USA — With the biggest fire in Los Angeles County history about 91% contained, concern is shifting to the threat of flash floods and mudslides.
The Station fire has blackened more than 160,000 acres in the San Gabriel Mountains, leaving barren and eroded hillsides towering over Los Angeles. When the winter rains start, there is little vegetation left in the burn areas to prevent water, silt, rocks and branches from coursing down steep canyons and ravines toward thousands of homes.
Although mudslides are a regular occurrence after fires, authorities are worried that this years flows could contain larger amounts of debris than usual. The San Gabriel mountains are made up of rock types that easily shatter and crumble.
A lot of these slopes are already shedding substantial debris down into the channels fine sediments and rock falls, said Richard Hadley, spokesman for a team of scientists, engineers and other experts assembled by the U.S. Forest Service to assess the effect of the fire.
Debris flows off of these hills can be almost like flowing concrete, he said. And on these really steep slopes, there can be a lot of power behind it. The intense heat from the blaze can also cause the soil to effectively seal itself, resulting in even larger and faster flows, he said.
The so-called Burn Area Emergency Response Team is studying satellite imagery and conducting soil tests to determine where the effects of the fire have been most severe. With that information, it will model water flows, in order to identify the communities that are most at risk and recommend measures to protect lives and property. Possible safety measures include putting in temporary barriers to divert the flows, officials said.
The Los Angeles County Department of Public Works oversees an elaborate flood-control system, including 14 major dams, about 500 miles of open channels and 3,000 miles of storm drains that dump water into the Pacific Ocean.
Nine basins have also been carved into the hills in the affected area to trap silt, rocks and vegetation before storm flows reach urban areas, said Mark Pestrella, public works deputy director. The system works very well every year, year in and year out, Pestrella said. But he cautioned that some residences, particularly those located at the foot of burned slopes, could still be at risk.