The good news about landslides: A new, first-of-its kind state study found 38,000 fewer Pierce County properties are thought imperiled than before.
The bad news, however, is as palpable as the downed trees and shiny mounds of gray clay still along the Kopachuck State Park beach from a bad slide earlier this year.
More than 80,000 buildings in the county, and about a quarter of the road network, are confirmed as vulnerable to landslides. The total dollar value of property with a moderate or higher risk of landslide tops $10 billion.
The study is the beginning of a statewide attempt to pinpoint potential landslide threats in the wake of the sudden 2014 Oso mudslide that killed 43. Pierce County got to go first because officials already had compiled high-quality data, state officials said.
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In Gig Harbor, Jackie Murphy was surprised earlier this year by the landslide on the bluffs south of Kopachuck Park. It began as a 2-inch crack creeping across her neighbor’s yard. Within days, about a foot of land was slipping away each day.
Now that neighbor has a 30-foot dropoff so close to his porch that the house is being moved, and the Murphys’ downhill route to Puget Sound waterfront is impossibly steep.
“It’s kind of unsettling at times,” said Murphy, whose family has lived in the same spot for decades. “... Nobody had any idea this was considered a landslide area.”
The study, which used the county’s data from a laser surveying tool called lidar, is intended to prevent such surprises.
State and county official said the findings work two ways: by eliminating false positives, thousands of parcels in the county can be built on without requiring expensive geotechnical analyses, if the county adopts the new map. And its precision in identifying previously unknown landslide hazards will help people and officials protect dangerous zones.
The best news from the study is that nowhere in Pierce County has the same mass-disaster vulnerability that enabled the Oso landslide, officials said.
“The geology is very different in the Oso area,” said Stephen Slaughter, the state Department of Natural Resources landslide hazard program coordinator.
Although Pierce County terrain varies from Cascade Range mountains to the lowlands around the Sound, the county’s main landslide vulnerability comes from having most of its population living along the river-sliced, glacier-deposited soils of the western half of the county.
At particular risk, the study says, are high waterfront bluffs such as the Murphys live on. There, sand and gravel sit atop thicker silt and clay. Add water to the mix and the pressure can move things around.
In the eastern part of the county, the landslide risk has a different basis, the study notes: much of the topsoil lies atop an arrangement of volcanic and other rocks aren’t packed tightly enough to always stay in place.
The study analyzed nearly 1,100 square miles of land in the county, including cities, but cut out Mount Rainier National Park.
It identified three areas of potential risk: known past landslides, areas susceptible to shallow landslides — think rocks, trees, mud and surface structures all moving downhill fast — and areas susceptible to deep underground slides, which might first be noticed with a crack in a building’s foundation.
“It’s not a life safety issue,” Slaughter said of deep landslides. “It’s more a property issue.”
The county had previously cobbled together its landslide maps from an array of other sources, none as comprehensive or precise.
“The DNR has done the county a pretty big favor by doing this mapping,” said Mitch Brells, Pierce County’s development engineering manager.
Homeowners and the rest of the public will be able to use the new maps to pinpoint the county’s risk factors later this year on the DNR site, state officials said.
Agency officials made the study’s mapping data available on its geology blog, but turning that into a map requires sophisticated mapping software that most of the public doesn’t use.
Its precision trims the areas where the county had feared potential landslides considerably. It showed 121 known shoreline landslide areas, 71 fewer than previously thought. And it cuts the amount of known landslide hazard and susceptibility area by 51 percent from what the county’s official hazard maps currently show.
The analysis found only one important public facility sits within the area of a known landslide: a slope behind a sports field area at Eatonville High School.
“If the school wants to build something out there, they now have a map that would guide them,” said Kate Mickelson, a DNR senior landslide hazard geologist.