New technology helps predict landslides

The laser system can identify areas that are prone to damage

By John Denny

The Clackamas Review, Jan 27, 2009, Updated Oct 30, 2009

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The Newell Creek Apartments in 1996; the home in the west hills of Portland in October; the Lake Oswego house a month ago — for an area where homes are built on steep slopes and rain doesn’t stop falling for half the year, landslides such as these are more or less an inevitability.

But officials are now working on substantially more accurate maps than they’ve had before to determine not only levels of risk for a given area, but types of probable landslides in hopes of preventing future development in more prone areas from sliding and allow current landowners to understand their risk.

Previously, in cities like Oregon City, developers were often required to have a geotechnical study done by an expert to determine the landslide threat. But even the most practiced experts are essentially making best guesses based on topography, dirt samples and their own surface observations.

Now, starting with Oregon City, the state is trying to improve that process using LIDAR, what James Roddey described as an airplane-mounted laser imaging system, to produce susceptibility maps.

Roddey is the earth sciences information officer with the state Department of Geology and Mineral Industries (DOGAMI). He said that using computer software, they can strip away buildings and vegetation from the LIDAR maps to see what the earth actually looks like underneath.

“A geotechnical engineer is going to be able to take this data and make a much better determination on an area,” Roddey said. “Before, they (could) go out and look at something, but if it’s all covered in blackberry bushes and trees, your guess is as good as ours. Obviously they’re a little bit better at guessing … LIDAR’s able to strip all that away and you’re able to look at bare earth and make much more critical and accurate determinations on slope stability and things like that.”

The state has maintained landslide maps for years. But with LIDAR, those maps are becoming exponentially more accurate.

“For landslides, we’re seeing literally, in a quadrant, thousands of landslides that we’ve never seen before,” Roddey said. “This is like spy satellites; you can see a pack of cigarettes (from the plane). It’s accurate in elevation to within a couple of centimeters. We’re gathering data that’s more accurate than anywhere in the country right now.”

How will this help?

Roddey said landslides will never be predictable in the same way floods and hurricanes are.

“Where the water’s going to flow is pretty obvious on a bare earth model,” Roddey said. “How much water it takes to get things moving is a much more complicated calculation, and that’s where susceptibility maps” come in. “A susceptibility map can’t say, if it rains six inches, your slope is going to fail. It can only say, here is a heightened probability area that, given perfect conditions for landslides, this slope might fail.”

The first round of maps, which cover Oregon City, also show levels of susceptibility for various types of landslides — shallow slides, deep-seated slides, debris flows, rock falls and more.

“For the local government, they’ll be able to take this information, and (for) any new developments they’ll have this in hand and be able to work with developers to mitigate the risks, if they even let people build in some of these areas,” he said.

The maps may even show areas that shouldn’t be developed. They could see a spot where, “you dig into the toe, the bottom of the landslide, you may actually reactivate the landslide because that may be holding it in place,” Roddey said.

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