Reservoir is drained for rebuild project
The largest reservoir in county will remain nearly empty until 2024
Anderson Reservoir, the largest in Santa Clara County, is nearly empty.
The Santa Clara Valley Water District, which owns the reservoir located east of Highway 101 between Morgan Hill and San Jose, said Tuesday that its crews have finished draining nearly all of its water — leaving it just 3% full — as part of a historic, $576 million earthquake repair job.
Draining began Oct. 1. Construction workers now can begin a 10-year seismic safety project to rebuild Anderson’s 240-foot earthen dam, which was built in 1950. The reservoir will remain nearly empty until 2024, water district officials said, while a 1,700-footlong tunnel, up to 24 feet in diameter, is built on the north side of the dam. It is expected to increase by fivefold the rate at which water can be released during major storms or after an earthquake that could damage the dam.
After that, the rest of the water will be drained out for six years, until about 2030 or 2031, while the dam is rebuilt to more modern standards. During that time, most of Anderson Lake County Park will be closed.
Seismic problems at the dam first were identified 12 years ago. But the district’s delays in beginning the repair work prompted the Federal Energy Regulatory Commission in February to order the reservoir drained.
For the past three months, the massive lake was drained at the rate of about 200 acrefeet, or 65 million gallons, a day — the equivalent of 98 Olympic swimming pools every 24 hours. The water wasn’t wasted, however. Most of it went to drinking water treatment plants and was served to the public, some was percolated back into underground aquifers for storage and some was released into Coyote Creek for fish.
Federal regulators criticized the water district for taking so long to undertake the project. The agency first learned of the seismic problems in 2009, and
subsequently released engineering studies showing that a 6.6 magnitude quake on the Calaveras Fault directly at Anderson Reservoir, or a 7.2 quake centered 1 mile away, could cause the reservoir’s huge earthen dam to slump and fail.
Although the chances of that are extremely slim, if such a disaster had occurred when the reservoir was full, it could have sent a 35-foot wall of water into downtown Morgan Hill in 14 minutes. The waters would have been 8 feet deep in San Jose in three hours, potentially killing thousands of people.
Coincidentally, on Tuesday
morning, two earthquakes occurred 1 mile east of Anderson Reservoir on the Calaveras Fault — a magnitude 3.7 and 3.5. No damage was reported. Some local residents wondered whether the draining of the reservoir, which had been 18% full Oct. 1, caused the earthquakes by shifting the amount of weight on the nearby fault.
No, said federal geologists. “The recent earthquakes occurred along a known fault patch that typically has several similarly sized, naturally occurring earthquakes per year,” said Rob Skoumal, a research geophysicist, with the USGS Earthquake Sci
ence Center at Moffett Field.
When full, Anderson Reservoir holds 89,278 acre-feet of water — more than all other nine dams operated by the Santa Clara Valley Water District combined.
The district, which serves 2 million people, will make up for the lost water during construction by pumping more local groundwater, using recycled water, and importing water from the Delta and from the Semitropic Water Storage District in Kern County, where it has been storing water underground for years.