Types of Mass Wasting
Reference: Dept of Conservation, State of California
DMG Note 50 Definition.
This kind of slide is characterized by large blocks of material and a failure plane that is relatively deep. The movement is forms straight or almost straight lines downhill. It occurs in relatively cohesive, homogeneous soils and rock. The soil mantle may be greater than 5 feet thick, but sliding is not restricted to the zone of weathering. Failure commonly occurs along bedrock bedding planes that are deep-seated and dip in the same direction as the slope surface. In saturated conditions, incompetent clayey bedrock material may fail under overburden weight and high pore pressures, resulting in a deep-seated rotational-type failure. Translational slides commonly are controlled structurally by surfaces of weakness such as faults, joints, bedding planes, and contacts between bedrock and overlying deposits.
Translational/rotational slides generally form a concentric (like) near vertical scarp in the head region and, occasionally, along the lateral margins of the slide. Slide materials are characterized by hummocky topography consisting of rolling, bumpy ground, frequent benches, and depressions. The toe of the slide may be steep where slide material has accumulated. Steep crownscarps and margins of the translational/rotational slide and toe areas of large slides may be subject to debris sliding.
Diagrammatic sketch of a translational/rotational landslide by Janet Appleby,
Richard Kilbourne, and Thomas Spittler; modified from Varnes (1978)
An earthflow is a landslide resulting from slow to rapid movement of saturated soil and debris in a liquidy state. After initial failure, the earthflow may move, or creep, seasonally in response to destabilizing forces. Earthflows are composed of clay-rich materials that swell when wet, causing a reduction in friction between the soil particles. When saturated, the fine-grained, clay-rich matrix may carry larger, more resistant boulders with them in slow, creeping movements. Slide materials erode easily, resulting in gullying and irregular drainage patterns. The irregular, hummocky ground characteristic of earthflows is generally bare of trees. Failures commonly occur on slopes that are gentle to moderate, although they may also occur on steeper slopes where vegetation has been removed. Undercutting of the toe of an earthflow is likely to reactivate downslope movement.
Diagrammatic sketch of an earthflow by Janet Appleby
and Richard Kilbourne; modified from Varnes (1978)
A debris slide is characterized by unconsolidated rock and soil that has moved downslope along a relatively shallow failure plane. Debris slides form steep, unvegetated scars (depressions) in the head region and irregular, hummocky deposits (when present) in the toe region. Debris slide scars are likely to ravel and remain unvegetated for many years. These scars can be recognized by the nature of the slope, steepness of the slope, and the light bulb-shaped form left by many mid- and upper-slope failures.
Debris slides are most likely to occur on slopes greater than 65 percent where unconsolidated colluvium overlie a shallow soil/bedrock. The shallow slide surface is usually less than 15 feet deep. The probability of sliding is low where bedrock is exposed, except, where weak bedding planes and extensive bedrock joints and fractures parallel the slope.
Diagrammatic sketch of a debris slide by Janet Appleby
and Richard Kilbourne; modified from Varnes (1978).
Debris Flow/Torrent Track
Debris flow and debris torrent tracks are characterized by long stretches of bare, generally unstable stream channel banks that have been scoured and eroded by the extremely rapid movement of water-laden debris. They commonly are caused by debris sliding or the failure of fill materials along stream crossings in the upper part of a drainage during high intensity storms.
Debris flow/torrent tracks are formed by the failure of water-charged soil and organic material down steep stream channels. They are often triggered by debris slide movement on adjacent hill slopes and by the mobilization of debris accumulated in the stream channels themselves. Debris flows and torrents commonly entrain large quantities of inorganic and organic material from the stream bed and banks. Occasionally, the channel may be scoured to bedrock. When momentum is lost, scoured debris may be deposited as a tangled mass of large organic debris in a matrix of sediment and finer organic material. Such debris may be reactivated or washed away during subsequent events. The erosion of steep debris slide-prone streambanks below the initial failure may caused further failure downstream.
Diagrammatic sketch of a debris flow/torrent
track.by Janet Appleby and Richard Kilbourne.
For a look at landslides, earthflow, debris slide and debris flow, check out the Landslide Photos