Construction has begun on the second phase of construction for the roughly $600 million Isabella Dam safety modification project in Kern County, Calif. The U.S. Army Corps of Engineers in Sacramento is overseeing the project, with a Flatiron/Dragados/Sukut joint venture of Benicia, Calif., working on a $204-million contract to lead construction.

For the job, construction crews will raise both the main and auxiliary dams 16 ft to help minimize the risk of overtopping and add filters and drainage to both dams to increase dam stability. They will also construct improvements to the existing spillway, and build a new 300-ft-wide emergency spillway. 

David Serafini, the corps’ lead engineer on the project, told me constructing critical dam safety modifications to an operational reservoir, while maintaining existing flood damage reduction benefits, is an interesting challenge.

“Modifications to raise the main dam control tower require close coordination between operations, construction, engineering and the contractor,” says Serafini. “Modifications to the main and auxiliary dams require work to be performed during specific work windows because of reservoir operations during flood seasons to account for rainfall and snowmelt changes in lake levels.”

Phase one of the project was completed in fall 2017 with the completion of two U.S. Forest Service facilities – an administrative building and warehouse in Kernville, and a fire station in Lake Isabella – to replace the former facilities that sit in the project’s excavation footprint. Phase two began April 3 and is slated to complete in 2022.

Phase two work on the main and auxiliary dams will incorporate modern dam design practices and construction techniques. “Both dams will include a crest raise to address the hydrologic overtopping deficiency and will incorporate of full height filter and drain zones designed to modern standards addressing other failure modes,” says Serafini.

For a project of this magnitude, there will of course be a significant amount of concrete involved, with the majority being placed for the Labyrinth Weir in the planned emergency spillway. This section has more than 60,000 cu yds of concrete planned for placement between the foundation, slab, weir and walls. “Concrete will be batched on site using aggregates produced from the Emergency Spillway excavation.  Placement of the concrete will vary from mass placements to formed placements,” says Serafini.

In the course of construction crews will have excavated more than 3.5 million cu yds of earth between the emergency spillway, auxiliary dam and main dam. Other fun factoids include the placement of more than 67,000 cu yds of concrete between the emergency spillway, service spillway, main dam control tower raise, and state Route 155 right abutment; and nearly 2.5 million cu yds of processed rock to be placed on the main and auxiliary dams, with the remainder placed on Engineers' Point.

Completed in 1953, Isabella Lake Dam is located approximately 40 miles northeast of Bakersfield. The reservoir is impounded by two earthen dams on the Kern River and Hot Springs Valley. Today, Isabella Lake and its dams reduce flood risk for Bakersfield and the surrounding region, and is a primary water source for water users throughout Kern County. The Isabella Lake Dam safety modification project will address overtopping, seismic and seepage issues identified with Isabella Lake’s main and auxiliary dams to reduce the likelihood of dam failure.