93 dam spillways that could be at-risk.