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 Southern California’s San Gabriel River floodplain. An estimated one million people live and work along the floodplain.
According to an agency report, unusually heavy rains could trigger a premature opening of the dam’s massive spillway. “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,” the report said. “Depending on the size of the discharge, flooding could extend from Pico Rivera, immediately downstream of the dam, to Long Beach.”

The Corps recently reclassified the dam as one of its highest priority safety projects in the nation. Engineers have also found that the mile-long earthen structure could fail altogether if water were to flow over its crest or if seepage eroded the sandy soil underneath.

The dam provides water conservation storage but the primary purpose of Whittier Narrows Dam is flood risk management — it is the central element of the LA County Drainage Area (LACDA) flood control system. Both the San Gabriel River, and the Rio Hondo, a tributary of the Los Angeles River, flow into the Whittier Narrows reservoir. The Pomona Freeway (CA-60) passes through the reservoir flood control basin and the San Gabriel River Freeway (I-605) passes along the eastern boundary of the basin.

With a retrofit not likely to begin until 2021, according to a Corps spokesman, LA 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 retrofit operation is expected to include replacing the existing spillway with a system less likely to malfunction, as well as shoring up the dam’s foundation to reduce erosion and prevent subsidence that could result in floodwaters spilling over the top.

Source: Los Angeles Times