Calero Dam Could Fail in an Earthquake; Water District Takes Precautions

Officials will cut in half the water storage in the Calero Reservoir after discovering that its dam might fail in a large earthquake, according to an announcement today from the Santa Clara Valley Water District.

If the Calero Dam were to fail, water from the 3-billion-gallon reservoir just south of San Jose would escape north along the Calero Creek and flood the southeast part of the city, according to a study for the Association of Bay Area Governments. 

After drilling into the 75-year-old earthen dam, scientists discovered the same gravely material as in the recently restricted Anderson Reservoir. In a large earthquake, the dam could “slump” below the reservoir's surface and rapidly erode from the cresting water, according to Marty Grimes, spokesman for the water district.

“It’s unstoppable,” Grimes said. “Once that water gets going, there’s nothing you can do.”

 While the findings are preliminary, Grimes said the district chose to reduce the water level by 20 feet as a safety measure until consultants finish the study in 2012.

Engineers identified three other potentially at-risk dams: Stevens Creek, Anderson and Guadalupe, according to a January report to the water board of directors. 

"If we lose our local water supply, we're in trouble," said Don Gage, chairman of the water board.

The water from Calero goes directly to the Santa Teresa Water Treatment Plant for distribution around the county, Grimes said. Twenty-five percent of the county's water comes from local reservoirs. 

“Calero Dam has never experienced earthquake damage, but keeping the reservoir level low is a prudent safety precaution until we can be sure the dam can withstand a major quake,” said the district’s chief executive officer, Beau Goldie, in today's press release.

The Shannon Fault, one mile from the dam, can generate up to a 6.7-magnitude earthquake, according to the release. The 1989 Loma Prieta earthquake, in comparison, was a 6.9.

The district has evaluated the integrity of its dams since 2009, applying modern engineering standards to the aging system, according to the release.

"Most of those dams were built in the '30s and '50s," the board's Gage said. "The standards then weren't what they are today."

While the potential cost of revamping the Calero Dam is unknown, the district’s Capitol Improvement Plan estimated the Anderson project at $110 million, Grimes said. If changes are necessary, options include fortifying the existing dam or building an entire new one.

Calero collects little runoff, but it accumulates water through a canal that connects it to the smaller and more active Almaden Reservoir to the west, said the spokesman. It represents 6 percent of the total capacity of the district’s 10 reservoirs.

In addition to water from reservoirs, the county collects 25 percent of its water from underground aquifers. The remaining amount is brought in from other regions, according to Grimes and material from the district.

A series of consulting firms from around the country are collecting the data: URS, AMEC Geomatrix, Terra and GeoPentech, according to the recent report.