Landslide Glossary and Terms

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.

Bedding  surface/plane  In  sedimentary  or  stratified  rocks,  the  division  planes that  separate  each  successive  layer  or  bed  from  the  one  above  or  below.  It  is commonly marked by a visible change in lithology or colour.

Bedrock  The  solid  rock  underlying  gravel,  sand,  clay,  and  so  forth;  any  solid rock exposed at the surface of the earth or overlain by unconsolidated superficial material.

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

Check dams Check dams are 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  or natural habitat (such as a camp or unique spawning area) lays downslope.

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.

Debris  basin  (sometimes  called  catch  basins)  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  are  often  used  as debris basins.

Delta-front  landsliding  Delta  fronts  are  where  deposition  in  deltas  is  most active—underwater  landsliding  along  coastal  and  delta  regions  due  to  rapid sedimentation  of  loosely  consolidated  clay,  which  is  low  in  strength  and  high  in pore-water pressures.

Digital Elevation Model (DEM) A digital elevation model (DEM) is a digital file consisting of terrain elevations for ground positions at regularly spaced horizontal intervals. (A commercial definition – new technology)

Digital  Terrain  Model  (DTM)  The  term  used  by  United  States  Department  of Defense and other organizations to describe digital elevation data.

Drawdown  Lowering  of  water  levels  in  rivers,  lakes,  wells,  or  underground aquifers due to withdrawal of water. Drawdown may leave unsupported banks or poorly packed earth that can cause landslides.

Early  Warning  System  (EWS)  The  set  of  capacities  needed  to  generate  and disseminate  timely  and  meaningful  warning  information  to  enable  individuals, communities  and  organizations  threatened  by  a  hazard  to  prepare  and  to  act appropriately and in sufficient time to reduce the possibility of harm or loss.

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.

Elements  at  risk  The  population,  buildings  and  engineering  works,  economic activities, public services utilities, other infrastructures and environmental values in the area potentially affected by the landslide hazard.

Epicentre  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  shift,  crack,  and break as soils shrink and subside or expand. Also known as swelling soils.

Extensometer  An  instrument  for  measuring  small  deformations,  as  in  tests  of stress.

Factor of safety  The factor of safety, also known as  Safety Factor, is used to provide  a  design  margin  over  the  theoretical  design  capacity  to  allow  for uncertainty in the design process. The uncertainty could be any one of a number of  the  components  of  the  design  process  including  calculations  and  material strengths for example. Commonly, a factor of safety of less than 1, for instance,
on  an  engineered  slope  indicates  potential  failure,  where  a  factor  of  safety  of greater than 1, indicates stability.

Geodesic/geodetic  measurements  The  investigation  of  any  scientific questions connected with the shape and dimensions of the Earth.

Fracture  Brittle  deformation  due  to  a  momentary  loss  of  cohesion  or  loss  of resistance to differential stress and a release of stored elastic energy. Both joints and faults are fractures.

Geographic  Information  System  (GIS)  A  computer  program  and  associated data bases that permit cartographic  information (including geologic  information) to  be  queried  by  the  geographic  coordinates  of  features.  Usually  the  data  are organized  in  “layers”  representing  different  geographic  entities  such  as hydrology,  culture,  topography,  and  so  forth.  A  geographic  information  system, or  GIS,  permits  information  from  different  layers  to  be  easily  integrated  and analysed.

Geologic  hazard  (or  geohazard)  A  geologic  condition,  either  natural  or manmade,  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.

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

Geomorphology The science that treats the general configuration of the Earth’s surface;  specifically,  the  study  of  the  classification,  description,  nature,  origin, and  development  of  landforms  and  their  relationships  to  underlying  structures, and the history of geologic changes as recorded by these surface features.

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.

Hazard  A  condition  with  the  potential  for  causing  an  undesirable  consequence. The description of landslide hazard should include the location, volume (or area), classification  and  velocity  of  the  potential  landslides  and  any  resultant  detached material, and the probability of their occurrence within a given period of time.

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

Hydrology The science that relates to the water of the Earth.

Inclinometer Instrument for measuring inclination to the horizontal.

Landslide  dam  An  earthen  dam  created  when  a  landslide  blocks  a  stream  or river.

Lahar Landslide, debris flow or mudflow, of pyroclastic material on the flank of a volcano; deposit produced by such a debris flow. 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, it is termed a hot lahar.

Liquefaction  The  transformation  of  saturated,  loosely  packed,  coarse-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.

Landslide inventory maps Inventories identify areas that appear to have failed by landslide processes, including debris flows and cut-and-fill failures.

Landslide  susceptibility  map  This  map  goes  beyond  an  inventory  map  and depicts 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.

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.

Landslide  risk  map  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 socioeconomic effects on an area and (or) community.

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.

Loess  A  widespread,  homogenous,  commonly  nonstratified,  porous,  friable, slightly  coherent,  usually  highly  calcareous,  fine-grained  blanket  deposit (generally  less  than  30  m  thick)  consisting  predominantly  of  silt,  with subordinate grain sizes ranging from amounts of clay to fine sand.

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.

Mudslide  An  imprecise  but  popular  term  coined  in  California,  USA,  frequently used  by  the  general  public  and  the  news  media  to  describe  a  wide  scope  of events,  ranging  from  debris-laden  floods  to  landslides.  Not  technically  correct. Please see “mudflow,” next Glossary entry.

Mudflow  A  general  term  for  a  mass-movement  landform  and  process characterized  by  a  flowing  mass  of  predominately  fine-grained  earth  material possessing  a  high  degree  of  fluidity  during  movement.  The  water  content  may range up to 60 percent.

Perched ground water Unconfined ground water separated from an underlying main body of ground water by an unsaturated zone.

Piezometer  An  instrument  for  measuring  pressure  head  in  a  conduit,  tank,  or soil  —  it  is  a  small  diameter  water  well  used  to  measure  the  hydraulic  head  of ground water in aquifers.

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  or  by  direct  measurement  of  the  pressure  by  means  of  piezometers. 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.

Pore water, or interstitial water Subsurface water in an interstice, or pore.

Quick clay A clay that loses nearly all its shear strength after being disturbed; a clay that shows no appreciable gain in strength after remoulding.

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.

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

Risk  The  probability  of  occurrence  or  expected  degree  of  loss,  as  a  result  of exposure to a hazard.

Rock  mechanics  The  theoretical  and  applied  science  of  the  mechanical behaviour  of  rocks,  representing  a  “branch  of  mechanics  concerned  with  the response of rock to the force fields of its physical environment.”

Sag  pond  A  small  body  of  water  occupying  an  enclosed  depression  or  sag formed  where  active  or  recent  fault  or  landslide  movement  has  impounded drainage.

Seepage  Concentrated  subsurface  drainage  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.

Sea cliff retreat A cliff formed by wave action, causing the coastal cliff to erode and recede toward land.

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

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.

Soil mechanics The application of the principles of mechanics and hydraulics to engineering  problems dealing with the behaviour 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.

Strainmeter  A  seismometer  that  is  designed  to  detect  deformation  of  the ground by measuring relative displacement of two points.

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  centimetre;  also,  by  extension,  the  external  pressure  that  creates  the internal force.

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  or  by  human  activity  such  as  subsurface  mining  or  the pumping of oil or ground water.

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.

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

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

Vulnerability  The  degree  of  loss  to  a  given  element  or  set  of  elements  within the area affected by the landslide. It is expressed on a scale of 0 (no loss) to 1 (total loss). For property, the loss will be the value of the damage relative to the value  of  the  property;  for  persons,  it will  be the probability that a particular  life (the  element  at  risk)  will  be  lost,  given  the  person(s)  is  (are)  affected  by  the landslide.

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.

Weathering,  differential  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.

Weathering, mechanical 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.

Zoning:  the  division  of  land  into  homogeneous  areas  or  domains  and  their ranking according to degrees of actual or potential landslide susceptibility, hazard or risk or applicability of certain hazard-related regulations.

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