Isabella Reservoir Dam Safety Modifications

Isabella Reservoir, located an hour outside of Bakersfield, California, was authorized to be constructed under the Flood Control act of 1944 and was completed in 1953. The reservoir consists of an earthfill dam and auxiliary dam across the Kern River and Hot Springs valley providing flood-risk management, irrigation, and recreational benefits. In 2006, the US Army Corps of Engineers (USACE) began a dam safety modification study to address seismic, hydrologic, and seepage issues at the dams. USACE has a dam safety rating scale ranging from 1 to 4. 1 for the being the dam is safe and 4 for being the dam is a safety risk. The dams for Lake Isabela were given a dam safety rating of 4. This rating required that operational restrictions be implemented thus limiting the reservoir’s normal storage capacity until modifications could be made.

In September of 2017, USACE awarded the joint venture of Flatiron/Dragados/Sukut (FDS) a $323 million contract to complete the Phase II Dam Safety Modifications. The modifications included raising the main dam and auxiliary dam 16-ft, creating a 300-ft long auxiliary spillway, and a 1300-ft long, 28-ft tall labyrinth weir anchored to the bedrock walls of the spillway. Nicholson Construction Co. was retained by FDS to provide pattern rock anchors, tiedown anchors, spot anchors, shotcrete dowels, and j-hooks to tie the Labyrinth weir and auxiliary spillway to the bedrock surface.

During a period of intense construction activities, from April 2021 to July 2021, Brierley Associates was retained by Nicholson Construction to serve as their lead on-site Quality Control Manager overseeing the preparation of submittals to FDS and USACE for the rock pattern anchors, tiedown anchors, and spot anchors related to the construction of the Labyrinth Weir.

In March of 2023, the Phase II Dam Safety Modifications were completed, now lifting the restriction levels and allowing USACE to perform necessary testing with an initial fill, capturing the spring runoff which is exceeding 400% of average snowpack. Lake Isabella reservoir level is continuing to climb, currently at 438,182 acre-ft of the 568,000 acre-ft of capacity.