Mudslides in Snohomish County are so common, especially along the rail route between Everett and Seattle, that we give little thought to what that term really means. Turns out it’s somewhat of a misnomer, at least as far as scientists are concerned.
The U.S. Geological Survey is calling the Oso event a debris flow, “also commonly referred to as a mudslide or mud flow.”
The USGS has an entire section of its website devoted to resources and facts about landslides at landslides.usgs.gov. An exploration of the mountain of texts there finds many definitions for a landslide, depending on the physics and material involved.
While we await a professional, thorough on-site assessment by geologists, here are some key USGS definitions:
Debris flow: “A form of rapid mass movement in which loose soil, rock and sometimes organic matter combine with water to form a slurry that flows downslope.”
Debris avalanche: “Essentially a large, extremely rapid, often open-slope flow formed when an unstable slope collapses and the resulting fragmented debris is rapidly transported away from the slope.”
Rotational landslide: “A landslide on which the surface of rupture is curved upward (spoon-shaped) and the slide movement is more or less rotational about an axis that is parallel to the contour of the slope. The displaced mass may, under certain circumstances, move as a relatively coherent mass along the rupture surface with little internal deformation. The head of the displaced material may move almost vertically downward, and the upper surface of the displaced material may tilt backwards toward the scarp.”
Mudslide: “An imprecise but popular term coined in California, frequently used by laymen and the news media to describe a wide scope of events, ranging from debris-laden floods to landslides. Not technically correct.”
Chuck Taylor: 425-339-3429; ctaylor@heraldnet.com.
Talk to us
> Give us your news tips.
> Send us a letter to the editor.
> More Herald contact information.
