Long-Term Displacement Monitoring of Slow Earthflows by Inclinometers

geosciences

Article Long-Term Displacement Monitoring of Slow Earthﬂows by Inclinometers and GPS, and Wide Area Surveillance by COSMO-SkyMed Data

Roberto Vassallo 1, Stefano Calcaterra 2 , Nicola D’Agostino 3 , Jacopo De Rosa 1,*, Caterina Di Maio 1 and Piera Gambino 2 1 Scuola di Ingegneria, Università della Basilicata, Viale dell’Ateneo Lucano, 10, 85100 Potenza, Italy; [email protected] (R.V.); [email protected] (C.D.M.) 2 Istituto Superiore per la Protezione e la Ricerca Ambientale (ISPRA), Servizio Geologico d’Italia, Via Brancati, 48, 00144 Roma, Italy; [email protected] (S.C.); [email protected] (P.G.) 3 Istituto Nazionale di Geoﬁsica e Vulcanologia (INGV), Centro Nazionale Terremoti, Via di Vigna Murata, 605, 00143 Roma, Italy; [email protected] * Correspondence: [email protected]; Tel.: +39-340-5974380

Received: 29 February 2020; Accepted: 6 May 2020; Published: 8 May 2020

Abstract: With reference to two slow earthflows in structurally complex clayey formations of the Italian southern Apennines, this paper shows the results of a long-term displacement monitoring using integrated systems of inclinometers and GPS, and their comparison with PSInSAR data. A fixed-in-place and traversal inclinometer system, first installed in 2004, recorded both the shear displacements along the slip bands, and the internal deformations of the landslide masses. A GPS network of permanent stations and benchmarks, installed in 2006–2007 in 23 strategic points of the slope, allowed for the temporal continuity of displacement monitoring. The two long series of data allowed to evaluate the factor scaling of the PSInSAR COSMO-SkyMed data, although the component of the displacement vector along the line of sight (LOS) was small. PSInSAR data allowed for the monitoring extension to houses and rigid structures that acted as reflectors. The joint data analysis allowed for the comprehension of the main features of the landslides’ kinematics.

Keywords: landslide; monitoring; displacement; inclinometer; GPS; DInSAR

1. Introduction Earthflows in structurally complex clayey formations [1] are widely diffused in the Italian Apennines. Earthflows are the second most common type of landslides in wide hill and mountain areas of the northern Apennines, where they prevalently occur in argillaceous lithologies [2]. The widespread diffusion of earthflows in clayey and flysch sequences has been reported for some areas of the southern Apennines by Revellino et al. [3] and Pellegrino et al. [4] who, in an area of 70 km2, recognized 484 landslides, 95% of which were of the earthflow type. Earthflows are frequently characterized by alternate phases of surges and very slow movements or quiescence [5–8]. In very slow conditions, an almost steady-state motion occurs, with limited seasonal variations, generally due to changes in pore water pressures [9–13]. Many very large earthflows remain in a dormant state for many decades, allowing roads and buildings to be built on them [6]. The aim of studies such as that presented in this paper is to identify the main landslide mechanisms, which can give an indication of triggering factors and possible effective mitigation measures. To this aim, the monitoring of displacements plays a fundamental role. However, due to the very slow movements, a long monitoring period can be necessary. Comprehensive and long monitoring can be achieved by combining different methods: airborne LIDAR and aerial photographs [14,15], radar interferometric

Geosciences 2020, 10, 171; doi:10.3390/geosciences10050171 www.mdpi.com/journal/geosciences Geosciences 2019, 9, x FOR PEER REVIEW 2 of 17 photographsGeosciences 2020 [14,15],, 10, 171 radar interferometric technologies (DiNSAR), and Global Positioning2 ofSystem 17 (GPS) [16–18], inclinometers and DiNSAR with GPS data [19] or topographic measurements [20], DiNSARtechnologies and statistical (DiNSAR), elaboration and Global [21]. Positioning System (GPS) [16–18], inclinometers and DiNSAR withWith GPS the data aim [ 19of] enriching or topographic the technical measurements database [20], DiNSARon very andslow statistical earthflows, elaboration and analyzing [21]. their kinematics,With this the aimpaper of enrichingpresents thesome technical experimental database onresults very slowon the earthflows, displacements and analyzing of two their clayey landslideskinematics, occurring this paper in the presents Costa some della experimentalGaveta slope, results close onto the displacementscity of Potenza, of twoItaly: clayey the homonymous landslides Costaoccurring della Gaveta in the earthflow,Costa della Gaveta and slope,an earthflow close to theoccurring city of Potenza, in the Italy:western the homonymoussector of theCosta Varco della d’Izzo landslideGaveta systemearthflow, (Figures and an earthflow1 and 2). occurringThe movements in the western of both sector the oflandslides the Varco d’Izzoare verylandslide slow; system however, they(Figures cause severe1 and2). damage The movements over time of to both houses the landslides and infrastructure. are very slow; Some however, houses they must cause be severeevacuated, damage over time to houses and infrastructure. Some houses must be evacuated, and the highway and the highway and railway must be continuously monitored and frequently maintained. To reduce and railway must be continuously monitored and frequently maintained. To reduce the landslides’ the landslides’ risk, since 2005 the slope is monitored by systems of piezometers and tensiometers, risk, since 2005 the slope is monitored by systems of piezometers and tensiometers, inclinometers, GPS inclinometers,stations, total GPS stress stations, cells [22 total–24], stress distributed cells fiber-optic[22–24], distributed strain sensors fiber-optic [25], and otherstrain systems sensors specific [25], and otherfor systems the chemical specific characterization for the chemical of the characterization subsoil [23,24]. of the subsoil [23,24]. In thisIn this paper, paper, the the displacement displacement data data of of the the long-termlong-term monitoring monitoring carried carried out out by aby large a large number number of traversingof traversing and and fixed-in-place fixed-in-place inclinometer inclinometer prob probeses areare reportedreported together together with with those those obtained obtained by by continuouscontinuous GNSS GNSS (Global (Global Navigation Navigation Satellite Satellite System) stationsstations and and discrete discrete GPS GPS surveys. surveys. The The data data integrationintegration provides provides a 15-yeara 15-year uninterrupted uninterrupted robust displacement robust displacement series. Furthermore, series. interferometric Furthermore, interferometricPSInSAR satellite PSInSAR dataare satellite examined. data The are spatial examin anded. temporal The spatial extension and of temporal the dataset extension allowed theof the datasetconstruction allowed ofthe the construction field of displacement of the field rate of and displa the evaluationcement rate of itsand evolution. the evaluation of its evolution.

FigureFigure 1. Geological 1. Geological map map of ofthe the CostaCosta della della Gaveta Gaveta earthﬂowearthflow (redrawn (redrawn from from [24 [24]),]), and and the theVarco Varco d’Izzo d’Izzo landslidelandslide system system (red (redrawnrawn from from [26]). [26]).

2. Geological and Geomorphological Outline

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Figure 2. Costa della Gaveta and Varco d’Izzo earthflows and location of inclinometers, GPS stations and Figure 2. Costa della Gaveta and Varco d’Izzo earthﬂows and location of inclinometers, GPS stations and benchmarks, and area of topographic measurements. benchmarks, and area of topographic measurements.

2. Geological and Geomorphological Outline Section AA’ - Costa della Gaveta The landslides under examination occur in the southern slope of the Costa della Gaveta hill, on the I12b left flank of the Basento river valley, east of PotenzaI12 city (Figure1). The geologicalInclinometers structure with of the hill is mainlyI10 linked to Miocene orogenic transport that deformed indication of the depth highway of failure I9 the Mesozoic Lagonegro successions,50 m creating wide folds withI8 the axis approximately oriented NS, EW. I7 Basento river The folds are truncatedInclinometers by100 of faults otherm past EW and thrusts along the eastern and western flanks of the main mountains [27,28].geotechnical The tectonic surveys phases of Pliocene and Pleistocene resulted in further folds truncated by a high angle fault system with Apennine and anti-Apennine orientation [29]. The complex fault system also involved the Costa della Gaveta hill, and theSection outcropping BB’ - Varco d’Izzo sequence of Variegated Clay and Corleto Perticara formations [30,31]. The two formationsI6 present a high degree of fractures and joints and are cut by a system of faults50 m with NW–SE orientation intersecting a system with NE–SW orientation. The Variegated Clay Formation (Upper Cretaceous–Lower Miocene), first described by 0 I4 Ogniben [30], is comprised of a succession of chaotic, heterogeneous,I3 highway severely tectonized scaly clays and marly clays,0 100 m passing upwards to calcareous marls, calcilutites,I2 tunnel andBasento calcarenites. These I1 river terrains outcrop from the top to the bottom of the slope, where they are covered by the Quaternary deposits of the Basento river alluvial terraces. The Corleto Perticara Formation (Eocene–Oligocene), first described by Selli [31], which is partially heteropic to the terrains of the Variegated Clays, consists Figure 3. Sections AA’ and BB’ with inclinometers of recent and past surveys, depth of failures, and of alternating layers and benches of marly limestone, and massive calcilutites with both plane-parallel hypothesized slip surfaces. and wavy laminations. Between the two geological formations, a transition zone outcrops (Figure1), constituted by different lithological horizons, where limestone and calcareous marl layers progressively 3. Inclinometer Measurements prevail on marl and clay layers [32]. 3.1. MethodsThe Costa della Gaveta and Varco d’Izzo earthflows are a mixture of the materials of the two formations: a clayey matrix incorporates rock fragments, blocks, and disarranged strata of marly limestoneInclinometers and calcarenite. were installed The fine matrixto determine controls the landslides’magnitude, behavior; rate, direction, its clay fractiondepth, c.f.and ranges time evolution of the landslides’ deformations and movements [35]. The inclinometer system consists of 20 devices, installed from 2004 to 2018, which recorded data over different time intervals in each zone depending on the displacement rates. At the end of 2004, 11 boreholes were drilled to 50 m depth,

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Geosciences 2020, 10, 171 4 of 17 between 25% and 50%, illite–muscovite, and kaolinite are the main mineral components. The liquid limit wL ranges between 50% and 100%. The residual friction angle of the clayey matrix ranges between 6◦ and 13◦, depending on the clay fraction, clay mineral composition, and pore fluid composition [24]. The systems of faults have most likely influenced the first slope failures and the subsequent evolution of the landslides [24,32]. The main body of the Costa della Gaveta earthflow develops along the NW–SE fault (Figure1). In the source zone, markedly depleted, localized erosion, rock falls, slides, and earth-debris flows periodically occur along the NE–SW fault, and feed the main landslide body [24,32]. This latter is characterized by an average inclination of about 10◦, approximately 900 m length, 100 to 600 m width, and a maximum slip surface depth of about 40 m (Figure3); the landslide volume is approximately 3 106 m3. According to Guida and Iaccarino [5], who analyzed a large number of landslides in the × Basento river valley, and Urciuoli et al. [6], this landslide is one of the earthflows of the valley which has had three key stages. After a first stage A characterized by mobilization and flow, and a second stage B of flow within a well-defined lateral shear surface, the earthflow is currently in a slow–extremely slow slidingFigure phase 2. (stage Costa della C). AGaveta similar and long-term Varco d’Izzo evolution earthflows of and active location earthflows of inclinometers, is described GPS bystations Mackey and and Roeringbenchmarks, [14] for a and different area of geological topographic context. measurements.

Section AA’ - Costa della Gaveta

I12b I12 Inclinometers with I10 indication of the depth highway of failure I9

50 m I8 I7 Basento river Inclinometers100 of otherm past geotechnical surveys

Section BB’ - Varco d’Izzo

50 m

0 I4 I3 highway 0 100 m I2 tunnel Basento I1 river

FigureFigure 3. 3.Sections Sections AA’ AA’ and and BB’ BB’ with with inclinometers inclinometers of of recent recent and and past past surveys, surveys, depth depth of of failures, failures, and and hypothesizedhypothesized slip slip surfaces. surfaces.

3. InclinometerEast of the Costa Measurements della Gaveta earthflow, and in the same geological formations, the Varco d’Izzo landslide system develops. It is a huge and complex system (Figure1) characterized by approximately 1.53.1. km Methods maximum width. The system consists of several minor unstable bodies including slides (of earth,Inclinometers rock blocks, and were even installed olistolites to updetermine to some 10the m), magnitude, and earthflows. rate, direction, The main earthflow,depth, and in time the westernevolution and of mostthe landslides’ urbanized def zoneormations [33,34], developsand movements along a[35]. NW–SE The faultinclinometer almost parallelsystem consists to that of of Costa20 devices, della Gaveta installed. It isfrom characterized 2004 to 2018, by which an average recorded inclination data over of different about 10 time◦, approximately intervals in each 1250 zone m length,depending 150 toon 300 the m displacement width, and multiple rates. At slip the surfacesend of 2004, (Figure 11 3boreholes). In the accumulation,were drilled to a 50 local m slidedepth, occurs in correspondence to a bend of the Basento river.

3. Inclinometer Measurements

3.1. Methods Inclinometers were installed to determine the magnitude, rate, direction, depth, and time evolution of the landslides’ deformations and movements [35]. The inclinometer system consists of 20 devices, Geosciences 2020, 10, 171 5 of 17

Geosciences 2019, 9, x FOR PEER REVIEW 5 of 17 installed from 2004 to 2018, which recorded data over different time intervals in each zone depending on the displacement rates. At the end of 2004, 11 boreholes were drilled to 50 m depth, i.e., well i.e., well below the potential zone of movement suggested by borehole-core interpretation, and below the potential zone of movement suggested by borehole-core interpretation, and equipped with equipped with inclinometer casings. Further tubes were installed all over the monitoring period to inclinometer casings. Further tubes were installed all over the monitoring period to replace those that replace those that had gone out of use for excess of displacements. Possible random or systematic had gone out of use for excess of displacements. Possible random or systematic errors were minimized errors were minimized by following recommended procedures for installation, measurement, and by following recommended procedures for installation, measurement, and data processing [36,37]. data processing [36,37]. Traversing servo-accelerometer probes with a precision of 1 mm per 20 m Traversing servo-accelerometer probes with a precision of 1 mm per 20 m were used. Readings were were used. Readings were generally taken in steps of 0.5 m (equal to the distance between the two generally taken in steps of 0.5 m (equal to the distance between the two spring-pressured wheels) and spring-pressured wheels) and of 0.1 m in correspondence to concentrated deformations [22]. The of 0.1 m in correspondence to concentrated deformations [22]. The spiral correction was verified to be spiral correction was verified to be unnecessary. In some boreholes (I9, I9b, I12, I12b), after the slip unnecessary. In some boreholes (I9, I9b, I12, I12b), after the slip surface had been detected by discrete surface had been detected by discrete inclinometer surveys, fixed-in-place probes with automated inclinometer surveys, fixed-in-place probes with automated data acquisition equipment were installed. data acquisition equipment were installed. The casings were mainly installed along the longitudinal Theaxes casingsof the earthflows’ were mainly bodies installed (Figures along 2 and the longitudinal3). Some of them axes ofwere the installed earthflows’ along bodies the transversal (Figures2 andsections3). Some (Figure of them 4). were installed along the transversal sections (Figure4).

Displacement (cm) a) Displacement (cm) Displacement (cm) 051005100510 0 0 0

5 5 5

10 10 10

15 15 15 I9c (2012-20) 20 20 20

25 25

Depth (m) I9b (2012-20) 30 30 35 40 I9 (2005-13) 45 50 m b) lateral landslide borders I10 I9c I9b - I9 - I8 50 m - - I7 I7b I10 - - - I9 I8 50 m I7

Figure 4.4. Costa della Gaveta earthflow:earthflow: inclinometer profiles profiles I9, I9b, and I9c ((a)a);; longitudinallongitudinal sectionsection with inclinometer profilesprofiles relativerelative toto thethe samesame timetime periodperiod andand transversaltransversal sectionssections ((b)b)..

3.2. Results Figure3 3 reports reports thethe longitudinallongitudinal sectionssections ofof thethe two landslides hypothesized from inclinometers. Figure4 4aa showsshows thethe cumulativecumulative displacementdisplacement profilesprofiles determineddetermined inin somesome boreholesboreholes inin thethe samesame transversal section of the Costa della Gaveta earthflow.earthflow. The inclinometer profiles profiles show that the tube deformations areare concentratedconcentrated in in a a thin thin band—which band—which is consideredis considered to correspondto correspond tothe to the slip slip band—and band— theand internalthe internal deformations deformations are negligible. are negligible. A thickness A thickness of the of band the band of about of about 0.5 m 0.5 was m inferred was inferred from inclinometerfrom inclinometer readings readings in 0.1 in m 0.1 steps. m steps. In I9c,In I9c, close close to to the the lateral lateral landslide landslide channel channel whichwhich collectscollects superficialsuperficial water, andand probablyprobably becausebecause ofof the consequentconsequent higher water content of the 3 m m thick thick cover, cover, the highesthighest internalinternal deformations deformations among among the the profiles profil ofes thisof this landslide landslide occur occur [22]. [22]. Notwithstanding Notwithstanding this, concentratedthis, concentrated sliding sliding along along a thin a slipthin band slip band is still is the still prevailing the prevailing movement. movement. Analogous Analogous profiles profiles were providedwere provided by all theby inclinometersall the inclinometers of the landslide, of the landslide, consistently consistently with data reportedwith data in thereported literature in the for theliterature slow/extremely for the slow/extremely slow stage of this slow type stage of landslides of this [type5–7] andof landslides of shallower [5–7] earthflows and of [38shallower–40]. earthflows [38–40]. Figure 4b, which shows some displacement profiles along the longitudinal axis and some transversal sections, shows that the displacement rates decrease in the downslope direction, whereas the depth of the slip surface and the area of the transversal sections increase. The transversal sections were reconstructed following Di Maio et al. [41], by generating the slip surface by two families of

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planes. The planes interpolate the ground surface of the flanks of the landslide channel and intersect the inclinometer verticals in correspondence to the depth of failure. Figure 5a,b, which reports respectively the cumulative deep and superficial displacements against time, illustrates that the differences between the two sets of data are small, i.e., internal deformations are negligible. Both the figures show an almost linear time trend of displacements, with Geosciences 2020, 10, 171 6 of 17 the average slopes of the time series, i.e., the average displacement rates, decreasing in the downslope direction. FigureThe inclinometer4b, which showsprofiles someof the displacementVarco d’Izzo earthflow profiles (Figure along the6) indicate longitudinal the presence axis and of two some transversalmain slip surfaces, sections, important shows that internal the displacement deformations rates, and decreasedisplacement in the rates downslope higher than direction, those of whereas the theCosta depth della of Gaveta the slip earthflow. surface and The the longitudinal area of the section transversal BB’ sectionsintercepts increase. several Theinclinometers transversal which sections wereindicate reconstructed multiple slip following surfaces (Figure Di Maio 3). et In al. correspondence [41], by generating to the railway the slip tunnel, surface two by slip two surfaces families of planes.were detected, The planes at the interpolate depths of theabout ground 20 m and surface 40 m, of respectively. the flanks of The the earthflow landslide develops channel andalong intersect the theshallowest inclinometer slip surface verticals [42]. in correspondenceIn the accumulation, to the in depth correspondence of failure. to the entrance of the railway tunnel, and of a bend of the Basento river, a local landslide occurs. Figure 7, which reports the Figure5a,b, which reports respectively the cumulative deep and superficial displacements against displacements against time recorded in several inclinometers installed since 1993, shows the time, illustrates that the differences between the two sets of data are small, i.e., internal deformations variability of displacement rate from place to place and along a single vertical. The inclinometer I1, are negligible. Both the figures show an almost linear time trend of displacements, with the average in the accumulation, provides a sufficiently long series with almost constant yearly displacement slopes of the time series, i.e., the average displacement rates, decreasing in the downslope direction. rates on both the slip surfaces.

35 a) I12c 30 I12b .. 25 B A I12 20 I9b 15

10 I9c I9 AB displacement (cm) AB displacement I10 I8 5 I7 0

5 6 8 1 3 6 8 9 0 0 0 07 0 09 1 1 14 1 1 1 2 35 n n n n n n a Jan Jab) J Jan Ja Jan 10 Jan Jan 12 Jan Ja Jan 15 Ja Jan 17 Jan Ja Jan D 30 I12c

.. I12b 25 A I9b 20 I12

15 I9c I10 10 I9 AD displacement (cm) AD displacement I8 5 I7

2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020

FigureFigure 5. 5.Time Time seriesseries ofof cumulativecumulative deep displacements AB AB ( (aa)) and and superficial superﬁcial displacements displacements AD AD (b (b) ) of Costaof Costa della della Gaveta Gavetalandslide, landslide, obtained obtained from from inclinometer inclinometer data. data.

The inclinometer profiles of the Varco d’Izzo earthflow (Figure6) indicate the presence of two main slip surfaces, important internal deformations, and displacement rates higher than those of the Costa della Gaveta earthflow. The longitudinal section BB’ intercepts several inclinometers which indicate multiple slip surfaces (Figure3). In correspondence to the railway tunnel, two slip surfaces were detected, at the depths of about 20 m and 40 m, respectively. The earthflow develops along the shallowest slip surface [42]. In the accumulation, in correspondence to the entrance of the railway tunnel, and of a bend of the Basento river, a local landslide occurs. Figure7, which reports the displacements against time recorded in several inclinometers installed since 1993, shows the variability of displacement rate from place to place and along a single vertical. The inclinometer I1, in the accumulation, provides a sufficiently long series with almost constant yearly displacement rates on both the slip surfaces. Geosciences 2020, 10, 171 7 of 17 Geosciences 2019, 9, x FOR PEER REVIEW 7 of 17 Geosciences 2019, 9, x FOR PEER REVIEW 7 of 17

Displacement (cm) a) Displacement (cm) Displacement (cm) a) Displacement (cm)DisplacementDisplacement (cm) (cm) 05100510 05100510 0 0 0 0 5 5 Jul 06 Oct 05 5 5 Dec 05 Jul 06 Oct 05 10 10 Dec 05 10 10 b) 15 15 I3 (2005-06) I4 b) 15 I3 (2005-06) I4 highway 15 I3 highway 20 20 I3 tunnel Basento 20 20 I2 tunnel Basentoriver I2 I1 river 25 25 I1

Depth (m) Depth 25 25 cm

Depth (m) Depth 30 30 000,1cm 0.1 30 30 000,10.1 35 m

35 m 35 35 38 40 40 38 40 I1 (2005-12) 40 45 I1 (2005-12) 45 45 45 Figure 6. Inclinometer profiles I1 and I3 (a) and longitudinal section of the lower Varco d’Izzo earthflow FigureFigure 6. 6.Inclinometer Inclinometer profiles profiles I1 I1 and and I3 I3 ( a(a)) and and longitudinal longitudinal section section of of the the lower lowerVarco Varco d’Izzo d’Izzoearthflow earthflow (b). I3 went out of use in a year along the shallower slip surface, when the displacements on the deeper (b(b)).. I3 I3 went went out out of of use use in in a a year year along along the theshallower shallower slip slip surface, surface, when when the the displacements displacements on on the the deeper deeper slip surface were small (as visible in the zoom relative to the profile of December 2005). slipslip surface surface were were small small (as (as visible visible in in the the zoom zoom relative relative to to the the profile profile of of December December 2005). 2005).

12 12 I3 D I3 C D I3, displ. A'D FSC N C I3, displ. A'D 10 FSCB25 N B'' FS5 10 S1f B25 A'' B'' FS5 I2 S1fB24 FS7 A'' I1, displ. A'D 8 I2 B24 FS7FSB B' I1, displ. A'D FS6 FSBl A' 8 ne FS5 B' FS6un l A' B23 S2f t ne FS5 un 6 B23 S2f t B22 I1 I3, displ. A''B'' 6 ay I1 ilw B22 I3, displ. A''B'' ra ay ilw 4 0r 50a 100 m 4 0 50 100 m Displacement (cm) Displacement I1, displ. A''B'' Displacement (cm) Displacement S1f, FSB, I1, displ. A''B'' 2 B22-25 S1f, FSB, 2 B22-25 S2f FS6, FSC S2f I1, displ. A'B' FS6, FSC 0 I1, displ. A'B' 0 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 shear displacement along the slip bands (depth reported in brackets) shear displacement along the slip bands (depth reported in brackets) B22 (34m) B23 (19m) B23 (29m) B24 (17m) B25 (12m) B22 (34m) B23 (19m) B23 (29m) B24 (17m) B25 (12m) FS5 (21m) FS6 (32m) FSC (27m) S1f (22m) S2f (26m) FS5 (21m) FS6 (32m) FSC (27m) S1f (22m) S2f (26m) FSB (28m) I3 (11m) I1 (20m) I1 (34m) FSB (28m) I3 (11m) I1 (20m) I1 (34m) total displacement total displacement I1 I3 I1 I3

Figure 7. Deep and total displacements in different areas of the Varco d’Izzo earthflow, obtained from Figure 7.Deep Deep and and total total displacements displacements inin didifferentfferent areas areas of of the the Varco d’Izzoearthflow, earthflow, obtained obtained from from inclinometerinclinometer datadata (redrawn(redrawn fromfrom DiDi MaioMaio etet al.al. [[42]).42]). inclinometer data (redrawn from Di Maio et al. [42]). 4. GPS Data 4. GPS Data 4.1. Methods 4.1. Methods 4.1. Methods In 2006, GPS benchmarks were installed for the measurement of surface displacements by means In 2006, GPS benchmarks were installed for the measurement of surface displacements by means of discreteIn 2006, surveys GPS benchmarks and, one year were later, installed were for integrated the measurement with continuous of surface GNSS displacements (cGNSS) by stations means of discrete surveys and, one year later, were integrated with continuous GNSS (cGNSS) stations (Figureof discrete2). surveys and, one year later, were integrated with continuous GNSS (cGNSS) stations (Figure 2). (FigureThe 2). first network of 10 benchmarks surveyed using GPS technique was installed in 2006 in various The first network of 10 benchmarks surveyed using GPS technique was installed in 2006 in zonesThe of the first slope network and annual of 10 campaignsbenchmarks were surveyed carried using out. FollowingGPS technique the first was inclinometer installed in results,2006 in various zones of the slope and annual campaigns were carried out. Following the first inclinometer 6various cGNSS zones stations of werethe slope installed and inannual some campaigns sectors to better were understandcarried out. theFollowing kinematics the offirst the inclinometer landslides. results, 6 cGNSS stations were installed in some sectors to better understand the kinematics of the Theyresults, were 6 cGNSS installed stations in July 2007were andinstalled are still in insome use. sectors In total, to 17 better benchmarks understand have the been kinematics installed soof far,the landslides. They were installed in July 2007 and are still in use. In total, 17 benchmarks have been withlandslides. 14 still inThey use. were installed in July 2007 and are still in use. In total, 17 benchmarks have been installed so far, with 14 still in use. installed so far, with 14 still in use.

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The reference permanent station (master) was installed on the top of the hill, in an area considered stable, upslope from the landslides. It consists of a dual-frequency GNSS receiver and a choke ring antenna. Its position is periodically controlled relative to permanent stations of the EUREF network. Figure 8 shows its displacement trend and those of two other permanent stations of the same Region in the IGS14 coordinate system, which is consistent with the movement of the Southern Italian Apennines [43]. Three permanent stations were installed on concrete columns (F1, F3, F5), one on a concrete wall (F2), and one on a building (F4) with shallow foundations [44]. These permanent stations are equipped with a dual-frequency geodetic antenna. Continuous data, recorded at a 30 s sampling rate, are automatically transmitted to the head office of ISPRA. Here, the quality of raw data is controlled, RINEX files are created and the daily displacements, relative to the master station, are provided as an output by the Leica GPS Spider software. In this paper, the RINEX data available in 24 h batches were reprocessed to obtain three component daily positions. GPS data were reduced using the NASA Jet Propulsion Laboratory (JPL) GIPSY-OASIS II software. Precise point positioning mode was applied to the ionospheric-free carrier phase and pseudo-range data [45] using JPLs final fiducial-free GPS orbit products. Ambiguity resolution was applied using the wide lane and phase bias (WLPB) method, which phase-connects individual stations to IGS stations in common view [45]. Resolving ambiguities significantly reduces the scatter mostly in the east component time series. The ocean loading was computed from the FES2004 tidal model coefficients provided by the Ocean Tide Loading Provider [46] and applied as a station motion model. Satellite orbit and clock parameters were provided by JPL, which determined them using a subset of the available IGS core stations as tracking sites. The fiducial-free daily GPS solutions were aligned to IGS14 [47] by applying a daily seven-parameter Helmert transformation (three rotations, three translations, and a scale component) obtained from JPL [45]. GeosciencesThe2020 discrete, 10, 171 GPS surveys, carried out in static mode, use the top of inclinometer tubes by 8an of 17 adapter constructed ad hoc [44], rigid walls (CS01 and CS14), and low concrete walls (CS13, CS06, and CS12). During the surveys, several geodetic receivers are employed using a 3D forced centering systemThe reference in order to permanent warrant the station repetitiveness (master) wasof measurements. installed on the The top static of the positioning hill, in an technique area considered was stable,used, upslope with three from site the occupations landslides. and It consistsat least six of ahours’ dual-frequency acquisition. GNSS The data receiver processing and a chokewas ringperformed antenna. using Its position Bernese software is periodically version controlled5.2 [48] and relative the coordinates to permanent of the stations stations were of the obtained EUREF network.by constraining Figure8 shows some itsEUREF displacement permanent trend stations and those with of a two processing other permanent strategy stationsfor regional of the GPS same Regionnetworks. in the The IGS14 average coordinate precision system, (RMS) which achieved is consistent was 2–4 withmm and the movement5–10 mm for of the the horizontal Southern Italianand Apenninesvertical components [43]. respectively.

FigureFigure 8. 8.Displacement Displacement time time series series in in the the IGS14IGS14 coordinatecoordinate system of of the the master master station station and and two two other other permanentpermanent stations stations located located in in the the same same sector sector ofof thethe SouthernSouthern Italian Apennines: Apennines: MATE MATE (operated (operated by by ASI—ItalianASI—Italian Space Space Agency) Agency) and and TITO TITO (operated (operated byby CNR—NationalCNR—National Research Council Council of of Italy). Italy).

Three permanent stations were installed on concrete columns (F1, F3, F5), one on a concrete wall (F2), and one on a building (F4) with shallow foundations [44]. These permanent stations are equipped with a dual-frequency geodetic antenna. Continuous data, recorded at a 30 s sampling rate, are automatically transmitted to the head office of ISPRA. Here, the quality of raw data is controlled, RINEX files are created and the daily displacements, relative to the master station, are provided as an output by the Leica GPS Spider software. In this paper, the RINEX data available in 24 h batches were reprocessed to obtain three component daily positions. GPS data were reduced using the NASA Jet Propulsion Laboratory (JPL) GIPSY-OASIS II software. Precise point positioning mode was applied to the ionospheric-free carrier phase and pseudo-range data [45] using JPLs final fiducial-free GPS orbit products. Ambiguity resolution was applied using the wide lane and phase bias (WLPB) method, which phase-connects individual stations to IGS stations in common view [45]. Resolving ambiguities significantly reduces the scatter mostly in the east component time series. The ocean loading was computed from the FES2004 tidal model coefficients provided by the Ocean Tide Loading Provider [46] and applied as a station motion model. Satellite orbit and clock parameters were provided by JPL, which determined them using a subset of the available IGS core stations as tracking sites. The fiducial-free daily GPS solutions were aligned to IGS14 [47] by applying a daily seven-parameter Helmert transformation (three rotations, three translations, and a scale component) obtained from JPL [45]. The discrete GPS surveys, carried out in static mode, use the top of inclinometer tubes by an adapter constructed ad hoc [44], rigid walls (CS01 and CS14), and low concrete walls (CS13, CS06, and CS12). During the surveys, several geodetic receivers are employed using a 3D forced centering system in order to warrant the repetitiveness of measurements. The static positioning technique was used, with three site occupations and at least six hours’ acquisition. The data processing was performed using Bernese software version 5.2 [48] and the coordinates of the stations were obtained by constraining some EUREF permanent stations with a processing strategy for regional GPS networks. The average precision (RMS) achieved was 2–4 mm and 5–10 mm for the horizontal and vertical components respectively. Geosciences 2020, 10, 171 9 of 17

GeosciencesGeosciences 20192019,, 99,, xx FORFOR PEERPEER REVIEWREVIEW 99 ofof 1717 In order to increase the monitoring points in the most urbanized zone of the earthﬂow, close to InIn orderorder toto increaseincrease thethe monitoringmonitoring pointspoints inin ththee mostmost urbanizedurbanized zonezone ofof thethe earthflow,earthflow, closeclose toto inclinometer I3 and GPS F3, the displacements of 8 benchmarks installed on rigid structures were inclinometerinclinometer I3I3 andand GPSGPS F3,F3, thethe displacementsdisplacements ofof 88 benchmarksbenchmarks installedinstalled onon rigidrigid structuresstructures werewere evaluated, by a polar method, elaborating the data obtained by an electronic theodolite (Kolida KTS evaluated,evaluated, byby aa polarpolar method,method, elaboratingelaborating thethe datadata obtainedobtained byby anan electronicelectronic theodolitetheodolite (Kolida(Kolida KTSKTS 445) with 0.1” angular resolution and 2 mm 2 ppm/km linear precision. 445)445) withwith 0.10.1″″ angularangular resolutionresolution andand 22 mmmm ±±± 22 ppm/kmppm/km linearlinear precision.precision. 4.2. Results 4.2.4.2. ResultsResults The GNSS system provided data almost continuously since its installation. Figure9a,b shows TheThe GNSSGNSS systemsystem providedprovided datadata almostalmost continuouscontinuouslyly sincesince itsits installation.installation. FigureFigure 9a,b9a,b showsshows aa a good agreement, in the overlapping monitoring period, between GPS displacements and surface goodgood agreement,agreement, inin thethe overlappingoverlapping monitoringmonitoring period,period, betweenbetween GPSGPS displacementsdisplacements andand surfacesurface inclinometer displacements obtained in the same areas. The GPS data can thus be used to extend the inclinometerinclinometer displacementsdisplacements obtainedobtained inin thethe samesame areaareas.s. TheThe GPSGPS datadata cancan thusthus bebe usedused toto extendextend thethe displacementdisplacementdisplacement timetime seriesseries when when inclinometers inclinometers gogo go outout out ofof of use.use. use. InIn In particular,particular, particular, itit itisis is worthworth worth notingnoting noting thatthat that thethe the GPSGPSGPS F3 F3F3 allowed allowedallowed for forfor the thethe monitoring monitoringmonitoring of ofof the thethe fastest fastestfastest zone zonezone ofofof Varco VarcoVarco d’Izzod’Izzod’Izzo earthﬂow earthflowearthflow afterafter thethe inclinometerinclinometer I3 wentI3I3 wentwent out out ofout use. ofof use.use. Itsdisplacement ItsIts displacementdisplacement rate rate israte similar isis similarsimilar to that toto of thatthat inclinometer ofof inclinometerinclinometer I3 and I3I3 also andand to alsoalso the ratestoto thethe detected ratesrates bydetecteddetected the topographic byby thethe topographictopographic surveys (Figure surveyssurveys9 b). (Figure(Figure 9b).9b).

a) b) a) 4040 b) 4040 100100 F5F5 F3F3 F3, I3F3, displacement (cm) F3, I3F3, displacement (cm) I3I3 .. .. I9I9 3030 3030 F1F1 7575 GPS8GPS8 I1I1 I8 I8 theodolitetheodolite 2020 2020 5050 Displacement (cm) Displacement Displacement (cm) Displacement 1010 1010 2525 .. .. F1, I1 displacement (cm) displacement F1, I1 F1, I1 displacement (cm) displacement F1, I1

00 00 00 2005 2010 2015 2020 2005 2010 2015 2020 2005 2010 2015 2020 2005 2010 2015 2020 FigureFigureFigure 9. 9.9. ComparisonComparison of: of: inclinometerinclinometer andand and GPSGPS GPS datadata data ofof of CostaCostaCosta delladella della GavetaGaveta Gaveta earthflowearthflowearthﬂow ((aa);); (a inclinometer,inclinometer,); inclinometer, GPS,GPS,GPS, and andand theodolite theodolitetheodolite datadata ofof VarcoVarco d’Izzo d’Izzo earthflowearthﬂowearthflow (( (bb).).).

a) b) a) 4040 b) 4040 100100 GPS10GPS10 F3F3 GPSI2bGPSI2b F1F1 F3 displacement (cm) GPS12cGPS12c F3 displacement (cm)

.. F2

.. F2 F5 30 3030 F5 30 GPSGPS CS06CS06 7575 GPS8GPS8 GPSGPS CS12CS12 GPSGPS CS14CS14 GPS4 F4F4 GPS4 GPS CS13 2020 GPS9bGPS9b 2020 GPS CS13 5050 GPS9cGPS9c Displacement (cm) Displacement Displacement (cm) Displacement Displacement (cm) Displacement Displacement (cm) Displacement 10 1010 25

10 25 .. ..

00 00 00 2005 2010 2015 2020 2005 2010 2015 2020 2005 2010 2015 2020 2005 2010 2015 2020 FigureFigureFigure 10. 10.10. DisplacementsDisplacements ofof of GPSGPS GPS statstat stationsionsions andand and benchmarksbenchmarks benchmarks ofof CostaCosta of Costa delladella della GavetaGaveta Gaveta ((aa)) andand(a) VarcoVarco and Varco d’Izzod’Izzo d’Izzo ((bb)) (earthflows.bearthflows.) earthﬂows.

TheTheThe available availableavailable GPSGPS datadata are are shown shown in in FigureFigure 1010 fo foforrr thethe the twotwo two landslides.landslides. landslides. TheThe The curvescurves curves showshow show that:that: that: thethe the stationsstationsstations upslope upslopeupslope fromfrom thethe highway highway areare slightlyslightly deceleratingdecelerating decelerating (F3,(F3, (F3, F5,F5, F5, GPS8);GPS8); GPS8); thosethose those downslopedownslope downslope fromfrom from itit it areareare undergoing undergoingundergoing displacementsdisplacements with with coco constantnstantnstant yearlyyearly yearly ratesrates rates (F1(F1 (F1 andand and F2)F2) F2) oror or withwith with slightslight slight accelerationacceleration acceleration (F4).(F4). (F4). SomeSomeSome stations stationsstations (GPS(GPS(GPS CS06,CS06, GPS4) GPS4) have have notnot movedmoved inin in thethe the lastlast last 44 4 years.years. years. SeasonalSeasonal Seasonal datadata data oscillationsoscillations oscillations havehave have aaffectedaffectedﬀected the thethe GPS GPSGPS stationsstationsstations installedinstalled on on concreteconcrete columnscolumns columns withwith with shallowshallow shallow foundations,foundations, foundations, reasonablyreasonably reasonably duedue due toto to seasonalseasonalseasonal swelling swelling/shrinkageswelling/shrinkage/shrinkage ofof the the foundation foundation soilsoil [49].[49]. [49].

Geosciences 2020, 10, 171 10 of 17 Geosciences 2019, 9, x FOR PEER REVIEW 10 of 17

5.5. PSInSAR PSInSAR DataData and Comparison with with GPS GPS and and Inclinometer Inclinometer Data Data COSMO-SkyMedCOSMO-SkyMed satellite satellite PSInSAR PSInSAR data data are areavailable available for the for area the from area 2011 from to 2011 2014.to They 2014. were They wereprocessed processed within within the PST-A: the PST-A: a high a highprecision precision plan planof remote of remote sensing sensing of the of theItalian Italian Ministry Ministry of of EnvironmentEnvironment [ 50[50].]. SuchSuch data data provide provide the the displacement displacement component component along along the the satellite satellite line line of of sight sight (LOS) which(LOS) inwhich this casein this is approximatelycase is approximately WE and WE related and torelated an SN to ascendingan SN ascending orbit. Since orbit. the Since landslides’ the displacementslandslides’ displacements are approximately are approximately NS, just a NS, small just component a small component of the total of the displacement total displacement is visible is by thevisible satellite by the and satellite thus the and displacement thus the displacement rates range rates between range between very low very values low andvalues are and rather are dispersedrather dispersed (Figure 11). However, the availability of the long inclinometer and GPS data series makes (Figure 11). However, the availability of the long inclinometer and GPS data series makes it possible to it possible to evaluate the conversion factor from the LOS component to the horizontal displacement. evaluate the conversion factor from the LOS component to the horizontal displacement. The evaluation The evaluation is based on the hypothesis that: (a) the azimuth of the scatterer displacement is the is based on the hypothesis that: (a) the azimuth of the scatterer displacement is the same as that of the same as that of the closest inclinometer or GPS station, and (b) the inclination of displacements to the closest inclinometer or GPS station, and (b) the inclination of displacements to the horizontal is 10 , i.e., horizontal is 10°, i.e., equal to the slope inclination. Based on these assumptions, and considering the◦ equallocal tocharacteristics the slope inclination. of the satellite Based observation on these assumptions,(i.e., azimuth and and inclination considering to the horizontal local characteristics of the ofLOS the and satellite the observationpass being ascending), (i.e., azimuth the and conversi inclinationon factors to the were horizontal estimated of theby LOStrigonometric and the pass beingcalculations, ascending), following the conversionDi Maio et factorsal. [20]. wereTheir estimatedvalues are byquite trigonometric high (approximately calculations, from following3 to 5 Didepending Maio et al. on [20 the]. Theirconsidered values zone), are quite and higheven (approximately a 5° azimuth difference from 3 to implies 5 depending a variation on the of considered up to zone),40%. and even a 5◦ azimuth diﬀerence implies a variation of up to 40%. TheThe PSInSARPSInSAR data, data, referred referred to toas CSK as CSK data data in the in following, the following, were analyzed were analyzed in detail infor detailthe zones for the zones“a”–“j” “a”–“j” of Figure of Figure 11. For 11 the. Forzones the “a”-“d”, zones “a”–“d”, they are compared they are compared to the GPS to and the inclinometer GPS and inclinometer data in dataFigure in Figure12. In the 12 .Costa In the dellaCosta Gaveta della landslide, Gaveta landslide, a very good a very agreement good agreement of data can of be data observed can be (Figure observed (Figure12a,b). 12Ina,b). the Varco In the d’IzzoVarco landslide, d’Izzo landslide, practically practically the same the displacement same displacement rate is inferred rate isinferred in zone in“c” zone “c”from from the the two two types types of data of data (Figure (Figure 12c). 12 c).In zone In zone “d”, “d”, located located between between the thehighway highway and andthe railway the railway tunnel,tunnel, the the displacementdisplacement rates from from CSK CSK data data are are slightly slightly higher higher than than that that of GPS of GPS F2 (Figure F2 (Figure 12d) 12 andd) and ratherrather uniform uniform (Figure(Figure 1313a).a). TheThe CSK CSK data, data, having having been been validated validated by GPSby GPS and inclinometers,and inclinometers, can be can used be to used obtain to information obtain ininformation the areas where in the other areas data where is limited, other suchdata asis zoneslimited, “e–“h”. such Figureas zones 13 b–e,“e–“h”. which Figure reports 13b–e, displacements which reports displacements against time, show linear time trends corresponding to horizontal against time, show linear time trends corresponding to horizontal displacement rates between 1 cm/y displacement rates between 1 cm/y and 6 cm/y. and 6 cm/y.

FigureFigure 11.11. Average rates of of the the line line of of sight sight (LOS) (LOS) displa displacementcement of ofmonitored monitored sca scattererstterers in the in period the period MayMay 2011–March2011–March 2014 evaluated from from COSMO-SkyMed COSMO-SkyMed ascending ascending data. data.

Geosciences 2020, 10, 171 11 of 17

GeosciencesGeosciences 2019 2019, ,9 9, ,x x FOR FOR PEER PEER REVIEW REVIEW 1111 of of 17 17 . . . . 2020 1515 a)a) b)b) inclincl I12 I12 1515 inclincl I9b I9b 1010 1010 CSKCSK CSKCSK 55 55 inclincl I9 I9 0 0

Horizontal displacement (cm) 0 0 Horizontal displacement (cm) displacement Horizontal Horizontal displacement (cm) Horizontal displacement (cm) displacement Horizontal 2007 2009 2011 2013 2015 2017 2011 2013 2015 2017 2007 2009 2011 2013 2015 2017 2011 2013 2015 2017 ...... 8080 2020 c)c) d)d) GPS F3 6060 GPS F3 1515 GPSGPS F2 F2 CSK 40 10 CSK 40 CSKCSK 10

2020 55

00 00 Horizontal displacement (cm) displacement Horizontal Horizontal displacement (cm) displacement Horizontal Horizontal displacement (cm) displacement Horizontal Horizontal displacement (cm) displacement Horizontal 2006 2008 2010 2012 2014 2016 2018 2020 2006 2008 2010 2012 2014 2016 2018 2020 2006 2008 2010 2012 2014 2016 2018 2020 2006 2008 2010 2012 2014 2016 2018 2020 FigureFigureFigure 12. 12.12.Comparison ComparisonComparison of displacementsofof displacementsdisplacements obtained obtainedobtained from fromfrom PSInSAR PSInSARPSInSAR data, data,data, GPS, GPS,GPS, and an inclinometersandd inclinometersinclinometers in zones inin (a),zoneszones (b), ( ca a),, ,b andb, ,c c, ,and (andd) ofd d of Figureof Figure Figure 11 11. .11.

Figure 13. (a) LOS displacement rate against latitude along DD’; (b–e) LOS displacement time series FigureFigure 13. 13.(a) ( LOSa) LOS displacement displacement rate rate against against latitude latitude along along DD’; DD’; (b (–be–)e LOS) LOS displacement displacement time time series series of of the groups of scatterers “e”–“j” (location in Figure 11). theof groups the groups of scatterers of scatterers “e”–“j” “e”–“j” (location (location in Figure in Figure 11). 11).

Geosciences 2020, 10, 171 12 of 17

6. Displacement Field The field of the average rates of superficial displacement evaluated in the monitoring period was obtained by using all the available inclinometer, GPS, and PSInSAR data (Figure 14). The results of the topographic measurements are also reported. Figure 14, as a whole, shows the main kinematic Geosciencesfeatures 2019 of, 9, the x FOR two PEER earthflows. REVIEW 13 of 17

FigureFigure 14. Vectors 14. Vectors of average of average rates rates of of horizontal horizontal displaceme displacementsnts obtained obtained from from inclinometer, inclinometer, GPS, GPS, and and PSInSARPSInSAR data data in the in the period period May May 2011–March 2011–March 20142014 and and from from topographic topographic measurements measurements carried carried out in out in 2010–20112010–2011 (a) (a;); zoom zoom (b) (b) and longitudinallongitudinal section section (c) (c) of Varcoof Varco d’Izzo d’Izzoearthﬂow earthflow accumulation. accumulation.

The yearly average displacement rates in the Costa della Gaveta earthflow determined by a) b) inclinometer,600 GPS, and PSInSAR data are all consistent500 with each other, both in value and time trend. On the basis of inclinometer profiles (Figure4), the superficial displacement rates can be also

considered500 representative of the deep displacement. So, the rate ﬁeld reportedrain in Figure 11 canCumulative rain (mm be )

) 450 7800 3 I12 considered representative of the movements of the3 entire landslide body all over the 15-year monitoring period.400 The displacement rates range from a few cm/y to a few mm/y, decreasingI9b in the downslope 400 direction and becoming negligible in the accumulation. The movement seems continuous along 300 Qs the main axis. The soil discharge crossing the diﬀerent transversal sections of the landslide can be 350 I8 6400 evaluated200 from the curves of soil volumes of Figure 15. The volumes were obtained by multiplying the I8 I9 Volume of soil (m soil of Volume

areas of the sections (Figure4) by the corresponding (m soil of Volume inclinometer displacements (Figure5) assumed I9b I10 300 uniform100 in each section. The curvesI12 relativeI12b to the sections through the diﬀerent inclinometers are very) close to each other (Figure 15a).I12c Thus, it seems reasonable to hypothesize a mechanism of movement 0 250 5000 at constant soil discharge in the landslide channel. The average soil discharge is provided by the slope of the line 2005 interpolating 2008 2011 the 2014 volume 2017 series. 2020 Figure 15 2012b shows 2013 that the 2014Costa della2015 Gaveta 2016earthﬂow accelerates during rainy periods, with a fast response to rainfall. The way by which this occurs was evaluatedFigure 15. experimentallyCosta della Gaveta by Diearthflow: Maio et volumes al. [13]. Theof soils, correlation with an between indication displacement of the average rates soil and rainfalldischarge was Qs quantitatively, crossing the derivedtransverse by Vassallosections etthrough al. [51 ].the inclinometers (a), and comparison with cumulative rain (b).

7. Conclusions The movements of the Costa della Gaveta and Varco d’Izzo earthflows are very slow; however, they cause severe damage to houses and infrastructure. The comprehension of the main features of the landslides’ kinematics is vital for the definition of mitigation measures, and is pursued in this work by a composite monitoring system. A long-term displacement monitoring carried out by inclinometers and GPS stations along with PSInSAR and topographic data allowed for reciprocal data validation, spatial and temporal extension of the dataset, and elaboration of a field of displacement rate. The Costa della Gaveta landslide, an earthflow in a very/extremely slow sliding phase, is characterized by well-defined kinematic features. All over the monitoring period, the soil moved in a well-defined regular track, along a fault line. The displacement rates decrease from a few cm/y in the head zone to a few mm/y in the accumulation. The variation of the displacement rate along the longitudinal axis can be attributed to the geometry of the landslide channel, characterized by large

Geosciences 2019, 9, x FOR PEER REVIEW 13 of 17

Figure 14. Vectors of average rates of horizontal displacements obtained from inclinometer, GPS, and GeosciencesPSInSAR2020, 10data, 171 in the period May 2011–March 2014 and from topographic measurements carried out13 of 17 in 2010–2011 (a); zoom (b) and longitudinal section (c) of Varco d’Izzo earthflow accumulation.

a) b) 600 500

500 rain Cumulative rain (mm )

) 450 7800 3 I12 3 400 I9b 400 300 Qs 350 I8 6400 200 I8 I9 Volume of soil (m soil of Volume I9b I10 (m soil of Volume 300 100 I12 I12b ) I12c 0 250 5000 2005 2008 2011 2014 2017 2020 2012 2013 2014 2015 2016 Figure 15. Costa della Gaveta earthflow: volumes of soils, with an indication of the average soil discharge Figure 15. Costa della Gaveta earthflow: volumes of soils, with an indication of the average soil Qs, crossing the transverse sections through the inclinometers (a), and comparison with cumulative discharge Qs, crossing the transverse sections through the inclinometers (a), and comparison with rain (b). cumulative rain (b). The displacement rates of Varco d’Izzo reported in Figure 14 include the movements along the 7. Conclusions deepest slip surface; however, these latter are negligible percentages of the total displacements (FiguresThe6 movementsand7) and thus of the the Costa rate della field Gaveta of Figure and 14 Varco can bed’Izzo considered earthflows representative are very slow; of thehowever, earthflow they kinematics.cause severe The damage high to levels houses of anthropogenic and infrastructure modification. The comprehension in this sector of do the not main allow features for detailed of the observationlandslides’ kinematics and analysis is vital of the for earthflow the definition kinematic of mitigation zones as measures, in Parise [and52] andis pursued Guerriero in this et al. work [8]. Nevertheless,by a composite the ratemonitoring field of Figure system. 14 allowsA long-ter for them identification displacement of threemonitoring zones whichcarried move out with by diinclinometersfferent characteristics: and GPS stations (1) the along upper with zone PSInSAR around and I5 andtopographic I6 where data the allowed displacement for reciprocal rates are data in thevalidation, order of spatial about 1and cm temporal/y; (2) the extension zone around of the I4 thatdataset, underwent and elaboration negligible ofdisplacements a field of displacement over the monitoringrate. period; (3) the lower zone between I3 and the river, where the average yearly displacement ratesThe ranged Costa between della Gaveta 7 cm/y andlandslide, 1 cm/ yan along earthflow the main inlongitudinal a very/extremely axis, first slow decreasing sliding andphase, then is increasingcharacterized in the by downslopewell-defined direction. kinematic The features. overall Al behaviorl over the of themonitoring latter zone period, is probably the soil influenced moved in bya well-defined the interaction regular of the track, landslide along with a fault large line. concrete The displacement structures such rates as decrease retaining from walls a onfew deep cm/y pile in foundations,the head zone overpass, to a few mm/y tunnel, in and the buildings.accumulation. In particular, The variation the zoomof the fromdisplacement inclinometer rate along I3 to thethe Basentolongitudinal river axis (Figure can 14 beb) attributed shows that to the the displacement geometry of ratesthe landslide decrease approachingchannel, characterized the railway by tunnel, large and then increase downslope from it. This latter, which contrasts the earth movements, is a shallow tunnel re-built in 1992, protected by two sheet pile walls of contiguous piles, 1 m diameter of and 20 m length, crossing the upper slip surface. The structure, monitored by distributed fiber-optic sensors, underwent negligible deformations in the observation period [25], consistently with the inclinometer data. The zoom of Figure 14 also shows that the direction and rate of the movements in the landslide foot are influenced by the pronounced bend in the Basento river, where a local slide also occurs.

7. Conclusions The movements of the Costa della Gaveta and Varco d’Izzo earthflows are very slow; however, they cause severe damage to houses and infrastructure. The comprehension of the main features of the landslides’ kinematics is vital for the definition of mitigation measures, and is pursued in this work by a composite monitoring system. A long-term displacement monitoring carried out by inclinometers and GPS stations along with PSInSAR and topographic data allowed for reciprocal data validation, spatial and temporal extension of the dataset, and elaboration of a field of displacement rate. The Costa della Gaveta landslide, an earthflow in a very/extremely slow sliding phase, is characterized by well-defined kinematic features. All over the monitoring period, the soil moved in a well-defined regular track, along a fault line. The displacement rates decrease from a few cm/y in the head zone to a few mm/y in the accumulation. The variation of the displacement rate along the longitudinal axis can be attributed to the geometry of the landslide channel, characterized by large depth and width variations along its axis. The general mechanism of movement corresponds Geosciences 2020, 10, 171 14 of 17 to constant soil discharge along the main track. The average yearly displacement rate was almost constant over the monitoring period, with a slight decrease in the last few years. Seasonal variations overlap the regular general trend. As a remedial measure, a system of deep drains intercepting the slip surface [13,53] is currently under study. In the case of the Varco d’Izzo landslide system, multiple slip surfaces, internal deformations, and ruptures give the landslide more complex features. The homonymous earthflow occurs along a fault parallel to that of the nearby Costa della Gaveta earthflow. The movement along the longitudinal axis is heterogeneous. The most urbanized zone of the earthflow is also the fastest, with yearly displacement rates ranging between 7 cm/y and 1 cm/y. The variations seem to be influenced by the interaction with the built environment and with the Basento river. Stabilization of the river bank as a remedial measure is currently under study.

Author Contributions: Conceptualization, R.V. and C.D.M.; data curation, R.V., S.C., N.D., J.D.R. and P.G.; funding acquisition, C.D.M.; investigation, R.V., S.C., J.D.R., C.D.M. and P.G., methodology: R.V., S.C., N.D., C.D.M. and P.G.; validation, R.V., S.C., J.D.R., N.D., C.D.M. and P.G.; writing—original draft preparation, R.V., S.C., C.D.M. and P.G.; writing—review & editing, R.V., J.D.R. and C.D.M. All authors have read and agreed to the published version of the manuscript. Funding: The geotechnical investigation has been funded by the Basilicata Administrative Region and by the Italian Ministry of Education, University and Research (PRIN 2015: Innovative Monitoring and design strategies for sustainable landslide risk mitigation). The GPS network has been designed by the Geological Survey of Italy – ISPRA (Institute for Environmental Protection and Research), in collaboration with the University of Basilicata, with the financial support of the Basilicata Administrative Region – and with the National Civil Protection. Project carried out using CSK® Products, © ASI (Italian Space Agency), delivered under an ASI license to use, provided by the Italian Ministry of Environment. Acknowledgments: The authors wish to acknowledge M. Belvedere and A. Marotta for carrying out the inclinometer measurements. Particular thanks are given to some members of the ISPRA Institute: G. Buttinelli, G. Carcani, E. Mariani, D. Matarazzo, and B. Porfidia for their technical contribution, and to D. Niceforo for her participation in the GPS campaigns. Conflicts of Interest: The authors declare no conflict of interest.

References

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