Quake Could Break Dam, Kill 400, Study Says

A strong earthquake would likely turn the sandy soils at the base of Lake Casitas dam into a jiggling mass that could rupture the structure and release a 300-foot wall of water into Coyote Creek, killing about 400 people.

That worst-case scenario outlined in a final environmental report released Monday by the U.S. Bureau of Reclamation has inspired federal officials to press ahead with plans to shore up the dam that holds back Lake Casitas.

“If something did happen, there is a large potential for loss of life,” said William Pennington, Casitas Dam project manager for the bureau. “It would be an ugly situation.”

Under the scenario described in the report, a magnitude 7 quake on the Red Mountain fault would rupture the dam and release the wall of water, killing people along the lower Ventura River and causing $430 million in property damage by the time the torrent reached the ocean.

The dam is one of 40 that the bureau has identified as quake-threatened in California, Oregon and Nevada.

Beginning May 14, the agency plans to start work to strengthen the 334-foot-tall dam, removing spongy soil and replacing it with firm earth. The job will cost $42 million and be completed by December 2000, Pennington said.

Casitas Dam, built between 1956 and 1959, provides water to 40,000 users in Ventura County. The water it stores provides an estimated $30.6 million in annual benefits, one-third from recreational boaters and fishermen, and the rest from municipal, industrial and agricultural users.

Short-term remedies in place to reduce the quake risk include dewatering wells to help dry out soils and a reduction in the volume of water kept in a spillway catch basin.