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Steep slopes and rain make landslides common in Coast Range
12/13/2007
 
Contact: Contact: Kevin Weeks, 503-689-6879 or 503-325-5451
 
What caused this week’s landslide on Highway 30? In the Oregon Coast Range, there’s a fairly simple recipe: Landslides occur in steep terrain – often after heavy rainfall.
 
Soil becomes increasingly heavy and lubricated by the rain, and gravity pulls it down. An Oregon Department of Forestry (ODF) study in 1996 on storm impacts and landslides found that slopes of 70 percent to 80 percent steepness posed the highest hazard for shallow, rapid landslides. These slides are of most concern because of the safety risks they may pose.
 
Flood-swollen streams can add to the probability by acting as conveyor belts of energy that triggers landslides. Streams undercut and erode, increasing slope steepness.
 
The 1996 ODF study also looked at other factors, such as the age of stands, that could contribute to landslides. It found that both younger stands, ages zero to 9 years, and stands older than 100 years appeared to have more landslides than stands 10 to 100 years old.
 
In the case of the debris flow on Highway 30 near Clatskanie, all these elements – steep slope, heavy rain, flooded stream and possibly previous harvesting – came into play.   However, a major factor was the legacy of an old historic railroad fill. 
 
Two slides occurred earlier in the week in two draws that feed into a larger creek. One slide came from a 15-year-old stand of trees previously clear-cut, and the other from a five-year-old stand. Both stands were reforested as required by Oregon’s Forest Practices Act.
 
It’s unclear whether the age of the stands was a significant factor in these two earlier slides. The volume of rain – about a foot in 48 hours that flooded the residences days before the recent slide occurred – could have been enough to trigger slides in any age of stand.
 
The material from these two slides blocked the drainage underneath the old railroad fill,  resulting in a pond nearly 40 feet deep.  The fill material eventually became saturated and could not withstand the pressure of the water in the pond. This kind of failure is technically called a "dam-break flood."
 
“We don’t know if the previous harvesting affected the stability of the two initial sites,” said Jason Hinkle, an ODF geotechnical specialist investigating the slide. “There are a lot of unknowns. The whole hill is one big slide that’s been happening for eons – things are going to move with or without timber harvesting.”
 
Without the railroad fill, it is likely the debris and water from the earlier slides would not have had such a large impact.
  
The landowner, the family-owned Evenson Timberland Agency, worked with its own crews and a contract excavator to relieve pressure by pumping out water from behind the fill. Eric Evenson also warned downstream landowners and ODF, which brought in a geotechnical expert, and other agencies.
 
Despite those efforts, the graded area collapsed and released the water and debris in the dam-break flood that moved at about 30 to 45 mph to engulf the evacuated residences and cross the highway.
 
However, the landowner’s vigilance and the cooperative work with other agencies, including the Oregon Department of Transportation and local sheriff and other authorities, allowed for closure of the highway and evacuation of residences before the slide occurred.
 
“The landowner was very much heads-up,” ODF spokesman Rod Nichols said. “He notified the agencies, and they worked together, and lives may well have been saved.”
 
Landslides and floods are important factors in the geologic development of the Coast Range.  Steep slopes and rain are the major, but not the only factors, in these events. 
 
During heavy rainfall, homeowners and motorists should pay attention, especially if there are confined channels that can direct a debris flow at a house or road, and avoid being in those locations. 
 
Forestry Department staff continue to assess other areas along Highway 30 to determine the potential for more slides.
 
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