South Bay dam sits on earthquake faults, new project aims to reduce risk


MORGAN HILL (KRON) — At 235 feet high, the Anderson Dam, which looms large above hundreds of homes on the east side of Morgan Hill, sits more or less on top of the Calaveras and Coyote earthquake faults.

Studies have concluded that in a magnitude seven or larger earthquake nearby, the dam could slump and fail, putting tens of thousand of people downstream at risk, according to Valley Water.

“When Anderson is full, that’s a pretty large body moving down Coyote Creek, if it was completely full there could be a 30 foot wave of water that would inundate the community below all the way to the bay,” said Chris Hake with Valley Water.

Geo-technical and archaeological work is already underway and on schedule. In a project that could take five years to complete, once construction begins in 2022, crews will strip the earthen dam down to it’s clay core and replace what could turn into jello in a big quake.

“There is some material at the base of the dam that would get kind of slushy for lack of a better term, they call it liquefiable and so the earthquake shakes it and it gets mushy and the dam cold slump down and the water would go over the top,” said Hake.

Plans are in the works to address neighbors worries about truck traffic on nearby streets, dust and other impacts from construction.

For now, the water level in the reservoir is being kept below 58 percent for safety’s sake, but eventually it must be drained so crews can get at the base of the dam.

Taking the valley’s largest reservoir off-line is also a concern but there’s a plan for that too says Hake.

“We import about 55 percent of our water and we have the ability to increase that and we can pump ground water and we can use the South Bay aqueduct that is a source as well as the San Luis Reservoir which fills the needs through our federally contracted water, so we will have planned in advance to meet the needs of the community,” he said.

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