• Massive landslides are part of our past

  • After the tragic landslide in Washington, I've had people ask whether it can happen in our valley. The short answer is yes "» but it depends.
    • email print
  • After the tragic landslide in Washington, I've had people ask whether it can happen in our valley. The short answer is yes "¦ but it depends.
    Landslides take many forms (such as rock falls, slumps, flows and slope creep). Common characteristics are over-steepened slopes, weak rock or soil and abundant subsurface water. Other factors are whether the bedrock is inclined in the same direction as the slope and vegetative cover. Recognizing these factors allows scientists to identify areas of potential failure.
    The Stillaguamish River in Washington cut into its bank below a steep cliff. That over-steepened the slope and removed the natural retaining wall ("toe") of an earlier landslide. That, combined with evidence of earlier landslides in the same area and the fact the town was built on a low-lying river terrace opposite the steep slope created a clear recipe for disaster.
    Bedrock type determines the type of landslide. For example, deep bedrock landslides occur in weak rock and soil derived from clay-rich volcanic rocks, such as those along the northeast side of the Bear Creek Valley. Huge landslides occurred in the prehistoric past when the climate was wetter. One cascaded from near Grizzly Peak east of Ashland as a mass of broken rock and clay. It tumbled more than four miles to what is now Eagle Mill road near Ashland. The Port of Entry Station on I-5 is located near the end of the landslide, terrain largely without trees.
    Another huge landslide broke from the cliffs north of Dead Indian Memorial Road and surged more than five miles down Walker Creek. Will such huge failures occur in the near future? Not unless we have abnormally high rainfall far beyond historic records.
    Recognition of deep bedrock slides is vital for public safety. Earth scientists look for features on slopes such as headwalls (scoop-like arcuate failure surfaces), step-like rotated blocks, bumpy, rolling terrain, and seasonal springs, especially at the toe (end) of the slide. After failure occurs, the headwalls are over-steepened. Curved "tension fractures" progressively open behind the headwall, indicating where the next slide will start. Deep bedrock slides and the broken debris within them will continue to move until they reach an "angle of repose" wherein the slope is low enough to become stable "¦ until a road, housing pad or other feature is cut into the slope or where groundwater flow increases.
    Very different rocks occur south of Talent where granitic rocks of the Mount Ashland pluton underlie the terrain. Such rocks fail in a very different manner. Rock falls (seen along the Mount Ashland road where the slopes are steep and soil is absent) and "debris torrents" in drainages typify failure in granitic rocks. Moreover, granitic rocks are conducive to granular decay, breaking into sand- and gravel-sized material that collects in low areas. Debris torrents occur in drainages where the shallow groundwater table mimics the topography. Groundwater and surface water concentrate in stream drainages, filling the spaces between grains and reducing friction that holds the slopes in place. When sufficient water pressure builds up, debris collected in the stream channels is freed like laxative releases "¦ well, you know what.
    The resulting failure follows the narrow stream channel, taking out everything until it stops. Those stoppages are prime areas for the next debris torrent. Such debris torrents occur every year rainfall and snowmelt exceed the norm, such as in 1996.
    The Forest Service has maps of such failures in the Ashland watershed. Debris torrents can also occur in the volcanic terrain northeast of the valley, as were seen in the 1996 storm event, where rock debris collects in shallow drainages.
    Water is the common factor in all landslides: water reduces frictional forces holding grains together and increases the weight of the slope allowing gravity to do its thing. Although many landslides occur during or at the tail end of intense rainstorms, failure can occur weeks or months later as moisture slowly percolates and weakens rock and soil. Such failures commonly occur near the coast, moving large masses of unstable rock and soil toward the ocean.
    Vegetation loss, such as after a fire, can increase the chances of soil removal during intense rainstorms and lead to landslides. Water, normally taken up by the roots of plants and transpired, seeps into the soil and rock. Landslide-prone areas can be recognized in forested terrain where trees can't become established before the slide moves again. Think of snow avalanche chutes, which mimic landslides.
    Common remediation practices include controlling drainage (subsurface drains or diverting water from an unstable area), choosing vegetation that doesn't require much watering (don't over-water lawns on steep slopes), building retaining walls and monitoring rainfall accumulation.
    Just because a home is on a hillside doesn't mean it is imminent danger. Yet people generally are unaware of whether they might be living on or below landslides or if those landslides could become active. How do you know if you live in an unstable area? Recognizing some of the features mentioned here can be instructive. Some general help is available from Oregon's Department of Oil, Gas and Mineral Industries in the form of the recent geologic map of the Bear Creek Valley (2011) or the Land Use Geology of Jackson County (1977). Local geotechnical consulting firms are available for a price. Forewarned is forearmed.
    Jad D'Allura is emeritus professor of the former Southern Oregon University Geology Department. Reach him at rockit@dishmail.net.
Reader Reaction
      • calendar