Landslide classification
There have been known various classifications of landslides. Broad definitions include forms of mass movement that narrower definitions exclude. For example, the McGraw-Hill Encyclopedia of Science and Technology distinguishes the following types of landslides:
- fall
- fall
- slump
- rockslide
- earthflow
- sinkholes, mountain side
- rockslide that develops into rock avalanche
The causes of landslides are usually related to instabilities in slopes. It is usually possible to identify one or more landslide causes and one landslide trigger. The difference between these two concepts is subtle but important. The landslide causes are the reasons that a landslide occurred in that location and at that time and may be considered to be factors that made the slope vulnerable to failure, that predispose the slope to becoming unstable. The trigger is the single event that finally initiated the landslide. Thus, causes combine to make a slope vulnerable to failure, and the trigger finally initiates the movement. Landslides can have many causes but can only have one trigger. Usually, it is relatively easy to determine the trigger after the landslide has occurred.
Classification factors
Various scientific disciplines have developed taxonomic classification systems to describe natural phenomena or individuals, like for example, plants or animals. These systems are based on specific characteristics like shape of organs or nature of reproduction. Differently, in landslide classification, there are great difficulties because phenomena are not perfectly repeatable; usually being characterised by different causes, movements and morphology, and involving genetically different material. For this reason, landslide classifications are based on different discriminating factors, sometimes very subjective. In the following write-up, factors are discussed by dividing them into two groups: the first one is made up of the criteria utilised in the most widespread classification systems that can generally be easily determined. The second one is formed by those factors that have been utilised in some classifications and can be useful in descriptions.A1) Type of movement
This is the most important criterion, even if uncertainties and difficulties can arise in the identification of movements, being the mechanisms of some landslides often particularly complex. The main movements are falls, slides and flows, but usually topples, lateral spreading and complex movements are added to these.A2) Involved material
, earth and debris are the terms generally used to distinguish the materials involved in the landslide process. For example, the distinction between earth and debris is usually made by comparing the percentage of coarse grain size fractions. If the weight of the particles with a diameter greater than 2 mm is less than 20%, the material will be defined as earth; in the opposite case, it is debris.A3) Activity
The classification of a landslide based on its activity is particularly relevant in the evaluation of future events. The recommendations of the WP/WLI define the concept of activity with reference to the spatial and temporal conditions, defining the state, the distribution and the style. The first term describes the information regarding the time in which the movement took place, permitting information to be available on future evolution, the second term describes, in a general way, where the landslide is moving and the third term indicates how it is moving.A4) Movement velocity
This factor has a great importance in the hazard evaluation. A velocity range is connected to the different type of landslides, on the basis of observation of case history or site observations.B1) The age of the movement
dating is an interesting topic in the evaluation of hazard. The knowledge of the landslide frequency is a fundamental element for any kind of probabilistic evaluation. Furthermore, the evaluation of the age of the landslide permits to correlate the trigger to specific conditions, as earthquakes or periods of intense rains. It is possible that phenomena could be occurred in past geological times, under specific environmental conditions which no longer act as agents today. For example, in some Alpine areas, landslides of the Pleistocene age are connected with particular tectonic, geomorphological and climatic conditions.B2) Geological conditions
This represent a fundamental factor of the morphological evolution of a slope. Bedding attitude and the presence of discontinuities or faults control the slope morphogenesis.B3) Morphological characteristics
As the landslide is a geological volume with a hidden side, morphological characteristics are extremely important in the reconstruction of the technical model.B4) Geographical location
This criterion describes, in a general way, the location of landslides in the physiographic context of the area. Some authors have therefore identified landslides according to their geographical position so that it is possible to describe "alpine landslides", "landslides in plains", "hilly landslides" or "cliff landslides". As a consequence, specific morphological contexts are referred characterised by slope evolution processes.B5) Topographical criteria
With these criteria, landslides can be identified with a system similar to that of the denomination of formations. Consequently, it is possible to describe a landslide using the name of a site. In particular, the name will be that of the locality where the landslide happened with a specific characteristic type.B6) Type of climate
These criteria give particular importance to climate in the genesis of phenomena for which similar geological conditions can, in different climatic conditions, lead to totally different morphological evolution. As a consequence, in the description of a landslide, it can be interesting to understand in what type of climate the event occurred.B7) Causes of the movements
In the evaluation of landslide susceptibility, causes of the triggers is an important step. Terzaghi describes causes as "internal" and "external" referring to modifications in the conditions of the stability of the bodies. Whilst the internal causes induce modifications in the material itself which decrease its resistance to shear stress, the external causes generally induce an increase of shear stress, so that block or bodies are no longer stable. The triggering causes induce the movement of the mass. Predisposition to movement due to control factors is determining in landslide evolution. Structural and geological factors, as already described, can determine the development of the movement, inducing the presence of mass in kinematic freedom.Types and classification
In traditional usage, the term landslide has at one time or another been used to cover almost all forms of mass movement of rocks and regolith at the Earth's surface. In 1978, in a very highly cited publication, David Varnes noted this imprecise usage and proposed a new, much tighter scheme for the classification of mass movements and subsidence processes. This scheme was later modified by Cruden and Varnes in 1996, and influentially refined by Hutchinson and Hungr et al.. This full scheme results in the following classification for mass movements in general, where bold font indicates the landslide categories:Under this definition, landslides are restricted to "the movement... of shear strain and displacement along one or several surfaces that are visible or may reasonably be inferred, or within a relatively narrow zone", i.e., the movement is localised to a single failure plane within the subsurface. He noted landslides can occur catastrophically, or that movement on the surface can be gradual and progressive. Falls, topples, spreads, flows, and creep are all explicitly excluded from the term landslide.
Under the scheme, landslides are sub-classified by the material that moves, and by the form of the plane or planes on which movement happens. The planes may be broadly parallel to the surface or spoon-shaped. Material may be rock or regolith, with regolith subdivided into debris and earth.
Nevertheless, in broader usage, many of the categories that Varnes excluded are recognised as landslide types, as seen below. This leads to ambiguity in usage of the term.
The following clarifies the usages of the various terms in the table. Varnes and those who later modified his scheme only regard the slides category as forms of landslide.
Falls
Description: " the detachment of soil or rock from a steep slope along a surface on which little or no shear displacement takes place. The material then descends mainly through the air by falling, bouncing, or rolling".Secondary falls: "Secondary falls involves rock bodies already physically detached from cliff and merely lodged upon it"
Speed: from very to extremely rapid
Type of slope: slope angle 45–90 degrees
Control factor: Discontinuities
Causes: ''Vibration, undercutting, differential weathering, excavation, or stream erosion''
Topples
Description: "Toppling is the forward rotation out of the slope of a mass of soil or rock about a point or axis below the centre of gravity of the displaced mass. Toppling is sometimes driven by gravity exerted by material upslope of the displaced mass and sometimes by water or ice in cracks in the mass"Speed: extremely slow to extremely rapid
Type of slope: slope angle 45–90 degrees
Control factor: Discontinuities, lithostratigraphy
Causes: ''Vibration, undercutting, differential weathering, excavation, or stream erosion''