Slippery Slope : Scientists Survey Fire-Charred Hills Near Castaic Lake for Mudslide Risk

Legions of exhausted firefighters have headed home after halting one of Los Angeles County’s worst fires in years. But the work is just starting for Glenn Harris, one of the scientists in a damage-assessment team dispatched to the area.

Harris, who has been evaluating fire damage for 22 years with the federal Bureau of Land Management, needed hardly more than a quick look to conclude that there is plenty to worry about.

“This is really a tough nut. There are going to be problems,” he said, standing on a charred hillside north of Castaic Lake over the weekend.

Harris and the other researchers--hydrologists, soil scientists, geologists and botanists from around the country--have the tricky responsibility of calculating the likelihood of mudslides on slopes blackened and stripped of vegetation by the Marple fire. This is a common hazard in the aftermath of chaparral fires.

Later, ecologists will also survey the harm the fire wreaked on plant and animal life. But for now, they are most concerned with how to prepare for the first rainfall and potential landslides in the 22,000-acre burn area.

The team is concerned about the fire’s having swept through one of the region’s most important utility corridors, Harris said.

For starters, Castaic Lake--now surrounded by naked, black slopes--is a key reservoir for the Metropolitan Water District. If the slopes come crashing down into the lake, the utility will end up with a storage basin full of sediment.

Then there’s the Castaic power plant at the north end of the lake, one of the Los Angeles Department of Water and Power’s largest. Muck from collapsed slopes could clog its machinery.

Unstable slopes also could topple electric towers. Natural gas lines could become exposed and damaged, said Harris.

He only had to glance at his feet to prove his point. A long crack in the ground alongside the roadway showed that natural erosion had started even before the fire hit.

“Look, this whole thing is slumping. We are standing right on it!” he said. “This is going to come down,” he added, eyeing the power plant below. “They had better be ready.”

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The scientists will likely recommend that the catch basin around the power plant be expanded and nearby culverts widened. Assuming such measures are taken, the chances that a catastrophe will befall the power plant would be remote, said Harris.

But there is always uncertainty.

The scientists base their recommendations on sophisticated calculations of “sediment loads” and “peak flows,” measures of land and water movement. But ecosystems are too vast and varied to submit easily to empirical measure, said Julie Tupper, a National Forest Service hydrologist on the team. Much of what they do involves educated guesswork.

They rely on their wits and experience, compare impressions and feel relieved if their conclusions match at the end of the day.

Landslide potential is measured in part by checking water repellency--which is done by scraping away dirt at the surface and simply dribbling water on it.

When Harris did this on a patch of burned ground near the freeway, the water balled up and skittered across the ground like oil in a frying pan--not a drop was absorbed into the soil. Minutes later, as he prepared to leave, the water remained beaded on the ground.

When it rains, wet dirt on top is likely to shear off the water-resistant layer beneath and tumble downhill, Harris said.

Making matters worse is that the hills around Castaic are highly prone to landslides--even when they aren’t burned, Harris said. With the vegetation stripped away, cracks, creases and buckled slopes all stand out in sharp relief.

“It was a bad situation. It’s going to be worse,” he said.

But Harris and his team also found some good news. Already, they were predicting the area would recover rapidly.

That’s because most of the fire area appears to have only burned at low or moderate heat, he said. Sifting the soil through his fingers, he pointed out bits of twigs and grass, blackened but intact, a sign that the flames passed over quickly and lightly.

They spared root crowns and seeds of shrubs in many areas, and left leaves on the tops of some plants. Yuccas on the slopes above the lake still have a green cast to them.

And the dominant vegetation in the area--a shrub called chamise--is a fast grower that should have little problem springing back. The plant “has more than one seed--one that requires fire and one that doesn’t. It hedges its bets,” said botanist Mike Gonella.

Next year should be great for wildflowers, Harris added. “There’s going to be stuff blooming here we haven’t seen in 50 years.”

Harris and his team will finish their report in the coming days.