No matter how much it rains this winter, one of the Bay Area’s largest reservoirs won’t fill up. Seismic safety concerns are already restricting the amount of water that can be stored in Anderson Reservoir, and now the discovery of new “trace faults” near the dam have further stalled a $193 million project to strengthen it.
State regulators ordered a few years ago that the vast lake near Morgan Hill in Santa Clara County — which holds more water than the other nine reservoirs in the county combined — could not be filled any more than 68 percent full because geologic tests found that in a major earthquake, its 240-foot high earthen dam could slump, releasing a wall of water that could generate a trail of death and destruction all the way to San Jose.
With the plan to shore up the dam already a year behind schedule, officials at the Santa Clara Valley Water District, which owns Anderson Reservoir, announced earlier this month that instead of breaking ground in early 2017, they now hope to begin construction in early 2018. That means work won’t be finished until 2021 or 2022.
“We have to get it right,” said Katherine Oven, deputy operating officer at the Santa Clara Valley Water District. “We want that dam to stay intact, operate and provide water supply for the next 50 years. We want to be well-informed of the potential seismic dangers.”
The timing of the project has always posed a problem as California suffers through a historic drought: Strengthening the reservoir will require draining desperately needed water during construction.
But the latest hitch in the schedule surfaced when engineering crews last year dug 44 bore holes and eight trenches to obtain a detailed picture of the geology around the dam. They found several “trace faults” that had previously not been known to exist.
“These are right through the gut of the dam,” said Chris Mueller, an engineer with Black & Veatch, a Kansas-based engineering firm working on the project, at a recent public meeting.
Those faults required more digging, study and computer modeling to make sure that the design to retrofit the dam against earthquakes is sufficient.
The earthquake risks at the aging dam have been known now for six years.
In 2009, the water district released engineering studies showing that a magnitude 6.6 quake on the Calaveras Fault directly at Anderson Reservoir, or a 7.2 quake centered one mile away, could cause the dam to slump and fail.
Although the chances of it happening are extremely slim, a complete failure of Anderson Dam when the reservoir is full could send a 35-foot wall of water into downtown Morgan Hill within 14 minutes, state emergency maps show. The waters would be 8 feet deep in San Jose within three hours, potentially killing thousands.
Anderson isn’t the only dam in the Bay Area that has had earthquake issues.
In 1979, San Pablo Reservoir, east of Richmond, was drained by the East Bay Municipal Utility District so that the 1920s-era structure could be strengthened. But when more earthquake retrofitting was needed in 2008, the district was able to avoid draining it again by pumping concrete underneath it and building a new buttress on the downstream side.
Similarly, Calaveras Dam, on the Santa Clara-Alameda county border, was found to be vulnerable in a major quake. So state regulators ordered repairs, which are now underway and expected to be finished in 2018. The San Francisco Public Utilities Commission decided to build a new dam a few hundred feet downstream from the old one, allowing crews to avoid draining the lake.
But those fixes won’t work at Anderson, officials at the Santa Clara Valley Water District say. Unlike Anderson, Calaveras is in a narrow canyon where building a second dam is feasible.
When Anderson Dam was built in 1950, scientists thought the nearby Calaveras Fault was inactive. And water officials once believed that the dam was anchored in bedrock. An engineering firm performing tests required by federal regulators in 2009, however, found that the dam’s foundation contains sand and gravel, which could shift in a major quake.
When full, the reservoir holds 90,000 acre feet of water — roughly 29 billion gallons. But when state dam safety officials learned of the seismic risks, they ordered the water district to fill it to no more than 62,000 acre feet, to reduce the weight of water on the thinnest part at the top of the dam.
That unfilled reservoir space — 28,000 acre feet — has mattered during California’s ongoing drought. Lost is the opportunity to store enough water for 140,000 people for a year until the dam is fixed.
Currently the reservoir is only 34 percent full, holding about 31,000 acre feet of water. But if El Niño rains bring soaking storms this winter, as many meteorologists expect, the water district will be unable to fill Anderson to the brim.
The project to shore up the dam calls for widening its base by 70 feet each on the downstream side and the upstream side with new rock buttresses. Workers also will raise the height of the dam by about 7 feet and its spillway walls by up to 15 feet, along with expanding the outlet pipes.
Apart from finding the new trace faults, work also was delayed when crews found a rare plant: a type of shrub called coyote ceanothus.
Biologists at the state Department of Fish and Wildlife required the water district to conduct studies and obtain permits, making sure the soil disruption from trenching and building access roads would minimize harm to the plant.
“We’ve found out that the problems at Anderson are more complex than we knew,” said Gary Kremen, chairman of the water district’s board.
Originally, state officials required the dam to be fixed by 2018. Given the newly discovered faults and the potential for having to redesign the project, they say it’s acceptable that deadline won’t be met.
“With these big projects, you always have an aggressive schedule, and you try to meet it, but we haven’t seen anything we would question on their schedule,” said Christopher Dorsey, a senior engineer with the state Division of Safety of Dams in Sacramento.
Still looming is when the water district will drain Anderson Lake to do the work. Oven said that will probably happen in the fall of 2017 and be done over a six-month period. District officials say more than half of Anderson’s water will probably be sent to homes and businesses or pumped back into the ground to store in aquifers.
Kremen said, however, that if the drought drags into a fifth and sixth year, the water district may delay the draining because it will need every drop.
“Draining it during a drought is problematic,” he said. “You’re damned if you do, damned if you don’t. But it’s got to be fixed.”
Paul Rogers covers resources and environmental issues. Contact him at 408-920-5045. Follow him at Twitter.com/paulrogerssjmn.
