Mass Wasting Chapter Prepared by Iggy Isiorho for Dr. Isiorho Mass Wasting
Mass Wasting • Mass Wasting – Movement, caused by gravity, in which bedrock, rock debris, or soil moves downslope in bulk. • Landslides – The general term for a slowly to very rapidly descending mass of rock or debris.
Classification of Mass Wasting • The classification system used here and summarized in Table 9.1 is based on (1) rate of movement, (2) type of material, and (3) nature of the movement.
Rate of Movement • A landslide like the one in Peru clearly involves rapid movement. Just as clearly, movement of soil at a rate of less than a centimeter a year is slow movement. Between these extremes is a wide range of velocities. Back
Type of Material • Mass wasting processes are usually distinguished on the basis of whether the descending mass started as bedrock (as in a rockslide) or as debris. The term debris, as applied to mass wasting processes, means any unconsolidated material at the earth’s surface, such as soil and rock fragments of any size. • The amount of water (or ice and snow) in a descending mass strongly influences the rate and type of movement. Back
Type of Movement • In general, the type of movement in mass wasting can be classified as mainly flow, slide, or fall (Fig. 9.1). A flow implies that the descending mass is moving downslope as a viscous fluid. Slide means the descending mass remains relatively coherent, moving along one or more well-defined surfaces. A fall occurs when material free-falls or bounces down a cliff. • Two kinds of slip are shown in Fig. 9.1. In a translational slide, the descending mass moves along a plane approximately parallel to the slope of the surface. A rotational slide involves movement along a curved surface, the upper part moving downward while the lower part moves outward. Back
Gravity • Shear force – In mass wasting, component of gravitational force that is parallel to an inclined surface. • Shear strength – In mass wasting, the resistance to movement or deformation of material.
Water • Water is a critical factor in mass wasting. When debris is saturated with water (as from heavy rain or melting snow), it becomes heavier and is more likely to flow downslope. The added gravitational shear force from the increased weight, however, is probably less important than the reduction in shear strength. This is due to increased pore pressure in which water forces grains of debris apart. • Paradoxically, a small amount of water in soil can actually prevent downslope movement. When water does not completely fill the pore spaces between the grains of soil, it forms a thin film around each grain (Fig. 9.3). Loose grains adhere to one another because of the surface tension created by the the film of water and shear strength increases.
Common Types of Mass Wasting • Creep • Debris Flow • Rockfalls and Rockslides
Creep • Creep is very slow, downslope movement of soil or unconsolidated debris. Shear forces, over time, are only slightly greater than shear strengths. • Two factors that contribute significantly to creep are water in the soil and daily cycles of freezing and thawing. As we have said, water-saturated ground facilitates movement of soil downhill. What keeps downslope movement from becoming more rapid in most areas is the presence of abundant grass or other plants that anchor the soil. • Although creep does take place in year-round warm climates, the process is more active where the soil freezes and thaws during part of the year. Back
Debris Flow • The general term debris flow is used for mass wasting in which motion is taking place throughout the moving mass (flow). The common varieties Earthflow, Mudflow, and Debris avalanche are described in this section. Back
Earthflow • In an earthflow, debris moves downslope as a viscous fluid; the process can be slow or rapid. Earthflows usually occur on hillsides that have a thick cover of debris, often after heavy rains have saturated the soil. • A landslide may be entirely an earthflow, as in Fig. 9.7A, with debris particles moving past one another roughly parallel to the slope. Commonly, however, rotational sliding takes place above the earthflow as in Fig. 9.7Band Fig. 9.8. This examples is a debris slide (upper part) and an earthflow (lower part). In such cases, debris remains in a relatively coherent block or blocks that rotate downward and outward, forcing the debris below to flow. Debris Flow
Solifluction and Permafrost • Solifluction – Flow of water-saturated debris over impermeable material. • Permafrost – Ground that remains permanently frozen for many years. Back
Mudflow • Mudflow – A flowing mixture of debris and water, usually moving down a channel. • Mudflows are most likely to occur in places where debris is not protected by a vegetative cover. For this reason, mudflows are more likely to occur in arid regions than in wet climates. A hillside in a desert environment, where it may not have rained for many years, may be covered with a blanket of loose material. With sparse desert vegetation offering little protection, a sudden thunderstorm with drenching rain can rapidly saturate the loose debris and create a mudflow in minutes. Back
Debris Avalanche • The fastest variety of debris flow is a debris avalanche, a very rapidly moving, turbulent mass of debris, air, and water. • Some geologists have suggested that in very rapidly moving rock avalanches, air trapped under the rock mass creates an air cushion that reduces friction. This could explain why some landslides reach speeds of several hundred kilometers per hour. But other geologists have contended that the rock mass is too turbulent to permit such an air cushion to form. Back
Rockfalls and Rockslides • Rockfall – Rock falling freely or bouncing down a cliff. • Talus – An accumulation of broken rock at the base of a cliff. • Rockslide – Rapid sliding of a mass of bedrock along an inclined surface of weakness. • Rock avalanche – A very rapidly moving, turbulent mass of broken-up bedrock. Back