The Bureau of Reclamation awarded a $23.9 million contract Sept. 22 to Shimmick Construction Co. Inc. to modify and strengthen Stony Gorge Dam, on Stony Creek 25 miles west of Willows.
Shimmick, of Hayward, Calif., is to perform work intended to prevent the structure from failing in an earthquake. The work will involve construction of a concrete wall and additional supports between the dam’s vertical buttresses to provide increased stability during an earthquake.
BuRec said 2001 risk studies found the dam posed an unacceptable risk to downstream residents. Studies since that time indicated a large earthquake could cause the vertical concrete buttresses that support the dam to buckle, possibly leading to dam leakage or collapse.
BuRec said work on the 139-foot-tall, 868-foot-long dam is expected to begin by the end of this year. The project could take up to three years to complete.
The reservoir behind Stony Gorge Dam has a storage capacity of about 50,000 acre-feet. The dam regulates flows along the lower reaches of Stony Creek and stores water for irrigation, hydropower, and recreation.
Part of the Orland Project, Stony Gorge Dam is owned by BuRec and operated by the Orland Unit Water Users Association. The city of Santa Clara’s Silicon Valley Power utility operates a 4.9-MW hydro plant at the dam, Stony Gorge (No. 3193).
The government said the work would affect water releases and Santa Clara’s hydro plant minimally. Local water users will be responsible for reimbursing the U.S. government 15 percent of the cost of modifying the dam.
Stony Gorge Dam was completed in 1928; the hydropower plant was added in the mid-1980s. In 1986, the dam was modified to reduce danger posed by extreme flood events that could overtop the dam. At that time, it was modified to allow overtopping of one side of the structure and a concrete slab was placed downstream to prevent damage to the foundation.