A major earthquake near Anderson Lake could cause the
reservoir’s earthen dam to collapse and send a wall of water
rushing toward Morgan Hill and Gilroy.
A major earthquake near Anderson Lake could cause the reservoir’s earthen dam to collapse and send a wall of water rushing toward Morgan Hill and Gilroy.
That’s what the results of a preliminary seismic evaluation of Anderson Dam released Monday say, according to Santa Clara Valley Water District spokeswoman Susan Siravo. The study, conducted by AMEC Geomatrix, was part of the district’s ongoing program to monitor dam safety at its 10 reservoirs.
Specifically, the study says a magnitude 6.6 earthquake centered directly underneath the reservoir, or a 7.2 temblor centered within a mile or two, could cause the dam’s foundation to liquefy and collapse. If the foundation were to collapse, the top of the dam could slump down.
Such a failure “could likely result in an uncontrolled release of water from the dam” if the reservoir was at full capacity, potentially flooding the valley floor from Gilroy to north of San Jose, the study says.
Siravo said the study is only preliminary and is based on incomplete data, and more detailed engineering studies will be commissioned in the coming months.
Nevertheless, she added that given the reservoir’s current level at about two-thirds of its 90,000 acre-feet capacity, Morgan Hill is not in any immediate danger of flooding.
“Even if we were to experience a major earthquake, there’s not enough water in the reservoir to spill over the dam at this point,” Siravo said.
Furthermore, the district announced Monday that it will not allow the water level at Anderson Lake to exceed 87 percent of capacity, keeping the water level about 30 feet or more below the spillway, until more extensive studies can be completed.
Morgan Hill City Manager Ed Tewes said the city has figured the possibility of a “low risk, high impact” failure at Anderson Dam into its emergency preparedness scenarios since it was built in the 1950s.
However, he said the new study “indicates that the risks are a little greater than we had previously thought.”
City officials plan to attend the water district’s board of directors meeting Tuesday, Jan. 13. At that meeting the board will review the district’s dam safety program. Tewes said the city prefers a conservative approach that will minimize the threat to Morgan Hill as much as possible, and will thus ask the district to keep the reservoir at or below its current level.
Tewes said that property owners below the dam, including the commercial development Cochrane Commons, have long been notified of the potential for a dam failure via real estate disclosures. He added that the city’s emergency plan includes using an automated telephone message system to notify residents in the event of a failure of Anderson Dam.
