Draining a reservoir sounds like the simplest job in civil engineering. You open the outlet, the water runs down the creek, and you wait.<\/p>\n
Anderson Reservoir, the largest body of water in Santa Clara County, has been trying to do exactly that since October 2020. It is still not empty.<\/p>\n
The reason is plumbing. The only outlet through the bottom of the dam was a pipe four feet across, and it could not move water fast enough to matter.<\/p>\n
Federal regulators wanted that lake gone before the next earthquake. The lake did not have a big enough drain to leave through.<\/p>\n
So Valley Water dug a new one. Crews mined a tunnel through the hillside beside the dam, then pushed a second, smaller machine the final 347 feet and out through the floor of the reservoir, holding a line 30 feet below the waterline while the water was still sitting on top of it.<\/p>\n
That tunnel is now in its last stages. And on July 24, the district’s board awarded a $1.88 billion contract to tear down most of the 235-foot dam and build a new one, the largest construction contract in Valley Water’s history.<\/p>\n
Anderson Dam went up in 1950 across Coyote Creek, three miles east of U.S. 101 in Morgan Hill. It is an earthen dam, which means it is essentially a very large and very carefully built pile of compacted dirt, and it sits beside the Calaveras Fault.<\/p>\n
A seismic evaluation in 2011 found the embankments could go unstable in a large enough quake, and that a fault rupture underneath could damage the outlet pipe and the intake structure. California’s Division of Safety of Dams had already capped storage at 68 percent of capacity two years before that.<\/p>\n
Then on February 20, 2020, the Federal Energy Regulatory Commission stopped negotiating. Its order told Valley Water to hold the reservoir no higher than elevation 565 feet immediately, and to get it down another 77 feet to 488 feet, essentially deadpool, before October 1.<\/p>\n
The dam safety risk at the project was “unacceptably high,” wrote David Capka, then director of FERC’s Division of Dam Safety and Inspections. He added that the district’s actions to that point had not shown an appropriate sense of urgency.<\/p>\n
Valley Water started lowering on schedule. By mid-December 2020 the reservoir sat at 3 percent of capacity, which the district describes flatly as the lowest level reachable through the existing outlet tunnel.<\/p>\n
That was the floor. The old pipe could not do any better than that.<\/p>\n
Full capacity is 90,373 acre-feet, collected off roughly 195 square miles of the Coyote Creek watershed. It is the biggest of the district’s 10 reservoirs by a wide margin, and Valley Water likes to point out that all nine of the others would fit inside it. Filled, it holds enough to supply almost a million people for a year.<\/p>\n
What the federal regulator would not wait on is what happens downstream. A breach at full capacity, by the district’s own published assessment, would put water across land more than 30 miles northwest to San Francisco Bay and more than 40 miles southeast to Monterey Bay.<\/p>\n
The new tunnel runs 1,736 feet through the abutment next to the dam, just under a third of a mile. Most of that, about 1,400 feet, was mined conventionally at a bore of roughly 24 feet. Valley Water finished that excavation in February 2024.<\/p>\n
Twenty miles up the same valley, the machine built to dig San Jose’s BART extension<\/a> has a cutterhead 54 feet across. Anderson never needed anything like that. It needed a hole that could survive an earthquake and open into a lake.<\/p>\n The last stretch was the part with no undo button. Connecting the tunnel to the reservoir meant breaking through into the lake bottom while the lake was still there.<\/p>\n AECOM<\/a>, the lead designer, describes launching an 8-foot microtunnel boring machine from inside the tunnel itself, an approach it calls novel for a reason.<\/p>\n A normal boring machine braces against the rock around it and shoves itself forward, the way the 1,680-ton machines waiting in New Jersey<\/a> will when they start under the Hudson. This one had nothing to brace against. It was pushed from behind by a growing string of steel pipes, one section added at a time, while the cutterhead chewed rock at the front.<\/p>\n It ran 347 feet that way, roughly a football field including both end zones, holding 30 feet below the surface of the water above it, and broke through into the reservoir in the summer of 2024. Divers then went down, rigged the machine to a crane and lifted it out of the lake in sections.<\/p>\n Most boring machines that make the news are the size of a building and get handled in one piece, like the 1,500-ton borer that rolled across Charles de Gaulle on 34 axle lines<\/a>. This one went in small and came out underwater, in parts.<\/p>\n The finished tunnel carries a 13-foot outlet in place of the four-foot pipe. That is more than ten times the cross-section, and the district’s original drawdown plan put the gain in emergency release capacity at fivefold.<\/p>\n The lining is the other half of the job. It is reinforced concrete built with articulated joints, so the tube can deform rather than snap if the fault beneath it moves.<\/p>\n Ryan McCarter, Valley Water’s deputy operating officer for dam safety and capital delivery, told Engineering News-Record<\/a> the joints “can move actually a few feet without the tunnel collapsing.”<\/p>\n The original plan was a retrofit. What the district has now bought is a demolition followed by a new dam.<\/p>\n Under the plan FERC approved this spring, most of the existing embankment comes out and goes back in. That means a new downstream buttress, a new spillway, separate high-level and low-level outlets, foundation treatment and seepage control underneath, and permanent instrumentation to watch the thing afterward.<\/p>\n The dam’s 800-kilowatt hydroelectric plant, offline since 2018, gets decommissioned along the way.<\/p>\n State permitting documents say the rebuilt dam has to survive maximum credible earthquakes on both the Calaveras and Coyote Creek faults, pass the probable maximum flood, and still be able to drop the reservoir in a hurry during an emergency.<\/p>\n The Army Corps of Engineers issued its water resources permit on June 11. The board awarded the contract on July 24 and announced it on July 28<\/a>. “Rebuilding Anderson Dam is our top priority,” board chair Tony Estremera said.<\/p>\n The winner is the Flatiron Dragados Sukut ADSRP joint venture, selected under California’s Best Value process, which lets an agency weigh technical approach alongside price instead of handing the job to the low bid. GEI Consultants has construction management for the seven years.<\/p>\n Those three companies have done this exact job before, about 30 miles north. Dragados, Flatiron and Sukut built the Calaveras Dam replacement for the San Francisco Public Utilities Commission under a contract awarded in 2011.<\/p>\n That job tore out a 1920s earthen dam held under 40 percent capacity since 2001 and replaced it with a 220-foot dam engineered to take a rupture on the Calaveras Fault. Same fault. Same problem. Finished in 2018.<\/p>\n The $1.88 billion is the solid figure here, because it is a signed construction contract. The total program cost is where the reporting splits.<\/p>\n Valley Water puts the whole project at about $2.8 billion, and ENR used that number. The Mercury News, which broke the award, reported $3.2 billion. Nobody has published a reconciliation of the gap.<\/p>\n The direction of travel is not in dispute. When FERC ordered the drawdown in February 2020, the district’s own spokesman was quoting a $563 million project to ABC7. By late 2023 the estimate had reached $2.3 billion, which the Mercury News noted at the time was triple the number from two years earlier.<\/p>\nRebuilding won the argument against retrofitting<\/h2>\n
The numbers that are still moving<\/h2>\n