A wet winter could test some LA flood control dams choking with sediment

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Southern California could be in for above-normal rains this winter if predictions of a large El Niño system in the Pacific pan out. Flood control workers across Southern California have been preparing for possible torrential storms by clearing out debris basins and flood channels.

Big dams and the water and debris basins behind them are the first line of defense against flooding. There are scores of these dams stretching from Santa Barbara to Orange counties. Officials in those counties — along with counterparts in Riverside, San Bernardino and Ventura — all say their flood control systems are ready for whatever El Niño throws at them.

But Los Angeles County still has some work to do. Five of the county’s 14 flood control dams have been filling up with sediment washed down from mountainsides stripped of trees and vegetation by catastrophic wildfires, like the Station Fire in 2009. The sediment has limited the capacity of the dams to hold storm water.

Video

(Video courtesy of Jonathan Dietch, hang glider pilot and photographer)

Devil’s Gate Dam above Pasadena is in the worst shape. Its storage capacity is down by 60 percent. In a major storm, of the type that usually hits the region every 50 years or so, Devil’s Gate Dam would not be able contain all the water and debris coming in. Parts of the Arroyo Seco could overflow, flooding nearly 500 homes and businesses and closing the 110 Freeway.

This map shows the footprint of the 2009 Station Fire and the locations of the five dams that the Los Angeles County Flood Control District has identified as needing sediment removal. Residents near the dams know the work needs to be done, but many have challenged the flood control district over how.

Still, even if all opposition evaporated today, final planning, permitting and approvals for these projects are still needed. It’s not likely even a single grain of sediment will be removed before this winter.

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