The Anderson Reservoir in Morgan Hill held back by a 240-foot dam built in 1950 could be rebuilt following the State Assembly’s passage of AB 3005 in June. The bill authored by Assemblymember Robert Rivas, called the Expedited Dam Safety for Silicon Valley Act will help facilitate and expedite the Anderson Dam Seismic Retrofit Project. The project would cost about $576 million but still needs to pass through the State Senate.
“Today’s overwhelming vote of support on the Assembly floor underscores the critical importance of expediting the Anderson Dam project,” Assemblymember Rivas had said. “The clock is ticking on a catastrophic dam failure in case of a large earthquake, and we can’t allow bureaucratic delay to increase risks to public safety, water security, and environmental protections.”
The dam is owned by the Santa Clara Valley Water District, which has intentionally kept the water levels low as a previous study had shown that in the wake of a magnitude 7.25 earthquake on the nearby Calaveras fault or a magnitude 6.6 on the Coyote Creek fault beneath the dam, a catastrophic breach would result if water levels were at full capacity. In such an event devastating flood waters would extend more than 30 miles northwest to the Bay, impacting the cities of San Jose, Santa Clara, Sunnyvale and Milpitas, in addition to cities to the south including Morgan Hill, Gilroy and Watsonville.
“The Anderson Dam project will not only protect Silicon Valley and South County from the devastation of a dam failure, but it also will create thousands of good-paying jobs that are so badly needed in the wake of the Coronavirus pandemic,” said Valley Water Board Member John Varela, who represents the District 1 where Anderson Dam is located. “Getting this project construction underway as quickly as possible is Valley Water’s top priority, and that’s what AB 3005 will do.”
Beginning last fall, the Federal Energy Regulatory Commission (FERC) that has jurisdiction over the dam sent letters to Valley Water warning of risks related to dam failure and ultimately ordered draining of the reservoir to deadpool to start by Oct. 1. Matt Keller, media and public relations supervisor for Valley Water, said that there have been no delays in responding to the FERC order. He added that Valley Water continues to follow all legal orders and guidance from the County Health Office including as it relates to the COVID-19 pandemic.
Varela said that strengthening and retrofitting the dam is one of Valley Water’s top priorities. In addition to draining the reservoir to the lowest practical level or deadpool, the agency is planning to begin construction on a larger low-level outlet tunnel in early 2021, which will allow for better management of the reservoir’s water level. Once complete, the dam retrofit would begin in 2024 and take about seven years to complete.
“Valley Water will drain Anderson Reservoir in a safe manner, doing this gradually so as not to harm the creek channel, our water supply, or the environment,” Varela stated. “Valley Water plans to put the water currently in Anderson Reservoir to beneficial use by sending usable water to our three drinking water treatment plants and recharging the groundwater basins. Other measures that will be taking place as we build the low-level outlet include downstream flood protection mitigation to Coyote Creek. We want the creek to be able to handle greater flows from the reservoir once the larger outlet tunnel is complete.”
Varela explained that the dam strengthening work has faced years of delays because since efforts began in 2012, the scope of the work has significantly expanded due to a greater understanding of what was needed. In 2017, the California Division of Safety and Dams changed the design criteria for spillways, which necessitated that the Anderson spillway be replaced instead of merely repaired.