Stefano Devoto

A multidisciplinary approach for rock spreading and block sliding investigation in the north-western coast of Malta

Landslides are widespread along the north-western coast of the Island of Malta and are strictly linked to the structural setting. Exemplary cases of rock spreading and block sliding phenomena characterise this stretch of coast. They are favoured by the overposition of two different geological units widely outcropping there, the Blue Clay Formation and the Upper Coralline Limestone Formation. The latter forms a wide plateau, bordered by vertical cliffs. At the foot of the cliffs, clayey terrains crop out and develop gentle slopes covered by large blocks detached and moved by rock spreading and block sliding phenomena. These mass movements are favoured by the fragile behaviour of limestones, which cap clays, otherwise characterised by visco-plastic properties. In order to investigate the kinematics and the evolution of these types of coastal landslides, a multidisciplinary and multitechnical approach was applied on a study site, named Il-Prajjet, which provides a spectacular case of rock spreading evolving into block sliding. This paper illustrates the results achieved by means of different engineering geological and geophysical techniques allied with traditional detailed geomorphological survey and mapping. In particular, the surface displacements of the landslides were determined using long-term GPS observations, acquired approximately every 6 months, over a 4.5-year period. A network of GPS benchmarks were distributed on the edge of a limestone plateau affected by rock spreading and on a series of displaced blocks making up a large block slide, finally enabling the definition of the state of activity and the rates of movement to be performed. In addition, the results deriving from two continuous fissurimeters more recently installed at the edge of two persistent joints over the block sliding area are outlined, with reference to the correlation between variations of crack apertures and precipitation input. In order to identify main structural discontinuities and to reconstruct variability of underground surface contact between clays and overlying limestones, Resistivity Tomography profiles and GPR investigations were carried out. Finally, the results obtained by combining the outputs of geophysical surveys and different field monitoring activities can be considered a first step on which numerical models can be developed and validated, in order to assess landslide hazard and risk of this stretch of Maltese coastline.

Geomorphological map of the NW Coast of the Island of Malta (Mediterranean Sea)

This paper presents the results of geomorphological investigations carried along the north-western coast of the Island of Malta. Field surveys, accompanied by aerial photo-interpretation, have led to the production of a geomorphological map at 1:7500 scale which outlines the main processes and related landforms. The latter are the result of the complex interplay of structural, gravitational, coastal and karst processes. Particular attention was devoted to the recognition, identification and mapping of landslides which affect large coastal sectors of the study area, locally giving rise to hazardous conditions.

Landslide susceptibility modeling assisted by Persistent Scatterers Interferometry (PSI): an example from the northwestern coast of Malta

Abstract Persistent Scatterers Interferometry (PSI) techniques are widely employed in geosciences to detect and monitor landslides with high accuracy over large areas, but they also suffer from physical and technological constraints that restrict their field of application. These limitations prevent us from collecting information from several critical areas within the investigated region. In this paper, we present a novel approach that exploits the results of PSI analysis for the implementation of a statistical model for landslide susceptibility. The attempt is to identify active mass movements by means of PSI and to avoid, as input data, time-/cost-consuming and seldom updated landslide inventories. The study has been performed along the northwestern coast of Malta (central Mediterranean Sea), where the peculiar geological and geomorphological settings favor the occurrence of a series of extensive slow-moving landslides. Most of these consist in rock spreads, evolving into block slides, with large limestone blocks characterized by scarce vegetation and proper inclination, which represent suitable natural radar reflectors for applying PSI. Based on geomorphometric analyses and geomorphological investigations, a series of landslide predisposing factors were selected and a susceptibility map created. The result was validated by means of cross-validation technique, field surveys and global navigation satellite system in situ monitoring activities. The final outcome shows a good reliability and could represent an adequate response to the increasing demand for effective and low-cost tools for landslide susceptibility assessment.

Geomorphological identification, classification and spatial distribution of coastal landforms of Malta (Mediterranean Sea)

This paper presents the outcomes of a geomorphological investigation carried out along the coasts of the island of Malta and provides a detailed classification of the Maltese coastline based on the identification and definition of specific coastal geomorphotypes. The results of field surveys, supported by air-photo interpretation, have led to the production of a coastal geomorphological map at 1:30,000 scale which outlines the processes and related deposits and landforms. The latter are the result of the complex interplay of structural, gravitational, coastal and karst processes. Moreover, radiocarbon dates of marine organisms encrusted on boulders mapped along the NE coast are presented.

Landslides Along the North-West Coast of the Island of Malta

The paper shows the results of geomorphological and engineering-geological investigations carried out along the north-western coast of the Island of Malta, with special emphasis on landslides. Field surveys and aerial-photo interpretation allowed the recognition, identification and mapping of a series of landslides of different type and size, some of which showing evidence of activity. Coastal instability in the studied area is deeply controlled by structural factors, such as tectonic and stratigraphic ones. The research envisaged a multidisciplinary approach, which also included landslide monitoring in specific sites which were selected for detailed investigations, owing to the peculiarity of the instability processes occurring and for the related hazard and risk conditions. The paper outlines the research phases and the results achieved which proved to be fruitful thanks to the application of different methodologies for the study of coastal landslides. Particular attention has been paid to rock spreading phenomena, which are widespread along the north-western coast of Malta due to the superimposition of limestones over clayey terrains.

Advanced SAR Interferometric Analysis to Support Geomorphological Interpretation of Slow-Moving Coastal Landslides (Malta, Mediterranean Sea)

An advanced SAR interferometric analysis has been combined with a methodology for the automatic classification of radar reflectors phase histories to interpret slope-failure kinematics and trend of displacements of slow-moving landslides. To accomplish this goal, the large dataset of radar images, acquired in more than 20 years by the two European Space Agency (ESA) missions ERS-1/2 and ENVISAT, was exploited. The analysis was performed over the northern sector of Island of Malta (central Mediterranean Sea), where extensive landslides occur. The study was assisted by field surveys and with the analysis of existing thematic maps and landslide inventories. The outcomes allowed definition of a model capable of describing the geomorphological evolution of slow-moving landslides, providing a key for interpreting such phenomena that, due to their slowness, are usually scarcely investigated.

Geomorphological mapping of terrestrial and marine areas, northern Malta and Comino (central Mediterranean Sea)

ABSTRACT This paper presents an integrated geomorphological map of terrestrial and marine areas of northern Malta and Comino (central Mediterranean Sea) at 1:25,000 scale. The map is the result of terrestrial and marine analyses of different types of data such as aerial photos, digital terrain model of the seafloor and its acoustic backscatter image. The research is intended to emphasise the relationships and continuity of geomorphological features of both the land- and sea-scape, and outline their evolution. The map highlights the occurrence of fluvial, karst and gravity-induced landforms both on land and on the seafloor, such as palaeo-fluvial channels, sinkholes and landslides, which were shaped during sea level lowstands of the last glacial cycle.

Integrated Monitoring of Lateral Spreading Phenomena Along the North-West Coast of the Island of Malta

Landslides, and especially lateral spreading, are responsible for the main landforms observable along the north-western coast of the Island of Malta. A series of rock spreading phenomena, and associated mass movements, have been recognised in this area and the most significant cases have been investigated by means of a multi-technical approach which envisaged geomorphological survey and mapping, geophysical analyses and monitoring of displacements. The aim of the study was to define whether rock spreading phenomena are active along the investigated coastal stretch and, possibly, to determine the rate of movement of the most relevant cases. In this framework, a multidisciplinary approach was used and different techniques were tested and applied on a specific site, which was selected for the presence of an extensive rock spread phenomenon which may induce hazard conditions. The site is located at Il-Prajjet (Anchor Bay), a narrow inlet where a tourist attraction is located. Research activities have been carried out at Il-Prajjet since 2006 when a GPS network of eight benchmarks was installed. This technique was chosen because it proved to be a powerful tool in the study of similar ground deformation in coastal and mountain areas, showing high accuracy and reliability. Once GPS monitoring had showed that rock spreading was active, further techniques were applied in order to achieve an in-depth knowledge of the instability processes occurring at Il-Prajjet. The paper illustrates the results so far achieved thanks to GPS monitoring, tape extensometer measurements, SAR interferometric analyses and GPR investigations.

Long-term monitoring to support landslide inventory maps: the case of the north-western coast of the Island of Malta

The aim of this paper is to show the results of 8 years of research activities carried on the north-western coast of the Island of Malta, with particular reference to the outputs of monitoring and interferometric analysis as a support for the implementation of a landslide inventory. The study area is characterized by the presence of limestones overlapping clayey terrains, thus favoring the presence of large rock spreading phenomena, which often evolve into block slides. Field surveys resulted in approximately 86 landslides, distributed between the coastline and structural cliffs which flank limestone plateaus. In order to support geomorphological investigations, a GPS (Global Position System) monitoring network was deployed over two coastal sites. Slow rates of surface deformations varying from few millimeters per year to some centimeters per year were recorded. Since the extension of the GPS network over the entire north-western coast and for such a high number of landslides is not feasible in terms of cost-benefit relations, we applied PSI (Persistent Scatterers Interferometry) to validate the geomorphological map. DinSAR data analysis confirmed the displacement rates measured by the GPS monitoring network and provided useful information about the style of activity of a large number of landslides affecting the north-western coastline of Malta. The outcomes supported the satisfactory capabilities of an integrated approach, which coupled conventional geomorphological procedures with monitoring techniques to bring out an exhaustive landslide inventory, even for slow or very slow mass movements, essential for related hazard assessment.

Factors Triggering Sea Cliff Instability Along the Slovenian Coasts

ABSTRACT FURLANI, S., DEVOTO, S., BIOLCHI, S and CUCCHI, F., 2011. Factors Triggering Sea Cliff Instability Along the Slovenian Coasts. In: Micallef, A. (ed.), MCRR3-2010 Conference Proceedings, Journal of Coastal Research, Special Issue, No. 61, pp. 387–393. Grosseto, Tuscany, Italy, ISSN 0749-0208. The cliff retreat is the sum of the sustained action of marine and continental factors. Their interrelations depend on the geological and environmental settings of the area. The studied cliffs are cut in the Eocene Flysch, a turbiditic succession composed by centimetric—metric sandstones with millimetric-centimetric interbedded silty marlstones, almost horizontal in the study area. The low resistance of the rock mass allows the rapid retreat of the cliffs and the development of wide shore platforms. This work aims at evaluating sea cliff retreat and the factors that trigger the collapse of material and its removal from the cliff foot. Different methods have been used: a detailed characterization of the geomechanical properties and the quality of rock masses, the susceptibility to rock falls and a photographic surveying. The sea cliff retreat in five sites along the Slovenian coast, in the Northeastern Adriatic Sea, has been studied through the comparison of more than 7000 pictures collected since 1998, at precise time steps and under any weather conditions. These data have been subsequently compared with archaeological data, historical maps and the characterization of the geomechanical features of rock masses in order to determine the factors triggering the cliff retreat. Photographic surveying displays a rapid but complex behaviour of the cliff retreat: during long stable-weather periods, cliff modifications are very low and groundwater solution or landslides have been observed. Major changes in the cliff face have been observed after great storm events, because of a complex interaction between marine and non-marine factors. The integrated method proposed here, including the photographic surveying and the geomorphological-geomechanical characterisation of rock masses is a valuable method for understanding and evaluating the mechanisms of cliff retreat.