Nicola Casagli

Landslide monitoring by using ground-based SAR interferometry: an example of application to the Tessina landslide in Italy

Abstract An innovative technique, based on radar interferometry and implemented using ground-based instrumentation, has been applied for monitoring the Tessina landslide (Italy, Belluno). The technique has allowed us to derive multitemporal surface deformation maps of the entire depletion zone of the landslide with a high spatial resolution and accuracy. The portable device used in this application is known as Linear SAR (LISA), and it is able to provide measurements at 17 GHz with a synthetic aperture of up to 2.8 m. The results have been validated by comparing the recorded pixel displacements with independent measurements carried out by a motorized theodolite and Electronic Distance Meter (EDM) on two benchmarks.

Stability of streambanks formed in partially saturated soils and effects of negative pore water pressures: the Sieve River (Italy)

Abstract Streambanks of alluvial channels are usually composed of loose materials, which are unsaturated in ambient conditions. Unsaturated soils are subject to negative pore water pressures, which cause an apparent cohesion. The latter is the main factor in allowing the stability of near-vertical banks. Even during moderate in-bank flow events, the apparent cohesion can be strongly reduced as the material approaches full saturation; therefore, during the drawdown phase, as the confining pressure of the water in the channel disappears, a bank failure is likely to occur. Channel bed-level lowering along the Sieve River, Central Italy, has caused widespread bank instability. A geomorphological reconnaissance of forms and processes was followed by in situ tests to determine the shear strength of the banks. Interpretation of the tests and a streambank stability analysis were based on concepts of soil mechanics for unsaturated soils, in order to obtain relations between bank angle and height in limit equilibrium conditions. A stability chart was obtained with curves for different apparent cohesion values, and a stability analysis was performed taking into account the effects of flow events. In order to investigate the pore pressure effects, a series of piezo-tensiometers were installed in a streambank of the Sieve River. Data from a 1 year monitoring period show variations in pore water pressure and matric suction as a consequence of rainfall, evapotranspiration, and water stage variations. A planar failure with a tension crack occurred in the upper cohesive part of the bank during December 1996. The safety factor has been expressed as a function of the geometry of the bank and of the shear strength of the material. Safety factor variations through time are therefore shown as a function of seasonal variations in matric suction.

Object-Oriented Change Detection for Landslide Rapid Mapping

A complete multitemporal landslide inventory, ideally updated after each major event, is essential for quantitative landslide hazard assessment. However, traditional mapping methods, which rely on manual interpretation of aerial photographs and intensive field surveys, are time consuming and not efficient for generating such event-based inventories. In this letter, a semi-automatic approach based on object-oriented change detection for landslide rapid mapping and using very high resolution optical images is introduced. The usefulness of this methodology is demonstrated on the Messina landslide event in southern Italy that occurred on October 1, 2009. The algorithm was first developed in a training area of Altolia and subsequently tested without modifications in an independent area of Itala. Correctly detected were 198 newly triggered landslides, with user accuracies of 81.8% for the number of landslides and 75.9% for the extent of landslides. The principal novelties of this letter are as follows: 1) a fully automatic problem-specified multiscale optimization for image segmentation and 2) a multitemporal analysis at object level with several systemized spectral and textural measurements.

Persistent Scatterer Interferometry (PSI) Technique for Landslide Characterization and Monitoring

: The measurement of landslide superficial displacement often represents the most effective method for defining its behavior, allowing one to observe the relationship with triggering factors and to assess the effectiveness of the mitigation measures. Persistent Scatterer Interferometry (PSI) represents a powerful tool to measure landslide displacement, as it offers a synoptic view that can be repeated at different time intervals and at various scales. In many cases, PSI data are integrated with in situ monitoring instrumentation, since the joint use of satellite and ground-based data facilitates the geological interpretation of a landslide and allows a better understanding of landslide geometry and kinematics. In this work, PSI interferometry and conventional ground-based monitoring techniques have been used to characterize and to monitor the Santo Stefano d’Aveto landslide located in the Northern Apennines, Italy. This landslide can be defined as an earth rotational slide. PSI analysis has contributed to a more in-depth investigation of the phenomenon. In particular, PSI measurements have allowed better redefining of the boundaries of the landslide and the state of activity, while the time series analysis has permitted better understanding of the deformation pattern and its relation with the causes of the landslide itself. The integration of ground-based monitoring data and PSI data have provided sound results for landslide characterization. The punctual information deriving from inclinometers can help in defining the actual location of the sliding surface and the involved volumes, while the measuring of pore water pressure conditions or water table level can suggest a correlation between the deformation patterns and the triggering factors.

Determining grain size distribution of the material composing landslide dams in the Northern Apennines: sampling and processing methods

Abstract Landslide dams are a particular kind of natural dam which forms when an earth or rock mass reaches a river channel causing its complete or partial blockage. Since 1995, the Earth Sciences Department of the University of Firenze has been running a project involving the inventory of landslide dams in the Northern Apennines, which currently contains more than 60 case studies. It is known that the geotechnical characterisation of landslide dams is closely related to the grain size distribution of the materials composing the dams. In particular, grain size exerts a control on dam stability, influencing the strength and the permeability of the dam material. The main purpose of this work is to contribute to grain size characterisation of landslide dams by giving an outline of the possible methods that can be used for grain size assessment and showing the results obtained from the case studies selected in the Northern Apennines. Indeed, despite its importance, there are few quantitative studies directed at determining grain size distribution of debris dams, mainly because customary bulk sieve analyses are impractical when dealing with materials often ranging in size from blocks tens of cubic metres in size, to microscopic particles. This paper presents results derived from the application of two main sampling methods on 42 landslide dams, (a) volumetric analysis and (b) grid by number analysis (here presented in two variants). These techniques allow us to obtain very similar grain size distributions for the coarser part of the debris material, while some differences persist for the finer fraction. The paper would also like to highlight the importance of getting accurate grain size data, with particular reference to the determination of the hydrograph derived from a landslide dam failure. Many deterministic models attempt to approach this problem, by taking into account only the median percentile (D50) of the cumulate grain size distribution. However, in most of the selected cases, a marked bimodal distribution was found in the frequency diagrams, so the fiftieth percentile (D50) is often one of the less representative grain size classes. It should thus be taken carefully into account that this choice could be misleading, as it is not necessarily justified by the physical evidence.

Landslide monitoring by ground-based radar interferometry: A field test in Valdarno (Italy)

A ground-based Interferometric Synthetic Aperture Radar (InSAR) was installed to monitor a landslide in Valdarno (Italy). The aim was to field-test an innovative remote sensing instrument able to provide distributed information over sliding slopes with a rate of several images a day. Radar images and interferometric displacement maps projected on the Digital Elevation Model (DEM) of the test site are reported.

Updating landslide inventory maps using Persistent Scatterer Interferometry (PSI)

This work illustrates the contribution of persistent scatterer interferometry (PSI) from radar satellites ERS (European Remote Sensing satellite) and ENVISAT (Environmental Satellite) for the updating of a pre-existing landslide inventory (LSI) map: the main purpose is to change or confirm the landslide state of activity and geometry and to identify new landslides. Radar data have been integrated with optical images and ancillary data in a 1320 km2 wide river basin (Biferno Basin) located in the central-eastern part of Italy. The geological setting of the area is characterized by clay and alternated clayey, silt and sandy formations that are affected by slow landslides. Field validation confirmed the results and the capabilities of multi-interferometric synthetic aperture radar data, integrated and coupled with conventional techniques, to support landslide investigation at the regional scale thanks to the available archive of repeated satellite data, which provides measurements of ground displacements with a millimetre-scale accuracy. In the study area, about 9% of the pre-existing LSI has been modified by means of permanent scatterer (PS) information, 15% of which have changed the state of activity from dormant to active and 95 new landslides were detected. The radar interpretation method applied in Biferno Basin confirms its high capability of detecting and mapping landslides at basin the scale: the information acquired from radar interpretation is the basis of the proposed method to evaluate the state of activity and the intensity of slow landslides. However, it is clear that limitations exist and this method does not always support the updating of LSI for the whole study area. We consider this methodology and procedure as a portable and suitable one for different geological and geomorphological environments.

Pore water pressure and streambank stability: results from a monitoring site on the Sieve River, Italy

To investigate the role of pore water pressures in the stability of a streambank, a series of tensiometers and piezometers was installed in a bank of the Sieve River, Tuscany, Italy. Fluvial entrainment at the bank toe was monitored by repeated cross-profiling, erosion pins and marked pebbles. Fluctuations in matric suction measured at the tensiometers reflected the overall response of pore water pressures to rainfall, evapotranspiration, rising and drawdown of the river stage, and variations in water table. An expression was derived for the safety factor with respect to mass movement of the upper bank, incorporating the failure criterion for unsaturated soils and the normal Mohr–Coulomb criterion for saturated conditions. Variations in matric suction have important effects on the stability of the streambank. During low-flow periods, the shear strength term due to the matric suction allows the bank to remain stable at a steep angle. However, during rainfall and flow events, reduction in matric suction and increase in unit weight of the material from vertical and lateral infiltration may be sufficient to trigger a mass failure, without development of significant positive pore water pressures. During the rising limb of high-flow events, the factor of safety increases as a consequence of the stabilizing confining pressure of the water in the river, despite a reduction in matric suction. During drawdown in the river, when the suction values are still low and the confining pressure in the river decreases to zero, the factor of safety falls to lower values than those experienced prior to the runoff event. Measurements of fluvial entrainment reveal that, although the processes, mechanisms and the frequency of retreat of basal and upper bank zones differ significantly, the amount of retreat at the bank toe due to fluvial erosion is comparable to that of the upper portion of the bank due to mass failure. Copyright

Persistent Scatterers Interferometry Hotspot and Cluster Analysis PSI-HCA for detection of extremely slow-moving landslides

The synthetic aperture radar (SAR) interferometry (InSAR) technique has already shown its importance in landslide mapping and monitoring applications. However, the usefulness of traditional differential InSAR applications is limited by disturbing factors such as temporal decorrelation and atmospheric disturbances. The Persistent Scatterers Interferometry (PSI) technique is a recently developed InSAR approach. It generates stable radar benchmarks (namely persistent scatterers, PSI point targets) using a multi-interferogram analysis of SAR images. The PSI technique has the advantage of reducing temporal decorrelation and atmospheric artefacts. The PSI technique is suitable for the investigation of extremely slow-moving landslides due to its capability to detect ground displacements with millimetre precision. However, the interpretation of PSI outputs is sometimes difficult for the large number of possible persistent scatterers (PSs). A new approach of PSI Hotspot and Cluster Analysis (PSI-HCA) is introduced here in order to develop a procedure for mapping landslides efficiently and automatically. This analysis has been performed on PSs in hilly and mountainous areas within the Arno river basin (Italy). The aim is to use PSs processed from 4 years (2003–2006) of Radarsat images for identifying areas preferentially affected by extremely slow-moving landslides. The Getis–Ord Gi * statistic is applied in the study for the PSI-HCA approach. The velocity of PSs is used as weighting factor and the Gi * index is calculated for each single point target. The results indicate that both high positive and low negative Gi * values imply the clustering of potential mass movements. High positive values suggest the moving direction towards the sensor along the satellite line-of-sight (LOS), whereas low negative values imply the movement away from the sensor. Furthermore, the kernel function is used to estimate PS density based on these derived Gi * values. The output is a hotspot map which highlights active mass movements. This spatial statistic approach of PSI-HCA is considered an effective way to extract useful information from PSs at a regional scale, thus providing an innovative approach for rapid mapping of extremely slow-moving landslides over large areas.

Space‐borne and ground‐based SAR interferometry as tools for landslide hazard management in civil protection

In recent years, SAR interferometry has become one of the most popular emerging techniques for the assessment of ground displacements, and, as such, it is of great interest as a possible operational tool for civil protection institutions having to deal with landslide risk. The paper presents some of the results obtained in northern Italy during a research project aimed at testing the potentiality of the application of C‐band space‐borne interferometry and Ku‐band ground based interferometry during different specific civil protection activities. Main research objectives were the detection of the movements of complex earth and rock slides affecting built‐up areas during the 1990s, and the near real‐time monitoring of a reactivated rotational earth slide over an emergency period of 15 days. Results of space‐borne interferometry did qualitatively fit with the geological interpretation of the mass movements and with ground truths such as damaged buildings and in situ monitoring systems. However, this was not achieved in quantitative terms, suggesting that this technique should be used limitedly for displacement recognition and not monitoring. On the other hand, ground‐based interferometry proved valuable both for a qualitative and a quantitative estimate of slope movements. Nonetheless, the research has also enabled the limitations that are still to be tackled in order to bring these systems to an operational usage in civil protection to be highlighted.