County nearly finished updating dams for earthquakes

Big Tujunga Canyon Dam in the Angeles National Forest after the destructive 2009 Station Fire. Photo: LA County Department of Public Works

The mountains above the Los Angeles Basin and the San Fernando Valley through time have funneled enormous amounts of runoff water, boulders, mud and sand down canyons and onto the plains below. Landscape-altering floods in 1914 and 1938 led to the construction of dams across some of the canyons, but since the deadly Sylmar earthquake in 1971 it has been recognized that the dams were not built sufficiently strong to hold up reliably if a severe regional quake hit while the dams retained a full load of water. So there has been a major program underway to strengthen dams, such as the one built in 1931 in Big Tujunga Canyon. Now the dam retrofitting is nearly complete, the Los Angeles News Group reports.

Deep inside the Big Tujunga Dam in northern Los Angeles County, in a gallery just above the riverbed, engineer Michele Chimienti stands between the yellowish concrete face of the old dam and new light-gray concrete of the dam’s reinforcement wall.

“This dam can handle an 7.5-magnitude earthquake,” she says.

Four decades after the 1971 Sylmar earthquake prompted the state to invoke stringent standards for dams to withstand earthquakes, the Los Angeles County Department of Public Works is nearing completion of seismic rehab work on the county’s 14 major flood control dams.

The $94.8 million Big Tujunga seismic rehabilitation and spillway modification project was completed in July 2012. It was followed by a $2 million project to construct a riser — a reinforcement — on the Santa Anita Dam, completed in 2013.

There’s still valve and spillway work to finish on the Santa Anita Dam, work to seismically reinforce the Santa Anita Debris Basin Dam, seismic modification on the Eaton Wash Dam set for completion this month, and a study to determine whether work should be done on the Puddingstone Dam in Bonelli Regional Park....