In late September, the Santa Clara Valley Water District, in California, reached a major milestone in its Anderson Dam Seismic Retrofit Project, a $1.2-billion initiative to secure drinking water and protect the community from flood risks. Crews successfully removed a specialized micro-tunnel boring machine from the bottom of Anderson Lake, marking the completion of a 1,736-foot tunnel. It was a sight to see.
The tunneling work will increase water release capacity from the dam (and maintain water supply from the reservoir during the retrofit project), but it’s only part of a broader strategy to strengthen the crown jewel of Santa Clara County’s water infrastructure.
Valley Water Director John Varela called the Anderson Dam retrofit project “the most critical project in our capital improvement list.”
The new diversion tunnel will play an important role in managing water flow during emergencies; with a 20-foot diameter housing an internal pipe 13 feet in diameter, the tunnel can release up to 6,000 cubic feet of water per second into Coyote Creek, a significant increase from the dam’s previous 49-inch outlet capacity of 450-500 cfs.
“The amount of water capacity that can flow in a high-water season, to evacuate the dam as necessary, as quickly as we can, is important for safety purposes and flood protection,” Varela said. “That’s what this dam is all about, in addition to water storage for potable drinking water as well.”
The tunnel will ultimately connect to the reservoir floor through a “lake tap” procedure, enabling direct water intake from 30 feet below the surface. Though this tunneling work alongside a major dam is not uncommon, the high-stakes boring of a drop shaft beneath the reservoir presented a unique project, says Ryan McCarter, Valley Water deputy operating officer and Anderson Dam project manager. Crews installed a reinforced soil nail wall, with 244 rebar nails embedded 120 feet deep, to provide the tunnel with stability. Each piece of rebar weighs about 1,600 pounds.
“Projects like this don’t happen every day,” Varela said. This is an all-encompassing affair that will change the water supply landscape of the Santa Clara Valley for a very long time. Indeed, the complex infrastructure work that began more than a decade ago is slated to continue to completion in 2032.
Looking Back
Built in 1950, Anderson Dam stands approximately 210 feet tall at its highest point, holding back the Coyote Creek watershed and capturing rainfall within the area. The reservoir has a storage capacity of up to 90,000 acre-feet of water, though it’s been drained to about 3% of that capacity for the work under way now.
In the late 1980s, a pipeline was added to link Anderson Dam, enabling the movement of imported water in and out of the reservoir. This upgrade cemented Anderson Reservoir as a key part of the Valley Water supply system, allowing stored water to be distributed throughout the county as needed.
Then the future changed. The California Division of Safety of Dams identified seismic concerns in 2009, setting this project in motion.
The ultimate retrofit work, set to begin next year in earnest, includes the construction of a new, seismically resilient earthen dam embankment. The cost of inaction here: The existing dam could slump during a major earthquake, risking catastrophic flooding to the surrounding communities. Anderson Reservoir lies parallel to the Calaveras Fault; this is not a minor risk. Without a full retrofit, McCarter says the risk of liquefaction in the earthen dam material itself would be extremely high. A flooding event would inundate communities from Morgan Hill northwest to the San Francisco Bay.
Every infrastructure project in the U.S. is unique, but each one carries its own weight of risk management and future-proofed investment.
Reflecting on the history of Anderson Dam, Varela emphasized how dramatically the water management landscape has evolved since the dam’s construction.
In the 1950s, projects like this just didn’t face the same regulatory hurdles as today; the dam was built in less than two years. Nowadays, each phase of a given infrastructure project inevitably involves coordination with state and federal agencies, which provide critical guidance but also add layers of complexity to the schedule. This is a sprawling project management job. Still, this level of oversight is important for ensuring both community safety and responsible environmental management in the long run. This oversight is how we’ve come to embrace a growing industry culture of transparency and accountability.
The rehabilitation and renovation of aging water infrastructure in the U.S. is an opportunity to connect the nuts and bolts of projects like the Anderson Dam retrofit to the basic curiosities of the general public. Where does our drinking water come from? It’s a basic question that a taxpayer might ask, but one riddled with complexity. For residents in Morgan Hill, located just beneath the dam, and the Bay Area, the work at Anderson is a chance to connect dots and illuminate the ecosystem in which we live.
As an example of that connection, the Anderson Dam retrofit is taking place under the publicly funded auspices of Valley Water’s Safe, Clean Water and Natural Flood Protection Program, originally passed at the ballot in 2000 and renewed ever since.
Looking Ahead
Where do we go from here?
Now that the tunneling phase is complete, the full Anderson Dam seismic retrofit project is set to begin construction in late 2025, with completion projected by 2032.
This phase will see crews carefully dismantle and rebuild the dam embankment tier-by-tier to bring it up to current safety standards. The methodical construction process will ensure the dam can withstand significant seismic activity. Alongside the 15,000-sq.-ft. embankment replacement, a new emergency spillway—a massive concrete structure—will be constructed to handle overflow via that newly installed diversion outlet safely in high-water situations.
The scale of this project will involve hundreds of workers on-site, large machinery operations, and extensive material handling for both dirt and concrete. When complete, these updates will not only enhance the dam’s stability but also secure a critical water resource at full capacity for the county’s long-term safety and resilience. Those 90,000 acre-feet of water are enough to supply nearly a million residents with water for an entire year.
For Santa Clara County, the Anderson Dam Seismic Retrofit Project represents a vital investment in resilience, safeguarding both water resources and community safety well into the future.
