I. Mass wasting is gravity-driven erosion. Generally mass wasting is better known by such terms as landslides, rock avalanches, slumps, etc.
- A. What affects mass wasting?
- 1. Steep slopes generally result in slope failure more readily than gentler slopes. For loose sediment, there is generally a slope angle that marks the difference between a tendency for slope failure and slope stability. This is called the angle of repose. For loose sand, this angle is roughly 34 degrees. This angle can vary for other materials.
- 2. Adding water increases slope instability. It lubricates rock fragments so that they slide down slope more easily. Mass wasting occurs more readily in rainy seasons for this reason. It turns out that a small amount of water in the soil can aid slope stability because of increased surface tension. Too much water, of course, eliminates any effects of surface tension.
- 3. Vegetation increases slope stability by anchoring sediments by root growth. Plants can also take up some excess water that might lead to slope failure. Too much water in the soil, however, can't be stopped by slope vegetation.
- 4. As mentioned in earlier lectures, earthquakes generally increase likelihood mass wasting in landslide prone areas.
- 5. Lots of loose debris on the slope is likely to worsen mass wasting.
- 6. High relief (vertical distance between high and low elevations) generally helps landslides to pick up speed once the slope fails.
- B. Classifying types of mass wasting. Different types of mass wasting can be distinguished by the speed of travel, and the material that flows. Generally that material will be either wet or dry.
- 1. Slow flows
- a. Creep is ground movement of roughly one centimeter/year or less. It is generally only possible to observe creep over many years. Usually the tilting of trees and human structures are the best indicators of creep. Freeze-thaw cycles generally aid creep, as well as occasional water saturation.
- b. A special type of creep is solifluction. Solifluction takes place in areas where permafrost exists. In North America, these would typically be in Alaska or northern Canada. The upper layer of permafrost melts during spring and summer months, saturating the ground with water. This saturated ground will slide down hill on steeper slopes, then refreeze during the winter. Solifluction involves movements of centimeters to meters per year.
- 2. Medium flows
- a. Slumping involves abrupt land subsidance, often in response to rains. Slumping usually produces a steep cut or scarp.
- b. Slumps are related to earthflows, in which part of a water saturated slope flows down hill. Earthflows usually produce a rotational slide, with the slump forming at the upper end, and the earthflow at the lower toe.
- 3. Fast flows
- a. Rapid, water-saturated flows are called mud flows. These often occur in steep sloped valleys and desert environments where there is little vegetation holding the sediment down. These are often catastrophic events, resulting in instantaneous destruction. Southern California in the San Gabriel mountains has debris flows which are described in John McPhee's book, Control of Nature.
- b. Rock falls and rock avalanches are free falling rock debris that usually falls off a steep cliff or slope. Often a cliff that is undercut by erosive forces will result in this type of mass wasting. Usually rock debris rests at the bottom of the slope in an apron of rock debris called talus.
II. Preventing mass wasting
- A. Improve water drainage to prevent water saturation along slopes
- B. Avoid over-burdening slopes or cliffs (by building houses or roads).
- C. Maintain vegetation.
|