Evidence of Rock Slope Breathing Using Ground-Based Insar

GEOMOR-05677; No of Pages 18 Geomorphology xxx (2016) xxx–xxx

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Geomorphology

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Evidence of rock slope breathing using ground-based InSAR

Line Rouyet a,c,⁎, Lene Kristensen b, Marc-Henri Derron a, Clément Michoud a,d, Lars Harald Blikra b, Michel Jaboyedoff a, Tom Rune Lauknes c a Institute of Earth Sciences, Risk Group, Géopolis, Quartier Mouline, University of Lausanne, 1015 Lausanne, Switzerland b Norwegian Water Resources and Energy Directorate (NVE), Ødegårdsvegen 176, 6200 Stranda, Norway c Norut, P.O. Box 6434, Forskningsparken, 9294 Tromsø, Norway d Terranum Ltd., 35 bis rue de l'industrie, 1020 Bussigny, Switzerland article info abstract

Article history: Ground-Based Interferometric Synthetic Aperture Radar (GB-InSAR) campaigns were performed in summer Received 16 May 2015 2011 and 2012 in the Romsdalen valley (Møre & Romsdal county, western Norway) in order to assess displace- Received in revised form 30 June 2016 ments on Mannen/Børa rock slope. Located 1 km northwest, a second GB-InSAR system continuously monitors Accepted 4 July 2016 the large Mannen rockslide. The availability of two GB-InSAR positions creates a wide coverage of the Available online xxxx rock slope, including a slight dataset overlap valuable for validation. A phenomenon of rock slope breathing is detected in a remote and hard-to-access area in mid-slope. Millimetric upward displacements are recorded in Keywords: Rock slope August 2011. Analysis of 2012 GB-InSAR campaign, combined with the large dataset from the continuous station, Stability shows that the slope is affected by inflation/deflation phenomenon between 5 and 10 mm along the line-of-sight. Rockslide The pattern is not homogenous in time and inversions of movement have a seasonal recurrence. These seasonal Ground-based InSAR changes are confirmed by satellite InSAR observations and can possibly be caused by hydrogeological variations. SAR interferometry In addition, combination of GB-InSAR results, in situ measurements and satellite InSAR analyses contributes to a Groundwater effect better overview of movement distribution over the whole area. © 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction parameters. For the last decades, the development of remote sensing has had a major impact in this field (e.g. Mantovani et al., 1996; The detection, characterization and assessment of unstable slopes Metternicht et al., 2005; Michoud et al., 2010; Derron et al., 2011; require a multidisciplinary approach, including mapping, field and Jaboyedoff et al., 2012). In Norway, active optical and microwave laboratory measurements, modelling, etc. (e.g. Solheim et al., 2005; remote sensing techniques are used for the detection, mapping and Jaboyedoff et al., 2005). The characterization of movement rate, monitoring of unstable slopes (e.g. Lauknes et al., 2010; Dehls et al., distribution and evolution is a fundamental part in order to understand 2014; Oppikofer, 2016).They have proved being particularly valuable the behaviour of unstable slopes and forecast potential collapse events for operational reasons, due to its large coverage capability, high (e.g. Crosta and Agliardi, 2003; Petley et al., 2005; Blikra, 2008; resolution and accuracy, and the possibility to document areas difficult Federico et al., 2012; Blikra and Kristensen, 2013; Froese and Moreno, to access. 2014). In Norway, a classification system has been developed by the The contribution of active microwave remote sensing for detection, Norwegian Geological Survey (NGU) and is currently used to systemat- mapping and monitoring of ground displacements using Synthetic ically evaluate the hazard and risk of unstable slopes. Among the nine Aperture Radar Interferometry (InSAR) has got an international main criteria used to score the hazard, two are related to the slide scientific recognition at the beginning of the nineties (Gabriel et al., velocity and changes in displacement rates (Hermanns et al., 2012, 1989; Massonnet et al., 1993; Zebker et al., 1994). Firstly developed 2013a).Various tools and instrumentation are used to measure these for space borne platforms, InSAR devices were then developed for ground-based acquisitions (GB-InSAR). At the end of the nineties, a prototype of outdoor portable SAR system LISA (LInear Synthetic Aperture radar) was available (Tarchi et al., 1999; Luzi, 2010). Since ⁎ Corresponding author at: Institute of Earth Sciences, Risk Group, Géopolis, Quartier then, the use for mapping and monitoring of slope instabilities has Mouline, University of Lausanne, 1015 Lausanne, Switzerland. quickly increased and numerous case studies have been reported (e.g. E-mail addresses: [email protected] (L. Rouyet), [email protected] (L. Kristensen), Tarchi et al., 2003; Noferini et al., 2007; Herrera et al., 2009; Gischig [email protected] (M.-H. Derron), [email protected] (C. Michoud), [email protected] (L.H. Blikra), [email protected] (M. Jaboyedoff), et al., 2009; Barla et al., 2010; Casagli et al., 2010; Del Ventisette et al., [email protected] (T.R. Lauknes). 2011; Bozzano et al., 2011; Herrera et al., 2011; Intrieri et al., 2012;

http://dx.doi.org/10.1016/j.geomorph.2016.07.005 0169-555X/© 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Monserrat, 2012; Schulz et al., 2012; Agliardi et al., 2013; Mazzanti et al., 2.2. Mannen rockslide 2015). This paper provides an analysis of GB-InSAR measurements The main unstable area of Mannen rockslide is about 500 m wide on Mannen/Børa rock slope, in western Norway, where a large and 600 m long. It is located between 1290 and 600 m.a.s.l.; between rockslide is continuously monitored using a wide multi-device network 1230 and 540 m above the valley bottom. It is delimited at the top by (Kristensen and Blikra, 2013). Among these devices, two GB-InSAR a steep backscarp of about 25 m (Fig. 2,top). systems are operating, one permanently and one during intermittent The instability is known from the end of the nineties. Since this time, campaigns. The presence of these two sensors makes possible a large various studies have been carried out in order to know its geometry and coverage in unmonitored and hard-to-access areas. Moreover, the its activity. These include structural analyses (Henderson and Saintot, areas imaged by the GB-InSAR systems are partly overlapping, which 2007; Dahle et al., 2008, 2010; Saintot et al., 2011a, 2012), Terrestrial allows the consistency of the measured displacements to be checked. Laser Scanning (TLS) (Longchamp et al., 2010; Saintot et al., 2011a), In addition, GB-InSAR results are complimented by in situ measure- boreholes (Saintot et al., 2011a; Elvebakk, 2012), 2D resistivity survey ments and satellite InSAR. The results contribute to a better overview (Dalsegg and Rønning, 2012), run-out modelling and risk assessment of the movement distribution over the whole area. It especially high- (Dahle et al., 2011). lights a peculiar seasonal inversion of movement, hereby called rock From field campaigns, it appears that the foliation has an average dip slope breathing. This phenomenon is discussed thereafter in terms of direction to S-SE and a penetrative steep dip angle. However, the area is variations of groundwater pressure. located in a high-grade metamorphic unit intensively folded and is thus affected by significant variations of the foliation (Saintot et al., 2011a, 2. Mannen/Børa rock slope 2012). The area is highly fractured and includes several subvertical sets, as well as penetrative discontinuities that wedge-shape the 2.1. Context upper part of the instability (Henderson and Saintot, 2007; Dahle et al., 2010; Saintot et al., 2011a, 2012). In addition to field measure- The Mannen/Børa rock slope is located on the southwestern side of ments, structural analysis based on Terrestrial Laser Scanning (TLS, Romsdalen valley in Rauma Municipality in Møre & Romsdal county point spacing: ≤7 cm) and Aerial Laser Scanning (ALS, DEM resolution: (western Norway) (Fig. 1,A–B). The glacially-shaped valley (Fig. 1, 1 m) datasets has been performed (Longchamp et al., 2010) using the C) is characterized by extremely steep slopes and includes the highest Coltop3D software (Terranum Ltd.) designed to identify sets of discon- vertical cliff in Europe (Trollveggen) with a drop of 1700 m from the tinuities from point clouds (Jaboyedoff et al., 2007). The results are top to the bottom of the valley. Mannen is an active rockslide and shown as stereonets in Fig. 3. Comparing them to the field data, J3 can Børa is a large plateau located directly on the southeastern side of be identified as the foliation plane, J1/J1′–2′ as main sets involved in Mannen and showing extensive signs of instability (Fig. 2).The rock the wedging and sliding processes and J5/J6 as two major subvertical slope is north-east facing and has a mean slope of 40–50° in its upper sets back-shaping the instability (Rouyet, 2013). part. Based on these investigations, several possibly unstable volumes and Geologically the area is included in the Western Gneiss Region, a corresponding collapse scenarios were outlined (Dahle et al., 2010). The large basement window of Precambrian crystalline rocks reworked instability A has a failure surface estimated at 40–80 m deep and a during the Caledonian orogeny (Mosar, 2000; Smelror et al., 2007). volume of 2–4Mm3, while the second instability (B) has an estimated Mannen/Børa area is composed by metamorphic units, including 70–110 m deep failure surface and a volume of 15–25 Mm3. A third dioritic-granitic gneiss, amphibolites and pegmatites (Saintot et al., instability (C) includes a larger volume further southeast, estimated to 2011a, 2012). The Romsdalen valley has been affected by various events 80–100 Mm3 (Saintot et al., 2011a)(Fig. 2,bottom). of collapse and several other unstable areas were detected. This is due to Mannen is considered as a high risk rockslide (Blikra et al., 2010), a geological and geomorphological context unfavourable to slope combining high probability of occurrence and high potential casualties stability, due to the role of the inherited structures of the ductile and and damages (Dahle et al., 2011). It threatens houses, roads and a brittle tectonic history, the steepness of the slopes along the U-shaped railway track, either directly in the potential run out zone, or indirectly, valley, as well as the post-glacial debuttressing of the valley flanks in case of river damming and outburst (Hermanns et al., 2013b). Thus, (Saintotetal.,2011b). since 2009, a continuous real-time monitoring network including

Fig. 1. A) Location of Mannen/Børa site in western Norway (map google - Landsat 2015). B) Location of Mannen/Børa in Romsdalen valley (map google - Landsat 2015). The white rectangle corresponds to the extent of Fig. 2 (bottom). C) Picture from the top of Mannen rockslide (black dot in B), toward the North, giving an overview of the U-shaped valley (Rouyet, 11.08.2011).

Fig. 2. Edges of Mannen and Børa instabilities according to Braathen et al. (2004), Dahle et al. (2010), Saintot et al. (2011a) and Dahle et al. (2011). Top: Pictures of Mannen scarp corresponding to the upper edge of instability A (Rouyet, 11.08.2011) and Børa plateau with edges of small (full lines) and large (dashed line) instabilities and networks of cracks (dotted lines) (Rouyet, 20.08.2012). Bottom: Map of Mannen and Børa instabilities. Location in Fig. 1. Background: hillshade from 1 m DEM.

Fig. 3. Stereonets with the main discontinuity sets from Coltop3D analysis using TLS and ALS datasets (analysis: Rouyet, 2013; from 2008 TLS acquisitions: Longchamp et al., 2010). differential GPS (DGPS), laser-reflectors, extensometers, tiltmeters, a results from the Coltop3D analysis based on 2008 TLS dataset meteorological station and a GB-InSAR system (locations in Fig. 4)has (Longchamp et al., 2010) highlight overall the same elements (Fig. 3, progressively been implemented (Blikra et al., 2010; Kristensen and b), J6′ corresponding most likely to the foliation plane and J1 as flat- Blikra, 2013). The Norwegian Water Resources and Energy Directorate lying set. Results from 1 m ALS DEM are also presented in Fig. 3 (NVE) is in charge of the monitoring. In autumn 2014, significant accel- (c) and highlights overall the same sets of discontinuities (with some eration in an area with a volume estimated to 120,000–180,000 m3 was variations of the dip angles as a probable effect of the lower spatial recorded in the upper western part of instability B, called Veslemannen. resolution) (Rouyet, 2013). Inhabitants were evacuated and a high alarm level was maintained Due to fewer signs of activity and lower estimated probability of fail- several weeks before winter stabilization (Skrede et al., 2015). ure than for Mannen, fewer investigations were carried out on the plateau and no continuous monitoring is implemented. 2.3. Børa plateau 3. Data & methods Located at the southeastern edge of Mannen rockslide, Børa is an approximately 3 km long and 1 km wide plateau located at 3.1. GB-InSAR 950–1050 m.a.s.l., 890–990 m above the valley bottom. The largest outlined instability corresponds to a volume estimated at 50–200 Mm3 The current versions of GB-InSAR systems use generally a C, X or Ku (Braathen et al., 2004), and even at 300 Mm3 (Dahle et al., 2011). frequency band. The measuring head including transmitting and receiv- Three smaller unstable parts were highlighted (Dahle et al., 2011) ing antennas moves along a 2–3 m long rail in order to synthetize the (Fig. 2) and periodic dGNSS measurements (2003−2012) confirm SAR images (Luzi, 2010). Details about the available systems and their significant horizontal and vertical displacements at these locations, characteristics are available in recent reviews (Monserrat et al., 2014; with 3D rates between 5 and 14 mm/year (Oppikofer et al., 2013). Caduff et al., 2014). The present research analyses data acquired by Structural analyses (Braathen et al., 2004; Saintot et al., 2011a, LiSALab GB-InSAR systems (Ellegi Ltd.). It uses a Ku frequency band 2012), TLS (Longchamp et al., 2010), run-out modelling and risk (central frequency: 17.2 GHz; wavelength: 17.44 mm). The range reso- assessment (Dahle et al., 2011), periodical dGNSS measurements lution (depending on the bandwidth) is between 0.5 and 3 m and the (Oppikofer et al., 2013) and GB-InSAR surveys were performed azimuth resolution (depending on the rail size and the range) can (locations in Fig. 4). reach 1.5 m at 500 m. This system allows displacement rates from From the field campaigns, it appears that the plateau is affected by mm/year to a few m/day to be measured in near-real time, and up to large subvertical fractures with dip direction to N-NE/S-SW. A 4 km away from the sensor. It has to be noted that only 1D displace- subvertical foliation with a dip direction to ENE/WSW highly contrib- ments along the line-of-sight (LOS) can be detected. This leads to utes to shape the edge of the plateau. A flat-lying joint is also identified potential underestimation of the displacement if the LOS is oblique in and explained the development of the instability by a sagging respect to the real displacement vector. mechanism along flat-lying discontinuities and opening along the In early 2010, a permanent GB-InSAR system was installed in the vertical sets (Braathen et al., 2004; Saintot et al., 2011a, 2012). The valley to continuously monitor Mannen rockslide (location: Fig. 4, red

Fig. 4. Location of ground-based monitoring devices (in situ and GB-InSAR), coherent areas for GB-InSAR systems (here using 0.7 ﬁlter applied on 10–16.08.2011 interferogram). Background: 0.2 m orthophoto 2006.

star A). The distance to the backscarp is about 2100 m along the LOS. The Table 1 LOS has a SW orientation and a mean view angle (between the beam Summary of main GB-InSAR acquisitions parameters for the two available systems. and horizontal) of 35°. GB-InSAR parameters Station A Station B

In addition, a second station for intermittent GB-InSAR measure- Acquisition dates 02.2010–now 10.08–24.08.2011 ments of Børa area was built in summer 2011 (location: Fig. 4,yellow (Continuous) 21.09–18.10.2011a star B). Two campaigns were carried out at the station B, during 19.06–09.07.2012 15 days in August (10.08–24.08.2011) and 28 days in September– Maximum range 2700 m 2400 m October (21.09–18.10.2011). In 2012, a new campaign was performed Range resolution 1.9 m – Azimuth resolution 7.8 m at max. range 6.9 m at max. range during 21 days (19.06 09.07.2012). The objective was to get information 1.5 m at 500 m 1.5 m at 500 m about displacements in Børa area and create a data overlap with the per- Azimuth width 2200 m 2400 m manent GB-InSAR system in order to check the reliability of the results. Central frequency (wavelength) 17.2 GHz (17.44 mm) The distance along the LOS to the top of the slope is about 2000 m. The Bandwidth 80 MHz Number of freq. points 2001 LOS has a SW orientation and a mean view angle of about 31°. Synthetic aperture 3 m The main measurement parameters of the two GB-InSAR systems Number of steps inside the aperture 751 can be found in the Table 1. Pictures of the two stations and their Acquisition time 8 min measuring views are presented in Fig. 5 and the main coherent areas Revisiting time 10 min for the two sensors are displayed in Fig. 4. For sake of simpliﬁcation, a Campaign partly affected by snow.

Fig. 5. Left: Pictures of the two GB-InSAR stations. A: Permanent GB-InSAR station, B: Station for intermittent GB-InSAR measurements; Right: Respective views from the stations (Rouyet/ Derron, 20.08.2012). Acquisition parameters in Table 1 and locations in Fig. 4.

Fig. 6. Satellite InSAR mean coherence map from Multi-year Stacking method using TSX/TDX satellite InSAR data from ascending geometry (geocoding resolution: 10 m × 10 m). Areas in orange and red are respectively affected by layover and shadow effects.

A radar image is acquired every 10 min, but for comparisons 24 h- averaged images were used. This kind of data requires a procedure to remove the atmospheric component (e.g. Caduff et al., 2014; Kristensen et al., 2013). This has been performed by combining the 24 h-averaging and a supervised approach by manually selecting reference regions. The averaging contributes to reduce significantly the noise and phase component related to turbulent atmosphere. The supervised approach allows residual atmospheric disturbances to be corrected by selecting areas without any evidence of significant ground deformation. Different areas have been tested by different operators and do not show significant variations on the main areas of interest (b1 mm). In Mannen/Børa final results, the main moving areas show clear spatially and temporally progressive trends. In the overlapping area, they are detected by both GB-InSAR systems using two independent processing. Fig. 7. Baseline plot (spatial baseline vs. acquisition time) with the 24 selected TSX/TDX In Figs. 8–11 (cf. Section 4.1), negative values correspond to ascending scenes (high density in summer, gaps in winter). Scenes: black circles. Black movements toward the sensor (distance shortening along the LOS); lines: selected interferograms. positive values correspond to movements away from the sensor (distance increasing along the LOS). For sake of simplification and the two stations will be thereafter named station A (permanent GB- reader convenience, they are expressed thereafter as downward/up- InSAR) and station B (intermittent GB-InSAR measurements). In 2014, ward displacements. two new radar positions were built to carry out periodic measurements, but will not be included in this paper. Images acquired from station A were processed in order to get 3.2. Satellite InSAR information about displacements over one year in 2011–2012 (cf. Section 4.1.1). Because of the coherence loss during the winter season In addition to GB-InSAR data, satellite InSAR images acquired by due to snowfall, the reference periods were chosen in June. Datasets TerraSAR-X/TanDEM-X (TSX/TDX) sensors (Airbus Defense & Space - from the campaigns at the station B were also analysed. The results pre- Infoterra GmbH) were processed. The satellites use a X band (central sented in Section 4.1.2 correspond to the first 2011 campaign and the frequency: 9.6 GHz; wavelength: 3.11 cm) and have a repeat-pass 2012 campaign. The second 2011 campaign did not highlight significant time interval of 11 days. The dataset was acquired with a StripMap displacementsandwaspartlyaffectedbysnowfall.Imagesacquiredfrom mode and from an ascending orbit (LOS orientation: 75.2°N, incident the station A at the same periods were also analysed and the results com- angle: 21.4°) in order to get reduced geometrical distortions on east- pared with those of the station B. Using the large dataset acquired at the facing slopes. The LOS and the land cover provide a good coverage of station A, time series at specific locations between February 2010 and the plateau and the upper part of the slope. The bottom part of the December 2012 were extractedandareanalysedinSection 4.1.3. slope is partly masked out due to vegetation inducing low coherence The GB-InSAR data were processed using LiSALab software (Ellegi srl.). and layover effect from the other side of the valley (Fig. 6). Due to loss Theresultsare3×3multilookedand analysed using a 0.7 coherence filter. of coherence in case of snowfall, only 24 snow-free scenes between

Table 2 Summary of characteristics of satellite data and processing parameters.

Satellite data characteristics

Repeat- Number Track angle Incidence Satellite Band Dataset Time period pass of (LOS orient.) angle interval images

StripMap 07.2010 – -14.81° TSX/TDX X (λ:3.11cm) 11 days 24 21.4° Ascending 10.2013 (75,2°N)

InSAR processing parameters

Multilooking Max. SBAS Pixel selection Calibration (range x spatial & Selected spatial & (coh. & Processing type Generated point azimuth temporal interfer. temporal fraction of interfer. (UTM 32N) resolution) baselines filters image thr.)

Multi-years 6925508 69 57 - 0.4 & 0.6 Stacking Factors: 3 x 4 436468 600 m / (8.3 m x 8.3 88 d m) 500 m / (GPS 1 2013 SBAS 35 27 0.4 & 0.6 44 d stable)

Fig. 8. 2011 monitoring data on Mannen rockslide: GB-InSAR displacements (station A) in mm between June 2011 and June 2012 (geocoding resolution: 3 m × 3 m) and in situ displacements in mm between January 2011 and December 2011 (in pink are displayed in situ results with most signiﬁcant velocities). Location in Fig. 4. Background: hillshade from 1 m DEM.

July 2010 and October 2013 were used (Fig. 7). The satellite data charac- In Section 4.2 (Figs. 12–13), in order to have a straight-forward teristics are summarized in Table 2 (top). comparison with GB-InSAR results, signs convention is inversed from The results presented thereafter as maps were obtained using a the LOS-logic: negative values correspond to movements away from simple stacking of interferograms with small spatial (max. 600 m) the satellite (downward) and positive values correspond to movements and temporal (max. 88 d) baselines. This allows a large amount of toward the satellite (upward). interferograms from disconnected seasons to be processed together in order to reduce atmospheric effects and other noise sources (Zebker 4. Results and Villasenor, 1992; Zebker et al., 1997; Gabriel et al., 1989).A second processing using only the interferograms from 2013 has been 4.1. GB-InSAR results performed in order to get one connected set and be able to apply Small BAseline Subset (SBAS) method (Berardino et al., 2002; Lauknes 4.1.1. One year displacements on Mannen rockslide et al., 2011). This allows the time series presented in Section 4.2 to be The velocities recorded along the LOS between June 2011 and June retrieved. 2012 reached 18 mm/year toward the GB-InSAR in the upper part of The processing was performed using the Norut developed Mannen rockslide (Fig. 8). This is overall consistent with in situ data GSAR software (Larsen et al., 2005), using the national 10 m DEM that show 23 mm/year with a main vertical component for DGPS3and (provided by Kartverket) to remove the topographic component. The 18 mm/year for the laser-reﬂectors (Fig. 8, pink elements). A clear InSAR processing parameters can be found in Table 2 (bottom). The velocity contrast is noticeable between the upper southern part of the numbers of generated interferograms depend on the chosen spatial/ instability A and the rest of instabilities A and B. During this time inter- temporal baselines thresholds (column 4, InSAR processing parameters, val, 7 mm toward the GB-InSAR is measured on the lower southeastern Table 2) and a second manual selection is performed to remove interfer- part. In the lower northern part, mainly horizontal displacements ograms affected by low coherence, signiﬁcant atmospheric effects or ob- toward the north are recorded by DGPS. Due to an oblique LOS relatively vious unwrapping errors (column 5, InSAR processing parameters, to the displacement vectors, the GB-InSAR is not able to capture a signif- Table 2). icant part of the movement in this area. No information is available in

Fig. 9. Georeferenced and unwrapped GB-InSAR results (stations A and B). Geocoding resolution: 2 m × 2 m. The GB-InSAR results are expressed along the respective LOS of the two systems. In overlapping part, when data are available for both devices, results from the station A are displayed. Background: hillshade from 1 m DEM. Top: Displacements in period 10.08–23.08.2011. Bottom: Displacements in period 07.07–09.07.2012.

Fig. 10. Comparison of time series from GB-InSAR A & B at P1–P2 locations. Top: First campaign at the station B (10.08–23.08.2011). Bottom: 2012 campaign at the station B (19.06–9.07.2012) (points locations in Fig. 9). the western part of instability B located in a shadow area. In order to in active rockfall areas in the southeastern part of the dataset. Displace- overcome these problems, a new station for intermittent GB-InSAR ments are also detected in the lower part on debris cones affected by campaigns was set up further northwest, and measurements started active torrential processes and significant erosion/deposition cycles. in September 2014 (Skrede et al., 2015). The main interesting element is located in the overlapping part of the two GB-InSAR datasets (cf. Fig. 4). During August campaign 4.1.2. Intermittent Børa campaigns & corresponding Mannen results in 2011 an increase of sensor-to-target distance is detected, correspond- During the first Børa campaign in August 2011, considering the ing to upward displacements. The displacements exceed 8 mm in interval 10.08–23.08.2011 (14 days), no significant movement was 13 days (Fig. 9, top). In July 2012, this moving part is again clearly distin- detected on Mannen rockslide using GB-InSAR (station A). This is over- guishable, but this time the movements are inversed (shortening of all consistent with the in situ data, which show that main periods of sensor-to-target distance, i.e. downward displacements). The highest movements occurs during the thawing period earlier in the season. displacement rate is reached during 07.09–09.07.2012 interval (up to On Børa, the results of the station B highlight a slight downward trend 8 mm in 2 days) (Fig. 9, bottom). For both campaigns at the station B in the northwestern upper part that can be ascribed to residual (August 2011 and June–July 2012), GB-InSAR results are consistent atmospheric effects, but several fast moving small sectors are detected with station A in the overlapping part, and the georeferenced results

Please cite this article as: Rouyet, L., et al., Evidence of rock slope breathing using ground-based InSAR, Geomorphology (2016), http://dx.doi.org/ 10.1016/j.geomorph.2016.07.005 L. Rouyet et al. / Geomorphology xxx (2016) xxx–xxx 11 show a good spatial fitofthetwodatasets(Fig. 9). The moving part is movement and variations). In June–July 2012, the time series confirm well delineated and affected an area of approximately 1 × 0.5 km2. the movement inversion and the consistency of both datasets (Fig. 10, Time series are generated at two locations (P1 and P2) where bottom). The time series show two main stages of acceleration during coherent information is available for both GB-InSAR systems, in the 21.06–26.06.2012 (7 mm in 5 days) and 06.07–09.07.2012 (7 mm in overlapping part of the two datasets (Fig. 10, point locations in Fig. 9). 3days). The data are in the respective LOS for each GB-InSAR system. They confirm the upward trend in August 2011 (Fig. 10, top). For both points, 4.1.3. Inter-annual results maximal displacements are reached after 9 days. The measurements Using the whole GB-InSAR dataset acquired at the station A since show a relative stability the next 5 days. The two GB-InSAR systems February 2010 to the end of 2012, significant variations are confirmed provide measurements in good agreement (overall same rate of in the southeastern part (overlapping part with GB-InSAR B). Similar

Fig. 11. Left: Time series from GB-InSAR A at P2–P5 locations between May and October 2010, 2011 and 2012 (point locations in Fig. 9). Right: Meteorological data from the top of Mannen (station location in Fig. 4).

Fig. 12. Multi-years Stacking results using 2010–2013 TSX/TDX satellite data from ascending geometry (geocoding resolution: 10 m × 10 m). Black arrow: LOS, black star: calibration point. Background: hillshade from 1 m DEM. Top: Overview with location of zoom (white rectangle). Bottom: Zoom on Mannen rockslide and area affected by rock slope breathing. Blue stars: location of SBAS time series presented in Fig. 13.

Please cite this article as: Rouyet, L., et al., Evidence of rock slope breathing using ground-based InSAR, Geomorphology (2016), http://dx.doi.org/ 10.1016/j.geomorph.2016.07.005 L. Rouyet et al. / Geomorphology xxx (2016) xxx–xxx 13 trends are visible in the northwestern part of the monitored area but displacements during these periods can thus lead to an overestimation with lower amplitude (P4–P5) (Fig. 11, point locations in Fig. 9). The of the values. movement cycles have a seasonal recurrence. In 2010, 2011 and 2012, In the southwestern part of the instability B (Veslemannen), move- movement inversions occur abruptly between June and July (from ments are detected. Previously this area was not documented by downward to upward displacements) and the fastest upward peaks ground-based instruments, but after significant movements and occur every year between mid-June and August (vertical dashed acceleration were measured by GB-InSAR in September–October 2014, lines, Fig. 11). For P2, the upward displacement reaches 13 mm extensometers and geophones were installed at this location and an in 72 days in summer 2011.From the meteorological data(Fig. 11, automatic LiDAR system was put in the valley. The Veslemannen site graphs on the right. Station at the top of Mannen, cf. Fig. 4), it is is now closely monitored (Skrede et al., 2015). In the southeastern possible to see that the downward trend is clearly related to the part (Børa), some displacements are also detected, especially in the thawing period and its duration is affected by the abruptness of the lower part (debris cones affected by torrential processes and erosion/ transition (short duration and sharp transition in 2011 vs longer dura- deposition cycles). tion and smoother transition in 2012). This inflation/deflation phenom- The area subjected to rock slope breathing in the overlap of the GB- enon, hereafter so called rock slope breathing, is discussed more into InSAR datasets is not totally well documented by TSX/TDX satellites due details in Section 5.2. to the lower coherence downslope. The LOS of the sensor can also lead to potential misestimating of displacements in the area due to oblique 4.2. Satellite InSAR results or even perpendicular view in respect to the real vectors. However, mean rates up to 28 mm/year downward are detected from the Multi- The TSX/TDX multi-years Stacking processing using maximal year Stacking processing. Thanks to SBAS method using interferograms temporal baselines of 88 days (cf. Table 2) shows rate of movement from 2013, it is also possible to retrieve time series between July and up to 40 mm/year and 58 mm/year downward (distance increasing October 2013. Results at different locations on the area affected by along the LOS) recorded for some small sectors in the upper part of rock slope breathing are presented in Fig. 13 (top). For sake of compar- the instabilities A and B, while the main moving area has values ison and reliability check, other time series from the plateau (supposed between 15 and 25 mm/year (Fig. 12). It has to be noted that the results as stable) and on Mannen rockslide can be seen in Fig. 13 (bottom). The expressed in annual mean velocity correspond to an extrapolation locations of the selected points are displayed on the zoom in Fig. 12 of the snow-free seasons (no inter-annual interferogram). Large (bottom). The results highlight three different clear trends: 1) round

Fig. 13. SBAS time series in snow-free season 2013 (point locations: Fig. 12). Top: B1–4 on area affected by rock slope breathing. Bottom: Others examples of time series on the plateau and on Mannen rockslide. Note the difference of scale in y-axis.

Please cite this article as: Rouyet, L., et al., Evidence of rock slope breathing using ground-based InSAR, Geomorphology (2016), http://dx.doi.org/ 10.1016/j.geomorph.2016.07.005 14 L. Rouyet et al. / Geomorphology xxx (2016) xxx–xxx zero displacements on the plateau, 2) quite linear displacements on the 2) A second active part in the upper western part of the instability B rockslide to 18 mm in 3 months, 3) small displacements with inversions (Veslemannen), not documented by in situ data but highlighted by in August on the area affected by rock slope breathing, that confirm the satellite data (to 58 mm/year) (Fig. 12), and recently by the new GB-InSAR results. 2014 GB-InSAR station (Skrede et al., 2015). 3) A slower part including the other in situ devices. The DGPS highlight- 5. Discussion ed rates of displacement between 7 and 13 mm/year in 2011, the GB-InSAR data velocities to 6 mm/year (Fig. 8) and 10 mm/year for This section is organized in two parts. In Section 5.1, the results from satellite InSAR (Fig. 12). The especially low values recorded by GB- available measuring systems (GB-InSAR, in situ and satellite InSAR) are InSAR can be explained by an oblique LOS orientation in respect to summarized and main findings about the spatial distribution of the movement orientation documented by the DGPS (main horizon- movement rate are described. As main focus of the article, the rock tal component with a north orientation). slope breathing phenomenon and the current assumptions about the factors explaining this effect are discussed in Section 5.2. At Børa, GB-InSAR system did not highlight significant moving patterns during the intermittent campaigns, except in the area 5.1. Mannen rockslide & Børa plateau discussed in Section 5.2. No clear deformation patterns are measured by GB-InSAR at the locations of three identified small instabilities in fi The combined results of GB-InSAR, in situ and satellite InSAR Børa (Fig. 2). In these sectors, signi cant displacement rates were contribute to have a good overview of the Mannen rockslide be- measured by periodic dGNSS measurements between 2003 and 2012 haviour and affine the extents of the different instabilities especial- (Oppikofer et al., 2013). However, the mean annual rates revealed by fi ly in hard-to-access steep slopes. Overall the results are consistent dGNSS are between 5 and 14 mm/year, which explain the dif culty to and complementary. They allow the area to be divided in three be highlighted by GB-InSAR during short-term intermittent campaigns. parts: GB-InSAR detected nevertheless some small sectors along the upper steep part of the slope and on debris cones downslope (Fig. 9). This 1) A main active part in the upper southern part of the instabilities A. It is seems consistent with the satellite InSAR results which highlight also documented by DGPS 3 (23 mm/year in 2011), two lasers-reflectors some localized moving areas (Fig. 12). (to 18 mm/year in 2011), the extensometer 2 located in the backcrack (44 mm/year in 2011), GB-InSAR data (to 18 mm/year be- 5.2. Rock slope breathing tween June 2010–2011) (Fig. 8) and TSX/TDX satellite InSAR (to 40 mm/year) (Fig. 12). According to DGPS results, this moving The GB-InSAR campaigns at station B highlighted the presence of an area has a main vertical component and a northeast horizontal area subjected to variations of movement in the overlapping part with orientation. the data from the continuous GB-InSAR system (station A). The

Fig. 14. Focus on the area subjected to rock slope breathing. A: GB-InSAR results (here wrapped) of August 2011campaignat station B and the corresponding results of station A. B: Picture of the slope (Rouyet, 20.08.2012). C: Zoom on sector subjected to rock slope breathing (Rouyet, 20.08.2012). D: Siphon phenomenon in Kråkenesvatnalakes (NVE, 09.09.2011).

Fig. 15. Simpliﬁed 3D models of Mannen/Børa summarizing the geometries of the remote sensing systems, the main geological structures and discussing the rock slope breathing effect. Background: 0.2 m orthophoto 2006.

Please cite this article as: Rouyet, L., et al., Evidence of rock slope breathing using ground-based InSAR, Geomorphology (2016), http://dx.doi.org/ 10.1016/j.geomorph.2016.07.005 16 L. Rouyet et al. / Geomorphology xxx (2016) xxx–xxx combination of the GB-InSAR datasets from two stations showed that stresses related to hydrogeological cycles can be referred as hydrome- the movements fluctuate in time and has a seasonal recurrence chanical fatigue. Its contribution to rock slope deformation has been (observed during three consecutive years of monitoring). SBAS time further analysed and discussed in Cappa (2006), Preisig et al. (2016) series from TSX/TDX satellite data confirm these variations. and Gischig et al. (2016). Based on field experiments and simulations Looking at the location map (Fig. 14, top right), we can see that this of relations between fluid pressures and displacements, Cappa (2006) part is located below the two Kråkenesvatna lakes. These lakes have no performed a sensitivity study showing that the hydraulic aperture, visible outlets, but at their northern edge, a siphon phenomenon can be the stiffness and the network geometry of the fractures are key param- observed during the summer (Fig. 14, D). This testifies a strong water eters of hydromechanical processes in a fractured rock mass. In addi- infiltration in this area. Tracing tests from Kråkenesvatna lakes were tion, the temporal shift in the pressure-deformation response and the performed in October 2007 and concluded to a complex drainage impact of local heterogeneities are highlighted and contribute to the system involving multiple pathways of water in the rock mass. The complexity of the processes. Preisig et al. (2016) and Gischig et al. measurements at 19 source locations in the valley revealed indeed (2016) showed in addition that hydromechanical fatigue is effective that the peaks of responses curves are multiple and vary between 3 when high degree of damage pre-exists in the rock mass. Due to its and 10 days according to the location (Kvakland, 2009). Considering gradual and multi-factors nature, the phenomenon has to be consid- the time of transit through the rock mass the assumption of rock slope ered at several spatial and temporal scales and require advanced breathing caused by variations of groundwater charge is likely. The modelling techniques. For further work, applying principles of Biot's order of magnitude of 10 days fits well with the variations detected theory of poroelasticity (Biot, 1955) seems to be the most relevant ap- from GB-InSAR (Fig. 11). In Fig. 9, we can moreover see that the moving proach to model this complex fluid-to-solid coupling behaviour area is located just below and between the main delineated instabilities. (Wang, 2000). It probably contributes to a convergence/concentration of groundwater In this context, our research does not claim to explain the hydrome- from the large fractures at the basal and lateral edges of the unstable chanical processes involved in Mannen/Børa rock slope but provides areas. Thus, the current assumption to explain the displacements new measurements that evidence a breathing phenomenon and measured by GB-InSAR is that the area is subjected to large variations shows that remote sensing techniques can contribute to detect, map in groundwater pressure. At the beginning of the thawing season, an and monitor small-scale movement fluctuations due to water pressure increase of charge due to high water infiltration and possible presence in natural slopes. of residual ice in fractures lead to millimetric inflation of the rock slope, and thus downward displacements. During the summer, the re- 6. Conclusion & prospect duction of water inputs and complete melting of residual ice in fractures make possible a water evacuation and desaturation of the slope, which The Mannen/Børa rock slope is an interesting case study due to the leads to a deflation, and thus upward displacements. The assumption is availability of two GB-InSAR systems imaging the same rock slope. represented as simplified schemes in Fig. 15. The displayed structures They cover a large stretch of the slope and a slight data overlap allows are based on the approximate results of TLS/ALS analysis using Coltop3D the reliability of the results to be checked. They have the advantage to software (Fig. 3). provide information on the weakly monitored Børa plateau and in The remotely measured phenomenon highlights the need of further steep mid-slope where in situ devices are difficult to install. Thanks research in order to document and understand the rock slope breathing. to GB-InSAR large coverage, high temporal frequency, high resolution Seasonal variations can indeed have an impact on the significance of and accuracy, an approximately 1 × 0.5 km2 sector subjected to periodic displacement measurements and induce errors in the millimetric variations and inversion of movement (inflation/deflation) estimation of mean velocities (under-/overestimation of the values if has been highlighted. Satellite InSAR time series from TSX/TDX data the measurements are performed at wrong periods). In order to evalu- confirm these variations. Analysing the large GB-InSAR dataset from ate correctly the deformation rates of unstable areas and integrate the continuous station A, this rock slope breathing is proved to have a relevant information in hazard classification systems (Hermanns et al., seasonal recurrence. The phenomenon is ascribed to hydrogeological 2012, 2013a), seasonal trends have to be known and taken into account. variations. The rock slope breathing can moreover have a long-term effect on In addition and in order to overcome some limitations of GB-InSAR rock slope stability. In research on landslides, role of groundwater technique (1D measurements along the LOS, intermittent campaigns, variations is well known and considered in term of pore pressures atmospheric effects removing, coherence loss, etc.), analyses of in situ variations affecting the factor of safety (e.g. Collins and Znidarcic, monitoring data and satellite InSAR were performed. The combination 2004; Eberhardt et al., 2007). Effects of water table level fluctuations in- of in situ measurements, terrestrial and space borne remote sensing ducing subsidence and uplift of rock mass were studied to explain provides a large amount of complementary information. In this way, deformation related to tunnelling and hydroelectric infrastructure the edge and velocities of the main Mannen unstable areas can be better (e.g. Zangerl et al., 2008; Strozzi et al., 2011). Using satellite InSAR, defined. water loading fluctuations inducing land subsidence and uplift rebound These preliminary results highlight the need for further research in were detected, but they are mainly related to cases of excess pumping of this site, which can be summarized in two parts. Firstly, further investi- groundwater (e.g. Amelung et al., 1999; Schmidt and Bürgmann, 2003; gations are required to fully understand the hydrogeological processes Chen et al., 2007; Reeves et al., 2011). In case of rock instabilities, and the subsurface structures of the rock slope, especially to understand seasonal effects related to temperature (Gischig et al., 2011), precipita- the rock slope breathing phenomenon and confirm the assumption of tions, snow-melting (Coe et al., 2003) and/or freeze-thaw (Nordvik the water pressure influence. This includes further investigations et al., 2010; Blikra and Christiansen, 2014) have also been measured combining surface and subground data to depict its complex geometry. using conventional or/and remote-sensing methods. These studies In order to better understand and constrain the behaviour of the rock focus usually on the understanding of the triggers and controlling masses in the area stress-strain numerical analysis should be performed factors of the instabilities. and included in further models. Secondly, a complete and systematic The elastic or semi-elastic behaviours of the rock mass associated to analysis of the large GB-InSAR dataset from station A, as well as a ground water variations and potential fatigue mechanism related to more complete integration with the other GB-InSAR (intermittent these cycles are still not well covered in geohazards studies, although measurements from station B and new 2014 stations), in situ and satel- mentioned in several papers (e.g. Jaboyedoff et al., 2009; Mazzanti and lite InSAR results would be valuable to provide information about Brunetti, 2010; Salvini et al., 2015). The progressive weakening and movement directions and contribute to confirm the likelihood of degradation of rock mass from repeated (e.g. seasonal) fluctuations of mechanisms. This could overcome the intrinsic limitation of InSAR

Please cite this article as: Rouyet, L., et al., Evidence of rock slope breathing using ground-based InSAR, Geomorphology (2016), http://dx.doi.org/ 10.1016/j.geomorph.2016.07.005 L. Rouyet et al. / Geomorphology xxx (2016) xxx–xxx 17 technique which provides only 1D deformation measurements along Coe, J.A., Ellis, W.L., Godt, J.W., Savage, W.Z., Savage, J.E., Michael, J.A., Kibler, J.D., Powers, P.S., Lidke, D.J., Debray, S., 2003. Seasonal movement of the Slumgullion landslide the LOS. determined from global positioning system surveys and field instrumentation, In addition to the elements of prospect related to Mannen/Børa rock July 1998–March 2002. Eng. Geol. 68, 67–101. http://dx.doi.org/10.1016/S0013- slope, this work underlines the need of research on the locations, 7952(02)00199-0. Collins, B.D., Znidarcic, D., 2004. Stability analyses of rainfall induced landslides. amplitudes, causes and consequences of rock slope breathing in moun- J. Geotech. Geoenviron. 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Dahle, H., Anda, E., Sætre, S., Saintot, A., Bæhme, M., Hermanns, R., Oppikofer, T., Dalsegg, The research would not have been possible without the great job of E., Rønning, J.S., Derron, M.-H., 2011. Risiko- og sårbarheitsanalyse for fjellskred i the team working at the NVE monitoring center in Stranda. Thanks to Møre of Romsdal. Report. Fylkesmannen i Møre og Romsdal. Møre og Romsdal the intensive work of each member of the crew, impressively huge fylkeskommune & Norges Geologiske Undersøkelse (NGU), Norway (In Norwegian). Dalsegg, E., Rønning, J.S., 2012. Geofysiske målinger på Mannen i Rauma kommune, Møre and exciting datasets from the performant monitoring network of og Romsdal. NGU Report 2012.024. Geological Survey of Norway, Trondheim, Mannen/Børa are available for research. Many thanks also to LiSALab Norway (In Norwegian). team (Ellegi Ltd.) and especially Carlo Rivolta for his support during Dehls, J.F., Lauknes, T.R., Hermanns, R.L., Bunkholt, H., Grydeland, T., Larsen, Y., Eriksen, H.Ø., Eiken, T., 2014. Use of satellite and ground based InSAR in hazard classification GB-InSAR data processing. The TerraSAR-X data has been provided by of unstable rock slopes. In: Sassa, K., Canuti, P., Yin, Y. (Eds.), Landslide Science for a the German Aerospace Center (DLR) under the TSX-AO project contract Safer Geoenvironment, Vol.2. Springer International Publishing, pp. 389–392. #GEO0764. In addition, we acknowledge the works of the landslide http://dx.doi.org/10.1007/978–3–319-05050-8_60. group of the Geological Survey of Norway (NGU) that contribute to Del Ventisette, C., Intrieri, E., Luzi, G., Casagli, N., Fanti, R., Leva, D., 2011. 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Please cite this article as: Rouyet, L., et al., Evidence of rock slope breathing using ground-based InSAR, Geomorphology (2016), http://dx.doi.org/ 10.1016/j.geomorph.2016.07.005