MONTEBELLO (CBSLA.com) – The U.S. Army Corps of Engineers has determined that the 60-year-old Whittier Narrows Dam is structurally unsafe and poses a potentially catastrophic risk to the working-class communities along the San Gabriel River floodplain, it was reported Friday.

With a retrofit not likely to begin until 2021, according to a Corps spokesman, Los Angeles County and municipal officials are working with the federal government to develop emergency plans that can be implemented if necessary before repairs to the dam are completed, The Los Angeles Times reported.

In June of 2016, the USACE changed the Whittier Narrows Dam’s risk characterization from high urgency to very high urgency.

An agency report based on research conducted last year says unusually heavy rains could trigger a premature opening of the dam’s massive spillway, The Times reported.

“Under certain conditions, the spillway on the San Gabriel River can release more than 20 times what the downstream channel can safely contain within its levees,” according to the Corps report cited by The Times. “Depending on the size of the discharge, flooding could extend from Pico Rivera, immediately downstream of the dam, to Long Beach.”

Engineers also found that the mile-long earthen structure could fail if water were to flow over its crest or if seepage eroded the sandy soil underneath. The agency said Thursday that it is developing measures to address problems at the dam, which the corps recently reclassified as one of its highest priority safety projects in the nation.

The San Gabriel River plunges 9,900 feet, from forks in the mountains down to Irwindale, then meanders in a channel to Whittier Narrows at the southern boundary of the San Gabriel Valley. From there, it flows in a concrete-covered channel for most of its final journey to the Pacific Ocean. The river and its aquifers serve more than 3 million people in the San Gabriel Valley and southeast Los Angeles County, according to The Times.

An estimated 1 million people live and work along the floodplain.

The planned retrofit is expected to include replacing the existing spillway with a system less likely to malfunction and shoring up the dam’s foundation to reduce erosion and prevent subsidence that could result in floodwaters spilling over the top, officials said.

Until then, “we cannot approve any sort of construction projects in the floodplain under our jurisdiction,” said a Corps spokesman quoted by The Times.

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