Landslides in the Pacific Northwest
Introduction
Landslide is a general term which encompasses all types of downward movement of material. Landslides usually start on steep slopes and move downward by the force of gravity, accelerating to speeds of up to 35 miles per hour.
Landslides occur in every state of our country and are annually responsible for an estimated 25 to 50 deaths and $1 to $2 billion in property damage (FEMA 1995). Researchers suggest that approximately 40 percent of the United States' population is either directly or indirectly affected by landslide events.
What Are the Characteristics of a Landslide?
The Pacific Northwest is particularly susceptible to landslides due to the significant amount of precipitation and the earthquake potential of the Cascadia region. These two natural agents act as triggering events for landslides.
The principal natural factors, which play a role in landslide potential, are topography, geology, and precipitation. Areas with steep slopes are more susceptible to landslides than flat areas. In general the more precipitation an area experiences the greater the potential for landslides.
The type of material that is most prone to landslides, especially in the Pacific Northwest, is a loose type of sediment composed of silt, sand, gravel, and cobble stones called colluvium. This type of material is characteristic of areas with high precipitation and steep slopes.
Anthropogenic (human) factors also influence landslide potential. The major human induced factors are mining, forestry practices, construction of highways, buildings, and railroads.
Are There Various Types of Landslides?
There are several different types of landslides; some of these are mudslides, debris flows, rock falls, debris avalanches, debris slides, and earth flows. Landslides also can be grouped by the way in which they move. Landslide movement can be divided into four different types:
- Slides: Slides are characterized by the downward movement of material along one or more failure surfaces.
- Flows: Flows are similar to slides but differ in the fact that they are characterized by high water content and move similar to fluids.
- Lateral spreads: Lateral spreads are usually associated with loose, sandy soils with high liquefaction potential. Lateral spreads can occur on even very gentle slopes.
- Falls and topples: Falls and topples are movements in which masses of rock or other material fall from cliffs or other steep slopes. Earthquakes commonly trigger this final type of movement.
Source: Federal Emergency Management Agency (FEMA).1995. Multi-hazard Identification and Risk Assessment: A Cornerstone of the National Mitigation Strategy. Washington, D.C.,