It is “a moving mass of loose mud, sand, soil, rock, water and air that travels down a slope under the influence of gravity.” That’s the definition of a Debris Flow (more commonly called a Mudslide by the media) according to the Colorado Geological Society. Whichever name you use they have been in the news lately causing death and destruction. They list the actual definition this way “A Mud Slide (mud flow) is a mass of water and fine-grained earth that flows down a stream, ravine, canyon, arroyo or gulch.” To be considered a mudflow more than half of the particles must be sand sized or smaller. Mudflows are the sandy, more watery counterparts of debris flows.”
Rather than giving you a, pardon the expression, watered down explanation I’ll let an article from NationalGeographic.org give definition. “A landslide is the movement of rock, earth or debris down a sloped section of land. Landslides are caused by rain, earthquakes, volcanoes, or other factors that make the slope unstable.”
Geologists say there are three causes of landslides which are geology, morphology, and human activity. Here is how NationalGeographic.com explains each one. “Geology refers to characteristics of the material itself. The earth or rock might be weak or fractured, or different layers may have different strengths and stiffness. Morphology refers to the structure of the land. For example, slopes lose their vegetation to to fire or drought are more vulnerable to landslides. Vegetation holds soil in place, and without the root systems of trees, bushes, and other plants, the land is more likely to slide away. A classic morphological cause of landslides is erosion, or weakening of earth due to water.
Human activity such as agriculture and construction, can increase the risk of a landslide. Irrigation, deforestation, excavation, and water leakage are are some of the common activities that can help destabilize or weaken, a slope.”
A good reason to discuss landslides is that they have been prominent in the news lately. The death toll has risen to 20 due to the landslide in Montecito, California and 3 people were reported as still missing. Torrential rainfall caused the slide which destroyed 128 homes. The 11 debris fields cover an area over 1,000 feet wide. The largest rocks displaced by the slide weigh up to 5 tons and will be extremely difficult to move.
Highway 101 is being cleaned up as each day trucks are filling up and hauling away tons of debris. The also need to remove debris from every creek channel and clear all bridges to be prepared for the next rainfall.
Besides unstable land the main reason for the enormity of the landslide is the wildfires that denuded large hillsides of all vegetation. When the heavy rains came there was nothing to absorb all of that moisture, so the landslide was inevitable.
I’m sure you remember the Chetco Bar Fire in Curry county this summer. When the rain season got under way a flash flood watch and a landslide warning had to be issued due to the heavy rain that was causing serious runoff which could trigger landslides.
If you are interested in where landslides and mudflows have occurred in Oregon there is a source to find out. It’s the Statewide Landslide Information Database For Oregon (SLIDO). According to the Oregon Department of Geology and Mineral Studies website (on Oregongeology.org) “SLIDO is a compilation of landslides in Oregon that have been identified on published maps. The data base contains only landslides that have been located on these maps. Many landslides have not yet been located or are not on these maps and therefore are not in this data base.” The map is interactive containing identified landslides in Oregon shows the location, type and other attributes related to the landslides. To use the interactive map just go to Oregongeology.com.
You might want to check the interactive map link above to see if there have been any landslides in your immediate area.
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