201810.01
0

Southern California, parched and wary, hopes for rain, readies for flash floods

by in News

Southern California got ready Monday for the first storm of the new rain season, due midweek, while also preparing for possible debris flows as the threatening remnants of tropical storm Rosa rolled through the mountain and desert areas.

For a time Monday afternoon, authorities issued a voluntary flood evacuation for San Jacinto Mountain communities in the area of the Cranston fire that burned near Idyllwild as Rosa’s clouds headed north from Baja. That voluntary evacuation notice was lifted early Monday evening

The notice had advised residents of Hurkey Creek, Lake Hemet, Apple Canyon, Fleming Ranch that they “should go now, this is a safe time to leave,” according to a tweet from the Riverside County Emergency Management Department.

The National Weather Service also extended a flash flood watch advisory until midnight Monday for Riverside and San Bernardino county mountains and deserts.

It also issued a short-term flash flood watch scheduled to end Monday night for Alberhill, a community west of Lake Elsinore and near the burn area of the Holy fire.

And along the coast, high surf warnings will continue into Tuesday morning.

Mixed with the concerns over a possible heavy downpour launching debris flows and flooding was the need for rain through parched Southern California, where annual rainfall totals for several cities were far below normal for the rain year that just ended.

And as the threats of debris flows and flash floods in the eastern Inland desert and mountains  from the remains of Rosa were expected to weaken Tuesday, a separate Pacific storm was heading for Southern California, due to arrive sometime Wednesday.

And along the edge of the footprint that the Holy fire that burned into Riverside County hillsides and canyons as it roared in from Orange County’s Trabuco Canyon in August, workers were busy preparing to stop possible debris flows from hitting neighborhoods.

At Horsethief Canyon Road and Mountain Road, in the Horsethief Canyon development south of Corona, crews were working to divert and control possible debris flows from the hillsides that burned in the Holy fire.

“That intersection is right on the fire’s fringe,” said Robert Cullen,  assistant chief engineer for the Riverside County Flood Control and Water Conservation District.

Debris flows —  a mix of ash, rocks and boulders, burned material and loose soil —  is feared because they can overwhelm storm drainage systems, then move through neighborhoods.

“The best way to stop the debris is upstream of the flood control facilities, and that is what we are attempting to do,” Cullen said. The strategy is to produce a sump area for sediment and boulders, with the relatively debris-free water going into the storm drain system, he said.

Curbed roads are the secondary protection from flood waters reaching homes when drainage systems fail, but Horsethief Canyon Road, which runs downhill out of the development and is most direct exit for residents, is vital, Cullen said.

One extra layer of protection will be a cage-like covers for the storm drain inlets along the road, to prevent larger materials from entering them, Cullen said.

Experts learned from the deadly Thomas fire debris flooding of Montecito in January that even over-sized flood control inlets can get plugged, Cullen said.

Other work includes clearing the McVicker debris basin near the Holy fire’s footprint in Lake Elsinore, Cullen said. Planning for the work began shortly after the fire, with the county in consultation with the California State Geologic Survey identifying vulnerable areas, he said.

And there are other projects. There are about 220 employees in the flood control agency, about half of them maintenance workers, Cullen said “Nearly all of our maintenance staff and maybe a third of our office staff” has been devoted to debris-control work, he said.

Rainfall figures for the September-to-September 2017-2018 season were still being gathered Monday, but the numbers for some Southern California cities were available, and none looked good against the averages, according to the NWS.

Los Angeles got 4.72 inches, against an average of 14.93 inches; Long Beach 3.53 inches versus 12.26 inches; Santa Ana 2.66 inches compared with the average of 12.15, and Riverside 4.25 inches against an average of 12.40. .

“I think our season is off to a promising start,”  NWS San Diego office meteorologist Philip Gonsalves said by phone Monday, commenting on the approach of a Pacific storm so early in the new rain season.

While forecasts are for an equal chance of more or less than average rainfall for the year,  “if this pattern holds in next four weeks or so, we will get another good chance” for more rain that could edge the overall year toward at least above average, he said.

He also noted the long-range pattern will probably change more than once between now and the end of the rainy season at the end of March.