The project to upgrade a reservoir that serves as the San Francisco Bay area's main backup water supply has suffered a $200 million setback.
Crews building a new dam on the Calaveras Reservoir have come across an ancient, buried landslide that has forced them to redesign the project, the San Francisco Chronicle reported on Tuesday.
The changes will increase the costs from $416 million to $620 million and push back completion by more than 2 years to 2017.
The existing dam has been deemed seismically unsafe.
"It's an ancient landslide thousands of years old, buried below the surface and masked from view,'' said Dan Wade, the regional project manager for the San Francisco Public Utilities Commission's $4.6 billion plan to overhaul the Hetch Hetchy water system and make it seismically safe.
The system relies primarily on the Hetch Hetchy Reservoir in Yosemite National Park and serves about 2.6 million customers in the Bay area.
The dam replacement is among dozens of projects that are part of the Hetch Hetchy overhaul.
The additional costs for the Calaveras Reservoir will not affect ratepayers because the public utilities commission saved money on other projects in the Hetch Hetchy overhaul, said Julie Labonte, the public utilities commission official overseeing the dam replacement.
Labonte was expected to ask commissioners to approve $55 million more for the project on Tuesday.
The Calaveras Reservoir, located near the Bay area city of Milpitas, is among several local reservoirs that supply the region.
It is "critical'' in case the supply of water from Hetch Hetchy is interrupted by an earthquake or other event, Labonte said.
The current dam there was completed in 1925, but was deemed seismically unsafe in 2001.
Regulators ordered the reservoir to be drained to about a third of its capacity. The new reservoir will be filled to its historic level of 31 billion gallons. The discovery of the landslide will force crews to remove an additional 5 million tons of rock and dirt and make a deeper cut and a flatter, more gradual slope above the new dam, the Chronicle reported.
"The landslide was massive enough that it would have created an unsafe situation if workers were to continue with the original cut,'' Wade said.