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Analisi della risposta di corpi di frana a fattori perturbanti di tipo idrogeologico per mezzo di dati di monitoraggio ad alta frequenza
No full textLa disponibilità di dati di monitoraggio ad alta frequenza ha aperto nuove prospettive nell’ambito dello studio e della comprensione delle dinamiche che governano la rosposta di corpi di frana alle forzanti meteo-idrogeologiche.
L’analisi della relazioni esistenti tra input meteo-idrogeologico e spostamenti registrati in corpi di frana tramite dati di monitoraggio ad alta frequenza è il tema di questo Dottorato di Ricerca. L’obiettivo finale è quello di sfruttare l'informazione contenuta in dati di monitoraggio ad alta frequenza nell'ottica della gestione del rischio e l'allertamento basati su sistemi di monitoraggio.
Tale scopo è stato raggiunto in tre siti di monitoraggio tramite lo sviluppo di analisi riproducibili basate sull'applicazione di metodi statistici quali la Funzione di Cross-Correlation (CCF), l'Analisi delle Componenti Principali (PCA) e le Receiver Operative Curves (ROC). Questi metodi, usualmente utilizzati in altre branche scientifiche, sono stati applicati a serie di monitoraggio di parametri meteo-idrologici e degli spostamenti registrate usando stazioni meteo, piezometri e strumenti topografici (GPS, Stazioni Totali Automatizzate ed Estensimetri a fondo foro), permettendo di sfruttare nell'ambito della geologia applicata le informazioni contenute in queste serie di monitoraggio.
I casi studio analizzati consistono in frane di medie e grandi dimensioni (scivolamenti e colate in terra e scivolamenti in roccia) per le quali sono disponibili dati di monitoraggioche ricoprono più di un ciclo stagionale. Nello specifico si tratta delle frane di Col Piagneto (Appennino Reggiano, Italia), di Corvara in Val Badia (Dolomiti, Italia) e di Vallcebre (Pirenei, Spagna).
Nell'ambito del caso studio di Col Piagneto le curve ROC sono state usate per determinare e qualificare le precipitazioni (in termini di durata e relativa quantità cumulata) responsabili per il superamento di determinate soglie critiche di velocità o accelerazione nel corpo di frana. La serie storica giornaliera degli spostamenti è stata acquisita da un Staziome Totale Automatizzata operativa per 4 anni nel sito mentre le precipitazioni sono state registrate da una vicina stazione meteo. Le soglie utilizzate corrispondono a tassi di spostamento limite che possono portare al collasso della parte frontale della frana con l'innesco di scivolamenti in detrito, come accaduto nel 1972 con il danneggiamento della sottostante stada statale. I risultati rappresentano un dato utile per la definizione di sistemi di allertamento.
Per la frana di Vallcebre l'analisi di CCF è stata applicata sul record pluviometrico, gli spostamenti e le variazioni di piezometria registrati a cadenza oraria in tre punti della frana. Gli spostamenti sono stati misurati con estensimetri a fondo foro. I risultati evidenziano i tempi di latenza tra le variabili permettendo una migliore comprensione dei meccanismi responsabili per le riattivazioni della frana. Il mutuo effetto dell'erosione al piede del torrente Vallcebre e della variazione della superficie di falda in relazione alle precipitazioni è stato dimostrato.
Nel caso della frana di Corvara in Badia il metodo delle PCA applicato a serie temporali di spostamenti ottenute da misure GPS fatte ogni 15 giorni ha permesso di valutare l'effetto destabilizzante delle precipitazioni (neve e pioggia) sul corpo di frana a scala stagionale usando 1a componente principale. Inoltre l'effetto di specifici periodi caratterizzati da condizioni meteo avversee viene evidenziato dalla 2a componente principale.
Nel complesso la ricerca svolta dimostra quindi come l'analisi statistica di dati di monitoraggio ad alta frequenza di frane permetta una migliore comprensione dei meccanismi che determinano variazioni dei loro tassi di movimento.The availability of high frequency monitoring data opens new perspectives for the study and comprehension of dynamics and processes governing landslide’s response to meteo-hydrological factors in terms of unsteady displacement rates.
The main topic of this PhD dissertation is thus the analysis of the relationships between meteo-hydrological inputs and displacements recorded in a landslide by mean of high-frequency monitoring data recorded with various types of sensors. The final objective is to quantitatively define the relationships between displacement rates and precipitation so to support the development of early-warning rules and risk management systems based on monitoring.
Such aim has been achieved in three monitored test sites by developing replicable analysis based on the application of statistical methods such as the Cross Correlation Function (CCF), Principal Component Analysis (PCA) and Receiver Operative Curves (ROC). Such methods, consolidated in use in other disciplines, have been applied to meteo-hydrological and displacement time-series recorded by mean of meteorological stations, piezometers and several measuring systems (GPS receiver, Automatic Total Station and in-hole extensometer), and have allowed the information contained in such high-frequency monitoring data to be exploited in an applied hydro-geological perspective.
The analysed case studies include large to medium dimension landslides, in particular earthslides, earthflows and rockslides, for which monitoring datasets covering several seasonal cycles were available. The case studies are the Col Piagneto rockslide (Northern Apennines, Italy) the Corvara in Badia earthslide-earthflow (Dolomites, Italy) and the Vallcebre earthslide (Pyrenees, Spain).
In the Col Piagneto case study, the ROC curves were used to determine and define the amount of rainfall (in term of duration and related cumulated value) determining velocity or acceleration values exceeding critical thresholds. The daily displacement dataset is based on a Total Station operated for more than 4 years on the site while the rainfall dataset is based on a rain gauge nearby the site. The velocity and acceleration thresholds are statistically rare values that might lead to the collapse of the frontal portion of the landslide and its transition to debris slide, as it has occurred in 1972 damaging the national road 63 running at the base of the slope. The results obtained represent a useful information for the set-up of warning systems based on monitoring data.
In the Vallcebre case study, the CCF analysis was applied to rainfall, displacement and piezometry hourly time-series recorded in three distinct points of the landslide. Displacement, in particular, was obtained by down-hole wire extensometers. Results have highlighted time lags between these variables that allow a deeper comprehension of mechanisms responsible for re-activation of the landslide to be achieved. The combined effect of the Vallcebre stream toe-erosion and the variation of groundwater depth in relation to rainfall inputs was demonstrated.
In the Corvara in Badia case study, the PCA method applied to a displacement dataset, obtained by GPS surveys repeated at 15 days interval, has allowed the effects on the landslide velocity of precipitations at a seasonal scale (snow and rainfall) to be highlighted by means of the 1st principal component of the time series. Moreover, the effect of specific periods characterised by extreme weather conditions was highlighted by 2nd component of the displacement time series.
As a whole, the research demonstrates how the statistical treatment of high-frequency monitoring data can allow a better comprehension of the mechanisms determining variation of movement rates within landslide bodies to be achieved
Use of ROC curves for early warning of landslide displacement rates in response to precipitation (Piagneto landslide, Northern Apennines, Italy)
Full text linkActive landslides are generally characterized by variations
in displacement rate in response to cumulated precipitation.
Velocities that are only exceeded in a limited number of days
during the year might be considered as critical events, since they
might determine, or prelude to, a significant evolution of the
landslide. The purpose of this paper is to present a novel approach
based on the use of receiver operating characteristic (ROC) curves
for assessing cumulated precipitation thresholds that can provide
early warning for the occurrence of critical events such as the
exceedance of rare displacement rates. The approach has been
developed and tested in the Piagneto landslide, an active complex
rock slide—debris slide in the Northern Apennines of Italy, for
which a 5-year continuous surveying monitoring dataset is available.
On the basis of the first 4 years of monitoring data (training
dataset), threshold curves relating cumulative precipitation (mm)
to precipitation moving windows (days) have been generated by
using different benchmarks that, in literature, are used to estimate
the maximum predictive performance of ROC curves. These
threshold curves have been successfully validated using the last
1 year of monitoring data (validation dataset). They have then
been used to simulate how they might help defining different early
warning levels in due advance. The proposed methodology can be
replicated in any landslide for which a monitoring dataset that
includes recurrent acceleration events in response to precipitation
is available
Retrieving high resolution deformation patterns of slow moving landslides by COSMO-SkyMed SAR data: a sustainable long term monitoring system using artificial reflectors
No full textRetrieving high resolution deformation patterns of slow moving landslides by COSMO-SkyMed SAR data:
a sustainable long term monitoring system using artificial reflector
Acceleration of large active earthflows triggered by massive snow accumulation events : evidences from monitoring the Corvara landslide in early 2014 ( Dolomites , Italy )
No full textIn the Dolomites of Italy, snowfall during winter 2013/2014 was exceptionally abundant. Major snowfall events occurred from late December 2013 to mid-March 2014. Snow accumulation in Badia Valley peaked in early February: from 2 to 4 meters with a positive gradient respect to altimetry and accordingly to wind accumulation zones. Below 2000 m asl, due to the mild temperatures recorded before the onset of snowfall, the relatively dry snow cover was mostly deposited on top of unfrozen soils. The Corvara landslide is a large active earthflow located close to Corvara in Badia, at an elevation from 2000 to 1600 m. It’s displacement rate before, during and after the exceptional snowfall period was monitored at high temporal frequency. Surface displacement was measured bi-weekly by differential GPS in several benchmarks in the source, track and accumulation zone. Deep displacement was monitored semi-continuously by two in-place inclinometers at 48 m depth in the accumulation zone, across the main deep-seated sliding surface. Results show an acceleration of movements, both at surface and at depth, soon after the massive snow accumulation event of 31st January to 2nd February 2014, which suddenly increased snow thickness from 1 to more than 2 metres. Short time lags between the onset of the acceleration of movements in the source, the track and the accumulation zones were also recorded. The landslide then maintained a relatively constant velocity during the high snow cover period extended to earlyApril and underwent a progressive deceleration during the snowmelt period that lasted until mid-June. The fact that the acceleration of the Corvara earthflow was triggered by a massive and rapid snow accumulation event, provides a quite different perspective from the generally adopted one that considers the destabilizing effect of snow only in relation to the increase of groundwater level during rapid snowmelt. A full explanation of the processes associated to the dynamics observed in Corvara is undoubtedly still an open issue. However, it can be tentatively speculated that in the some sectors of the source and track zone, where sliding surfaces are relatively shallow -around 15 m deep -, the weight of the copiously fallen snow induced a distributed undrained loading in the already fully saturated and confined landslide mass. Or, alternatively, that snow accumulation over the unfrozen soil induced groundwater levels above the ground. To explain how acceleration of movements occurred as deep as 48 m in the accumulation zone, it might be argued that the mass and/or the pore pressure transfer from the track to the accumulation zone - evidenced by the time lag of velocity peaks- can have played a role in indirectly transferring to the accumulation zone the acceleration induced by massive snowfall in the track zone. To provide more robust answers, further monitoring data collection and analysis is needed. Thus, while waiting for other massive snowfall events, three continuous GPS receivers and a water pressure transducer in the soil havebeen added to the monitoring network during 2014
Cross-correlation and time-lag analysis of high frequency monitoring data of the Vallcebre landslide (Eastern Pyrenees, Spain) to reveal cause-effect relationships between variables governing slope instability
No full textThe Vallcebre landslide is a slow moving large landslide located 140 km north of Barcelona in the Eastern Pyrenees. Monitoring data of the Vallcebre landslide represent a singular case of multi parameter high-frequency monitoring system set up in 1996 and still ongoing. Data of movements and groundwater levels are measured and recorded with a frequency of 20 minutes in 6 boreholes distributed in the landslide, each one equipped with a wire extensometer and a piezometer, while rainfall is recorded by a specific gauge at the site. Data from 3 boreholes recorded during three full years of measurements (from 1999 to 2001) have been analyzed by means of a cross-correlation function in order to determine the reciprocal interdependency and the relative time lag between rainfall, groundwater and movement rate maxima and, ultimately, to evidence cause-effect processes occurring along the slope. It should be pinpointed that, in this specific case, rainfall is also a proxy for the discharge level of the stream eroding the toe of the landslide, that is believed to be one of the main instability factors. The cross-correlation function is a quite simple signal processing tool for measuring similarities of waveforms as function of an applied time-lag. Specifically, it was applied to study: i) the relations between rainfall and movement rate, so to highlight the relative time lag for rainfall to produce an effect in different points of the landslide; ii) the inter-dependencies between different movement rates in the three boreholes in order to determine the pattern of mobilization of the landslide (from up to down slope and vice-versa); iii) the response of groundwater with respect to rainfall, which reflects the local permeability; iv) the evolution of groundwater levels in the three monitoring points. Altogether, results confirm and constrain in time the retrogressive trend of movements in the landslide (in agreement with a 2D numerical model previously developed by Ferrari et. al. 2011) and the driving role of stream undercutting the toe of the landslide, as the acceleration of movements at the lower borehole has the shortest time lag with respect to rainfall, disregard the fact that groundwater level raise earlier in the upper piezometers. Finally, the in-depth analysis of time-lags between movement rates along the slope, allowed highlighting compression and tensile phases in different zones along the longitudinal cross section of the landslide
Prediction of landslide velocity at given cumulated rainfall values based on analysis of continuous monitoring data using ROC curves : application to the Piagneto landslide ( Northern Apennine , Italy )
No full textA novel approach to predict landslide velocity at given cumulated rainfall values based on the analysis of continuous displacement and rainfall monitoring data by using ROC curves has been developed and tested in the Piagneto landslide (northern Apennines, Italy). It is an active rock slide for which a velocity dataset covering the period October 2009 to December 2014 is available thanks to a total station that has been monitoring tens of prisms at duty cycles of 2 hours. Over the same time frame, an hourly rainfall dataset is available from rain gauges located just a few km away from the landslide. The ROC curve (Receiver Operating Characteristic) is a well-known and widely used method to assess the efficiency of a binary classifier. In this case, it is used to assess the efficiency of different values of cumulated rainfall to determine a given value of velocity in the landslide. Operatively, the daily velocity distributions of selected monitoring prisms is plotted in order to assess upper values at given levels of occurrence probability, i.e. velocity values at the upper 1st,2nd, 3rd quartile and 2 sigma. These velocity values are then classified with respect to daily rainfall cumulated over different time windows (from 1 to 120 days), that are considered singularly or in combination one another. The area under the ROC curves, as well as the max distance from the random line, is used as indicator of performance in order to assess the cumulated rainfall (in terms of amount and duration) showing the higher performance in predicting a defined landslide velocity level. The values obtained with the retrospective analysis of monitoring data, can then be used for prediction of expected velocity levels. This has been verified by using the conventional approach of dividing the dataset in a training and a validation subsets. Results underline the added value of the analysis of long time-series of continuous landslide monitoring data by mean of operational research tools, such as in this case the ROC curves, for extracting information useful for improving the comprehension and the prediction of slope dynamics that, in specific cases, can be of great support to risk management
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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