Mass Wasting

Most people would not consider an inch of rain to be much of a problem, but for the residents of Durango, Colorado last week, just over an inch of rain fell and became a serious problem. This summers wildfires near Durango left the hillsides bare of cover, the soil exposed, and primed for disaster. On Saturday, September 7, 2002, 1.14 inches of rain fell around Durango. As the rain fell, it carried large boulders, mud, and fallen trees down the slopes. The slides destroyed houses ranging from mobile homes to multi-million dollar homes, and closed an 8-mile section of County Road 250. This disaster may be the direct result of mountain slopes left bare after this summer's fires, but landslides like this one are common in every state and territory in the US and Canada, and around the world. The US Geological Survey estimates that $1 to 2 billion dollars of damager and over 25 lives are lost every year in the United States due to landslides and other mass wasting events.

Landslides and other mass wasting events are classified on several factors. The first area of classification is the type of material that makes up the mass wasting event. A mass can be composed of bedrock (as in a rockslide), or more commonly they are composed of various amounts of debris. The term debris is a catchall term and refers to all types of unconsolidated material from rock, soils, and even trees.

The types of mass wasting movement can be categorized as one of three types. Flow is a term used to describe a mass of debris moving downslope as a viscous fluid. Slip refers to a mass that descends a slope relatively coherently, moving along a well-defined surface. There are two types of slips that can be encountered. The first is a slide, where the debris mass moves along a plane parallel to the slope of the surface. Slumps involve the movement of the debris mass along a curved surface. The last type of movement is a fall. Falls generally occur on vertical or near vertical surfaces and are usually rock debris that bounce or fall down a cliff face.

There are many factors that control mass wasting events. These factors are often unique for each location, but there are some factors that always influence mass wasting events. Slope affects how often a mass wasting event might occur. Areas of steep slopes, like mountains, are out of equilibrium with respect to gravity, and mass wasting and other erosional factors work to bring these slopes into equilibrium. Areas with steep slopes tend to have more mass wasting events, while areas of shallow slopes have fewer mass wasting events.

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