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Castaic Dam Rides Out Small Quake, But A Massive One Could Unleash A Catastrophe

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The area that would be flooded if the main dam at Castaic Lake fails (California Department of Water Resources)
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The 3.5 magnitude earthquake that hit early Friday morning near Interstate 5 was only a mile away from Castaic Dam. That’s important, because the dam has one of the lowest safety ratings in Los Angeles County — due to seismic vulnerabilities.

The dam was inspected after the earthquake and no problems were detected, said Department of Water Resources spokeswoman Maggie Macias. When it was completed in 1974, it was built to a standard meant to withstand a much bigger earthquake.

But a massive quake near the dam could have catastrophic consequences if the main dam failed. Within 18 minutes, the rush of water could flood an area less than 2 miles away. Within an hour, it would be under 104 feet of water.

That’s according to the inundation map on file with the state Department of Water Resources.

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Everything in the flood path from Magic Mountain to the Pacific Ocean in Oxnard could be washed away.

HOW THINGS COULD UNFOLD

Castaic Lake can hold up to 323,700 acre-feet of water — enough to fill the Rose Bowl stadium 703 times.

If Castaic dam failed and released all its water, a tall wave of water would move south to Castaic Junction, where Interstate 5 meets state Highway 126.

Then it would follow gravity west in a broad destructive path along the Santa Clara River and put Oxnard under 5 feet of water.

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It would take the flood wave just 10 hours to reach the Pacific.

HOW WE KNOW

That scenario is public today, and available online, because of the extensive flooding that occurred after the Oroville emergency spillway failed in February 2017.

That spurred state officials to force dam owners — including the owners of Castaic dam — to take a closer look at whether they, too, had faulty spillways, and make more data public, including annual dam safety ratings (posted online), new emergency action plans and inundation maps.

Castaic was already one of a handful of dams in Los Angeles and surrounding counties rated by the state to be in “fair” condition. The dam fell one level short of the top rating of “satisfactory” because its outlet towers and the bridge to access them could be vulnerable in an earthquake. Also, the dam’s left abutment might be unstable, so that’s being evaluated.

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The dam is due for repairs that the state characterizes as a “modernization” to bring up the safety rating.

A bridge that workers use to reach a water intake tower needs a seismic retrofit. The state is also evaluating the condition of a 60-foot wide concrete spillway and running earthquake analyses on some other components of the dam. Once they finish evaluating what work is to be done, the repair and upgrade work could take another decade to finish.

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