We hear a lot about landslides in Oregon, especially in the rainy season and most especially in western Oregon where steep slopes and wet, water saturated soils mix together to make the earth move.
What can you do about landslides before and during an intense storm? Click here for some helpful tips.
Geologists define landslides as the downslope movement of rock, soil, or related debris. Geologists use the term “mass movement” to describe a great variety of different landslides, such as rock fall, creep, slump, mudflow, earth flow, debris flow, and debris avalanche, but in most mass movement events, water plays a pivotal role by assisting in the loosening and lubricating of rock and soil surfaces to help begin the earth moving. Different types of landslides are composed of different materials, such as rock, sand, clay, and water and the proportions of these materials will dictate how fast a landslide moves and how much area it will cover.To learn more about landslides in Oregon, read our Landslide Fact Sheet.
To learn more about debris flows, a dangerous type of fast moving, destructive landslide, click here.
Although landslides are propelled by gravity, they can be triggered by other natural geologic events or human activity. Volcanic eruptions and earthquakes can initiate earth movement on a grand scale. A variety of debris flows called “lahars”—a mixture of volcanic ash and water— are specific to volcanic activity and are often the major hazard experienced in a volcanic episode. Although earthquakes can initiate debris flows, the major causes of landslides in the northwest are continuous rains that saturate soils.
In December of 2007, a powerful debris flow roared down a steep drainage near the town of Clatskanie, destroying home and businesses and closing Highway 30. Oregon Department of Geology landslide specialist Bill Burns visited the site and put together this presentation:
Landslides can also be the direct consequence of human activity. Seemingly insignificant modifications of surface flow and drainage may induce landslides. In an urban setting, improper drainage is most often the factor when a landslide occurs.
Many unstable, landslide prone areas can be recognized. Tip-offs include scarps, tilted and bent (“gun-stocked”) trees, wetlands and standing water, irregular and hummocky ground topography, and oversteepened slopes with a thick soil cover. The technology of spotting landslides by use of aerial photography and new laser based terrain mapping called lidar (Light Detection and Ranging) is helping the Oregon Department of Geology develop much more accurate and detailed maps of areas with existing landslides and we are now able to create landslide susceptibility maps, that is, maps that that show where we think different types of landslides may occur in the future. You can learn more about mapping landslides with lidar here.
In 1998, following the deadly landslides that occurred during the storms of 1996, a task force formed by the State Legislature to look at the problem of landslides in Oregon, presented its results. You can read the report here from the Joint Interim Task Force On Landslides and Public Safety.
For additional background, you can read the Report to the Seventy-first Legislative Assembly on the Implementation of 1999 Senate Bill 12 Relating to Public Safety and Rapidly Moving Landslides
In 2006 and 2007, the Oregon Department of Geology hosted two conferences in Portland to discuss the status of landslide mitigation and preparation in the state:
Oregon Department of Geology and Mineral Industries
800 NE Oregon Street, Suite 965, Portland, OR 97232-2162
(971) 673-1555, FAX (971) 673-1562
email us at DOGAMI