Permanent Scatterers for landslide investigations: outcomes from the ESA-SLAM project

Within the SLAM project (Service for Landslide Monitoring), launched in 2003 by the European Space Agency (ESA) the Permanent Scatterers (PS) technique, a multi-image interferometric approach, coupled with the interpretation of aerial-photos and optical satellite images, was carried out for landslide investigations. The PS analysis was applied at a regional scale as support for landslide inventory mapping and at local scale for the monitoring of single well-known slope movements. For the integration of the PS measurements within a landslide inventory the Arno river basin (Italy) was chosen as test site for the presence of a high number of mass movements (to date about 300 areas at high landslide risk and more than 27,000 individual landslides mapped by the institutional authorities). About 350 SAR images have been interferometrically processed by means of the PS technique, with the detection of about 600,000 PS. The use of optical images contributed spatial meaning to the point-wise information provided by the PS, making it easier to identify terrain features related to slope instability and the landslide boundaries. Here we describe the employed methodology and its impact in the updating of a preexisting landslide inventory. 6.8% of the total number of landslides were characterized by ground displacement measurements from the PS: 6.1% of already mapped landslides and 0.8% of new unstable areas detected through the PS analysis. Moreover, most of the PS are located in urban areas, showing that the proposed methodology is suitable for landslide mapping in areas with a quite high density of urbanization, but that over vegetated areas it still suffers from the limitations induced by the current space-borne SAR missions (e.g. temporal de-correlation). On the other hand, the use of InSAR for the monitoring of single slow landslides threatening built-up areas has provided satisfactory results, allowing the measurement of superficial deformations with high accuracy on the landslide sectors characterized by a good radar reflectivity and coherence.     

Permanent Scatterers for landslide investigations: outcomes from the ESA-SLAM project

Within the SLAM project (Service for Landslide Monitoring), launched in 2003 by the European Space Agency (ESA) the Permanent Scatterers (PS) technique, a multi-image interferometric approach, coupled with the interpretation of aerial-photos and optical satellite images, was carried out for landslide investigations. The PS analysis was applied at a regional scale as support for landslide inventory mapping and at local scale for the monitoring of single well-known slope movements. For the integration of the PS measurements within a landslide inventory the Arno river basin (Italy) was chosen as test site for the presence of a high number of mass movements (to date about 300 areas at high landslide risk and more than 27,000 individual landslides mapped by the institutional authorities). About 350 SAR images have been interferometrically processed by means of the PS technique, with the detection of about 600,000 PS. The use of optical images contributed spatial meaning to the point-wise information provided by the PS, making it easier to identify terrain features related to slope instability and the landslide boundaries. Here we describe the employed methodology and its impact in the updating of a preexisting landslide inventory. 6.8% of the total number of landslides were characterized by ground displacement measurements from the PS: 6.1% of already mapped landslides and 0.8% of new unstable areas detected through the PS analysis. Moreover, most of the PS are located in urban areas, showing that the proposed methodology is suitable for landslide mapping in areas with a quite high density of urbanization, but that over vegetated areas it still suffers from the limitations induced by the current space-borne SAR missions (e.g. temporal de-correlation). On the other hand, the use of InSAR for the monitoring of single slow landslides threatening built-up areas has provided satisfactory results, allowing the measurement of superficial deformations with high accuracy on the landslide sectors characterized by a good radar reflectivity and coherence.     

Investigating landslides with space-borne Synthetic Aperture Radar (SAR) interferometry

This paper is addressed to readers without advanced knowledge of remote sensing. It illustrates some current and potential uses of satellite Synthetic Aperture Radar interferometry (InSAR) for landslide assessment. Data acquired by SAR systems can provide 3D terrain models and be used to assist in regional scale investigations, e.g. aimed at evaluation of susceptibility of slopes to failure. Under favourable environmental conditions, the innovative Permanent Scatterers (PS) technique, which overcomes several limitations of conventional SAR differential interferometry (DInSAR) applications in landslide studies, is suitable for monitoring slope deformations with millimetric precision. The PS technique combines the wide-area coverage typical of satellite imagery with the capability of providing displacement data relative to individual image pixels. With the currently available radar satellites, however, only very slow ground surface displacements can be reliably detected and measured. The presented case study of a landslide from the Liechtenstein Alps indicates that the most attractive and reliable contribution provided by this remote sensing technique lies in the possibility of (i.) wide-area qualitative distinction between stable and unstable areas and (ii.) qualitative (relative) hazard zonation of large, slow landslides based on the identification of segments characterised by different movement rates. Since only the radar line of sight projection of the displacements can be detected, a quantitative exploitation of the PS data is possible only where sufficient ground truth is available. In site specific or single landslide investigations the PS data can represent a very useful complementary data source with respect to the information acquired through ground based observations and in situ surveying. However, the difficulties associated with the feasibility assessments of the applicability of SAR data to local scale problems, as well as with the interpretation of PS results, require a close collaboration between landslide experts and specialists in advanced processing of radar satellite data. The interpretation of the exact geotechnical significance of small, radar sensed ground surface deformations is challenging, especially where ground truth is lacking. Although any ground deformation is potentially of interest to an engineering geologist, detection of movements in both vertical and horizontal directions is needed in the case of landslides to evaluate slope failure mechanisms. With their high radar viewing angles, however, the current space-borne systems can detect only a fraction of the horizontal component of movement. It is expected that the upcoming SAR dedicated missions with new sensors and different acquisition geometries, combined with the rapid developments in the field of advanced radar data processing, will allow a full 3D reconstruction of deformation data and help to further reduce the current limitations of the PS and similar DInSAR approaches.     

Survey and monitoring of landslide displacements by means of L-band satellite SAR interferometry

This paper illustrates the capabilities of L-band satellite SAR interferometry for the investigation of landslide displacements. SAR data acquired by the L-band JERS satellite over the Italian and Swiss Alps have been analyzed together with C-band ERS-1/2 SAR data and in situ information. The use of L-band SAR data with a wavelength larger than the usual C-band, generally considered for ground motion measurements, reduces some of the limitations of differential SAR interferometry, in particular, signal decorrelation induced by vegetation cover and rapid displacements. The sites of the Alta Val Badia region in South Tyrol (Italy), Ruinon in Lombardia (Italy), Saas Grund in Valais (Switzerland) and Campo Vallemaggia in Ticino (Switzerland), representing a comprehensive set of different mass wasting phenomena in various environments, are considered. The landslides in the Alta Val Badia region are good examples for presenting the improved performance of L-band in comparison to C-band for vegetated areas, in particular concerning open forest. The landslides of Ruinon, Saas Grund, and Campo Vallemaggia demonstrate the strength of L-band in observing moderately fast displacements in comparison to C-band. This work, performed with historical SAR data from a satellite which operated until 1998, demonstrates the capabilities of future planned L-band SAR missions, like ALOS and TerraSAR-L, for landslide studies.     

Retrieval of time series three-dimensional landslide surface displacements from multi-angular SAR observations

A major limitation for wide application of Synthetic Aperture Radar (SAR) remote sensing in mapping landslide surface displacements is the intrinsic gap between the ultimate objective of measuring three-dimensional displacements and the limited capability of detecting only one or two-dimensional displacements by repeat-pass SAR observations of identical imaging geometries. Although multi-orbit SAR observations of dissimilar viewing geometries can be jointly analyzed to inverse the three-dimensional displacements, the reliability of inversion results might be highly questionable in case of continuous motion because of the usually asynchronous acquisitions of multi-orbit SAR datasets. Aiming at this problem, we proposed an approach of retrieving time series three-dimensional displacements from multi-angular SAR datasets for step-like landslides in the Three Gorges area in this article. Firstly, time series displacements of a common ground target in the azimuth and line-of-sight (LOS) direction can be estimated using traditional methods of SAR interferometry (InSAR) and SAR pixel offset tracking (POT), respectively. Then, a spline fitting and interpolation procedure was employed to parameterize the displacement history in the sliding/dormant periods of step-like landslides and estimate displacements from multi-angular observations for identical date series. Finally, three-dimensional displacements can be inverted from these synchronized multi-angular measured displacements in traditional ways. As a case study, the proposed method was applied to retrieve the three-dimensional displacements history of the Shuping landslide in the Three Gorges area, China. Comparisons between SAR-measured displacements and measurements of global positioning system (GPS) showed good agreement. Furthermore, temporal correlation analyses suggest that reservoir water level fluctuation and rainfall are the two most important impact factors for the Shuping landslide stability.     

Deep-seated gravitational slope deformations triggered by extreme rainfall and agricultural practices (eastern Michoacan,Mexico)

The study of deep-seated gravitational slope deformations (DSGSD) in Mexico is scarce; therefore, their localization and causes are highly overlooked. The present paper examines the characterization of the DSGSD of Jungapeo and Las Pilas in eastern Michoacan state, currently active and endangering their inhabitants. An integrated study, including detailed lithology, morpho-structural inventories, analysis of land use, and pluviometric regime, was performed and complemented with differential global positioning system monitoring networks. Both landslides developed over highly weathered volcano-sedimentary rocks. On the one hand, the Jungapeo landslide has an estimated volume of 990,455 m³ with steady decreasing velocity rates from 41 to 15 cm/month in the first monitoring period to 13–3 cm/month in the last one. On the other hand, the Las Pilas landslide estimated volume is about 1,082,467 m³ with a stable velocity rate of 1.3 to 0.1 cm/month. Despite the multi-storeyed style of activity, two behaviors of instability were distinguished: slow deformation and secondary landslide stages. The conditioning factors for slow deformation in both DSGSD are the combination of weathered lithology with clay- and sand-rich content, and the shift toward intensive monoculture. The triggering factor is related to excess water produced by an inefficient flood-irrigation system that also generates an atypical acceleration behavior in both landslides during the dry season. The DSGSD activity thus predisposes the generation of tension cracks and secondary scarps from which the collateral landslides are triggered by atypical rainfall, such as that of 2010.     

时序InSAR技术三峡库区藕塘滑坡稳定性监测与状态更新

坡体表面形变是表征坡体稳定性的重要信息,因此,非常有必要对滑坡多发区域进行时序常规变形监测.近年来,星载合成孔径雷达数据由于其覆盖范围大、形变监测精度高的特点,被越来越多的用于山区滑坡识别与探测.首先介绍了联合分布式目标与点目标的时序InSAR方法,并将该方法应用于分析覆盖三峡藕塘滑坡的2007年至2011年的19景ALOS PALSAR数据和2015年至2018年的47景Sentinel-1数据,提取了数据覆盖时间段内的藕塘地区的变形速率.发现相比于2007年至2011年,2015年至2018年新增三处不稳定斜坡.进一步对滑坡的时序变形分析表明,降雨和水位变化是坡体稳定性最大的两个影响因素.实验证明时序InSAR方法可以作为常规形变手段来识别与监测三峡库区等地区潜在的滑坡,为防灾减灾提供支持与依据.      

Ground-based SAR interferometry for monitoring mass movements

An innovative technique for the remote assessment of ground displacements, based on radar interferometry and implemented using ground-based instrumentation (GB-InSAR), has been tested in recent years on a number of selected case sites. The system, known as LISA, developed by the Joint Research Centre (JRC) of the European Commission, is a ground-based radar interferometer specifically designed for field use. It is composed of two radar antennas mounted on a linear rail which horizontally slides to form a synthetic aperture. Coherent SAR processing converts the raw data into an image containing, for each pixel, information on the wave phase, which depends on the target-sensor distance. Consecutive couples of SAR images can be cross-correlated to form interferograms representing phase variations which can be directly related to ground displacement along the sight-line of the radar system, since they are acquired from exactly the same position. Several applications of the system have been conducted on a number of mass movements located in Italy, in order to validate the technique for the monitoring of landslides. GB-InSAR has proved its potential for the measurement of the superficial ground displacements of different landslide types, in terms of failure mechanism, materials involved, kinematics, water content and deformation rates. In particular conditions, such as fast-moving phenomena and inaccessible areas, the technique can be employed directly as a monitoring tool, providing multi-temporal displacement maps of the observed area. Additionally, some applications of the GB-InSAR have provided a fundamental support to decision makers during landslide emergencies, allowing the civil protection authorities to assess the risk and to manage an effective emergency response.     

The new landslide inventory of Tuscany (Italy) updated with PS-InSAR: geomorphological features and landslide distribution

In this paper, the updating of the landslide inventory of Tuscany region is presented. To achieve this goal, satellite SAR data processed with persistent scatter interferometry (PSI) technique have been used. The updating leads to a consistent reduction of unclassified landslides and to an increasing of active landslides. After the updating, we explored the characteristics of the new inventory, analysing landslide distribution and geomorphological features. Several maps have been elaborated, as sliding index or landslide density map; we also propose a density-area map to highlight areas with different landslide densities and sizes. A frequency-area analysis has been performed, highlighting a classical negative power-law distribution. We also explored landslide frequency for lithology, soil use and several morphological attributes (elevation, slope gradient, slope curvature), considering both all landslides and classified landslide types (flows, falls and slides).     

Long-term measurements using an integrated monitoring network to identify homogeneous landslide sectors in a complex geo-environmental context (Lago,Calabria, Italy)

Mass movements of different thicknesses and types affect the slope where the Greci and Piscopie villages lie (Lago, Calabria, Italy). The study concerns a moderately urbanized area where the lithological, structural, and hydrogeological conditions have produced a large and complex unstable area. In the study area, a multi-temporal field survey has allowed for detailed landslide inventory mapping and definition of the geometric and kinematic characteristics of mass movements with long-term monitoring. In particular, an integrated monitoring network was progressively put in place, since 1996 to measure superficial and deep displacements (GPS stations and inclinometers). The network dataset shows that the mass movements in the study area are characterized by an extremely slow or very slow rate of displacement. The elements acquired by a long-term monitoring of the deep displacements point out that the landslide bodies are prone to develop new failure surfaces progressively shallower, with a consequent increase of the hazard. Moreover, medium-deep and deep-seated landslides may rapidly accelerate in relation to rainfall conditions and significantly affect structures and infrastructures. The integration of the geological and geomorphological knowledges with the monitoring data allows for distinguishing six different homogeneous sectors on the slope, as well as the relative geometric and kinematic characteristics, and the type of mass movements. In the study area, which is representative of several aspects of other areas, the method proposed and the knowledge acquired by long-term monitoring could be useful to define mass movement mechanisms, geotechnical models, and risk mitigation strategies.     

Understanding the triggering mechanism and possible kinematic evolution of a reactivated landslide in the Three Gorges Reservoir

More than 5000 landslides or potential landslides have been induced in the Three Gorges Reservoir (TGR) region since the impoundment in 2003, which have caused great damage and remain a huge threat to the dam and people living in the reservoir area. Understanding the deformation characteristics and failure mechanism of the landslides can be helpful in stability evaluation and landslide prediction. The primary aim of this study is to research the characteristics of the landslide motion and its relationships with environmental triggers, taking the Quchi landslide, a large, slow-moving, reactivated landslide in the TGR region, as an example. The instability clearly showed visible signs of movements since 2002, and after that, the slope has been experiencing persistent deformation. By combining 4 years of meteorological, hydrological data with displacement measurements from open fractures, deep boreholes, and surface points, as well as in situ observations, this paper reports the geological and geotechnical investigations performed to define the movement. The deformation is believed to be governed by reservoir water levels, while the precipitation has a minor effect. Seasonally, the slope movement has a very distinctive pattern with large deformation starting abruptly right after reservoir drawdown in June and lasting into late summer (September). Then there is a rapid transition to constant deformation (almost no displacement) as the reservoir level rises. The slope displacements appear to gradually increase every year, which suggests very high possibility of the large and overall failure of the slide. Both monitoring results and geomorphological observations have highlighted that the two active slide masses Q1 and Q2 would probably collapse in different kinematic evolution modes, i.e., the multistage failure and whole sliding motion.     

Mapping of slow landslides on the Palos Verdes Peninsula using the California landslide inventory and persistent scatterer interferometry

Extremely slow landslides, those with a displacement rate <16 mm/year, may be imperceptible without proper instrumentation. These landslides can cause infrastructure damage on a long-term timescale. The objective is to identify these landslides through the combination of information from the California landslide inventory (CLI) and ground displacement rates using results from persistent scatterer interferometry (PSI), an interferometric synthetic aperture radar (InSAR) stacking technique, across the Palos Verdes Peninsula in California. A total of 34 ENVISAT radar images (acquired between 2005 and 2010) and 40 COSMO-SkyMed radar images (acquired between 2012 and 2014) were processed. An InSAR landslide inventory (ILI) is created using four criteria: minimum PS count, average measured ground velocity, slope angle, and slope aspect. The ILI is divided into four categories: long-term slides (LTSs), potentially active slides (PASs), relatively stable slopes (RSSs), and unmapped extremely slow slides (UESSs). These categories are based on whether landslides were previously mapped on that slope (in the CLI), if persistent scatterers (PSs) are present, and whether PSs are unstable or stable. The final inventory includes 263 mapped landslides across the peninsula, of them 67 landslides were identified as UESS. Although UESS exhibit low velocity and are relatively small (average area of 8865 m² per slide), their presence in a highly populated area such as the Palos Verdes Peninsula could lead to destruction of infrastructure and property over the long term.     

Monitoring of landslide displacements using UAS and control methods based on lines

This study describes a new methodology for monitoring landslide displacements using a set of linear elements included in the terrain. These linear elements are digitized using the products obtained from several photogrammetric flights implemented by means of unmanned aircraft systems in several dates. The control of the displacements between dates is carried out using some new metrics and methods based on lines. The results of the control of the lines are used to analyse the general and particular behaviour of the landslide. Using this type of elements allows us to study some singular behaviour of linear elements included in the landslides (e.g. an affected road can react as a block). The methodology has been tested with success using a real case of an active slope with several landslides which was studied on three separate dates. The results have demonstrated the viability of using this methodology for studying the evolution of the landslides and their consequences regarding certain elements included in the terrain.     

Structural control on the kinematics of the deep-seated La Clapière landslide revealed by L-band InSAR observations

The objective of this work is to document the deformation pattern of the deep-seated La Clapière landslide for the period 2007–2010 from the combination of L-band synthetic aperture radar (SAR) interferograms, ground-based total station measurements and identification of the slope geomorphological structures. The interferograms are calculated for pairs of ALOS/PALSAR images at a time interval of 46 days. The displacement field derived from the interferograms reveals a non-uniform displacement gradient from the top (subsidence) to the bottom (accumulation). Vertical velocities are calculated from the unwrapped phase values and are in good agreement with ground-based measurements. The results demonstrate the potential of L-band ALOS/PALSAR imagery for the monitoring of active landslides characterized by complex kinematic patterns and by important changes in the soil surface backscattering in time.     

Ground deformation monitoring by using the Permanent Scatterers Technique: The example of the Oltrepo Pavese (Lombardia, Italy)

The applicability of the Permanent Scatterers Synthetic Aperture Radar Interferometry (PSInSAR) technique for detecting and monitoring ground displacements was tested in the Oltrepo Pavese territory (Northern Italy, southern Lombardia), which could be representative of similar geological contexts in the Italian Apennines. The study area, which extends for almost 1100 km², is characterized by a complex geological and structural setting and the presence of clay-rich sedimentary formations. These characteristics make the Oltrepo Pavese particularly prone to several geological hazards: shallow and deep landslides, subsidence and swelling/shrinkage of the clayey soils. The PSInSAR technique used in this study overcomes most of the limitations of conventional interferometric approaches by identifying, within the area of interest, a set of “radar benchmarks” (PS), where very precise displacement measurements can be carried out. More than 90,000 PS were identified by processing Synthetic Aperture Radar (SAR) images acquired from 1992 to 2001 by the European Remote Sensing satellites (ERS). The PSInSAR application at a sub-regional scale detected slow ground deformations ranging from + 5 to − 16 mm/year, and resulting from various processes (landslides, swelling/shrinkage of clay soils and water pumping). The PS displacements were analysed by collecting data obtained through geological, geomorphologic field surveys, geotechnical analysis of the soils and the information was integrated within a landslide inventory and the damaged building inventory. Despite the limited number of landslide bodies with PS (7% of the inventoried landslides), the PS data helped to revise the state of activity of several landslides. Furthermore, some previously unknown unstable slopes were detected. Two areas of uplift and two areas of subsidence were identified.     

Measuring displacements of the Thompson River valley landslides,south of Ashcroft,BC, Canada,using satellite InSAR

The Thompson River valley, south of Ashcroft in British Columbia, Canada, has experienced several landslides since the mid-1800s. The national railways that run along the valley cross a number of these landslides. All the landslides occur in glacial deposits, typically sliding on weak clay layers. Some have failed rapidly to very rapidly and are currently inactive or showing deformation rates from a few millimeters to centimeters per year. An evaluation of satellite InSAR (Interferometric Synthetic Aperture Radar) using RADARSAT-2 images between September 2013 and November 2015 provides new insight into landslide displacements in the Thompson River valley. This information enhances the ongoing hazard management of unstable terrain. This paper presents the comparison of the InSAR measurements with other instrumentation (GPS and ShapeAccelArrays? -SAA) installed at one moving landslide and then addresses the extent and magnitude of the slope movements observed. InSAR was found to provide similar displacement values to those measured otherwise. The stable location showed displacements of +/?1 mm with an average near zero during the whole monitoring period. Six areas of slope movement were identified within the study area, all within or adjacent to the footprints of past landslides. The average line of sight (LOS) displacement rates range between 11 and 39 mm/year. Most of the landslides exhibited seasonal variations in velocity that corresponds to changes in river elevation in the valley.     

Displacement properties of landslide masses at the initiation of failure in quick clay deposits and the effects of meteorological and hydrological factors

This study aimed to identify displacement properties of landslide masses at the initiation of failure and factors that affect the landslides activities in areas where quick clay is found. We set up a research site in a quick clay deposit area in Norway and monitored the displacements of landslide masses and meteorological and hydrological factors for a long period of time using an automatic monitoring system. The system collected data for two landslides that occurred at the site from the start of their movement until their ultimate collapse.

The two landslides that were monitored showed definite secondary and tertiary creep stages before they collapsed. One of the landslides moved from the secondary stage to the tertiary creep stage when another landslide occurred nearby. The tertiary stage of this landslide showed reconstruction of short primary, secondary, and tertiary creep stages. These phenomena suggested that (1) the stress at the end of the landslide mass was released during the nearby landslide, and (2) a new stress distribution was formed in the landslide mass. The critical strain differed for 14 times between the two landslide masses we monitored. The difference was likely attributable to the difference in the contents of quick clay, which shows small critical stress against slope failure, as well as topological factors.

Our analyses of the effects of hydrological and meteorological factors on landslides showed that the precipitation of 3 and 10 days before six slope failures as the final stages of the landslides that had occurred in the research area was no different from the mean precipitation of periods that showed no slope failure, suggesting that precipitation had no direct effects on the collapse of the landslide masses. On the other hand, the traveling velocities of the landslide masses during the secondary creep stage, which was prior to their collapse, were affected by the water content of the soil and precipitation (and the amount of snowmelt water), but was little correlated with the pore-water pressure of the quick clay layer. We also found that the presence of snow cover scarcely affected landslide movements.     

Using wavelet tools to analyse seasonal variations from InSAR time-series data: a case study of the Huangtupo landslide

Synthetic aperture radar interferometry (InSAR) has proven to be a powerful tool for monitoring landslide movements with a wide spatial and temporal coverage. Interpreting landslide displacement time-series derived from InSAR techniques is a major challenge for understanding relationships between triggering factors and slope displacements. In this study, we propose the use of various wavelet tools, namely, continuous wavelet transform (CWT), cross wavelet transform (XWT) and wavelet coherence (WTC) for interpreting InSAR time-series information for a landslide. CWT enables time-series records to be analysed in time-frequency space, with the aim of identifying localized intermittent periodicities. Similarly, XWT and WTC help identify the common power and relative phase between two time-series records in time-frequency space, respectively. Statistically significant coherence and confidence levels against red noise (also known as brown noise or random walk noise) can be calculated. Taking the Huangtupo landslide (China) as an example, we demonstrate the capabilities of these tools for interpreting InSAR time-series information. The results show the Huangtupo slope is affected by an annual displacement periodicity controlled by rainfall and reservoir water level. Reservoir water level, which is completely regulated by the dam activity, is mainly in ‘anti-phase’ with natural rainfall, due to flood control in the Three Gorges Project. The seasonal displacements of the Huangtupo landslide is found to be ‘in-phase’ with respect to reservoir water level and the rainfall towards the front edge of the slope and to rainfall at the higher rear of the slope away from the reservoir.     

Landslides in the North of Lisbon Region (Portugal): Conditioning and triggering factors

Detailed geomorphological mapping carried out in 5 sample areas in the North of Lisbon Region allowed us to collect a set of geological and geomorphological data and to correlate them with the spatial occurrence of landslide. A total of 597 slope movements were identified in a total area of 61.7 km², which represents about 10 landslides per km².The main landslide conditioning factors are: lithology and geological structure, slope angle and slope morphology, land use, presence of old landslides, and human activity.The highest landslide density occurs in Cretaceous marls and marly limestones, but the largest movements are in Jurassic clays, marls and limestones.The landslide density is higher on slopes with gradients above 20 °, but the largest unstable area is found on slopes of 10 ° to 15 °, thus reflecting the presence of the biggest slope movements. There is a correlation between landslides and topographical concavities, a fact that can be interpreted as reflecting the significance of the hydrological regime in slope instability.Concerning land use, the highest density of landslides is found on slopes covered with shrub and undergrowth vegetation.About 26% of the total number of landslides are reactivation events. The presence of old landslides is particularly important in the occurrence of translational slides and complex and composite slope movements.20% of the landslide events were conditioned by anthropomorphic activity. Human's intervention manifests itself in ill-consolidated fills, cuts in potentially unstable slopes and, in a few cases, in the changing of river channels.Most slope movements in the study area exhibit a clear climatic signal. The analysis of rainfall distribution in periods of recognised slope instability allows the distinction of three situations: 1) moderate intensity rainfall episodes, responsible for minor slope movements on the bank of rivers and shallow translational slides, particularly in artificial trenches; 2) high intensity rainfall episodes, originating flash floods and most landslides triggered by bank erosion; 3) long-lasting rainfall periods, responsible for the rise of the groundwater table and triggering of landslides with deeper slip surfaces.     

A simplified method for predicting rainfall-induced mobility of active landslides

This paper deals with the landslides that are reactivated by a groundwater level increase owing to rainfall. These landslides are usually characterized by low displacement rate with deformations essentially concentrated within a narrow shear zone above which the unstable soil mass moves like a rigid body (i.e., with a horizontal displacement profile that is essentially constant with depth). In view of this evidence, a new method based on a simple sliding block model is proposed in the present study for a preliminary evaluation of landslide mobility. Unlike other existing methods that provide an evaluation of landslide mobility on the basis of groundwater level measurements, the present method directly relates landslide movements to rain recordings. This possibility constitutes a significant advantage from a practical viewpoint because it allows future displacement scenarios to be predicted from expected rainfall scenarios. In addition, the present method requires a limited number of parameters as input data, many of which can be obtained from conventional geotechnical tests. To evaluate the other parameters involved, an efficient calibration procedure is also proposed. Four case studies documented in the literature are analyzed to assess the capability of the present method to reproduce the main features of the slope response to rainfall. In all these case studies, both groundwater level variations and landslide displacements observed in field are well approximated by the method.