Two landslides on clear-cuts logged by Oregon State University's College of Forestry set into motion last week's torrent of mud and debris that inundated homes and U.S. 30 west of Clatskanie.
The slides from the 2,440-acre tract owned and managed by the College of Forestry were the first steps in a dominolike chain of events that sent thousands of truckloads' worth of mud and debris roaring downhill. Nobody was injured, but buildings and vehicles were heavily damaged, and one person reportedly escaped from a home by crawling through mud and out a window.
The collapses on OSU's land put the state's top forestry institution face to face with long-standing concerns that logging can increase the risk of landslides, especially on steep slopes.
College officials had notified state foresters before logging in 2004 that one area they planned to clear-cut had evidence of old slides and small failures of the slope, according to state documents. But the terrain was not steep enough, and was not thought to pose enough threat to property below, to trigger state rules restricting logging on landslide-prone slopes.
Intense rains in 1996 unleashed fatal landslides in Oregon, and studies afterward showed that landslides occur more often on recently clear-cut slopes. The Oregon Department of Forestry later adopted new logging rules intended to prevent landslides, but many conservation groups argue the rules remain far too weak.
Forest landowners have been found liable in some cases for damage caused by slides that begin on their land, said Marilyn Heiken, a Eugene attorney involved in past landslide cases.
OSU's College of Forestry will review the circumstances of the slides, said Stephen Hobbs, executive associate dean at the college who is also chairman of the Oregon Board of Forestry.
"Whenever something happens, you always try to assess the circumstances around what happened," he said.
Always at risk
But he also said the terrain involved is steep, unstable and always at risk in extreme rainstorms.
"Mother Nature threw a curveball at us," he said. "It was a pretty intense storm event, so you're going to have unexpected things happen."
The slides apparently started in two clear-cuts, one logged in 2004 and the other about 15 years ago, and probably happened during the major storm that swept across Oregon two weeks ago, according to geologists and state foresters who inspected the area.
Debris from the slides flowed downhill about a mile, where it plugged culverts draining water beneath a wood and earthen berm where railroad tracks once crossed a creek, said Bill Burns, an engineering geologist with the Oregon Department of Geology and Mineral Industries.
That turned the berm into a dam and set the stage for catastrophe. Water and mud backed up behind the berm for as much as a week -- collecting until it was as much as 40 feet deep -- and finally burst on Dec. 11, loosing a flood of mud and debris into the small community of Woodson, about half a mile to the north.
The chain of events involved multiple mechanisms known to contribute to damaging slides, Burns said: rain-soaked ground collapsing, runoff pooling behind a dam, and accelerating erosion when the dam burst and everything rushed out.
"It's quite a combination of ways these landslides initiate and grow," said Burns, who is piecing together a step-by-step account of what happened.
He said the initial landslides, on their own, might not have traveled far enough downhill to reach Woodson and U.S. 30. The berm, by collecting extra water and debris, and unleashing it all at once, multiplied the volume, force and destruction of the slide, he said.
The College of Forestry's land where the initial slides began is known as the Blodgett Tract because the Blodgett Timber Co. donated it to the college in 1929 for research into reforestation. It's now managed for intensive timber production that generates revenue for the college, according to OSU's plan for the acreage.
The college has invested more than $2.8 million since 1996 taking actions such as upgrading culverts on its lands to provide drainage and prevent landslides, said Dave Lysne, director of the college's forests. But he said the Blodgett Tract appeared to endure more rain in this month's storm than even in the 1996 storm that caused slides around the state.
"The ground is really steep up there," he said. "It's like a lot of the ground up in that country."
When heavy rains soak the ground, it can liquefy and flow as a landslide. But Lysne said there was no sign -- such as tilted trees -- that the ground was turning unstable.
"I cannot find any particular reason for this slide occurring where it did," he said.
Documents the College of Forestry filed with the state before the most recent clear-cutting noted signs of past slides in the same area where one of the landslides began last week. But the records also estimate the slope at 36 degrees to 65 degrees, which in most cases is not steep enough to bring state logging limits into play.
The limits generally apply on steep slopes when homes or other property below is close enough to be damaged by a slide. But state officials consider the risk of landslides under normal conditions, not extreme storms, said Malcolm Hiatt of the Oregon Department of Forestry who reviewed the OSU project at the time.
"We look at normal weather events, not catastrophic weather events," he said. "This was certainly a catastrophic event."
No new roads
The logging was well-planned and did not involve new roads, which can exacerbate the risk of a slide, he said. Both clear-cuts were replanted as state law requires.
Research in Oregon's Coast Range shows that clear-cutting increases the risk of landslides, especially in the first decade after logging. Logging kills tree roots that help hold slopes together, and removes the forest canopy that catches rain and slows how fast it reaches the ground, researchers have found.
That's important, because a rapid influx of water during storms soaks the ground faster than it can drain, leading to slides, said Arne Skaugset, a professor of forest engineering at OSU.
Skaugset served on the panel that wrote the management plan for the Blodgett Tract in 1999, and he recalled no signs of past landslides there. The larger concerns at the time were areas where roads had been built across streams without drainage.