Rebuilding a dam in the East Bay that was not designed to withstand a major earthquake will allow additional drinking water to be stored for Bay Area residents and protect the area near the reservoir from flood damage after a temblor.
A multibillion-dollar, 12-year project by the San Francisco Public Utilities Commission is overhauling the Hetch Hetchy water system to help protect drinking-water supplies after earthquakes for the 2.5 million customers served by the agency.
One of the major projects is rebuilding the Calaveras Dam, which is the second-largest drinking-water reservoir in the SFPUC system.
Many of San Francisco’s reservoirs were built over seismic faults in the early 20th century because the fault lines naturally create concave depressions in the earth, and Calaveras Reservoir is one of those.
When it was built in the 1920s, the 210-foot Calaveras Dam was designed to hold 31 billion gallons of drinking water to help supplement the main storage facility at Hetch Hetchy Reservoir in Yosemite National Park and in smaller dams throughout The City’s long water network.
Since the dam is not designed to withstand a major temblor that could strike the fault on which it sits, the water levels of the reservoir have been dramatically lowered.
“The existing dam was constructed using an antiquated method of construction,” Project Manager Dan Wade said.
Additionally, initial efforts to build Calaveras Dam failed and the structure that exists today was built on top of some of the rubble from the initial failure, which reduces the dam’s ability to withstand an earthquake, according to Wade.
An earthquake on the Calaveras Fault could cause ground beneath the reservoir to temporarily liquefy and lead to 20 to 30 feet of sediment piling up on its floor, Wade said.
The sediment would quickly raise the water level, potentially causing it to overflow the dam, which would erode the earthen structure and possibly lead to its total collapse. That would cause a flood of water to be released into Alameda Creek.
Federal officials in 2001 ordered San Francisco to reduce the reservoir’s water level by 60 percent to minimize flood damage to downstream cities, such as Sunol and Fremont, should the dam be damaged by an earthquake.
In an effort to restore the reservoir to full capacity, which helps ensure year-round drinking water supplies that are harvested in late winter and spring from melting snow, the San Francisco Public Utilities Commission will build a new dam wall nearby the existing one, which will then be removed.
As well as holding Hetch Hetchy water, the reservoir catches most of the water that flows through the Upper Alameda Creek, impacting trout, salmon and other species that rely on the habitat.
The dam’s estimated cost has grown from $257 million to $410 million, in part because of measures needed to release water into the creek, according to SFPUC General Manager Ed Harrington.
Costs also increased because naturally occurring asbestos in the serpentine soil required more worker protection measures than originally anticipated, according to Harrington.
Unlike higher-profile dams that are made of sheer concrete walls, such as the Hoover Dam, Calaveras Dam is a mound of clay, rocks and other fill material which is safer to use in earthquake zones than concrete walls.
“If anything happens to the rest of our system, we and our 2.5 million customers will be dependent on water storage on this side of the hills,” Harrington said.
Upgrade will make Peninsula reservoir safer
The primary water storage reservoir for the Peninsula will be rebuilt to protect the surrounding area from floods.
The Lower Crystal Springs Reservoir, an oasis that was constructed in the mountains of San Mateo County, was lowered 8 feet in the early 1980s after California dam safety officials ruled that it posed a flood risk.
The reservoir’s spillway will be widened to reduce flooding risks and other work will also be undertaken.
Spillways are built next to reservoirs to catch overflowing water to prevent floods from deluging rivers or creeks.
The $36 million project is forecast to begin in early 2011 and finish a year later.
Protecting water delivery
The Water System Improvement Program is a $4.4 billion to $4.6 billion, 12-year effort to rehabilitate and protect the system of pipes, reservoirs and equipment from earthquakes. The entire system delivers water — largely from the Hetch Hetchy Reservoir in Yosemite National Park — to 2.5 million Bay Area customers.
Calaveras Reservoir by the numbers
7 million gallons Water drawn daily from Calaveras Reservoir
$410 million Latest projected cost of Calaveras Dam replacement project
31 billion gallons Normal storage capacity of Calaveras Reservoir
12 billion gallons Current storage capacity of Calaveras Reservoir
210 feet Height of existing and replacement dam at Calaveras Reservoir
1,200 feet Length of crest existing Calaveras Dam
1,210 feet Length of crest for new Calaveras Dam
3.46 million cubic yards Volume of fill in existing Calaveras Dam
2.77 million cubic yards Volume of fill in planned new Calaveras Dam
Source: San Francisco Public Utilities Commission
The Hetch Hetchy system, which supplies water to millions in the Bay Area, is in the midst of a massive overhaul. The Examiner takes a look at the impact of the infrastructure changes to the vital resource.
Sunday: Multibillion-dollar project creates thousands of jobs
Today: Reservoirs filled to capacity after upgrades
Tuesday: First-ever tunnel beneath the Bay transports water
Wednesday: Upgrades add storage in San Francisco for water
Thursday: Water safer after new facility is built
See The Examiner's Hetch Hetchy series here.