Tuesday's record-setting rainfall in the last week of summer was the first test of how ready the region is to handle the large rains that could come in an El Niño winter.
El Niño brings not just rain, but the threat of flooding, heavy surf, mud slides, people getting swept away in turbulent storm channels, and a heightened danger of wildfires in the year following the rain.
Just as we prep our own homes for winter, Los Angeles County's emergency response departments went through their own pre-season storm preparation to-do list, reporting their state of readiness for El Niño to the Board of Supervisors Tuesday.
County workers have built sand berms at the beach to protect coastal areas from damage. They've stockpiled sand and sandbags for the public at accessible yards. The county is also working on an El Niño communication plan to get simple, accurate messages out to the public in several languages during an emergency.
Los Angeles County Public Works and Flood Control District officials say they're ready for storms. But that readiness comes with an asterisk.
Several of the county's 14 flood control dams have too much sediment in them, reducing the amount of storm water and sediment they can hold during a storm. Sediment removal projects are proposed for four dams — Devil's Gate, Pacoima, Big Tujunga and Cogswell.
Devil's Gate Dam is the worst off. In a big storm — the kind that happens only every 50 years or so — sediment could fill the reservoir and block its control gates, leaving dam keepers with no way to control the flow of water into the Arroyo Seco channel to the south, county officials said. Some 500 homes and businesses could be inundated, according to a county engineering report.
In its El Niño preparedness report, County Flood Control said it would use a warning plan to reach residents who live along the Arroyo Seco Channel below Devil's Gate Dam to help them evacuate if there is a danger of flooding.
Chris Stone toured the dam in the midst of Tuesday morning's downpour. As a County Flood Control assistant deputy director, he manages water resources, including Devil's Gate Dam. The dam filled with sediment during the rains that followed the 2009 Station Fire above Altadena.
"Right after the 2010 storms that came in after the fires, the sediment was basically covering all of the valve inlets, so we were almost to the point where we couldn't operate the dam," Stone said.
Enough was cleared to operate the gates that control the water flow through the dam. But so much sediment remains, homes along the Arroyo Seco Channel south of here could flood out in a 50-year storm.
Highland Park resident Moira Clegg knows this. She walked through heavy rain Tuesday to a nearby bridge to check out the water in the Arroyo.
"It comes quite high up the sides and it makes you think if you live in the neighborhood that it's worth keeping your eye on it," she said.
She also points out a shredded blue tarp that once was put out to protect a hillside high above the arroyo. Part of the hillside slipped down to the edge of the Arroyo channel, and she worries that the channel could be blocked in a future slide, creating another flooding risk for nearby homes.
The county is pursuing a plan to clear some of the sediment from Devil's Gate Dam, but lacking permits from the Army Corps of Engineers and other agencies, it will be next year or later before the work can begin.
The county is still working on clearing out debris basins, which catch rocks, dirt, trees and sediment so they don't flow downhill and clog streets and storm drains, the report said.
Debris basins below the 2014 Colby Fire above Glendora are cleared out, the report said. A debris basin below the Calgrove Fire, which burned in Santa Clarita in June, still needs to be cleared out.