El Niño rains will test flood and mudslide prevention measures

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Local cities and counties will get the first test of their preparations  to protect Southern California homes and businesses from flooding and mudslides as the first El Niño-driven rains arrive this week. 

Local and state authorities have been working for months to shore up soil made unstable by wildfires in mountains and foothills and clear debris basins that catch sediment coming out of upland areas to reduce the chance of that storm channels will flood. Protective measures have also been taken along the coast.

Four years of drought also mean that much of the region's capacity to capture or channel large volumes of rain to the ocean has not been severely tested.

In San Bernardino County, officials were so concerned about the need to clear vegetation and sediment from long-dry waterways that they declared an emergency, a measure that enabled them to fast-track agreements with state and federal agencies to do needed work.

In early December, giant earth-movers had been working for a couple of weeks clearing sand and dirt that's settled along the San Timoteo Channel in Redlands. The idea was to dig out sediment that  increased the risk of flooding downstream. By the end of the project, enough material was removed to fill half of Pasadena's Rose Bowl stadium.

ExcavatorExcavators clear out debris and sediment that’s settled along the San Timoteo Channel in Redlands on Thursday morning, Nov. 19, 2015. (Maya Sugarman/KPCC)

For a guy who loved playing with Tonka Toys as a kid, Angel Lemus  grew up to have the perfect job.

"This is an excavator here, and he's loading out onto these Super-10 dump trucks,"said Lemus, an operations supervisor for San Bernardino County Public Works.
 
"If these basins fill up, and we can't control the flow of the water, we have a railroad in danger, we have San Timoteo Bridge that could overtopple, and we have a series of private homes that we could actually get flooded out," he said.

Soil DrainSan Bernardino County Department of Public Works crews push sediment to get water to drain from the San Timoteo Channel in Redlands on Thursday, Nov. 19, 2015. (Maya Sugarman/KPCC)

That danger is never far from Lemus' mind.

He was on duty Christmas Eve 2003, two months after a massive wildfire burned over much of the San Bernardino mountains. A storm dropped four inches of rain onto unstable mountain slopes in just a few hours.

A cascade of mud, boulders, and burned trees wiped out a church camp along Waterman Creek and a campground farther downstream. Sixteen people died, nine of them children.
 
"That particular incident was really horrific," Lemus said.
 
Communities located below recent wildfires are the most vulnerable to rain-triggered mudslides.
 
The newest risk area is in Ventura County. The Solimar Fire that broke out over Christmas burned hillsides that could come down on Highway 101, Pacific Coast Highway and a portion of the Union Pacific Railroad.
 
Other areas on the El Niño mudslide watch list are the Silverado Canyon area of Orange County, the Colby Fire area near Glendora, and the Powerhouse Fire area above the Leona Valley in Northern Los Angeles County.

I-5 over the Grapevine has had repeated closures in past months due to mudslides. That's expected to continue through the winter.

 (From the mountains, to the desert to the ocean, many parts of Southern California could be vulnerable to floods, mud and debris flows in heavy rains expected this winter. Click the pins for a description of the risk and links to more information about them. Compiled by Sharon McNary/KPCC)

El Niño's heavy storms can also overwhelm flood channels in low-lying areas like Carbon Creek in Anaheim. The channel runs next to some apartments near Chippewa Ave. It was built to withstand the kind of storms that hit once every 25 years, and it has repeatedly overflowed in heavy rains into the adjacent carports.
 
"We're trying some new technology to see how it holds up. It's called Tiger Dam and Muscle Wall," said A.J. Jaime, operations manager for Orange County Public Works.

 A Tiger Dam is a water-filled tube about three feet around.  A 50-foot section of Tiger Dam replaces about 500 sand bags, Jaime said. These Tiger Dams are faster to install, just a couple of workers can lay them out along a channel edge and fill them up with a pump or fire hydrant.
 
They connect to each other and also to something called Muscle Walls, which Jaime describes as  hollow plastic, water-filled  K-rails (similar in shape to those familiar freeway barriers) that interlock. They are covered with thick plastic and anchored with sandbags.
 
They work together like a big water-filled Lego system to raise the channel walls and direct storm runoff away from homes and streets, Jaime said.

Similar installations have been put up at 10 other locations in Orange County.
 
Among the areas most vulnerable to flooding are low-lying stretches of the coast. The warm water associated with this winter's El Niño has already raised the base sea level by as much as a foot.
 
Cities and counties have put up high sand berms to protect beachfront homes and businesses from storm surges. But it wasn't enough to keep some Ventura streets from flooding. High surf knocked a dozen pilings from the Ventura Pier December 11.
 
El Niño's risks in the mountains, valleys and beaches can be anticipated and planned for. But there are also the fluke floods.
 
A mattress left out on a Boyle Heights street blocked a storm drain  in October and filled up the surrounding neighborhood like a bathtub. So residents also need to be alert to these kinds of hazards and keep streets clear of trash and urban debris.

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