CORVALLIS, Ore. – The massive rainstorm that soaked the Pacific Northwest in early December caused major flooding, more than a dozen deaths and once again raised questions about the cause of the numerous, sometimes very destructive landslides which follow such events – including one significant slide on Oregon State University property.
The landslide answer, according to OSU experts, is primarily that this is a region with unstable geology and torrential rains that hit often enough to make landslides a way of life – more so than in most other mountainous regions. And in the early years after timber harvest, they say, there indeed may be a link to logging practices, although that link should not be overstated.
“Most people who have not studied this issue assume that any landslide that occurs in a harvested unit is caused by logging, and that’s all there is to it,” said Arne Skaugset, an associate professor of forest engineering at Oregon State University. “This is not true. It is not nearly that simple.”
For Skaugset – who has spent much of his career studying Coast Range forestry issues and did his doctoral thesis on landslides in the area – the issues are anything but simple. A careful review of many studies shows that logging does play a role, but it is small compared to the naturally unstable features of this region.
For instance, in an average square mile of Coast Range forests, scientists can identify about 259 “headwalls,” or areas of deposited sediments that are the result of a past landslide and other sediments that fill in behind it. Landslides will occur again in these same places, usually as a result of major storms such as the one that occurred this month and in 1996.
The problem, experts say, began millions of years ago when what is now the Coast Range was the ocean floor – mostly sandstone with inter-bedded siltstone and mudstone. If you fast-forward millions of years, those ocean floor sediments have been uplifted by geologic forces to form steep Coast Range mountains that are typified by soils of varying depth, sometimes as shallow as only a few feet deep. Water soaks through the soils, hits a less permeable layer and runs along it until it surfaces, forming natural seeps where water accumulates. Then – given the steepness of the land – the heavy, saturated soil slides.
“Erosion has always happened in the Coast Range, and landslides are the primary cause of it,” Skaugset said. “Most of the time, it has nothing to do with land management.”
Sometimes, however, there is a connection, he said. Poor road construction or drainage techniques – much more of a problem in the past than with modern engineering standards – can help lead to landslides. And some good studies on the relationship between landslides and clearcut logging were done following the major storms of 1996.
Research by the Oregon Department of Forestry on landslides resulting from the major 1996 storm found that logging caused about a 40 percent increase in landslides during the first 10 years, if there was a major storm event during that time. But even then, the range of variability was huge – some recently logged areas actually had fewer landslides while others had significantly more.
In established, previously harvested forests, there was no discernible connection between logging and landslides, and in some cases there actually was a decrease in the incidence of landslides. In older, un-harvested forests, an increase in the incidence of landslides was found in some cases.
“We can’t really be sure what the causative mechanism of this is, but there are two dominant hypotheses,” Skaugset said. “One is that there is a temporary loss of root strength in the early years after logging as tree roots decay. Another idea is that the intensity of rainfall can be moderated by a forest canopy.”
But considering the number of historic landslides in the Coast Range, Skaugset said, the numbers are compelling. Results from the same ODF study showed that of the theoretical 259 headwalls – mostly previous landslides – on every square mile of land, about 240 are “unconditionally stable,” meaning they will not fail during almost any storm event. Another 14 will almost certainly fail, no matter whether the land is managed or not. And five of the original 259 historic landslide areas may fail in the first 10 years after logging, if there is a major storm event during that time.
“We find that about three-quarters of the landslides will occur whether logging occurred or not,” Skaugset said. “And some of the remaining headwalls will have landslides that could be linked to a combination of logging and major storm events. But there’s no way to tell which areas will slide and which will not. They look identical.”
Even under the “no harvest” scenario, Skaugset said, many landslides would still occur naturally, and there are no guarantees. In past years, university officials say, there have been numerous landslides on university-owned forest lands, including one of the largest in an old-growth forest reserve that was not logged.
Among other countless landslides this month in the Pacific Northwest was one on lands owned by OSU. That slide potentially helped to trigger a large road fill failure, creating a flood of water, soil and woody debris onto U.S. Hwy. 30 near Clatskanie, Ore. Most of the OSU land that slid in this instance, officials say, had been logged 15 years ago, was on a ridgetop with no apparent past history of instability, and the land, roads and culverts were being managed in state-of-the-art forestry techniques. The primary OSU slide had a young forest on it that was beyond the 0-10 year age class, the age class that studies suggest are more vulnerable to a slide.
“Landslides in the Pacific Northwest Coast Range are natural and common, mostly due to the geology and climate,” Skaugset said. “They have always occurred and always will.”