Appendix A. Basic Information about Landslides 60 The Landslide Handbook—A­ Guide to Understanding Landslides

Part 1. Glossary of Landslide Terms

Full references citations for glossary are at the end of the list.

alluvial fan An outspread, gently sloping Digital Terrain Model (DTM) The term used Geographic Information System (GIS) A mass of alluvium deposited by a stream, by United States Department of Defense and computer program and associated data bases especially in an arid or semiarid region other organizations to describe digital eleva- that permit cartographic information (includ- where a stream issues from a narrow canyon tion data. (Reference 3) ing geologic information) to be queried onto a plain or valley floor. Viewed from drawdown Lowering of water levels in riv- by the geographic coordinates of features. above, it has the shape of an open fan, the ers, lakes, wells, or underground aquifers due Usually the data are organized in “layers” apex being at the valley mouth. (Reference 3) to withdrawal of water. Drawdown may leave representing different geographic entities such as hydrology, culture, topography, and bedding surface/plane In sedimentary or unsupported banks or poorly packed earth so forth. A geographic information system, stratified rocks, the division planes that sepa- that can cause landslides. (Reference 3) or GIS, permits information from different rate each successive layer or bed from the electronic distance meter (EDM) A device layers to be easily integrated and analyzed. one above or below. It is commonly marked that emits ultrasonic waves that bounce off (Reference 3) by a visible change in lithology or color. solid objects and return to the meter. The (Reference 3) meter’s microprocessor then converts the geologic hazard A geologic condition, either natural or manmade, that poses bedrock The solid rock underlying gravel, elapsed time into a distance measurement. a potential danger to life and property. sand, clay, and so forth; any solid rock Sound waves spread 1 foot wide for every Examples: earthquake, landslides, flooding, exposed at the surface of the earth or overlain 10 feet measured. There are various types faulting, beach erosion, land subsidence, by unconsolidated superficial material. available. pollution, waste disposal, and foundation and (Reference 3) epicenter The point on the Earth’s surface footing failures. (Reference 3) borehole A circular hole drilled into the directly above the focus of an earthquake. geologic map A map on which is recorded earth, often to a great depth, as a prospec- (Reference 3) the distribution, nature, and age relationships tive oil, gas, or water well or for exploratory expansive soils Types of soil that shrink of rock units and the occurrence of structural purposes. (Reference 3) or swell as the moisture content decreases or features. (Reference 3) check dams Check dams are small sedi- increases. Structures built on these soils may geomorphology The science that treats the ment storage dams built in the channels of shift, crack, and break as soils shrink and general configuration of the Earth’s surface; steep gullies to stabilize the channel bed. A subside or expand. Also known as swelling soils. (Reference 5) specifically, the study of the classification, common use is to control channelized debris- description, nature, origin, and develop- flow frequency and volume. Check dams are extensometer An instrument for measur- ment of landforms and their relationships to expensive to construct and are therefore usu- ing small deformations, as in tests of stress. underlying structures, and the history of geo- ally only built where important installations (Reference 3) logic changes as recorded by these surface or natural habitat (such as a camp or unique factor of safety The factor of safety, also features. (Reference 3) spawning area) lies downslope. (Reference 2) known as Safety Factor, is used to provide geophysical studies The science of the colluvium A general term applied to loose a design margin over the theoretical design Earth, by quantitative physical methods, and incoherent deposits, usually at the foot of capacity to allow for uncertainty in the with respect to its structure, composition, a slope or cliff and brought there chiefly by design process. The uncertainty could be any and development. It includes the sciences of gravity. (Reference 2) one of a number of the components of the dynamical geology and physical geography debris basin (sometimes called catch design process including calculations and and makes use of geodesy, geology, seismol- basins) A large excavated basin into which material strengths for example. Commonly, ogy, meteorology, oceanography, magnetism, a debris flow runs or is directed and where it a factor of safety of less than 1, for instance, and other Earth sciences in collecting and quickly dissipates its energy and deposits its on an engineered slope indicates potential interpreting Earth data. (Reference 3) load. Abandoned gravel pits or rock quarries failure, where a factor of safety of greater are often used as debris basins. (Reference 3). than 1, indicates stability. (Reference 6) hydraulic Of or pertaining to fluids in motion; conveying, or acting, by water; geodesic/geodetic measurements The delta-front landsliding Delta fronts are operated or moved by means of water, as investigation of any scientific questions con- where deposition in deltas is most active— hydraulic mining. (Reference 3) underwater landsliding along coastal and nected with the shape and dimensions of the delta regions due to rapid sedimentation of Earth. (Reference 3) hydrology The science that relates to the water of the Earth. (Reference 3) loosely consolidated clay, which is low in fracture Brittle deformation due to a strength and high in pore-water pressures. momentary loss of cohesion or loss of resis- inclinometer Instrument for measuring Digital Elevation Model (DEM) A digital tance to differential stress and a release of inclination to the horizontal. (Reference 3) elevation model (DEM) is a digital file con- stored elastic energy. Both joints and faults landslide dam An earthen dam created sisting of terrain elevations for ground posi- are fractures. (Reference 3) when a landslide blocks a stream or river. tions at regularly spaced horizontal intervals. (Reference 3) (A commercial definition – new technology) Part 1. Glossary of Landslide Terms 61 lahar Landslide, debris flow or mudflow, of mudslide An imprecise but popular term sag pond A small body of water occupying pyroclastic material on the flank of a volcano; coined in California, USA, frequently used an enclosed depression or sag formed where deposit produced by such a debris flow. by the general public and the news media active or recent fault or landslide movement Lahars are described as wet if they are mixed to describe a wide scope of events, ranging has impounded drainage. (Reference 3) with water derived from heavy rains, escaping from debris-laden floods to landslides. Not seepage Concentrated subsurface drain- from a crater lake, or produced by melting technically correct. Please see “mudflow,” age indicated by springs, sag ponds, or moist snow. Dry lahars may result from tremors of next Glossary entry. (Reference 5) areas on open slopes, and seepage sites along a cone or by accumulating material becom- mudflow A general term for a mass-move- road cuts. The locations of these areas of con- ing unstable on a steep slope. If the material ment landform and process characterized by centrated subsurface flow should be noted on retains much heat, it is termed a hot lahar. a flowing mass of predominately fine-grained maps and profiles as potential sites of active, (Reference 3) earth material possessing a high degree of unstable ground. (Reference 2) liquefaction The transformation of satu- fluidity during movement. The water content sea cliff retreat A cliff formed by wave rated, loosely packed, coarse-grained soils may range up to 60 percent. (Reference 3) action, causing the coastal cliff to erode and from a solid to a liquid state. The soil grains perched ground water Unconfined ground recede toward land. (Reference 3) temporarily lose contact with each other, and water separated from an underlying main the particle weight is transferred to the pore shear A deformation resulting from stresses body of ground water by an unsaturated water. (Reference 4) that cause contiguous parts of a body to slide zone. (Reference 3) relative to each other in a direction parallel to landslide inventory maps Inventories piezometer An instrument for measuring their plane of contact. (Reference 3) identify areas that appear to have failed by pressure head in a conduit, tank, or soil—it is landslide processes, including debris flows slurry A highly fluid mixture of water and a small diameter water well used to mea- and cut-and-fill failures. (Reference 4) finely divided material; for example, pulver- sure the hydraulic head of ground water in ized coal and water for movement by pipeline landslide susceptibility map This map goes aquifers. (Reference 3) or of cement and water for use in grouting. beyond an inventory map and depicts areas pore-water pressure A measure of the (Reference 3) that have the potential for landsliding. These pressure produced by the head of water in areas are determined by correlating some of soil mechanics The application of the a saturated soil and transferred to the base the principal factors that contribute to land- principles of mechanics and hydraulics to sliding, such as steep slopes, weak geologic of the soil through the pore water. This is engineering problems dealing with the behav- units that lose strength when saturated, and quantifiable in the field by the measure- ior and nature of soils, sediments, and other poorly drained rock or soil, with the past ment of free water-surface level in the soil unconsolidated accumulations; the study of distribution of landslides. (Reference 5) or by direct measurement of the pressure by the physical properties and utilization of soils, means of piezometers. Pore-water pressure especially in relation to highway and founda- landslide hazard map Hazard maps show is a key factor in failure of a steep slope tion engineering. (Reference 3) the areal extent of threatening processes: soil and operates primarily by reducing the strainmeter A seismometer that is designed where landslide processes have occurred in weight component of soil shear strength. to detect deformation of the ground by mea- the past, where they occur now, and the likeli- (Reference 2) hood in various areas that a landslide will suring relative displacement of two points. occur in the future. (Reference 5) pore water, or interstitial water Subsurface (Reference 3) water in an interstice, or pore. (Reference 3) landslide risk map Landslide hazards and stress In a solid, the force per unit area, the probability that they will occur, expressed quick clay A clay that loses nearly all its acting on any surface within it, and variously in statistical recurrence rates; risk maps may shear strength after being disturbed; a clay expressed as pounds or tons per square inch, show cost/benefit relationships, loss potential that shows no appreciable gain in strength or dynes or kilograms per square centimeter; and other potential socioeconomic effects on after remolding. (Reference 3) also, by extension, the external pressure that an area and (or) community. reconnaissance geology/mapping A creates the internal force. (Reference 3) lithology The physical character of a rock, general, exploratory examination or survey sturzstroms (German language term for “fall generally as determined at the microscopic of the main features of a region, usually stream”) A huge mass of rapidly mov- level, or with the aid of a low-power magni- preliminary to a more detailed survey. It ing rock debris and dust, derived from the fier; the microscopic study and description of may be made in the field or office, depend- collapse of a cliff or mountainside, flowing rocks. (Reference 3) ing on the extent of information available. down steep slopes and across low ground, (Reference 2) often for several kilometers at speeds of loess A widespread, homogenous, com- more than 100 km/hr. Sturzstroms are the monly nonstratified, porous, friable, slightly relief The difference in elevation between most catastrophic of all forms of mass coherent, usually highly calcareous, fine- the high and low points of a land surface. movement. (Reference 3) grained blanket deposit (generally less than (Reference 3) 30 m thick) consisting predominantly of silt, risk The probability of occurrence or subaqueous (submarine) landslide Condi- with subordinate grain sizes ranging from expected degree of loss, as a result of expo- tions and processes, or features and deposits, amounts of clay to fine sand. (Reference 3) sure to a hazard. (Reference 4) that exist or are situated in or under water. Generally used to specify a process that mitigation Activities that reduce or rock mechanics The theoretical and occurs either on land (the slide extending eliminate the probability of occurrence of a applied science of the mechanical behavior underwater) or that begins under water; for disaster and (or) activities that dissipate or of rocks, representing a “branch of mechan- example, slumping, gravitational slides. lessen the effects of emergencies or disasters ics concerned with the response of rock to (Reference 3) when they actually occur. (Reference 5) the force fields of its physical environment.” (Reference 3) 62 The Landslide Handbook—A­ Guide to Understanding Landslides subsidence Sinking or downward settling weathering, differential When weather- References for Glossary: of the Earth’s surface, not restricted in rate, ing across a rock face or exposure occurs at magnitude, or area involved. Subsidence may different rates mainly due to variations in the 1. Creath, W.B., 1996, Homebuyers’ guide be caused by natural geologic processes, composition and resistance of the rock. This to geologic hazards: An AIPG issues such as solution, compaction, or withdrawal results in an uneven surface with the more and answers publication: Department of of fluid lava from beneath a solid crust or resistant material protruding. (Reference 4) Natural Resources, Colorado Geological by human activity such as subsurface min- Survey, Miscellaneous Publication (MI) ing or the pumping of oil or ground water. weathering, mechanical The physical no. 58, 30 p. (Reference 3) processes by which rocks exposed to the 2. Chatwin, S.C., Howes, D.E., Schwab, surficial geology Geology of surficial weather change in character, decay, and J.W., and Swanston, D.N., 1994, A deposits, including soils; the term is some- crumble into soil. Processes include tem- guide for management of landslide- times applied to the study of bedrock at or perature change (expansion and shrinkage), prone terrain in the Pacific Northwest, near the Earth’s surface. (Reference 3) freeze-thaw cycle, and the burrowing activity 2d edition: Research Branch, Ministry of animals. (Reference 4) swelling soils These are soils or soft of Forests, Province of British Colum- bedrock that increases in volume as they get zonation A term used generally, even bia, Victoria, British Columbia, Crown wet and shrink as they dry out. They are also vaguely, for a region of latitudinal charac- Publications. commonly known as bentonite, expansive, or ter more or less set off from surrounding montmorillinitic soils. (Reference 1) 3. Jackson, Julia A., ed., 1997, Glossary of regions by some distinctive characteristic, for geology, fourth edition: Prepared by the tensile stress A normal stress that tends to instance, the Earth’s torrid zone, two temper- American Geological Institute, Alexan- pull apart the material on the opposite sides ate zones, and two frigid zones. For hazards, dria, Virginia, USA, Doubleday. of the plane on which it acts. (Reference 3) zones are geographic regions or designations 4. Jochim, Candice L., Rogers, William P., weathering The destructive process by that are differentiated through a variety of Truby, John O., Wold, Robert L., Jr., which earth and rock materials exposed to different criteria, for example, residential Weber, George, and Brown, Sally P., the atmosphere undergo physical disintegra- zones, zones of low hazard, zones of high 1988, Colorado landslide hazard tion and chemical decomposition resulting hazard. (Reference 3) mitigation plan: Department of Natural in changes in color, texture, composition, or Resources, Colorado Geological Survey, form. Processes may be physical, chemical, Bulletin 48. or biological. (Reference 4) 5. Shelton, David C., and Prouty, Dick, 1979, Nature’s building codes, geology and construction in Colorado: Depart- ment of Natural Resources, Colorado Geological Survey Special Publication No. 48, 72 p. 6. Turner, A. Keith, and Schuster, Robert L., 1996, Landslides— Investigation and mitigation: National Research Council, Transportation Research Board, Special Report 247, National Academy Press, Washington, D.C., 673 p. Part 2. Parts of a Landslide—Description of Features/Glossary 63

Part 2. Parts of a Landslide—Description of Features/Glossary

Crown cracks Original ground surface Crown

Main scarp Minor scarp Head Transverse cracks

Right flank Transverse ridges

Radial cracks

Surface of rupture Toe Main body

Foot Toe of surface of rupture

Surface of separation Figure A1. Parts of a landslide. (Modified from Varnes, 1978, reference 43). accumulation The volume of the displaced main body The part of the displaced mate- top The highest point of contact between material, which lies above the original rial of the landslide that overlies the surface the displaced material and the main scarp. ground surface. of rupture between the main scarp and the toe of surface of rupture The intersection toe of the surface of rupture. crown The practically undisplaced material (usually buried) between the lower part of still in place and adjacent to the highest parts main scarp A steep surface on the undis- the surface of rupture of a landslide and the of the main scarp. turbed ground at the upper edge of the land- original ground surface. slide, caused by movement of the displaced depletion The volume bounded by the zone of accumulation The area of the land- material away from the undisturbed ground. slide within which the displaced material lies main scarp, the depleted mass and the origi- It is the visible part of the surface of rupture. nal ground surface. above the original ground surface. minor scarp A steep surface on the dis- zone of depletion The area of the landslide depleted mass The volume of the displaced placed material of the landslide produced by within which the displaced material lies material, which overlies the rupture surface differential movements within the displaced below the original ground surface. but underlies the original ground surface. material. displaced material Material displaced original ground surface The surface of the from its original position on the slope by slope that existed before the landslide took Sources of information on movement in the landslide. It forms both the place. nomenclature: depleted mass and the accumulation. surface of separation The part of the flank The undisplaced material adjacent original ground surface overlain by the foot 1. Cruden, D.M., 1993, The multilingual to the sides of the rupture surface. Compass of the landslide. landslide glossary: Richmond, British Columbia, Bitech Publishers, for the directions are preferable in describing the surface of rupture The surface that forms IUGS Working Party on World flanks, but if left and right are used, they (or which has formed) the lower boundary Landslide Inventory in 1993. refer to the flanks as viewed from the crown. of the displaced material below the original foot The portion of the landslide that has ground surface. 2. Varnes, D.J., 1978, Slope movement types and processes, in Schuster, R.L., moved beyond the toe of the surface of rup- tip The point of the toe farthest from the and Krizek, R. J., eds., Landslides— ture and overlies the original ground surface. top of the landslide. Analysis and control: Transportation head The upper parts of the landslide along toe The lower, usually curved margin of Research Board Special Report 176, the contact between the displaced material the displaced material of a landslide, it is the National Research Council, Washington, and the main scarp. most distant from the main scarp. D.C., p. 11–23. 64 The Landslide Handbook—A­ Guide to Understanding Landslides

Part 3. Landslide Causes and Triggering Mechanisms

Physical Causes—Triggers Natural Causes

• Intense rainfall Geological causes Morphological causes • Rapid snowmelt • Weak materials, such as some • Tectonic or volcanic uplift • Prolonged intense precipitation volcanic slopes or unconsolidated • Glacial rebound • Rapid drawdown (of floods and marine sediments, for example • Glacial meltwater outburst tides) or filling • Susceptible materials • Fluvial erosion of slope toe • Earthquake • Weathered materials • Wave erosion of slope toe • Volcanic eruption • Sheared materials • Glacial erosion of slope toe • Thawing • Jointed or fissured materials • Erosion of lateral margins • Freeze-and-thaw weathering • Adversely oriented mass disconti- • Subterranean erosion (solution, • Shrink-and-swell weathering nuity (bedding, schistosity, and so forth) piping) • Flooding • Adversely oriented structural • Deposition loading slope or its crest discontinuity (fault, unconformity, • Vegetation removal (by forest fire, contact, and so forth) drought) • Contrast in permeability For further reading: • Contrast in stiffness (stiff, dense References 9, 3, and 45 material over plastic materials)

Human Causes

• Excavation of slope or its toe • Use of unstable earth fills, for construction • Loading of slope or its crest, such as placing earth fill at the top of a slope • Drawdown and filling (of reservoirs) • Deforestation—cutting down trees/logging and (or) clearing land for crops; unstable logging roads • Irrigation and (or) lawn watering • Mining/mine waste containment • Artificial vibration such as pile driving, explosions, or other strong ground vibrations • Water leakage from utilities, such as water or sewer lines • Diversion (planned or unplanned) of a river current or longshore current by construction of piers, dikes, weirs, and so forth