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Landslide Prediction an Improved but Imperfect Science

The Monterey Bay Area is in the middle of its wettest winter in years, putting hillsides from Davenport to Big Sur at greater risk for mudslides. While the conditions are ripe, it’s hard to know when and where the land will give way. But prediction is critical, because an unexpected  collapse can leave tragedy in its wake.

North of Ben Lomond in the Santa Cruz mountains, a makeshift memorial still rests in a grove of trees near what used to be the neighborhood Love Creek Heights. There’s a toy box, teddy bear, and baseball mitt— in memory of two young children entombed by a landslide on January 5, 1982. The neighborhood was wiped out 33 years ago, when the hillside gave way, killing 10 people, during a devastating winter along the Central Coast.

Mark Strudley, a hydrologist with the National Weather Service, pays his respects at the monument, then walks across the road, up an overgrown and forested trail. “You can see a mixture of rocks and what looks like some cement that was probably part of people's homes, or foundations,” he says, gesturing toward wood and concrete debris, jutting from the soil. “It's hard to tell what all this is, but we're standing on slide deposit right now.”

Strudley looks up, traces the side of the mountain, then points to a crest high above us. “So about 30 feet thick of soil and debris came rushing down the hill slope here,” he says.

The mountainside would have come down quickly. “It happens in an instant,” Strudley explains. “For slides like this, there's really no time to get out of the way.”

For slides like this, prediction is the only way to avoid disaster. After the catastrophic winter storms of 1982 and ‘83, management officials set to work on a network of rainfall monitors.

About 10 minutes from Love Creek, The Santa Cruz County ALERT Rain Gauge stands in an open clearing of woodland. It's a tall brown post with a wire antenna on top. The gauge collects water and reports back how much rain is falling over time. In general any more than three quarters to an inch of rain per hour is bad news. “That's when most managers including myself start to get concerned about debris flows and landslides,” Strudley says.

“Those thresholds vary all over the [San Francisco] Bay Area depending where you are,” adds Brian Collins, a civil engineer for the US Geological Survey. He says the scientists used past rainfall data to map regional thresholds— that is set rates of rain— that trigger an alarm when the land is wet enough to slide.

“So what that means,” Collins says, ”is when you look at those maps you can say OK, if this area gets this much rainfall in six hours, is it above or below the threshold that’s shown on the map?”

But those thresholds are not enough. So in the San Francisco Bay Area, the USGS also started monitoring new variables that show how storms have changed the soil deep beneath the surface. While they sharpen landslide prediction, it may never be perfect. 

Mark Strudley says the challenge is scale.“It’s gonna be a very long time, and maybe never, that we'll be able to pinpoint exactly when and where something like this would happen,” he says.

Researchers would have to know how much rain is falling on every hillslope at every moment, and how much water each slope can take. Two things that scientists may never determine at fine-enough scales to make a perfect prediction. So for all our advances since Love Creek, we still might not see the next one coming.