Crews will soon begin digging 24-foot tunnel into reservoir

Construction crews and Valley Water officials recently reached a “milestone” on the $576-million Anderson Dam Seismic Retrofit Project in east Morgan Hill, with the completion of a massive concrete and “soil nail” wall next to the existing dam, according to water district staff.

The wall is just outside the reservoir, just south of the existing Anderson Dam. The structure leaves an opening for a 24-foot outlet tunnel that will feed into Coyote Creek when the project is complete. The wall is composed of 244 rebar soil nails, arranged in a grid pattern, that were driven 120 feet deep into the ground before they were reinforced with concrete, Valley Water Engineer Chris Hakes told directors at a Sept. 27 board meeting.

“It’s for slope stability,” Hakes said.

At the bottom of the wall is a “diversion portal entrance” for water release when necessary after the entirety of the dam seismic retrofit project is complete in about 10 years, Hakes added. Also recently completed is an “outlet works drop shaft,” which, combined with the diversion tunnel, will allow Valley Water to quickly drain up to 30% of the reservoir’s capacity within seven days to prevent downstream flooding if heavy rains cause the water body to overflow in the future.

In 2017, Anderson Reservoir flowed over its emergency spillway during a series of strong rainstorms, resulting in widespread flooding in residential areas downstream in south San Jose. The larger tunnel is designed to prevent such calamities, as it will allow dam operators to release water from the reservoir substantially more quickly than the existing spillway and outlet tunnel.

Later this month, crews plan to dig the 24-foot tunnel all the way through the hillside from the concrete wall into the reservoir, according to Valley Water staff. Anderson Reservoir has been almost completely empty since the district drained it in 2020 in order to allow the current and ongoing construction.

The wall, tunnel and related infrastructure are just one of five separate projects that, together, comprise the Anderson Dam Seismic Retrofit Project. The other projects are the Cross Valley Pipeline Extension Project (CVPEP); Coyote Creek Flood Management Measures; Coyote Percolation Dam Replacement; and the Coyote Creek Stream Augmentation Fish Protection Measure.

In addition to the new tunnel, also currently under construction is the CVPEP. The others are expected to start construction in the coming years, according to Valley Water staff.

The water district received $5.8 million for the CVPEP from the California Department of Water Resources in June. This project “will support groundwater recharge in the Santa Clara Subbasin, which is a high-priority basin supplying drinking water to the southern San Francisco Bay Area, including San Jose, and more than 2,000 water supply wells,” says the DWR website.

The Cross Valley project has been under construction since this summer, and the DWR funds will help complete the construction of this pipeline later this year. Specifically, the CVPEP project is extending an existing pipeline by about 1.25 miles to feed water into Coyote Creek near the Coyote Creek Golf Club.

Currently, this water can only be released into Coyote Creek just below Anderson Dam, according to Valley Water staff. The purpose of extending the pipeline is to ensure Coyote Creek and the Coyote Percolation Pond in south San Jose have enough water to recharge the groundwater basin and support local wildlife and its habitat while Anderson Dam is being rebuilt.

The dam retrofit and associated projects became necessary after state authorities in 2009 determined that Anderson Dam would not withstand a major earthquake, and the crest of the dam could slump in such an event—leaving Morgan Hill underwater within minutes.

After the tunnel is complete, crews will rebuild the earthen dam at Anderson to make it bigger and sturdier. The project is expected to be complete by 2031, at which point the reservoir can return to full capacity and help augment Santa Clara County’s water supplies.

Hakes said the design of the dam rebuild is about 90% complete, and a draft Environmental Impact Report will soon be released for public comment.

Anderson Reservoir is the largest of Valley Water’s 10 reservoirs. Its total capacity when full is 90,000 acre feet of water.

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