MONTEBELLO, Calif. (AP) The Army Corps of Engineers has raised its estimation of flood risk from the Whittier Narrows Dam in eastern Los Angeles County.
The Corps said the risk characterization has been changed from high urgency to very high urgency.
The earthen dam constrains the San Gabriel River and the Rio Hondo. Both are normally dry but fill during storms.
Analysis found that during an unlikely but possible storm, water released through the spillway could exceed downstream channel capacity, flooding adjacent land.
Other failure modes involve erosion from water piping through the dam's foundation and from water overtopping the dam.
The Corps said interim measures will begin before next winter's rains and other measures are likely by the end of 2017. Construction of final modifications is expected to begin in 2020.