1.2 More information on ‘landslide activity’

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