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Home , Earthflow, Landslide, Mudflow, Sand, Silt, Slump (geology)

Landslide Types and Processes

andslides in the United States occur in all 50 States. The primary regions of occurrence and potential are the coastal and mountainous areas of , Oregon, Land Washington, the States comprising the intermountain west, and the mountainous and hilly regions of the Eastern United States. Alaska and also experience all types of . Landslides in the United States cause approximately $3.5 billion (year 2001 dollars) in - age, and kill between 25 and 50 people annually. Casualties in the United States are primar- ily caused by , slides, and flows. Worldwide, landslides occur and cause thousands of casualties and billions in monetary losses annually. The information in this publication provides an introductory primer on understanding basic scientific facts about landslides—the different types of landslides, how they are initiated, and some basic information about how they can begin to be managed as a .

TYPES OF LANDSLIDES porate additional variables, such as the rate of movement and the , air, or ice content of The term “landslide” describes a wide variety the landslide material. of processes that result in the downward and outward movement of slope-forming materials Although landslides are primarily associ- including rock, , artificial fill, or a com- ated with mountainous regions, they can bination of these. The materials may move also occur in areas of generally low relief. In by falling, toppling, sliding, spreading, or low-relief areas, landslides occur as -and- La Conchita, coastal area of southern Califor- flowing. Figure 1 shows a graphic illustration fill failures (roadway and building excava- nia. This landslide and occurred in of a landslide, with the commonly accepted tions), bluff failures, lateral spreading the spring of 1995. People were evacuated and terminology describing its features. landslides, collapse of mine-waste piles the houses nearest the slide were completely The various types of landslides can be dif- (especially coal), and a wide variety of slope destroyed. This is a typical type of landslide. ferentiated by the kinds of material involved failures associated with and open-pit Photo by R.L. Schuster, U.S. Geological Survey. and the mode of movement. A classification mines. The most common types of landslides system based on these parameters is shown in are described as follows and are illustrated in figure 2. Other classification systems incor- figure 3. SLIDES: Although many types of mass movements are included in the general term Crown cracks “landslide,” the more restrictive use of the term refers only to mass movements, where Crown there is a distinct zone of weakness that separates the slide material from more stable underlying material. The two major types of Minor scarp Main scarp slides are rotational slides and translational slides. Head Transverse cracks Rotational slide: This is a slide in which the surface of rupture is curved concavely upward and the slide movement is roughly rotational Transverse ridges about an axis that is parallel to the ground sur- face and transverse across the slide (fig. 3A). Translational slide: In this type of slide, the Radial landslide mass moves along a roughly planar cracks surface with little rotation or backward tilting (fig. 3B). A block slide is a translational slide Surface of rupture in which the moving mass consists of a single Toe unit or a few closely related units that move Main body downslope as a relatively coherent mass (fig. 3C). Foot Toe of surface of rupture FALLS: Falls are abrupt movements of Surface of separation masses of geologic materials, such as rocks and , that become detached Figure 1. An idealized - flow showing commonly used nomenclature for labeling the from steep slopes or cliffs (fig. 3D). parts of a landslide.

U.S. Department of the Interior Fact Sheet 2004-3072 U.S. Geological Survey Printed on recycled paper July 2004 TYPE OF MATERIAL

TYPE OF MOVEMENT ENGINEERING Predominantly coarse Predominantly fine

FALLS Rock fall Debris fall Earth fall TOPPLES Rock topple Debris topple Earth topple

ROTATIONAL

SLIDES Rock slide Debris slide Earth slide TRANSLATIONAL

LATERAL SPREADS Rock spread Debris spread Earth spread Rock flow Earth flow FLOWS (deep creep) (soil creep) COMPLEX Combination of two or more principal types of movement

Figure 2. Types of landslides. Abbreviated version of Varnes’ classification of slope movements (Varnes, 1978).

Separation occurs along discontinuities such and under saturated conditions. However, dry als on shallow slopes is usually progressive. as fractures, joints, and bedding planes, and flows of are also possible. The failure starts suddenly in a small area movement occurs by free-fall, bouncing, and d. : A mudflow is an earthflow con- and spreads rapidly. Often the initial failure rolling. Falls are strongly influenced by grav- sisting of material that is wet enough to flow is a slump, but in some materials movement ity, mechanical , and the presence rapidly and that contains at least 50 percent occurs for no apparent reason. Combination of interstitial water. -, -, and -sized . In some of two or more of the above types is known as instances, for example in many newspaper a complex landslide. TOPPLES: Toppling failures are distin- reports, and debris flows are com- guished by the forward rotation of a unit or monly referred to as “mudslides.” LANDSLIDE CAUSES units about some pivotal point, below or low e. Creep: Creep is the imperceptibly slow, in the unit, under the actions of gravity and steady, downward movement of slope-form- 1. Geological causes forces exerted by adjacent units or by fluids in ing soil or rock. Movement is caused by a. Weak or sensitive materials cracks (fig. 3E). shear stress sufficient to produce permanent b. Weathered materials deformation, but too small to produce shear c. Sheared, jointed, or fissured materials FLOWS: There are five basic categories of failure. There are generally three types of d. Adversely oriented discontinuity flows that differ from one another in funda- creep: (1) seasonal, where movement is (bedding, schistosity, , mental ways. within the depth of soil affected by seasonal unconformity, contact, and so forth) a. Debris flow: A debris flow is a form of changes in and soil temperature; e. Contrast in permeability and/or rapid mass movement in which a combina- (2) continuous, where shear stress continu- stiffness of materials tion of loose soil, rock, organic matter, air, ously exceeds the strength of the material; and water mobilize as a slurry that flows and (3) progressive, where slopes are reaching 2. Morphological causes downslope (fig. 3F). Debris flows include the point of failure as other types of mass a. Tectonic or volcanic uplift <50% fines. Debris flows are commonly movements. Creep is indicated by curved b. Glacial rebound caused by intense surface-water flow, due to tree trunks, bent fences or retaining walls, c. Fluvial, , or glacial of heavy precipitation or rapid snowmelt, that tilted poles or fences, and small soil ripples or slope toe or lateral margins erodes and mobilizes loose soil or rock on ridges (fig. 3I). d. Subterranean erosion (solution, steep slopes. Debris flows also commonly piping) mobilize from other types of landslides that LATERAL SPREADS: Lateral spreads are e. Deposition loading slope or its crest occur on steep slopes, are nearly saturated, distinctive because they usually occur on f. removal (by fire, ) and consist of a large proportion of silt- and very gentle slopes or flat (fig. 3J). g. Thawing sand-sized material. Debris-flow source areas The dominant mode of movement is lateral h. Freeze-and-thaw weathering are often associated with steep gullies, and extension accompanied by shear or tensile i. Shrink-and-swell weathering debris-flow deposits are usually indicated by fractures. The failure is caused by liquefac- the presence of debris fans at the mouths of tion, the process whereby saturated, loose, 3. Human causes gullies. that denude slopes of vegetation cohesionless (usually and a. Excavation of slope or its toe intensify the susceptibility of slopes to debris ) are transformed from a into a b. Loading of slope or its crest flows. liquefied state. Failure is usually triggered by c. Drawdown (of ) b. Debris : This is a variety of very rapid ground motion, such as that experi- d. rapid to extremely rapid debris flow (fig. 3G). enced during an , but can also be e. Irrigation c. Earthflow: have a characteristic artificially induced. When coherent material, f. “hourglass” shape (fig. 3H). The slope mate- either bedrock or soil, rests on materials that g. Artificial vibration rial liquefies and runs out, forming a bowl or liquefy, the upper units may undergo fractur- h. Water leakage from utilities depression at the head. The flow itself is elon- ing and extension and may then subside, gate and usually occurs in fine-grained materi- translate, rotate, disintegrate, or liquefy and als or clay-bearing rocks on moderate slopes flow. Lateral spreading in fine-grained materi- A B C Surface rupture

Surface of rupture

Toe

Rotational landslide Translational landslide Block slide

D E F

Speed Limit 65

Rockfall Topple Debris flow

G H I Source area Curved tree trunks

Tilted pole Main track

Depositional area

Soil ripples

Fence out of alignment Debris avalanche Earthflow Creep

J Firm clay

Soft clay with water-bearing silt Bedrock and sand layers Lateral spread

Figure 3. These schematics illustrate the major types of landslide movement that are described in the previous pages. For additional information on these processes and where to find photos, please see “Where to Go For More Information” at the end of this fact sheet. Although there are multiple types of Landslides and Volcanic Activity causes of landslides, the three that cause most Where to go for more information Landslides due to volcanic activity are of the damaging landslides around the world some of the most devastating types. Volcanic 1. The U.S. Geological Survey Landslide are these: may melt at a rapid rate, causing Program has information, publications, and a deluge of rock, soil, ash, and water that educational information on its Web site. Landslides and Water accelerates rapidly on the steep slopes of Please see: http://landslides.usgs.gov Slope saturation by water is a primary volcanoes, devastating anything in its path. or phone toll-free: cause of landslides. This effect can occur These volcanic debris flows (also known as 1-800-654-4966 in the form of intense rainfall, snowmelt, ) reach great distances, once they leave changes in ground-water levels, and water- the flanks of the , and can damage 2. For general information about slides, level changes along coastlines, earth , structures in flat areas surrounding the volca- debris flows, rock falls, or other types of and the banks of , reservoirs, canals, and noes. The 1980 eruption of Mount St. Helens, landslides in your area, contact your city . in Washington triggered a massive landslide or county geology or planning office. In Landsliding and flooding are closely allied on the north flank of the volcano, the largest addition, all 50 States have State Geologi- because both are related to precipitation, landslide in recorded times. cal Surveys that can be accessed through a runoff, and the saturation of ground by water. link at the USGS Web site, In addition, debris flows and mudflows usu- —How to Reduce http://landslides.usgs.gov ally occur in small, steep channels and the Effects of Landslides often are mistaken for ; in fact, these 3. For an assessment of the landslide risk two events often occur simultaneously in the Vulnerability to landslide is a to an individual property or homesite, same area. function of location, type of human activity, obtain the services of a State-licensed - Landslides can cause flooding by forming use, and frequency of landslide events. The technical engineer or engineering . landslide dams that block valleys and stream effects of landslides on people and structures These professionals can be found through channels, allowing large amounts of water can be lessened by total avoidance of land- the membership listings of two professional to back up. This causes backwater flooding slide hazard areas or by restricting, prohibit- societies, the American Society of Civil and, if the dam fails, subsequent downstream ing, or imposing conditions on hazard-zone Engineers (ASCE), http://www.asce.org flooding. Also, solid landslide debris can activity. Local governments can reduce and the Association of Engineering “bulk” or add volume and to otherwise landslide effects through land-use policies http://www.aegweb.org. Often, normal streamflow or cause block- and regulations. Individuals can reduce their personnel in State or county planning or ages and diversions creating conditions exposure to hazards by educating themselves engineering departments can refer compe- or localized erosion. Landslides can also cause on the past hazard history of a site and by tent geotechnical engineers or engineering overtopping of reservoirs and/or reduced making inquiries to planning and engineer- geologists. capacity of reservoirs to store water. ing departments of local governments. They can also obtain the professional services of an engineering geologist, a geotechnical 4. For more information about the design Landslides and Seismic Activity engineer, or a civil engineer, who can properly and of debris-flow mitiga- Many mountainous areas that are vulner- evaluate the hazard potential of a site, built or tion measures which may include debris able to landslides have also experienced at unbuilt. basins, debris fences, deflection walls, or least moderate rates of earthquake occurrence other protective works, consult your city in recorded times. The occurrence of earth- The hazard from landslides can be reduced or county engineer, local flood-control quakes in steep landslide-prone areas greatly by avoiding construction on steep slopes agency, or the U.S. Department of Agri- increases the likelihood that landslides will and existing landslides, or by stabilizing the culture, Natural Resources Conservation occur, due to ground shaking alone or shak- slopes. Stability increases when ground water Service: ing-caused dilation of soil materials, which is prevented from rising in the landslide mass http://www.ncgc.nrcs.usda.gov/ allows rapid of water. The 1964 by (1) covering the landslide with an imper- Great Alaska Earthquake caused widespread meable membrane, (2) directing surface water 5. For photos of landslide types please see: landsliding and other ground failure, which away from the landslide, (3) draining ground http://landslides.usgs.gov/html_files/nlic/ caused most of the monetary loss due to the water away from the landslide, and (4) mini- nlicmisc.html earthquake. Other areas of the United States, mizing surface irrigation. Slope stability is such as California and the Puget region also increased when a retaining structure and/ 6. For more detailed information: two in Washington, have experienced slides, or the weight of a soil/rock are placed excellent publications that very clearly lateral spreading, and other types of ground at the toe of the landslide or when mass is describe the processes of landslides were failure due to moderate to large . removed from the top of the slope. consulted for this fact sheet: Widespread rockfalls also are caused by loos- Varnes, D.J., 1978, Slope movement types ening of rocks as a result of ground shaking. and processes, in Schuster, R.L., and Worldwide, landslides caused by earthquakes Krizek, R.J., eds., Landslides—Analy- kill people and damage structures at higher sis and control: National Research rates than in the United States. Compiled by Lynn Highland Council, Washington, D.C., Transporta- Graphics and layout design by Margo tion Research Board, Special Report Johnson 176, p. 11–33. Turner, Keith A., and Schuster, Robert L., This fact sheet is available online at 1996, Landslides—Investigation and http://pubs.usgs.gov/fs/2004/3072/ mitigation: Transportation Research Board, National Research Council, National Academy Press.