Monday, May 7, 2012

Woodson Landslides

While going over aerial images of landslides in northwest Oregon and southwest Washington I spotted a set of landslides as I scrolled from one research/project area to another. The December 2007 storm that impacted northwest Oregon and southwest Washington caused a slide that closed US Highway 30 between Portland and Astoria. Several landslides and floods of debris and water impacted a the hamlet of Woodson, Oregon during the storm. Approximately a week later, a much larger slide and resulting debris poured down the mountainside closing the highway and destroying homes. No one was killed as there was a warning issued regarding the pending failure.

Beyond my own work on some slide damaged sites in northeast Oregon from that storm, I only knew that the slide associated with the highway closure was related to an old road via the news. The Oregon Department of Forestry (ODF) did a forensic-like investigation of the landslides and events that led up to the slides.

ODF_Rpt_07_Debris_Flow_Eilertsen_Crk_Woodson.pdf and

The studies are well detailed and contain great images of the slide sites and explains as well as possible the sequence of slide events as well as the underlying geology and history of slides in the area. Within the past year fairly good resolution images have been released showing many of the landslide areas. I marked up some of the images of the Woodson slide area.
Aerial of Woodson area showing some of the 2007 landslide locations and steep drainage

This was not the first time that slides had impacted the Woodson site. Woodson is located at the base of a steep mountain slope with steep incised drainages. And the December 2007 storm packed a lot of rain over a short time. It was an intense rainfall event and thousands of landslides took place in southwest Washington and a lesser but still significant damaging slides took place in northwest Oregon.

Slides at upper Olsen Creek above Woodson

As can be seen in the images, multiple slides took place above Woodson. Residents indicated that multiple surges of flood water came down the mountainside during the storm. In the case of Olsen Creek, there were several small shallow slides within the headwall area of the creek drainage. As the slide material flows down the drainage it picks up debris and bulks up as it heads down the creek ravine.
Source area of large slide

The really big slide that impacted Woodson took place nearly a week after the storm. Two slides took place in the headwater area: one was a shallow surface failure associated with an old deep-seated landslide, and the second was associated with a reactivated deep-seated slide. These slides took place during the storm, but the material from these slides did not reach the bottom of the hill during the storm event. The slides path down the drainage was blocked by an old forest railroad grade. The railroad had originally been constructed as a trestle across the creek, but was later filled such that it was a 60-foot high embankment with a couple of culvert pipes passing underneath. The slides plugged the culverts and the loose fill embankment tuned into a temporary dam that lasted about one week. When the embankment collapsed a slurry of mud and logs poured down to Woodson. Fortunately, this potential trouble spot was recognized before the failure took place and residences were evacuated.

Geologists will readily say that if you have steep mountain slopes that occasionally get whacked with big rain fall events there will be landslides. Beyond that landslides and how to deal with this risk moves into the policy arena and politics as well.

One of the ODF papers includes a section at the end with recommendations. One recommendation was regarding development regulations. Despite a history of debris flows at this site and other nearby sites, there were no development regulations regarding building in these obvious geologic hazardous areas. For the most part, Washington State counties recognize geology hazards such as these.

The ODF report had some other recommendations as well and discusses the dilemma of managing large tracts of steep forest land for timber harvests versus public safety. What was not mentioned in the recommendations was fish, but that may be due to these streams not being salmon bearing.

No comments: