Mudslides Pose Continuing Threat, Geologists Warn

These are the times that try hillside homeowners’ souls.

Days of foreboding gray clouds and nights of sopping showers soaking already soggy hillsides.

The trade-off for those staggering views becomes the nagging uncertainty of whether that mound of earth beneath the foundation or above the house might start shimmying downhill.

In short, the fear that El Nino’s rains could bring a mudslide. That fear, geologists say, is warranted.

The question, they say, is not whether a hill will slide. But when.

“Basically, yes, people who live on or below hills should be worried,” said Kelvin Kaup, a Thousand Oaks-based engineering geologist. “It is a fact of nature that hills are only there for as long as it takes for the weather to wear them down and carry them to the ocean. It’s a natural process.”

This past rainy winter, Ventura County experienced scores of mudslides, most of them minor, said Jim O’Tousa, the county’s contract geologist.

Slides have threatened homes in Casitas Springs, crushed apartments on Cedar Street in Ventura, ruptured gas and oil lines on deserted hillsides and repeatedly blocked roads in and out of the Ojai Valley.

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“It’s been a big landslide year,” O’Tousa said. “El Nino rains have caused some reactivation of older, or ancient, slides, and we’ve seen an awful lot of smaller debris flows or mudflows.”

But before this season’s rains make you ditch your hillside pad for the flatlands, there is some good news.

Mudslides are pretty courteous: The bigger ones RSVP before rumbling to your doorstep. And good drainage plus good landscaping go a long way toward stabilizing slopes.

So how do you know if your particular hill is vulnerable?

It’s complicated, said Hugh Robertson, an engineering geologist based in Westlake Village.

“Our industry is not good at predicting the future,” he said. “We assess risk. I can’t come out to your house and say your hill is going to fall down next Wednesday at 10 a.m. If I could do that, I’d be making a lot of money.”

That said, there are some general guidelines about determining which hills are likely to wobble, explained O’Tousa.

“All conditions being the same, the steeper slope is more prone to fail than the shallow slope--that’s the best we know,” he said. “And it will go faster and it will go sooner, usually. But we do have shallow slopes that do fail.”

A slew of other factors play a role, such as the hillside’s history, the underlying geologic formations and the things people have done to the hill.

Typically, hills that previously have slid--such as in La Conchita--may have more tumbling to do, O’Tousa said.

Land forms that can concentrate water--canyons or gullies, for example--can also trouble your terra, because they help liquefy underlying layers of soil, Robertson said.

Even if the top layer of earth has dried out, the coast isn’t necessarily clear, as recent mudslides in Laguna Nigel prove. Even months after rains, a soggy, liquefied layer of soil a dozen feet down can cause the topsoil to slide down a hill like a layer of angel food cake on slippery icing.

The mudslide risk is greater at the foot of a hill than at the top.

“I’d much rather be at the top of [the] hill than at the mouth of the gully,” said Kaup, a 20-year veteran of the field. “The geologic process brings earth down, not up.”

Mudflows such as the one that briefly threatened three homes in Casitas Springs on Wednesday are hard to anticipate, but bigger slides--such as the one that destroyed nine homes in La Conchita three years ago--usually give some advance warning.

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Before major failures, hillsides often exhibit cracks or fissures. Sometimes smaller rivulets of mud precede a big slide, geologists say.

But there are no guarantees.

“It’s not as absolute as people would like,” conceded Robertson, who has also done geologic engineering for two decades. “When the air hisses out of a tire, you’re pretty sure you have a flat tire. With hillsides, it’s not like that. You could get cracks and even fissures without a slide. There’s no absolute set of circumstances that mean it’s going to fall down. And there’s no assurance that it won’t.”

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If you’re worried about property value, you can take some precautions to protect your home, the geologists stress:

* Avoid over-watering plants. It is convenient to use a sprinkler on a timer to keep your landscaping lush, but it’s a bad idea during the rainy season, when every extra drop of saturation counts. Let Mother Nature tend to your plants for a while.

* Improve drainage. You don’t want water pooling on your hillside or near your foundation. A qualified engineer can help create drainage systems that protect your home without shunting water to neighbors next door or downhill.

* Plant hill-hugging vegetation. Look for plants with deep roots--to hold soil in place--and not too much mass above ground, so they won’t provide extra momentum for slides. Native plants, such as sage, manzanita, chaparral and even oleander are good choices.

* Add protection from slides. Retaining walls can keep some smaller slides from sloshing into your living room, but they aren’t fail-safe.

* Consider terracing. When done by a professional, terracing can help prevent mudslides. But if the terracing is too steep, it can lead to erosion; if too shallow, it can lead to water pooling.