It will be another year and a half or so before water district officials know if Anderson Dam in Morgan Hill is strong enough to hold back the biggest reservoir in Santa Clara County if a major earthquake shakes the region.
Contractors and engineers for the Santa Clara Valley Water District began collecting samples from within the dam’s downstream shelf last week, beginning a comprehensive “seismic analysis” to determine the earthen structure’s ability to withstand about a 6.6-plus magnitude quake.
Friday, about halfway up Anderson Dam, which is east of Morgan Hill on Cochrane Road, a rig and a crew from AMEC Geomatrix were drilling a six-inch diameter hole about 70 feet deep. The contractor hired by the SCVWD will drill at least 20 such holes throughout the dam in the coming months, preserving the dirt and rock collected in a cylindrical tube from each boring for further lab studies, according to SCVWD engineer Dave Hook. The depth of the borings will range from 50 to 200 feet.
While drilling, instruments also measure the pressure applied on the machine until it reaches the bottom of its sample. The engineers will analyze those measurements and the samples to see how much “alluvial material,” or soft dirt that could liquefy in the event of an earthquake, is inside the dam, which was built in the 1950s.
Hook said after the first six borings, staff noticed at least one that contained about seven feet of the potentially movable material. Hook explained that if that soft material were suddenly lost or liquefied, the dam’s clay core would have no support, essentially turning the dam into a landslide.
“We’ll continue to look and find out if it’s spread up and down the dam, and if it’s widespread (throughout the dam), we’ll look at a seismic retrofit,” Hook said. “If it’s isolated, the dam may be just fine.”
A seismic retrofit of Anderson Dam could become a major project, potentially requiring additional structural support in the form of more earthen buttresses and berms on the dam’s downstream side. Hook said the East Bay Municipal Utility District is in the process of seismically retrofitting one of its dams at a total cost of about $60 million.
The studies currently underway at Anderson Dam will cost about $3.5 million. A preliminary study conducted in January noted that a 6.6-magnitude temblor centered directly underneath Anderson Dam could cause the structure to slump, causing an “uncontrolled release” of up to 90,000 acre-feet of water.
In response to that study, undertaken as part of the water district’s ongoing seismic evaluation program for the eight dams it owns, the SCVWD board of directors ordered the more thorough analysis that began a couple of weeks ago. The analysis is expected to be complete within 18 months, Hook said.
Also in response to the January study, the California Division of Safety of Dams ordered the district to keep the water level in Anderson Lake, the district’s largest reservoir, at about 70 percent of its capacity or lower. The reservoir is currently at about 65 percent capacity.
It is not anticipated that any water will have to be released from the dam, Hook said.
SCVWD Director Rosemary Kamei, who visited the dam with district staff Friday, said the study is worth the cost.
“The board’s number one priority is public safety,” Kamei said. “The board felt strongly that even though it’s an expenditure of several million dollars we needed more information in order to ensure public safety.”
Hook said the study will be funded out of the district’s water utility fund, which is financed by water rates. He said water rates “could go a little higher” after the study is finished.
Water districts in California are required to keep updated “dam failure inundation maps” on file. The latest such map for Anderson Dam, published in 2003, shows that in a worst-case scenario in which the reservoir was completely full and the dam failed, downtown Morgan Hill would be under 35 feet of water in about 14 minutes, and Gilroy would be under 14 feet of water in about two and a half hours.
Hook said Friday that the inundation map is the result of a “very conservative” study that shows an unlikely scenario, but the district takes the possibility “very seriously.”
He noted that last Monday’s minor 4.3 earthquake in Morgan Hill caused no damage to Anderson Dam, but added that rate payers should be aware that the Santa Clara Valley sits in “earthquake central” where underground faults are constantly shifting.
Similar seismic analyses are also underway at the dams at Almaden, Calero, and Guadalupe reservoirs, which are also owned by the SCVWD.
