Granite Construction announced on July 22 that it has received a $50 million contract from Pacific Gas & Electric (PG&E) to replace the spillway at Tiger Creek Control Dam in Northern California, where construction began in May.
The work follows assessments of spillways that PG&E conducted after the Oroville Dam spillway failure in 2017 prompted federal and California dam safety regulators to require hydro operators to reassess aging spillways using updated hydrologic modeling and engineering analyses.
At Tiger Creek, the review identified deterioration of the existing spillway, uncertainty about whether the dam’s three spillway traps would activate as intended, and inadequate hydraulic capacity under likely peak flood conditions.
Hydraulic modeling found that the existing spillway could safely pass about 2,750 cubic feet per second, less than half of the site’s estimated maximum likely flood of 5,653 cubic feet per second. Because the 1931 dam was not designed to withstand overtopping, PG&E selected a replacement spillway designed to pass up to 6,000 cfs.
The Tiger Creek Dam is part of PG&E’s Mokelumne River Hydroelectric Project near Pioneer, California. State-certified project documents describe the nearly century-old structure as a 486-foot-long, 110-foot-tall slab and concrete buttress dam. Construction is scheduled to be completed in May 2028.
Granite said the contract includes construction of a new spillway crest structure and concrete channel, installation of temporary formwork, excavation, rock anchoring, concrete placement, abandonment of the existing spillway and construction of a permanent access road. The company previously completed an approximately $600,000 preconstruction contract that included constructability reviews and early project planning.
Project plans certified by the state call for building the replacement spillway along the right abutment of the dam rather than modifying the existing spillway. The design includes a 40-foot-wide spillway that terminates in a tilting bucket with divider blocks before the flows discharge into an excavated plunge pool.
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State-certified project plans show the Tiger Creek Regulator Dam project area and the Doakes Ridge staging and tailings site, where excavated material from the landfill replacement will be transported during construction.
Map courtesy of Calif. State Water Resources Control Board
The project also requires cutting a new opening through the dam’s right abutment to connect the reservoir to the replacement spillway before the original spillway is permanently abandoned, replacing rather than rehabilitating the existing flood release system.
Construction will take place behind temporary formwork installed during a planned drawdown of the reservoir, allowing the new spillway to be constructed in dry conditions while reservoir operations continue.
State-certified project documents estimate the work will require approximately 35,000 cubic meters of excavation and about 4,000 cubic meters of structural concrete, underscoring the scale of what is effectively a new hydraulic structure rather than a rehabilitation project.
The replacement spillway is one of several dam safety improvements made by California hydro operators since the Oroville incident prompted a broader reevaluation of aging spillway systems and their ability to accommodate updated flood estimates. At Tiger Creek, the solution was to build an entirely new spillway rather than modify the original design.
Granite and PG&E do not respond promptly to ENR’s requests for additional information.
