1. What is a landslide?

A landslide is a movement of a mass of rock, earth or debris down a slope due to gravity.

Landslides belong to a group of geological processes referred to as MASS MOVEMENT: mass movement involves the outward or downward movement of a mass of slope forming material, under the influence of gravity.

Landslide forms are distinguishable from other forms of mass movement by the presence of distinct boundaries and rates of movement perceptibly higher than any movement experienced on the adjoining slopes.

Image commonly used to illustrate the parts of a landslide (from Varnes, 1978)

The uppermost part is the depletion area (also: erosional, depletion or failure zone) where the slope material has failed and becomes displaced downslope. The displaced mass may remain close to the depletion area or it may continue to travel downslope, along a transport track, ending in an accumulation zone or acting as a supply to other geomorphic agents (e.g. river, sea, glacier etc.). The distance travelled by the landslide material (run out) is a specific characteristic of each type of landslide.

Landslide deposits range in texture from dislodged blocks of intact source material to highly fragmented sediments forming a poorly sorted, unstratified deposit.

Seven states of activity are recognized for ladslides and they can be classified by 2 main categories:

  1. Active: landslide has moved within the last twelve months,
  2. Inactive: landslide has not moved within the last twelve months.

In detail, landslides can be:

  • Active: landslide is currently moving.
  • Suspended: landslide has moved within the last twelve months but is not active at present.
  • Reactivated: landslide is active after a period of inactivity.
  • Dormant: inactive landslide that can be reactivated by its original causes or by other causes.
  • Abandoned: inactive landslide that is no longer affected by its original causes.
  • Stabilised: inactive landslide thanks to remedial/mitigation measures.
  • Relict: inactive landslide which developed under climatic or geomorphological conditions considerably different from the current ones. It is very unlikely that it can reactivate under the present conditions.

There are several morphological features that can be recognized to a greater or lesser extent in most landslides. The uppermost part is the depletion or concave zone (erosional, generating or failure zone) where slope material has failed and become displaced downslope. In some cases the displacement may be only a few metres while in others the failure zone will be completely evacuated to expose the surface of rupture and to leave a distinctive scar on the hillslope.

Fig. 1. Image commonly used to illustrate the part of a landslide (from Varnes, 1978)
Fig. 2. Translational rock slide, occured in Novembre 1994 in the Langhe Hills (Piemonte Region), in which can be clearly distiguished the main scarp, the flanks and the displaced material (photo from Archivio CNR-IRPI, Torino)

The displaced mass may remain close to the failure zone or it may continue to travel downslope leaving a transport track ending in a colluvial accumulation zone or acting as a supply to some other geomorphic agent (e.g. river, sea or glacier). The distance that landslide material travels (runout) is a characteristic of the type of land- slide. For example, controlled by the height of fall and volume, rock avalanches can travel at high velocity for several kilometres. Runout distance and velocity for other types of landslide are controlled by factors such as volume, slope angle and morphology, clay content, water content, and surface frictional characteristics of the runout pathway.

For the references herein and for knowing sources of didactic material go to 1.3 Selected references

Case of landslide mainly governed by climatic conditions:

1. Slope is stable: Water table is at its lowest level. The safety factor F is > 1 (the resisting forces are superior to the driving forces).

2. First failure phase: Due to the water table rises, the safety factor F is < 1. The Archimedes thrust lifts or lightens the soil. Thus, a decrease in the friction forces along the slip surface promotes movement. Rate of displacement can be more or less important. According to the climatic conditions, water table level could decrease and the slope could be “temporarily” stabilized (active or dormant!).

3. Second failure phase: In the event of rainy conditions, water table rises at a high level and major displacements occur. Main scarp is clearly observed. In the rear of the main scarp, on of the landslide crown, new cracks could appear.

4.Reactivation: If water table rises at its highest level, reactivation along extended slip surface occurs. Secondary scarp is now well observed. According to the predisposition factors (morphological features, geological factors, land cover), water table level (or triggering threshold) which provoke the instability is different for each slope.

Figure 1: Example of the Villerville-Cricqueboeuf landslide (>>see case study Villerville-Cricqueboeuf landslide): four states of instability in relation with piezometric variations and effective annual rainfall. Major displacements correspond to a repetition of these four phases with time (from Maquaire and Malet, 2006)

Note: delay between each phase can be very short (several days or months) or very long (several years or centuries!) – Notion of landslide activity

Landslide activity and morphological indicators:

Seven states of activity are recognized and can be classified in two categories:

  • Active: landslide has moved within the last twelve months,
  • Inactive: landslide has not moved within the last twelve months.

In detail, landslides can be:

  • Active: landslide is currently moving,
  • Suspended: landslide has moved within the last twelve months but is not active at present,
  • Reactivated: landslide is active but has been inactive.
  • Dormant: inactive landslide can be reactivated by its original causes or by other causes
  • Abandoned: inactive landslide is no longer affected by its original causes
  • Stabilised: inactive landslide has been protected from its original causes by remedial measures
  • Relict: inactive landslide which developed under climatic or geomorphological conditions considerably different from those at present. It can not be reactivated.

Figure 2 shows the seven states of activity of a landslide and morphological characteristics associated (from Dikau et al., 1996):

A. Active landslide: erosion at the toe of the slope causes the topple of a block
B. Suspended landslide: local cracking (red colour) in the crown of the topple
C. Reactivated landslide: another block topples, disturbing the previously displaced material
D. Dormant landslide: the displaced mass begins to regain its tree cover, scarps are modified by weathering
E. Abandoned landslide: fluvial deposition has protected the toe of the slope – F. Stabilised landslide: a wall protects the toe of the slope
G. Relict landslide: uniform tree cover has been established.

These stages can be successive in time. Figure 3 shows the block diagrams of morphological changes with time of a rotational slide, from active state to dormant state, in arid or semiarid climate (from Keaton and Degraaf, 1996). Morphological features are great indicators to assess the state of activity of a landslide:

1. Active or recently active landslide: features are sharply defined and distinct. Bedrock is freshly exposed in the main scarp or the flanks. Water is ponded in closed depressions caused by rotational movement or by blockage of original runoff paths.

2. Dormant-young landslide: features remain clear but are not sharply defined owing to slope wash and shallow mass movements on steep scarps. Although bedrock is still visible in many places, weathering has obscured the original structure. Drainage lines are not established.

3. Dormant-mature landslide: Drainage follows rifts and sags on slide mass, internal blocks are slightly dissected and material is eroded from slide mass.

4. Dormant-old landslide: features are weak and often subtle. Slide mass is almost completely removed, drainage network shows weak structural control, valley drainage re-establishes its pre-slide profile.

Figure 4 shows the block diagrams of morphological changes with time of a rotational slide, from active state to dormant state, in humid climate (from Keaton and Degraaf, 1996):

A. Active or recently active landslide: features are sharply defined and distinct. Bedrock is freshly exposed in the main scarp or the flanks. Many cracks exist above and subparallel to the main scarp; cracks extend across the slide; radial cracks occur within the toe portion. Original vegetation has been disrupted. The present orientation of vegetation indicates direction of principal movement and rotation (figure 5). Water is ponded in closed depressions caused by rotational movement or by blockage of original runoff paths.

B. Dormant-young landslide: features remain clear but are not sharply defined owing to slope wash and shallow mass movements on steep scarps. Although bedrock is still visible in many places, weathering has obscured the original structure. Cracks are no longer visible within or adjacent to the slide mass. Hydrophilic vegetation has established itself in the ponded areas. Minor scarps and transverse ridges have been modified and the ground has a distinctive hummocky appearance.

C. Dormant-mature landslide: features are modified by surface drainage, internal erosion and vegetation. Erosion has reduced the slopes of the scarp, flank and toe regions. Erosion has resulted in gullies and establishment of new drainage paths within and adjacent to the landslide. Likewise the original hummocky surface has been somewhat subdued.

D. dormant-old landslide: features are weak and often subtle. Slope breaks are indistinguishable from scarp, flank and toe regions. Neither vegetation nor developed runoff paths reflect original landslide boundaries. The disturbed area has been completely filled with silt and heavy vegetation has disguised its location.

Use of vegetation:

Vegetation is also used as indicator of landslide activity. It can permit to assess the degree of activity, age, type and component parts of a landslide. The term “dendrogeomorphology” (in Greek “dendro” means “tree”) is used to define the study of geomorphic processes using data gathered from trees. Tree growing on an instable slope is liable to develop a tilting or a curvature in the direction of the movement. Analysis of external deformations of the tree gives information on landslide activity.

Figure 5: Characteristically deformed tree on partially stabilized landslide (from Brunsden and Prior, 1984)

Trees that have been tilted from the vertical also undergo changes to internal tissue as they respond to the altered growing conditions. Coniferous trees support new growth by adding “compression tissue” on the lower side of the trunk, whereas deciduous trees develop “tension tissue” mainly on the upper side. Investigation of tissue disturbance in combination with analysis of the eccentricity of the tree annual rings can be used to determine dates of significant movement of the slope.

Figure 6: Mass movement induced movement of a coniferous tree, curvature of the trunk and consequent eccentric growth of annual rings (from Braam and al., 1987)

Dendrogeomorphology is used to assess slide activity, debris flow activity and also rock fall activity.

For the references herein and for knowing sources of didactic material go to 1.3 Selected references 

BRAAM R.R., WEISS E.E.J., BURROUGH P.A., 1987. Spatial and temporal analysis of mass movement using dendrochronology. Catena, vol. 14, 1987b. p. 573-584

BRUNSDEN D., PRIOR D., 1984. Slope instability. John Wiley & Sons Ltd, Chichester

CROZIER M.J., 2004. Landslide in GOUDIE A.S. (Ed.), 2004. Encyclopedia of Geomorphology. Vol. 2, 605-608. Routledge, London and New York

CRUDEN, D.M. & VARNES, D.J., 1996. Landslides Types and Processes. In: A.K. TURNER & R.L. SCHUSTER (Editors), Landslides: Investigation and Mitigation. Transportation Research Board, National Academy of Sciences, Washington D.C., Special Report 247, pp. 36-75.

DIKAU R., BRUNSDEN D., SCHROTT L., IBSEN M.-L. (eds.), 1996. Landslide Recognition: Identification, Movement and Causes. John Wiley & Sons Ltd, Chichester.

FLAGEOLLET, J.-C., MALET, J.-P., MAQUAIRE, O., SCHMUTZ, M., 2004. Integrated investigations on landslides: example of the Super-Sauze earthflow. In Margottini, C. (Ed): Natural Disasters and Sustainable Development, Springer-Verlag, Berlin, Chapter 14, 213-238.

GLADE, T., ANDERSON , M., CROZIER M., J. (ed.s), 2009. Landslide hazard and natural risks. John Wiley & Sons Ltd, Chichester.

HIGHLAND L. M., BOBWROSKY P., 2008. The Landslide Handbook – A Guide to Understanding Landslides. Downloadable at USGS website. http://pubs.usgs.gov/circ/1325/

IAEG (Commission on Landslides), 1990. Suggested nomenclature for Landslides. IAEG Bull., 41: 13-16.

KEATON, J.R., DEGRAAF, J.V., 1996. Surface Observation and Geologic Mapping. In: TURNER, SCHUSTER (EDS) “Landslides : investigation and mitigation”. Transportation Research Board – National Research Council, Special Report 247, Washington, D.C., National Academy Press, pp. 178-230.

MALET, J.-P., MAQUAIRE, O.. 2004. Hydrological behaviour of earthflows developed in clay-shales: investigation, concept and modelling. In Picarelli, L. (Ed): The Occurrence and Mechanisms of Flows in Natural Slopes and Earthfills, Patron Editore, Bologna, 175-193.

MAQUAIRE, O., 2005. Geomorphic hazards and natural risks, In: Koster, E., A. (ed.), The Physical Geography of Western Europe, Oxford Regional Environments, Oxford University Press, Chapter 18, 354-377.

MAQUAIRE, O., MALET J.-P., 2006. Shallow landslidings. In BOARDMAN, J. A, POESEN, J. (Eds): Soil Erosion in Europe, Wiley, Chapter 2.8, 583-598.

PASUTO A.& SCHROTT L. – 1997.  Eurolandslide. A visual journey through european mass movements. CNR – IRPI , Padova Italy. CD-ROM Windows-Macintosh

SASSA K. (ed.), 1999. Landslides of the World. Kyoto University Press, Kyoto,

TURNER A.K. & SCHUSTER R.L. (eds.),1996. Landslides: Investigation and Mitigation. Transportation Research Board, Special Report 247, National Academy Press, Washington D.C..

VALLARIO A., 1992. Frane e territorio. Liguori Editore, Napoli.

VARNES, D.J., 1978. Slope movements: types and processes. In: R.L. SCHUSTER & R.J. KRIZEK (Editors), Landslides: Analysis and Control. Transportation Research Board, National Academy of Sciences, Washington D.C., Special Report, 176, pp. 11-33.