The U.S. Army Corps of Engineers has determined that the earthen Whittier Narrows Dam is unsafe and could trigger catastrophic flooding in working-class cities along the San Gabriel River flood plain downstream between Pico Rivera and Long Beach.
The agency is developing measures to address significant structural problems at the 65-year-old structure that was recently classified as one of the highest-priority safety projects in its portfolio of 700 dams across the nation, officials said.
Federal authorities are particularly concerned that an extreme storm could trigger premature opening of the dam’s massive spillway, which is designed to open automatically when a pool of water behind it rises above 228.5 feet in elevation.
“Under certain conditions, the spillway on the San Gabriel River can release more than 20 times what the downstream channel san safely contain within its levees,” according to an Army Corps report. “Depending on the size of the discharge, flooding could extend from Pico Rivera, immediately downstream of the dam, to Long Beach.”
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