Modelling the dynamics of a large rock landslide in the Dolomites (eastern Italian Alps) using multi-temporal DEMs

Latest advances in topographic data acquisition techniques have greatly enhanced the possibility to analyse landscapes in order to understand the processes that shaped them. High-resolution Digital Elevation Models (DEMs), such as LiDAR-derived ones, provide detailed topographic information. In particular, if multi-temporal DEMs are available, it is possible to carry out a detailed geomorphic change detection analysis. This analysis may provide information about the dynamics of large landslides and may thus, be useful for landslide risk assessments. However, LiDAR-derived DEMs are mostly available only as post-event surveys. The technique is relatively recent, and local or national authorities only started widespread surveys in the last decade. Therefore, it is of a certain interest to analyse the effectiveness of DEMs derived from technical cartography to produce reliable volumetric estimates related to large landslides. This study evaluates the use of a multi-source DEM of Difference (DoD) analysis for the investigation of a large landslide –Le Laste–, which occurred on November 12, 2014 on Mount Antelao (eastern Italian Alps). The landslide initiated as a 365,000 m3 rockslide close to the summit of the mountain and transformed into a debris avalanche during its runout. The comparison of pre- and post-event DEMs allowed for the identification and quantification of erosion and deposition areas, and for the estimation of landslide volume. A sound back-analysis of the landslide with the 3D numerical model DAN3D was based on this comparison and on seismic records of the event. These seismic records proved to be remarkably useful, as they allowed for the calibration of the simulated landslide velocity. This ensured the reliability of the model notwithstanding the topographic datasets, intrinsic uncertainties. We found that using a pre-event DEM derived from technical cartography tends to slightly overestimate the volume with respect to the use of the more accurate LiDAR-derived DEM. In recent years, the landslide risk around Mt. Antelao has been increasing alongside the ever-growing population and human activities in the area. Sediment accumulations produced by the Le Laste landslide significantly amplified the debris flow hazard by providing new sediment sources. Therefore, it is crucial to delineate the distribution of this material to enable an adequate debris flow hazard assessment. The material properties derived from the back-analysis of the Le Laste landslide can be used to simulate the runout of possible future events, and to generate reliable hazard zone maps, which are necessary for effective risk mitigation

A multi-disciplinary investigation of the AFEN Slide: The relationship between contourites and submarine landslides

Contourite drifts are sediment deposits formed by ocean bottom currents on continental slopes worldwide. Although it has become increasingly apparent that contourites are often prone to slope failure, the physical controls on slope instability remain unclear. This study presents high-resolution sedimentological, geochemical and geotechnical analyses of sediments to better understand the physical controls on slope failure that occurred within a sheeted contourite drift within the Faroe-Shetland Channel. We aim to identify and characterize the failure plane of the late Quaternary landslide (the AFEN Slide), and explain its location within the sheeted drift stratigraphy. The analyses reveal abrupt lithological contrasts characterized by distinct changes in physical, geochemical and geotechnical properties. Our findings indicate that the AFEN Slide likely initiated along a distinct lithological interface, between overlying sandy contouritic sediments and softer underlying mud-rich sediments. These lithological contrasts are interpreted to relate to climatically-controlled variations in sediment input and bottom current intensity. Similar lithological contrasts are likely to be common within contourite drifts at many other oceanic gateways worldwide; hence our findings are likely to apply more widely. As we demonstrate here, recognition of such contrasts requires multi-disciplinary data over the depth range of stratigraphy that is potentially prone to slope failure

Micro-mechanics of weak layers: key role of sediment structure and composition

Submarine landslides are gravity-driven mass movements that occur in underwater slope settings worldwide. They are one of the volumetrically most important processes for transporting sediments from the continental margin into the deep ocean. Despite the hazard they pose to coastal communities and critical seafloor infrastructure, many aspects of submarine landslides remain poorly understood. Our understanding of submarine landslides is often based on hypotheses that are hard to test, and we tend to infer landslide behaviour rather than understand the reason behind their formation. Sufficient information regarding the internal structure and composition, i.e. from sediment cores and in-situ measurements is often missing. Therefore, some key questions still remain unanswered, which include why some areas fail while adjacent slopes do not, or how submarine landslides can fail on low angle slops (<2°). Many studies proposed that these phenomena and the large areal extend of submarine landslides may be explained by laterally-extensive weak layers within the slope stratigraphy. Our knowledge regarding weak layers, in particular their compositional and structural characteristics, as well as the processes that control and form them, however, is still very limited. This thesis makes use of a variety of datasets at different scales and resolution in order to both qualitatively and quantitatively investigate the role of sediment structure and composition on weak layer and submarine landslide formation. Furthermore, the role of the environmental setting on the formation of weak layers, and their control on the triggering mechanism are investigated. Establishing such a relation is crucial to identify conditions (i.e. failure mechanism) under which slope failure may occur. Part of this thesis is a comprehensive literature review of published submarine landslide studies that examine the failure planes and apparent weak layers of historic and ancient submarine landslides, to evaluate what types of sediment are capable of forming weak layers and to understand their global distribution. The results show that failure planes usually form in the vicinity of an interface between distinct lithologies that together comprise a weak layer. The review further demonstrates that different types of weak layers show an affinity to specific geographical and physiographical locations. These include contourite or turbidite systems that can create siliciclastic sediment sequences, areas of high productivity or upwelling where biogenic sediments may dominate, or regions that experience repeated ash deposition from proximal or distal volcanic sources. Weak layers are further investivated by means of two selected case studies, a cohesive submarine landslide that occurred in a low angle sheeted contourite drift (namely the AFEN Slide) and a coastal retrogressive submarine landside that initiated along a regional turbidite event bed (namely the Finneidfjord Slide). The AFEN Slide is investigated using a combination of geophysical, sedimentological, geochemical, and geotechnical data. These data reveal abrupt lithological contrasts characterised by distinct changes in physical, geochemical and geotechnical properties. The findings indicate that failure likely initiated along this distinct climatically-controlled lithological contrast, which marks the boundary between a sandy contourite and underlying softer mud-rich sediments. Whether climate change played a role in triggering slope failure remains unclear, however, the data demonstrate its role in dictating the location of the failure plane. Furthermore, the results highlight the necessity to integrate high-resolution sediment core analyses and information about the regional setting to identify potential weak layers over the depth range of stratigraphy. The second case study, the Finneidfjord Slide offshore Norway, is investigated by means of high-resolution 3D micro-Computed Tomography imaging. The results reveal clear compositional and structural differences between individual sub-units of the weak layer, as well as the background sediment. The pore space distribution is highly spatially variable. Such high variability may be masked by bulk porosity measurements. Bulk-porosity measurements work on a centimetre-scale, while the observed changes are found on a millimetre-scale. Such differences, however, may be crucial for the formation of weak layers as they appear to dictate the location of the failure plane. These findings have important implications for understanding how weak layers are formed and their influence on failure plane formation. The results further enable a better constraint on the relation between environmental setting and weak layer distribution, as well as triggering and failure mechanisms

A Hybrid Lister‐Outrigger Probe for Rapid Marine Geothermal Gradient Measurement

We have successfully constructed and tested a new, portable, Hybrid Lister‐Outrigger (HyLO) probe designed to measure geothermal gradients in submarine environments. The lightweight, low‐cost probe is 1‐3 m long, contains 4‐12 semiconductor temperature sensors that have a temperature resolution of 0.002 oC, a sample rate of 500 mbsl. Data are saved on solid‐state disks, transferred directly to the ship during deployment via a data cable, or transmitted via Bluetooth when the probe is at the sea surface. The probe contains an accelerometer to measure tilt, and internal pressure, temperature, and humidity gauges. Key advantages of this probe include (1) near‐real time temperature measurements and data transfer; (2) a low‐cost, transportable, and lightweight design; (3) easy and rapid two‐point attachment to a gravity corer, (4) short (3‐5 minute) thermal response times; (5) high temporal/spatial resolution and (6) longer deployment endurance compared to traditional methods. We successfully tested the probe both in lakes and during sea trials in May 2019 offshore Montserrat during the R/V Meteor Cruise 154/2. Probe‐measured thermal gradients were consistent with seafloor ocean‐drilling temperature measurements. Ongoing probe improvements include the addition of real‐time bottom‐camera feeds and long‐term (6‐12 month) deployment for monitoring. Key Points - We have designed, developed, and tested a low‐cost, portable hybrid Lister‐type probe to measure shallow thermal gradients - The probe consists of lightweight, quickly interchangeable/expendable components deployable to 2100 meters depth - The probe provides high vertical and temporal temperature resolution and rapid data transmission, reducing down‐tim