Landslides, as a major type of geological hazard, represent one of the natural events that occur most frequently worldwide after hydro-meteorological events. The occurrence of landslides depends on complex interactions among a large number of partially interrelated factors, such as geological setting, geomorphic features, seismicity, soil properties, land cover characteristics, hydrological and the effects and impacts of anthropogenic changes to the landscape. Landslide predisposing or preparatory variables making the slopes susceptible to failure include soil and rock geo-mechanical properties, slope gradient and aspect, elevation, land cover, lithology and drainage patterns; triggering or dynamic factors are those initiating landslide movements, and might be either natural or human-induced, or even any combination of both (Dai and Lee, 2002). Natural triggers include intense or prolonged rainfall, earthquakes, volcanic eruptions, rapid snowmelt and permafrost thawing, and slope undercutting by rivers or sea waves. Other factors capable of acting as triggers for landslide failures are human activities such as slope excavation and loading, land use changes (e.g. deforestation), rapid reservoir drawdown, blasting vibrations, and water leakage from utilities. Earthquakes are notorious for triggering landslides. Slow-moving landslides such as those caused by subsidence and large scale slope deformation are other forms of landslides to be considered.

Landslides represent a main hazard in mountainous and hilly regions as well as along steep riverbanks and coastlines, and their impacts depend largely on the area and volume involved, the motion velocity and intensity, number and distribution of elements at risk, their vulnerability and their exposure value.

Landslide hazard and risk analyses are important steps for urban planning in government strategies, but they are often challenging tasks that require expertise and availability of multiple datasets (Carrara et al., 2003; Fell et al., 2008). In the geo-hazard community, landslide risk is generally defined as a measure of the expected probability of a damaging event in a given area and it results from the product of three macro-factors: hazard, vulnerability and exposure of the elements at risk (Van Westen et al.2006; Sassa et al., 2005; Pellicani et al., 2014). Hazard is the temporal and/or spatial likelihood (susceptibility) of a potentially damaging landslide, occurring within a given area; vulnerability is the degree of loss to specific elements within the area affected by the landslide event; and exposure is an intrinsic value referred to the location, characteristics and cost of the elements at risk (Glade et al., 2005). The risk analysis, prior to risk management, is usually based on the computation of these sub-components.

The landslide hazard is usually defined as the probability of occurrence of a potentially damaging phenomenon within a certain area and in a given period of time (Varnes et al., 1984). Thus, a landslide hazard zonation requires identifying those areas which could be affected by a damaging landslide and assessing the probability of landslide occurrence within a time span (Bianchini et al., 2017). However, the indication of a recurrence time for landslides is very difficult to determine, even under ideal conditions. As a result, landslide hazard is often represented by the landslide susceptibility (Dai et al., 2002). The landslide susceptibility only identifies areas potentially affected by instability phenomena and does not imply an occurrence time for the events.

Taking into account the different objectives and tasks regarding landslide hazard in deferred, near-real and real time, the main possible actions and implementations can be divided overall in mainly scientific tasks and operational actions, and can be summarized as follows:

	Scientific actions	Operational actions
Deferred time	• Mapping and long-term monitoring • Typology and kinematics • Modelling and prediction • Vulnerability assessment and modelling	• Inventory (location, type, area) • State and style of activity • Magnitude (intensity, volume) • Monitoring of areas at higher hazard/risk • Forecasting
Near-real and Real Time	• Mapping landslide events and their consequences • Statistics of landslide event inventories • Definition of landslide triggers and related thresholds • Event vulnerability assessment and modelling	• Residual risk definition and mapping • Post-event motion assessment • Residual hazard and risk zonation

Table 1 - Main possible actions and implementations regarding landslide hazard and risk.

 

The spatial scale for landslide phenomena ranges between regional and local, i.e. varies from studies of landslide mapping over very wide areas (up to a few thousands of square kilometres) to analysis of isolated phenomena. For this reason, the technologies supporting landslide studies should guarantee both large area coverage and access to detailed information over very small areas (e.g. a few square meters), as well as very accurate ground motion characterization. 

Temporal scales for landslide hazards are strongly controlled by the intensity of the observed phenomena and may range from monthly observations for extremely slow processes to daily or even hourly observations for more rapid phenomena.

Spatial and temporal scales also vary from phase to phase of the landslide management cycle, which deals with different needs in terms of frequency and resolution of information. More detailed information is required during response and recovery phases, in terms of both spatial and temporal sampling of the observed phenomena; up to centimetre resolutions might be required, with temporal resolutions as high as every few minutes during emergencies.

Concerning the temporal characterization of landslide, the recurrence, which is the expected time for the repetition of an event, is evaluated studying historical records. Historical data however are seldom available and difficult to obtain for single landslides or landslide prone areas (Guzzetti et al., 1994, 1999; Ibsen and Brunsden, 1996). In addition, for first-time failures (Hutchinson, 1988) recurrence is not applicable. First-time landslides occur at or close to peak strength values, whereas reactivations occur between peak and residual conditions. Thus, first-time landslides provide little information on the behavior of reactivations. Additionally, each time a landslide occurs, the topographic, geological and hydrological settings of the slope change, often dramatically, giving rise to different conditions of instability. These changes allow geomorphologists to identify landslides and understand mechanisms and causes of failures, but limit their ability to forecast reactivations. Despite the lack of consensus on the reliability and usefulness of historic information, some investigators have attempted the reconstruction of historical records for single landslides or landslide prone regions. The results appear to be somewhat encouraging and useful for the evaluation of landslide hazard at various scales (Guzzetti et al., 1994, 1999; Ibsen and Brunsden, 1996; Cruden, 1997; Evans, 1997; Glade, 1998). Historical records may be integrated with temporal data derived from dendrocronology and other dating techniques which have been used by some investigators to date landslide deposits (Stout, 1977; DeGraff and Agard, 1984; Trustrum and De Rose, 1988).

 

 

last update: 23-July-2020