Landslide Hazard Overview

What is a Landslide?

The term landslide includes a wide range of ground movement, such as rock falls, deep failure of slopes, and shallow debris flows. Although gravity acting on an over-steepened slope is the primary reason for a landslide, there are other contributing factors:

  • erosion by rivers, glaciers, or ocean waves create oversteepened slopes
  • rock and soil slopes are weakened through saturation by snowmelt or heavy rains
  • earthquakes create stresses that make weak slopes fail
  • earthquakes of magnitude 4.0 and greater have been known to trigger landslides
  • volcanic eruptions produce loose ash deposits, heavy rain, and debris flows
  • excess weight from accumulation of rain or snow, stockpiling of rock or ore, from waste piles, or from man-made structures may stress weak slopes to failure and other structures

Slope material that become saturated with water may develop a debris flow or mud flow. The resulting slurry of rock and mud may pick up trees, houses, and cars, thus blocking bridges and tributaries causing flooding along its path.

Landslides occur when the strength of rocks or soil is exceeded by stress applied to those hillslope materials.  Common stresses are gravity, increased pore-water pressure, earthquake shaking, and slope modification.

Some of the most common terms are landslide, mudslide, and rockslide.  Other terms such as mass wasting, slope movement, and slope failure are also commonly used to discuss landslide phenomena.  Regardless of which term is used, all landslides share physical and mechanical (in rock and soil) processes that explain their occurrence.


Descriptions of Landslide Types

The following six descriptions are modified from Highland and Bobrowsky (2008).

  • Translational slides—In a translational slide, the slide mass moves down a relatively planar surface, often the contact between soil and underlying bedrock.
  • Rotational slides—Also called slumps, these slides are distinguished by an upwardcurved slide plane, which causes rotational movement
  • Flows—Types of flows include debris flow, debris avalanche, and earthflow.
  • Creep—Creep is an extremely slow type of flow (less than 1 m per decade) that can only be noticed by its effects.
  • Spreads—Spreads usually occur on very gentle slopes where soft, clay-rich layers undergo lateral extension, which spreads apart overlying firmer rocks and soil. Spreads can occur in clayey lacustrine and other glacial deposits.
  • Rockfalls and topples—Rock material of varying size can free-fall through the air from cliffs, roadcuts, or steep slopes.

Generally, more landslides occur when average statewide rainfall is higher. Landslides also typically occur more in the late winter and early spring months, when rainfall or snowmelt is high relative to the rest of the year.

Link to the Kentucky Hazard Mitigation PlanLandslide Risk Assessment for complete information and maps.


KGS Landslide Resources

KGS Landslide Information Map: http://kgs.uky.edu/kgsmap/kgsgeoserver/viewer.asp?layoutid=25.  This map is a qualitative evaluation of known landslides. The interactive and customizable map service shows data from the KGS statewide landslide inventory database within a geologic and geomorphic context. This map services assists users in investigations of landslide hazard areas by showing the known existing landslides. This information allows for better determination of where future landslides may occur and help develop hazard mitigation strategies.  Help file: http://kgs.uky.edu/kgsmap/helpfiles/landslide_help.shtm

KGS Landslide Inventory: http://kgs.uky.edu/kgsweb/olops/pub/kgs/IC31_12.pdf.  An information circular explaining the how’s and why’s of the landslide inventory work.

Landslides and Your Property: http://kgs.uky.edu/kgsweb/olops/pub/kgs/LandslidesBrochure.pdf.  A combined pamphlet-style publication by the KGS, U. of Cincinnati, the Indiana Geological Survey, and the Ohio Geological Survey.

The Geologic Context of Landslide and Rockfall Maintenance Costs in Kentucky: http://kgs.uky.edu/kgsweb/olops/pub/kgs/RI34_12.pdf.  A KGS report of investigations that assessed KYTC landslide and rockfall cost data and geology along roadways.

Using LiDAR to Map Landslides in Kenton and Campbell Counties, Kentuckyhttp://kgs.uky.edu/kgsweb/olops/pub/kgs/ri24_12.pdf.  KGS report of investigations that mapped existing landslides using LiDAR.

Geologic, geotechnical, and geophysical investigation of a shallow landslide, eastern Kentuckyhttp://kgs.uky.edu/kgsweb/olops/pub/kgs/RI29_12.pdf.  KGS report of investigations that characterized and monitored a shallow colluvial landslide near Ashland, KY.


USGS Landslide Links

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Landslides in Colluvium 1994, Fleming, Robert W.; Johnson, Arvid M. USGS Bulletin: 2059-B.  Link here for Full-text PDF or go to http://pubs.er.usgs.gov/publication/b2059B


USGS Fact Sheet  – USGS Emergency Response Resources

The USGS Emergency Response Resources Fact Sheet is available.   This is a quick reference guide to USGS response resources including:

  • real-time information for flooding, earthquakes, landslides, and volcanoes
  • flood and hurricane databases
  • geospatial data and services supporting emergency response
  • USGS offices supporting emergency response

Link to the fact sheet USGS Emergency Response Resources.