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.