WOODSON - Expert reviews of the December landslide that inundated U.S. Highway 30 and homes west of Clatskanie found that the chain of events started on slopes clear-cut by Oregon State University's College of Forestry but no evidence the logging caused the collapse.
Instead, the reviewers said extremely heavy rainfall reactivated ancient, deep rifts in the ground that existed prior to the logging and had slid long ago.
The reviews highlighted weaknesses in Oregon planning that leave homes in danger zones such as the area west of Clatskanie, where landslides have struck before and probably will again.
A separate administrative review by the Oregon Department of Forestry, almost finished, has found that when reviewing the OSU logging the state should have better recognized the history of landslides in the area and the homes in danger below.
"Clearly we didn't capture that - our tools weren't strong enough," said Mike Cafferata, policy unit manager at the Department of Forestry.
Even if logging did not contribute to the slides west of Clatskanie, state foresters might have identified other factors that raised the risk in different ways, he said.
For instance, an old railroad crossing that blocked a stream multiplied the force and damage of the landslide that hit U.S. 30. Water and debris from the OSU land plugged a culvert under the crossing, allowing water, mud and logs to back up in a small lake that finally broke loose and deluged the community of Woodson.
An independent geologist who examined the slide for the Department of Forestry recommended that the state identify other embankments similar to the railroad crossing to avoid pileups of water and debris during severe storms.
"If that embankment had not been there, the magnitude of the event would have been much smaller," said Gunnar Schlieder, an engineering geologist with Geoscience Inc. in Eugene.
He said Oregon must do more to keep people from living in areas in danger of severe landslides. The state launched an initiative to reduce landslide safety risks after people were killed by slides in 1996, but some elements of the plan were never carried out.
For example, maps of landslide hazard areas mandated by the Legislature were never officially disseminated.
Schlieder said there is a misperception that landslides can be avoided merely by controlling logging and other activities on the slopes where they start, when sometimes it's more effective to keep homes out of the danger zone down below.
In the case of Woodson, there is a long history of landslides barreling down the same drainage where the December slide hit, highlighting the danger. But some locals remained unaware of the risk around them.
"We have this problem where some of the houses at the bottom were rentals, and you probably have people there who were clueless as to what happened in the past," he said.
State officials have given the U.S. 30 corridor a high priority for new laser-based mapping that can better identify landslide-prone areas, but the initiative still needs funding, said Ted Lorenson, assistant state forester.
Schlieder concluded that OSU logging, in 1992 and 2004, of the slopes where the initial slides started was not a major factor in triggering the slides. Rainfall that approached or set records in the area loosened deep sections of earth that had already slid long ago.
"We just had more rain than the slope could handle," he said.
He also said the logged areas had endured other heavy rains without sliding, suggesting that the logging was not to blame. Evidence of the ancient slides would have been visible only to a trained eye, he said.
Clear-cutting has been found to increase the risk of rapidly moving landslides, also called debris flows, and the state enacted rules to limit logging on slide-prone slopes. The OSU land above Woodson was not steep enough to trigger those rules.
Another examination by Department of Forestry geotechnical experts did not say so clearly that logging did not contribute to the landslides. It said some research shows that logging can increase the risk of deep landslides like those above Woodson.
But it said that because the research is not clear and knowledge of the area where the slides occurred is limited, "it is not possible to state definitively the relative contribution of forest management" to the December slides.
Either way, both Schlieder and the Forestry Department team concluded that debris from the OSU land plugged the culvert intended to let water pass beneath the railroad crossing. That turned the crossing into a dam that caused water and debris to back up into a lake about 1.5 acres that finally broke loose, the Forestry Department team wrote.
The lake held enough water to fill about 10 Olympic-size swimming pools, plus logs and tree root wads carried from the logged lands.
Department of Forestry staff will present the reviews' findings to the Oregon Board of Forestry, which could then direct revisions or changes in state logging reviews to better identify the risk of slides.