In this section:
The California Geological Survey classifies landslides with a two-part designation based on Varnes (1978) and Cruden and Varnes (1996). The designation captures both the type of material that failed and the type of movement that the failed material exhibited.
Material types are broadly categorized as either
rock or
soil, or a combination of the two for complex movements. Rock refers to hard or firm bedrock that was intact and in place prior to slope movement. Soil, either residual or transported material, is used in the engineering sense to mean unconsolidated particles or poorly cemented rock or aggregates. Soil is distinguished further on the basis of texture as
debris (coarse fragments) or
earth (fine fragments). The distinction between rock and soil, and the further distinction between debris and earth is most often based on interpretation of geomorphic characteristics within landslide deposits, but can also be inferred from geologic characteristics of the parent material described on maps or observed in the field.
Landslide movements are interpreted from the geomorphic expression of the landslide deposit and source area, and are categorized as
falls, topples, spreads, slides, or
flows.
Falls are masses of soil or rock that dislodge from steep slopes and free-fall, bounce, or roll downslope.
Topples move by the forward pivoting of a mass around an axis below the displaced mass.
Lateral spreads, commonly induced by liquefaction of material in an earthquake, move by horizontal extension and shear or tensile fractures.
Slides displace masses of material along one or more discrete planes. In
rotational sliding the slide plane is curved and the mass rotates backwards around an axis parallel to the slope; in
translational sliding the failure surface is more or less planar and the mass moves parallel to the ground surface.
Flows mobilize as a deforming, viscous mass without a discrete failure plane. More than one form of movement may occur during a failure, in which case the movement is classified as
complex if movements occur sequentially and
composite if they do not.
Five of the 20 possible material/movement combinations are commonly found when preparing a landslide inventory map. These are
earth flows, debris flows, debris slides, rock slides, and
rock falls, and are described in more detail below.
Earth Flows
Diagram by J. Appleby, R. Kilbourne, and C. Wills after Varnes, 1978
An earth flow is a specific type of soil flow landslide where the majority of the soil materials are fine-grained (silt and clay) and cohesive. The material strength is low through much of the slide mass, and movement occurs on many discontinuous shear surfaces throughout the landslide mass. This movement along numerous internal slide planes disrupts the landslide mass leading to cumulative movement that resembles the flow of a viscous liquid characterized by a lumpy, or “hummocky” slope morphology. The lower parts of an earth flow usually bulge outward and are steeper than adjacent slopes.
Earth flows commonly occur on moderately steep slopes. Slope gradients are commonly from 10% to as steep as 30%, although steeper slopes may be found in headscarp and toe areas.
Earth flows typically are initiated by periods of prolonged rainfall and sometimes don’t initiate until well after a storm or the rainy season has passed. They are characteristically slow moving, in the millimeters or centimeters per day range, and may continue to move for a period of days to weeks after initiating.
Debris Flows
Diagram by J. Appleby and R. Kilbourne, CGS
A debris flow is a soil flow where the majority of the materials are coarse-grained (fine sand to boulder size particles) and non-cohesive. Debris flows are most often triggered by intense rainfall following a period of less intense precipitation, or by rapid snow melt. High pore water pressures cause the soil and weathered rock to rapidly lose strength and flow downslope. Debris flows can move very rapidly, at rates ranging from meters per hour to meters per second and travel relatively long distances, making them a significant threat to life and property.
Debris flows commonly begin as a slide of a shallow mass of soil and weathered rock. Their most distinctive landform is the scar left by the original shallow slide. The path of the debris flow may be marked by a small drainage that has been stripped of vegetation. The debris flow may not leave any deposit if it flows directly into a larger creek and is immediately eroded away. Many debris flow deposits are ephemeral, but in some cases successive debris flows may deposit material in the same area thereby forming a debris fan, which resembles a small, steep alluvial fan.
Individual debris flows typically are small in areal extent and their deposits are relatively thin. Evidence of past debris flow movements often is masked by vegetation growth which can cover the surface rapidly, sometimes within a few years, making them difficult to identify using aerial photographic and field reconnaissance methods. Therefore, only the larger and more recent debris flows typically are identified and included on landslide inventory maps.
Debris Slides
Diagram by J. Appleby, R. Kilbourne, and C. Wills after Varnes, 1978
A debris slide is a landslide of coarse-grained soil, most common in unconsolidated sandy or gravelly units, but also are common in residual soils that form from in-place weathering of relatively hard rock. Owing to the granular constituents, overall strength of the debris slide mass generally is higher than that of earth flows, but there may be a very low strength zone at the base of the soil or within weathered bedrock. Debris slides typically move initially as shallow intact slabs of soil and vegetation, but break up after a short distance into falls and flows. Movement of the slide mass as a shallow slab leads to a smooth, steep, commonly curved scar. The debris is deposited at the base as a loose hummocky mass, although the deposit may be rapidly removed by erosion.
Debris slides commonly occur on very steep slopes, as steep as 60% to 70%, usually in an area where the base of a slope is undercut by erosion. Debris slides form steep, un-vegetated scars which are likely to remain un-vegetated for years. Re-vegetated scars can be recognized by their steep slopes, and a shallow amphitheater morphology.
A single heavy rainstorm or series of storms may deliver enough rain to trigger debris slides. Individual debris slides may move at rates ranging from meters per day to meters per minute. Debris slide scars are extremely steep and therefore are very sensitive to renewed disturbance. Natural erosion at the base of debris slide scars may trigger additional slides. Cutting into the base of a debris slide scar may also trigger renewed slides. Even without additional disturbance, debris slide scars tend to ravel and erode, leading to small rock falls and debris slides from the same slope.
Rock Slides
Diagram by J. Appleby, R. Kilbourne, and T. Spittler after Varnes, 1978
A rock slide is a landslide involving bedrock in which the rock that moves remains largely intact for at least a portion of the movement. Rock slides can range in size from small and thin to very large and thick, and are subject to a wide range of triggering mechanisms. The sliding occurs at the base of the rock mass along one to several relatively thin zones of weakness, which are variably referred to as “slide planes,” “shear surfaces,” “slip surfaces,” “rupture surfaces,” or “failure surfaces.” The sliding surface may be curved or planar in shape. Rock slides with curved sliding surfaces are commonly called “slumps” or “rotational slides,” while those with planar failure surfaces are commonly called “translational slides,” “block slides,” or “block glides.” Rock slides that occur on intersecting planar surfaces are commonly called “wedge failures.
Rock slides commonly occur on relatively steep slopes in competent rocks. Slope gradients are commonly from 35% to as steep as 70%. Movement of an intact rock mass along a curved slide plane leads to a steep, arcuate headscarp at the upper boundary of the slide. Immediately below the headscarp is a block that is commonly rotated so that it is less steep than the surrounding hill slopes. Below the bench, the slide mass may be intact with a similar gradient to the surrounding slopes or may have additional scarps and benches. The lower parts of the slopes may bulge outward and be steeper that the surrounding slopes.
Rock Falls
Diagram modified after Colorado Geological Survey
A rock fall is a landslide where a mass of rock detaches from a steep slope by sliding, spreading or toppling and descends mainly through the air by falling, bouncing or rolling. Intense rain, earthquakes or freeze-thaw wedging may trigger this type of movement.
Rock falls occur on steep slopes of hard, fractured rock. The scar left by a rock fall on the slope may be no more apparent than an area of rock that is less weathered than the surrounding rocks. Rock fall deposits are loose piles of rubble that may be easily removed by erosion. Because neither the scar nor the deposit are distinctive, and because the most frequently occurring rock falls are typically small, individual rock falls are usually not shown on regional-scale (1:24,000 and smaller) landslide maps.
Though infrequent, moderate- to large-volume rock falls can be extremely dangerous and sometimes fatal. Large slabs of rock impacting a hard ledge after a long drop can rapidly break apart, leading to air entrainment and long runouts, induced airblasts, airborne projectiles (flyrock) and severe dust clouds.
