Landslides Glossary | U.S. Geological Survey

A

Alluvial fan

an outspread, gently sloping mass of alluvium deposited by a stream, especially in an arid or semiarid region where a stream issues from a narrow canyon onto a plain or valley floor. Viewed from above, it has the shape of an open fan, the apex being at the valley mouth.

B

Bedding suface/plane

in sedimentary or stratified rocks, the division planes which separate each successive layer or bed from the one above or below. It commonly marks a visible change in lithology or color. (American Geological Institute (AGI))

Bedrock

the solid rock underlying gravel, sand, slay, etc.; any solid rock exposed at the surface of the earth or overlain by unconsolidated material.

Borehole

a circular hole drilled into the earth, often to a great depth, as a prospective oil well, or for exploratory purposes.

C

Catch basin

see - debris basin

Check dams

small sediment storage dams built in the channels of steep gullies to stabilize the channel bed. A common use is to control channelized debris flow frequency and volume. Check dams are expensive to construct and are therefore usually only built where important installations, such as a camp or unique spawning area, lie downslope. Chatwin, et. al

Colluvium

a general term applied to loose and incoherent deposits, usually at the foot of a slope or cliff and brought there chiefly by gravity. Chatwin et. al

D

Debris basin

sometimes called "catch basin" - a large excavated basin into which a debris flow runs or is directed, and where it quickly dissipates its energy and deposits its load. Abandoned gravel pits or rock quarries can often be incorporated.

Debris flow

debris flows occur when masses of poorly sorted sediment, agitated and saturated with water, surge down slopes in response to gravity.

Differential weathering

when weathering across a rock face or exposure occurs at different rates, mainly due to variations in the composition and resistance of the rock. This results in an uneven surface with the more resistant material protruding.

Digital elevation model (DEM)

a digital elevation model is a digital file consisting of terrain elevations for ground positions at regularly-spaced horizontal intervals.

Digital terrain model (DTM)

a Three-dimensional model of digital elevation data (DEM - see previous definition) for cartographic representation. Terrain models are often displayed as grids, superimposed over topography maps to illustrate peaks and valleys.

Drawdown

lowering of water levels in rivers, lakes, or underground aquifers due to pumping or artesian flow. Drawdown may leave unsupported banks or poorly packed earth that can cause landslides.

E

Electronic distance meter (EDM)

a device that emits ultrasonic waves that bounce off solid objects and return to the meter. The meter's microprocessor then converts the elapsed time into a distance measurement. Sound waves spread 1 foot wide for every 10 feet measured. There are various types available.

Epicenter

the point on the earth's surface directly above the focus of an earthquake.

Expansive soils

types of soil that shrink or swell as the moisture content decreases or increases. Structures built on these soils may experience shifting, cracking, and breaking damage as soils shrink and subside or expand.

F

Fracture

breaks in rocks due to intense folding or faulting; can be caused by breaking oil-, gas-, or water-bearing strata by injecting a fluid under such pressure as to cause partings in the rock.

G

Geodesic/geodetic measurements

the investigation of any scientific questions connected with the shape and dimensions of the earth. AGI

Geographic Information System (GIS)

automated systems for the capture, storage, retrieval, analysis, and display of spatial data. (Clarke, 1990)

Geologic hazard

a geologic condition, either natural or man-made, that poses a potential danger to life and property. Examples: earthquake, landslides, flooding, faulting, beach erosion, land subsidence, pollution, waste disposal, and foundation and footing failures. AGI

Geologic map

a map on which is recorded the distribution, nature, and age relationships of rock units and the occurrence of structural features. AGI

Geomorphology

the science that treats the general configuration of the earth's surface; specifically, the study of the classification, description, nature, origen, and development of landforms and their relationships to underlying structures, and the history of geologic changes as recorded by these surface features. AGI

Geophysical studies

the science of the earth, by quantitative physical methods, with respect to its structure, composition, and development. It includes the sciences of dynamical geology and physical geography, and makes use of geodesy, geology, seismology, meteorology, oceanography, magnetism, and other earth sciences in collecting and interpreting earth data. AGI

H

Hydraulic

of or pertaining to fluids in motion; conveying, or acting, by water; operated or moved by means of water, as hydraulic mining. AGI

Hydrology

the science that relates to the water of the earth. AGI

L

Lahar

landslide or mudflow of pyroclastic material on the flank of a volcano; deposit produced by such a landslide. Lahars are described as wet if they are mixed with water derived from heavy rains, escaping from a crater lake or produced by melting snow. Dry lahars may result from tremors of a cone or by accumulating material becoming unstable on a steep slope. If the material retains much heat, they are termed hot lahars. AGI

Landslide

the movement of a mass of rock, debris, or earth down a slope. Earthquake shaking and other factors can also induce landslides underwater. These landslides are called submarine landslides. Submarine landslides sometimes cause tsunamis that damage coastal areas.

Landslide dam

an earthen dam created when a landslide completely blocks a stream or river.

Landslide hazard map

hazard maps show the areal extent of threatening processes: where landslide processes have occurred in the past, where they occur now, and the likelihood in various areas that a landslide will occur in the future. CO Survey, 1988

Landslide inventory maps

inventories identify areas that appear to have failed by landslide processes, including debris flows and cut-and-fill failures. CO Survey, 1988

Landslide risk map

these maps show landslide hazards and the probability that they will occur, expressed in statistical recurrence rates; risk maps may show cost/benefit relationships, loss potential and other potential socio-economic impacts on an area and/or community.

Landslide susceptibility map

these maps go beyond an inventory map and depict areas that have the potential for landsliding. These areas are determined by correlating some of the principal factors that contribute to landsliding, such as steep slopes, weak geologic units that lose strength when saturated, and poorly drained rock or soil, with the past distribution of landslides. CO Survey, 1988.

LIDAR (Light Detection and Ranging, also known as Airborne Laser Swath Mapping or ALSM)

a technology that employs an airborne scanning laser rangefinder to produce detailed and accurate topographic surveys. LIDAR can be used to accurately measure the topography of the ground, even where overlying vegetation is quite dense. (see Puget Sound LIDAR Consortium)

Liquefaction

the transformation of saturated, loosely packed, course-grained soils from a solid to a liquid state. The soil grains temporarily lose contact with each other and the particle weight is transferred to the pore water. CO Survey, 1988.

Lithology

the physical character of a rock, generally as determined at the microscopic level, or with the aid of a low-power magnifier; the microscopic study and description of rocks. AGI

Loess

a homogenous, nonstratified, deposit consisting predominantly of silt, with subordinate amounts of very fine sand and/or clay. AGI

M

Mechanical weathering

the physical processes by which rocks exposed to the weather change in character, decay, and crumble into soil. Processes include temperature change (expansion and shrinkage), freeze-thaw cycle, and the burrowing activity of animals. CO Landslide Mitigation Plan

Mitigation

activities that reduce or eliminate the probability of occurrence of a disaster and/or activities that dissipate or lessen the effects of emergencies or disasters when they actually occur. CO Survey, 1988.

Mudflow

a general term for a mass-movement landform and process characterized by a flowing mass of fine-grained earth material with a high degree of fluidity. The water content may range up to 60%; also spelled mudflow. AGI

Mudslide

an imprecise but popular term coined in California, USA, frequently used by laymen and the news media to describe a wide scope of events, ranging from debris-laden floods to landslides. Not technically correct. Please see "mudflow," previous glossary entry. CO Survey, 1988.

P

Perched ground water

unconfined ground water separated from the underlying main body of ground water by unsaturated rock. AGI

Pore water pressure

a measure of the pressure produced by the head of water in a saturated soil and transferred to the base of the soil through the pore water. This is quantifiable in the field by the measurement of free water surface level in the soil. Pore water pressure is a key factor in failure of a steep slope soil and operates primarily by reducing the weight component of soil shear strength. Chatwin and others, 1994.

Pore water, or interstitial water

subsurface water in an interstice, or pore. AGI

R

Reconnaissance geology/mapping

a general, exploratory examination or survey of the main features of a region, usually preliminary to a more detailed survey. It may be made in the field or office, depending on the extent of information available. Chatwin and others, 1994.

Relief

the difference in elevation between the high and low points of a land surface. AGI

Risk

the probability of occurrence or expected degree of loss, as a result of exposure to a hazard. CO Survey, 1988

Rock mechanics

the theoretical and applied science of the mechanical behavior of rocks, representing a "branch of mechanics concerned with the response of rock to the force fields of its physical environment."

S

Sag pond

a small body of water occupying an enclosed depression or sag formed where active or recent fault movement has impounded drainage. AGI

Sea cliff retreat

a cliff formed by wave action, causing the coastal cliff to erode and recede towards land. AGI

Seepage

and concentrated subsurface drainage are indicated by springs, sag ponds, or moist areas on open slopes, and seepage sites along road cuts. The locations of these areas of concentrated subsurface flow should be noted on maps and profiles as potential sites of active, unstable ground. Chatwin and others, 1994.

Shear

a deformation resulting from stresses that cause contiguous parts of a body to slide relatively to each other in a direction parallel to their plane of contact. AGI

Slurry

a highly fluid mixture of water and finely divided material; for example, pulverized coal and water for movement by pipeline, or of cement and water for use in grouting. AGI

Soil mechanics

the application of the principles of mechanics and hydraulics to engineering problems dealing with the behavior and nature of soils, sediments, and other unconsolidated accumulations; the study of the physical properties and utilization of soils, especially in relation to highway and foundation engineering. AGI

Stress

in a solid, the force per unit area, acting on any surface within it, and variously expressed as pounds or tons per square inch, or dynes or kilograms per square centimeter; also, by extension, the external pressure which creates the internal force. AGI

Sturzstroms

German for "fall stream". A huge mass of rapidly moving rock debris and dust, derived from the collapse of a cliff or mountainside, flowing down steep slopes and across low ground, often for several kilometers at speeds of more than 100 km/hr. Sturzstroms are the most catastrophic of all forms of mass movement.

Submarine and subaqueous landslide

deposits that occur from landslide processes under water. Submarine landslides can be for example, block slides, flow slides, mud slides. Large underwater landslides can displace water, sometimes rapidly, and cause tsunamis. Garrison and Sangrey, USGS, 1990.

Subsidence

sinking or downward settling of the earth's surface, not restricted in rate, magnitude, or area involved. Subsidence may be caused by natural geologic processes, such as solution, compaction, or withdrawal of fluid lava from beneath a solid crust; man's activity such as subsurface mining or the pumping of oil or ground water may also cause subsidence. AGI

Surficial geology

geology of surficial deposits, including soils; the term is sometimes applied to the study of bedrock at or near the earth's surface. AGI

Swelling soils

soils or soft bedrock which increase in volume as they get wet and shrink as they dry out. They are also commonly known as bentonite, expansive, or montmorillinitic soils.

T

Tensile stress

a normal stress that tends to pull apart the material on the opposite sides of the plane on which it acts. AGI

V

Vulnerability assessment

the susceptibility or exposure to injury or loss from a hazard.

W

Weathering

the destructive process by which earth and rock materials exposed to the atmosphere undergo physical disintegration and chemical decomposition resulting in changes in color, texture, composition, or form. Processes may be physical, chemical, or biological.

Z

Zonation

a term used generally, even vaguely, for a region of latitudinal character more or less set off from surrounding regions by some distinctive characteristic; for example, the earth's torrid zone, two temperate zones, and two frigid zones. For hazards, zones are geographic regions or designations that are differentiated through a variety of different criteria; for example, residential zones, zones of low hazard, zones of high hazard. AGI