The rains have returned to the Santa Clara Valley, and with them renewed anxiety over the capacity and stability of the county’s biggest body of water, the Anderson Reservoir.

The source of that anxiety isn’t likely to go away until after as many as nine rainy seasons, as the Santa Clara Valley Water District now says that a five-year, $550 million project to upgrade the earthquake safety of the Anderson dam may not begin until 2022 at the earliest, two years later than its official start date. The popular recreation lake will be drained for at least five years during the project.

New seismic data prompted the district, which owns the reservoir, to revise its plans for the “Anderson Seismic Retrofit” this fall. The project may take longer and cost more: the initial cost estimate was $400 million. The new data will require a complete reconstruction of the nearly 70-year-old earthen dam, according to the district.

In February 2017, water from the Anderson Reservoir flowed over the banks of Coyote Creek and gushed into a South San Jose neighborhood in one of the worst floods in San Jose’s history. The devastating flood prompted evacuation orders for 14,000 homes, left $100 million in damage and prompted litigation by more than 150 flood victims against the city, county and water district.

A dam reconstruction plan was first announced by the water district in 2013, about four years after authorities learned the current structure could collapse in a major earthquake.

The revised project was unveiled at a public meeting in Morgan Hill in late October, and is currently in the design phase. The water district board will be considering revising the project start date early next year. New geologic investigations in areas around the dam resulted in the discovery of “previously unidentified seismic deficiencies,” according to the water district report:

• The upstream embankment is “susceptible to liquefaction” during a “maximum considered earthquake,” an earthquake that is expected to occur once in approximately 2,500 years, or a 2 percent chance every 50 years.
• The special materials placed between the reservoir’s clay core and the rock fill were determined to be inadequate to prevent failure in the event of a “fault offset,” leading to seepage and erosion through the bedrock foundation beneath Anderson Dam during a major earthquake.

Even before these new findings, concerns about earthquake safety prompted the district in January 2017 to lower the reservoir’s water surface elevation limit an additional 10 feet. Anderson Reservoir is currently limited to about 52 percent of its capacity.

“The retrofit project which was originally planned to include large upstream and downstream buttresses has been modified to a nearly complete replacement of Anderson Dam in place,” the district said in its latest report. The project will return the reservoir to its original storage capacity.

The defeat of Proposition 3, for new water bonds, by the state’s voters Nov. 6 won’t affect the viability of the project, says the district.

District spokeswoman Gina Adriano said, “We’ve already allocated project costs within our budget. However, it does reduce the opportunity to lower the district’s project costs by utilizing what would have been potential Prop. 3 funding.”

Because the reduced capacity of the reservoir will extend for another two years, continuing the lingering possibility of another spillover, the district had considered using special floating pumps to reduce flood risks by pumping the water out of the reservoir over the spillway. The district concluded that “installing pumps on the dam or in the reservoir added risk and hazards to the operation of the dam,” and the same benefits “could be achieved through operational changes.”

Anderson Dam creates the county’s largest surface water reservoir—Anderson Reservoi—which stores local rainfall runoff and “imported” water from the Central Valley.

The reservoir is an important water source for treatment plants and the recharge of the groundwater basin. Besides restoring drinking water supplies, the upgrade also supports compliance with environmental regulations. The district’s regular reservoir releases ensure that downstream habitat has healthy flows and temperatures to sustain wildlife.

A breach of Anderson Dam at full capacity could have catastrophic consequences, including inundation of surrounding land more than 30 miles northwest to San Francisco Bay, and more than 40 miles southeast to Monterey Bay.

The district said it is completing a draft environmental impact report for public review, and plans what it calls “a permanent fix to the risks identified by the seismic study.”

In addition to rebuilding the dam, the project will:

• Replace the existing outlet pipe that runs below the dam to improve capacity and reliability
• Increase the wall height of the concrete spillway to approximately 9 feet and the height of the dam crest to 7 feet.

The new plan to replace the entire dam “will ensure the post-project facility has removed all liquefiable material in and beneath the embankments and will be built to the most modern design standards and with rigorous quality control,” according to the district. In addition, a new high-level outlet will be constructed to allow rapid drawdown of the upper portion of the reservoir in case of an emergency.

The water district is working with the state’s Division of Safety of Dams and the Federal Energy Regulatory Commission. Both agencies have jurisdictional authority over the dam and reservoir, and the water district must obtain their review and approval for all project design plans. In addition, the project is continuously overseen by an independent panel of dam experts. Environmental documents will be prepared to comply with federal and state regulations, and permits will be obtained from several regulatory agencies for water diversion activities during construction, including full draining of the reservoir.

The district cautioned that its 2022-2027 timeline depends on a few factors. Currently, engineering work is on track to be 90 percent complete this fall. A critical part of the schedule depends on the acquisition of environmental permits from state and federal agencies, such as National Marine Fisheries Service, U.S. Army Corps of Engineers, U.S. Fish and Wildlife Service, and California Department of Fish and Wildlife.

Current estimates put the project cost at $550 million. Of that total cost, 15 to 20 percent will be spent on planning and design, as well as on environmental studies and documentation, with the remaining spent on construction. These cost estimates may change as the project progresses.

The Safe, Clean Water and Natural Flood Protection Program, which Santa Clara County voters approved in November 2012, will fund about $65 million of this project’s cost. The remaining project costs will be funded by water rates. Upon completion of the project, the average household in the area of the county roughly north of Metcalf Road in Coyote Valley can expect an increase of $6.25 per month in their water rates. Households in the area south of Metcalf Road can expect to see an increase of about $3.50 per month.

The project will require the use of heavy equipment, which may generate traffic in multiple shifts. Residents living near Anderson Dam east of Morgan Hill can anticipate other impacts due to lighting, noise and dust.

The water district is working with the city of Morgan Hill, the county’s Department of Parks and Recreation and local residents to develop a program to minimize construction impacts. It is expected that the reservoir will be available for some recreational use until early 2022. For more information, click here.