Why'd the Mudslide Happen? Maybe the Wrong Kind of Logging.
By Ken Armstrong, Justin Mayo and Mike Baker
State regulators have been using outdated boundaries to restrict logging above the Snohomish County slope that collapsed March 22, failing to incorporate newer research that would have protected a swath of land that wound up being clear-cut, according to a Seattle Times analysis of documents and geographical data.
Because trees intercept and absorb water, removing them can contribute to the risk or size of a landslide by increasing the soil's saturation, according to geological reports. The impact can linger for years.
In 1997, a report commissioned by the state Department of Ecology used "newly developed computational tools" to map the plateau atop the unstable hill outside Oso. That report was prepared by geologist Daniel J. Miller and hydrologist Joan Sias; Miller's portion drew boundaries for where groundwater could feed into the slope and increase the risks of landslide.
When the Department of Natural Resources (DNR) issued logging restrictions later that same year, the agency cited the Miller-Sias report and treated it as state of the art, saying any future study should emulate its methods. But instead of adopting Miller's map, DNR used boundaries that had been drawn up in 1988.
"We did the work. It was cited in the prescriptions as what you should do. And it appears from your comparison of the maps that it didn't get done," Miller told The Seattle Times on Sunday. "I suspect it just got lost in the shuffle somewhere."
Had Miller's map been used for the plateau, called the Whitman Bench, an additional 12 acres to the west would have been placed under protection, according to a Seattle Times analysis.
In 2004, DNR approved the clear-cutting of 7 acres on the plateau _ about 5 of which would have been protected under Miller's boundaries.
Grandy Lake Forest, the owner of that property, finished harvesting the acreage by August 2005. The 7 acres took the shape of a pizza slice _ with its tip just touching the part of the slope that fell away this month, releasing millions of cubic yards of sand, silt and clay.
The Seattle Times previously reported the actual harvest appeared to extend past the permitted area, with about an acre cut in restricted land.
The hill that collapsed had a long history of slides, including ones in 1949, 1951, 1967 and 1988. Geologists studying the slope have cited multiple factors that could have contributed to the slides, from excessive precipitation to erosion of the hill's base by the Stillaguamish River to logging.
DNR issued its logging restrictions in 1997 as part of what's called a watershed analysis. Even then, the geologist who authored the section on the potential link between logging and landslides expressed doubts about using the boundaries from 1988.
Gerald Thorsen, who died five years ago, wrote that he considered it beyond his work's scope to "second guess" the 1988 study, but added: "However, I feel that there is enough additional information available now (since the 1988 study), and enough unanswered questions, to warrant a new look at this slide and its recharge area."
The "recharge area" refers to the part of the plateau at risk of sending groundwater into the slope.
DNR spokeswoman Diana Lofflin emailed a statement to The Times, saying: "The 1997 Watershed Analysis is a historical document and (it) will take some time to understand what data was used in the analysis and the underlying assumptions. This is further complicated by the fact that the report was prepared 17 years ago under a previous administration with different staff.
"Please be patient as we review the information and complete an investigation, and bear in mind that we are focused on the recovery efforts at this time."
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