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Sep 5 / Dave Schumaker

The Power of Landslides

The USGS has a fantastic page of frequently asked questions that is updated nearly every day. Two recent questions that have been posted to their website involve landslides.

The first:

Q: Do human activities cause landslides?
A: The causes of landslides are complex. The main causes of recent landslides in southern California were increased soil moisture and elevated ground-water levels that resulted from recent abnormally high rainfall. Detailed on-site investigation is required to determine the importance of human factors in causing any particular landslide.

The second:

Q: What is a landslide, and what causes them?

A: A landslide is defined as, the movement of a mass of rock, debris, or earth down a slope. (Cruden, 1991). Landslides are a type of “mass wasting” which denotes any down slope movement of soil and rock under the direct influence of gravity. The term “landslide” encompasses events such as rock falls, topples, slides, spreads, and flows (Varnes, 1996). Landslides can be initiated by rainfall, earthquakes, volcanic activity, changes in groundwater, disturbance and change of a slope by man-made construction activities, or any combination of these factors. Landslides can also occur underwater, causing tidal waves and damage to coastal areas. These landslides are called submarine landslides.

Failure of a slope occurs when the force that is pulling the slope downward (gravity) exceeds the strength of the earth materials that compose the slope. They can move slowly, (millimeters per year) or can move quickly and disastrously, as is the case with debris-flows. Debris-flows can travel down a hillside of speeds up to 200 miles per hour (more commonly, 30 – 50 miles per hour), depending on the slope angle, water content, and type of earth and debris in the flow. These flows are initiated by heavy, usually sustained, periods of rainfall, but sometimes can happen as a result of short bursts of concentrated rainfall in susceptible areas. Burned areas charred by wildfires are particularly susceptible to debris flows, given certain soil characteristics and slope conditions. More information can be found in USGS Fact Sheet numbers FS-071-00, “Landslide Hazards” (English Version), and FS-072-00, “Peligros de Deslizamientos” (Spanish Version.).

Recent hurricane activity has triggered landslides in the Caribbean islands, and Haiti in particular. The latest post on the Dave’s Landslide Blog (another great geology website) examines why Haiti is so prone to landslides.

So why is Haiti so vulnerable to hurricanes? Basically, Haiti is the most extreme illustration of the impact of deforestation on landslides and flash floods. Haiti is the poorest country in the Caribbean – over half the population live on less that $1 per day ($1 is the recognised mark of extreme poverty) and over 75% live on less that $2. More that 60% of the working population do not have formal employment. The consequence of this has been extreme deforestation, primarily for firewood to create charcoal.

We’ve written about the awesome power of landslides before, as well as shown some awesome footage of a landslide in Japan.

While searching for some more information on landslides, I discovered this fantastic video from National Geographic. (For some reason, the embedded file isn’t playing below. Click the link to view it).

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