OSO — Before the hillside above the Steelhead Haven neighborhood collapsed, killing 43 people, it was known as a site of periodic slides that did little more than divert the North Fork Stillaguamish River.
In the year since, scientists have been trying to understand why a slope that behaved one way for much of recorded history suddenly turned into a massive and deadly mudslide on March 22, 2014.
The main question, said Jonathan Godt, landslide coordinator for the U.S. Geological Survey, is why the slide traveled so far and so fast across the valley.
More than 10.8 million cubic yards of material, moving at about 60 mph, crossed the river, blasted through Steelhead Haven and sloshed back when it reached higher ground on the far side of the valley.
Scientists are working to learn what subsurface geology enabled that — why the slide’s runout was so fluid and deadly — but they are just getting started. There is a lot of data to study, more data to collect, and there are many theories to be tested.
Research so far
Geologists last summer mapped the debris field with help from Snohomish County officials and the state departments of Natural Resources and Transportation. They bored two test holes in the Whitman Bench behind the scarp of the slide, the core samples of which will be analyzed at the University of California at Berkeley.
The Oso slide was a “deep-seated” slide, meaning that the depth where the ground lost cohesion and slipped is well below surface soil and tree roots.
The slide exposed deep layers of earth and rock. The uphill extent of the collapse was 300 feet beyond the point of the next-oldest slide. Video shot by a Darrington resident minutes after the slide documents water pouring out of the exposed hillside.
Near-record rainfall in the weeks before the slide is the likeliest explanation for the deadly torrent of mud, according to a study published in January by USGS researchers in the journal Earth and Planetary Science Letters. They employed mathematical models based on field work to suggest that had five percent less water been in the ground, the slide would have stopped at the river, as it did in 2006.
Research still under way will build on that study, Godt said, employing data recorded by seismographs in the region, aerial photography and maps created with sophisticated airborne remote sensors.
“The ultimate goal of that is to look at those failure surfaces and the relationship of those to the seismic information that’s been reported,” Godt said.
After “what happened?” comes “where else can it happen?”
“Are there other characteristics in other valleys that might lead other slides to behave that way?” Godt said.
Finding the answer to the second question is going to take a lot of time and painstaking research.
What we don’t know
After the Oso slide, scientists turned to three-dimensional imaging technology to study the Stillaguamish valley. That technology, called light detection and ranging, or LiDAR, bounces laser beams from an aircraft off the terrain to create topographic maps.
Unlike standard cartography, the lasers are able to pierce vegetation, including tree canopies, to precisely render the surface in high resolution. Differences measured in fractions of an inch are discernible.
“We can take that and we can identify landslides pretty accurately,” said Dave Norman, the Washington state geologist.
LiDAR maps of the North Fork Stillaguamish valley, which the state Department of Transportation created, show a number of landslides surrounding Oso, some dating back thousands of years. But there are relatively few areas of the state where that kind of data is available.
Norman said that getting such data will depend in part on the Legislature approving a $6.6 million funding request by the Department of Natural Resources (DNR).
The priority is “areas where there are populations and infrastructure and known landslide risks,” Norman said.
DNR already has maps for some areas, notably Pierce and Lewis counties, parts of the Columbia River valley in Eastern Washington and the Pacific coastal areas.
The department does not have maps of many other areas, especially in the landslide-prone foothills of the Cascades in heavily populated metro Puget Sound.
That isn’t to say those maps don’t exist. County, state and federal agencies, universities and private businesses sometimes do their own mapping, to their own standards. But those maps don’t always get shared, said Joe Smillie, a spokesman for DNR.
For example, in Snohomish County, DNR only has LiDAR maps for parts of the Snohomish and Skykomish river valleys, and it only has imagery of the Stillaguamish valley because it obtained it from the Transportation Department after the Oso slide.
And having a LiDAR map of the entire state won’t be much help without expert interpretation.
Natural Resources’ $6.6 million request, which the state Senate approved in February, would pay to hire 14 people to create LiDAR images, analyze them and make them publicly available.
The department now has just five geologists dedicated to mapping and the study of natural hazards, Smillie said.
LiDAR mapping the state is just one step. Unanswered questions about the Oso slide are also applicable to other regions.
As for work on the ground, David Montgomery, a geomorphologist at the University of Washington, would like to see:
More drilling in and around the slide zone to understand the three-dimensional structure of the collapsed hillside,
More carbon-dating of other known landslides in the valley,
And more study of the groundwater and how it moves through the hillside.
Those are broad categories.
Examining ground hydrology alone might yield insight into how much rainfall is enough to trigger a slide. It might confirm or disprove popular speculation that logging on the Whitman Bench about a decade ago increased ground saturation.
Calculating risk
More research might also help determine if each previous slide changed the geology so much to make past events unreliable predictors of future ones.
“Do we need to look at reanalyzing their possibility of failure each time they fail?” Montgomery asked.
“And how many other places up and down the Cascades have similar long-runout landslides?”
Determining which unstable areas are prone to long, fluid runouts, and which are most dangerous, are questions that science can’t answer with the available data.
Yet those questions inform any assessment of risk.
How much risk are we willing to take on to live in a beautiful mountainous region?
The state of California and the nation of New Zealand have sought some answers.
California’s legislature passed a law in 1979 that allowed creation of Geologic Hazard Abatement Districts.
A GHAD is a taxing district to raise money to prevent, control or repair damage resulting from geologic events. Often, though not always, the money comes from a property tax levy on parcels within the district.
GHADs have the authority to bypass many regulations and bureaucracy to react quickly to events.
“They almost function like very efficient private entities but have the power of statewide agencies,” said Uri Eliahu, the president of the California Association of GHADs and of the geotechnical engineering firm Engeo.
GHADs can own land, can sue and can be sued. The law is broad enough that they have been created not just in landslide zones but along fault lines and in flood zones, and to address other surface-water problems.
A community in Malibu recently created a district to restore eroded beach, Eliahu said. Rather than wait years for the GHAD to collect enough money, the district borrowed against future property tax collections.
“It’s a very safe loan for a commercial entity to make,” Eliahu said.
There are more than 35 GHADs in California, with probably another 20 or so in various stages of development, Eliahu said.
In New Zealand, the Christchurch earthquake of Feb. 22, 2011, led to numerous slides and rockfalls in the city’s hilly neighborhoods, killing 185 people.
Local government took a different approach than California. Officials worked with the engineering community to develop a risk-assessment policy.
In the end, more than 1,500 homes* were deemed too dangerous to live in, said Chris Massey, an engineering geologist with GNS Science, which is New Zealand’s equivalent of the USGS.
About 600 property owners* were offered buyouts at pre-quake valuations — with the understanding that not accepting a buyout would mean the owner must build some sort of engineered mitigation before future sale. Mitigation might be so expensive that remaining in a home is cost-prohibitive, Massey said.
The whole program could only come about because scientists calculating risk had communicated closely with elected officials to develop that policy, Massey said.
“It’s up to the engineering geology technical community to articulate the risk levels to allow those policymakers to make informed decisions,” he said.
Here in Washington, Snohomish County has offered buyouts to some property owners in the Oso slide area.
A detailed parcel-by-parcel risk map could only happen if scientists can identify landslide-prone areas, said Norman, the state geologist.
Debating points
Meanwhile, scientists are still debating the Oso mudslide.
For example, seismic data collected by a regional network of monitoring stations was interpreted in the USGS report to mean there was just one, extended collapse of the slope. That’s contrary to a report issued last July by a consortium of scientists called Geotechnical Extreme Events Reconnaissance, or GEER. They deduced that there were two distinct collapses, as did a paper published in December in Natural Hazards and Earth System Sciences by three Columbia University researchers.
Montgomery, the UW geomorphologist, was on the GEER team that collected data about the Oso slide. He hopes that understanding what happened there will enable him and other scientists to assess other areas prone to landslides.
Similar slide zones will likely be found in remote areas, far from civilization. As the Oso mudslide showed, there are populated areas that also might be at risk.
“A giant landslide like this in the wilderness is a curiosity,” Montgomery said. “In a subdivision, it’s a catastrophe.”
Chris Winters: 425-374-4165; cwinters@heraldnet.com. Twitter: @Chris_At_Herald.
Correction, March 12, 2015: After the 2011 New Zealand earthquake, more than 1,500 homes were deemed too dangerous to live in and about 600 property owners were offered buyouts at pre-quake valuations, according to Chris Massey, an engineering geologist with GNS Science. The numbers were incorrect in an earlier version of this story.
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