Rock-avalanche geomorphological and hydrological impact on an alpine watershed

    Rock-avalanche geomorphological and hydrological impact on an alpine watershed P. Frattini, F. Riva, G.B. Crosta, R. Scotti, L. Greggio, F. Brardinoni, N. Fusi PII: DOI: Reference:

S0169-555X(16)30102-7 doi: 10.1016/j.geomorph.2016.03.013 GEOMOR 5545

To appear in:

Geomorphology

Received date: Revised date: Accepted date:

21 September 2015 10 March 2016 11 March 2016

Please cite this article as: Frattini, P., Riva, F., Crosta, G.B., Scotti, R., Greggio, L., Brardinoni, F., Fusi, N., Rock-avalanche geomorphological and hydrological impact on an alpine watershed, Geomorphology (2016), doi: 10.1016/j.geomorph.2016.03.013

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ACCEPTED MANUSCRIPT Rock-avalanche geomorphological and hydrological impact on an

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Alpine watershed

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Frattini P.a, Riva F. a, Crosta G.B. a, Scotti R. a, Greggio L. a, Brardinoni F. a,b, Fusi N. a Department of Earth and Environmental Sciences, Università degli Studi di Milano - Bicocca, Piazza della Scienza 4, 20126

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Milano, Italy

b Department of Biology, Geology and Environmental Sciences (BiGeA), University of Bologna, Via Zamboni, 67, 40126 Bologna,

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Italy

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ABSTRACT

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Rock avalanches are large flow-like movements of fragmented rock that can cause extensive and rapid topographic changes, for which very few quantitative data are available. This paper analyses the geomorphological and hydrological impact of the 3 million m3 Thurwieser rock avalanche (2004, Italian Central Alps) by using Terrestrial Laser Scanner, airborne Lidar and GNSS data collected from 2005 to 2014. Sediment yield with respect to the normal valley regime, the dynamic and mass balance of affected glaciers, and the reorganization of superficial and groundwater flow networks are quantified. In the middle portion of the avalanche deposit, a natural sediment trap collected sediments from a new stream channel developed along the upper portion of the deposit and from a lateral drainage basin. This made possible to assess the 10-year impact of the rock avalanche on the sediment yield, which increased from about 120 to about 400 t km-2 a-1.

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ACCEPTED MANUSCRIPT The rock avalanche partially covered a glacier with a shallow debris layer that acted as a thermal insulator, limiting ice ablation and producing a 10-m high scarp between the free

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surface of the glacier and the debris-covered portion. A reduction of 75% of ice ablation was

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observed due to thermal insulation. The rock avalanche filled a tributary valley, splitting the

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original drainage basin in two. Under ordinary flows, seepage occurs within the avalanche deposit along the old valley axis. During high flow conditions, a new stream channel is activated

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along the middle and lower margin of the deposit, which has produced a new alluvial fan on the

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main valley floor. The fan evolution is described up to the present volume of about 2,000 m3.

Short and medium term quantitative geomorphological evolution in an alpine valley

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HIGHLIGHTS

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after a large rock avalanche. -

Adjustment of valley hydrology in response to rock avalanche and sediment deposition.

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Denudation rates, debris erosion and transport.

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Reduced ice ablation due to thermal insulation of rock-avalanche debris cover on glacier.

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GNSS and TLS techniques for detailed geomorphological surveys.

KEYWORDS Rock avalanche; Sediment yield; Hydrological impact; Glacier mass balance.

1. Introduction 2

ACCEPTED MANUSCRIPT Rock avalanches are rapid, massive, flow-like movements of fragmented rock generated from a large rockslide or rock fall. They contribute significantly to erosion and sediment transfer in

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mountain areas at the regional scale (Post, 1967; Whitehouse, 1983; McSaveney, 2002; Korup

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et al., 2004; Hermanns et al., 2006; Crosta et al., 2007; Korup et al., 2007; Hewitt et al., 2008;

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Korup and Clague, 2009). Specifically, they can produce an abrupt increase in sediment supply to the drainage network, inducing a rapid transient response via fluvial reworking of the newly-

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deposited material, which ultimately translates into extensive topographic change in a

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relatively short time. Despite their geomorphic significance, a limited number of studies have addressed the evolution of avalanche deposits at relatively high temporal and spatial resolution

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trying to quantify their effects (King et al., 1989; Crosta, 2001; Korup et al., 2004; Mikos et al.,

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2006).

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A variety of short-term post-landslide responses have been observed in relation to valley geometry, the amount of debris deposited, and the transport capacity of the receiving fluvial system (Korup and Crozier, 2002; Korup et al., 2004; Brardinoni et al., 2009; Davies and Korup, 2010; Huggel et al., 2012):

1 – A landslide can suddenly transform into a debris flow by water entrainment or liquefaction, thus delivering a large amount of sediments downstream in a single event (Voight and Sousa, 1994; Crosta, 2001; Hauser, 2002; Boultbee et al., 2006; Mikos et al., 2006; Xu et al., 2012; Huggel et al., 2012). 2 – A landslide can dam a valley, generating different scenarios according to the volume of sediment and the morphology of the host valley. Accordingly, landslide damming can cause lake impoundment and deposition of sediment upstream of the dam (Hermanns et al., 2006; Cossart

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ACCEPTED MANUSCRIPT and Fort, 2008), lateral channel avulsions, and the formation of epigenetic gorges (Hewitt, 1988; Semenza and Ghirotti, 2000; Korup et al., 2006; Pratt-Situala et al., 2007; Ouimet et al.,

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2008). When landslide dams fail, outburst floods release down valley large amounts of

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sediments (Kojan and Hutchinson, 1978; Costa and Schuster, 1988, Selby, 1988, King et al.,

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1989; Clague and Evans, 1994; Clague and Evans, 2000; Schuster, 2000; Dunning et al 2006). In other cases, fluvial reworking of the landslide deposit causes a peak of sediment mobilization

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within a few years after the event (Ohmori, 1992; Ries, 2000; Sutherland et al., 2002; Korup et

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al., 2004; Hancox et al., 2005; Chen et al., 2006).

3 – A landslide may affect only a slope or involve low-order tributaries, with a minor or indirect

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interaction with the main drainage network (McSaveney et al., 2000). The landslide deposit can

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2006).

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be eventually remobilized and transported downstream by debris flow activity (Mikos et al.,

When rock avalanches hit glaciers they tend to travel faster and for longer run-out distances (Evans and Clague, 1988; Schneider et al., 2011; Sosio et al., 2012; De Blasio, 2014). On the other hand, deposition of rock avalanche debris can alter the glacier's mass balance and the relevant glacier flow dynamics (Reid, 1969; McSaveney, 1978; Deline 2009; Hewitt 2009, Vacco et al, 2010; Shugar and Clague, 2011; Deline et al., 2014). In this paper, we examine the effects of the Thurwieser rock avalanche on a glacierized alpine tributary valley. In particular, we aim to identify and quantify the effects of rock avalanching on local topography, sediment yield, hydrologic pathways, and glacier mass balance.

2. The Thurwieser rock avalanche 4

ACCEPTED MANUSCRIPT 2.1. Geological setting Punta Thurwieser (3652 m a.s.l.) is located in the upper Zebrù River Valley, an NE trending 10

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km long valley, located 12 km East from Bormio (Central Italian Alps, Northern Italy, Fig. 1),

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ranging in elevation between 1350 and 3859 m a.s.l. The valley follows the Zebrù thrust, on

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which the Mesozoic covers of the Ortles Nappe have been displaced above the Paleozoic metamorphic basement of the Campo Nappe during the Alpine orogeny. The Ortles Nappe

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outcrops on the right-hand side of the valley, and consists of folded and faulted dolostones with

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intercalations of limestones (Dolomia Principale Fm., Late Norian), locally crossed by Tertiary intrusions. The Campo Nappe, on the other side of the valley, consists of phyllites and

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paragneiss, with intercalations of marbles, amphibolites and porphyries (Fig. 1a). The Punta

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Thurwieser peak belongs to the Ortles Nappe and is composed of well-stratified black

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limestones (including marly limestone, shale and breccias), sometimes tectonically laminated and intensely deformed, intercalated with grey dolostones, showing large-scale folds and abrupt interruptions. The bedding attitude and the different mechanical behavior between laminated and massive-strata portions of this limestone played a key role in the failure of 2004 rock avalanche (Sosio et al., 2008). The Zebrù River Valley was intensively carved by glaciers, which are still present above 3000 m a.s.l. The U-shaped valley hangs on the Valfurva Valley, and glacial landforms like cirques, moraines and striated rocks are widespread. Because glaciers retreat after the Last Glacial Maximum, fluvial processes partly reshaped the valley with deposition of fluvio-glacial deposits in debris alluvial fans. The rock avalanche impacted the Zebrù Glacier and the East Zebrù Glacier. The Zebrù Glacier (1.02 km2 in 2007) flows for 2.5 km from the northern side of Monte

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ACCEPTED MANUSCRIPT Zebrù (3734 m a.s.l.) and terminates just beside Punta Thurwieser. The East Zebrù Glacier (0.97 km2 in 2007) flows for 0.8 km from Cima della Miniera (3405 m a.s.l.) and ends up in proximity

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of the Zebrù Glacier terminus, to which the ice was connected until 1990 (Bonardi et al., 2012)

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(Fig. 1a).

Fig. 1. The Thurwieser rock avalanche. a) Geological map of Marè Valley. b) Comparison of Punta Thurwieser before (2001) and after (2006) the event (photos: Giuseppe Cola and Michele

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ACCEPTED MANUSCRIPT Bariselli). c) Photo of the rock avalanche the day after the event (photo: Massimo Ceriani). d)

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Location of the study area.

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2.2. The 2004 rock avalanche

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On 18 September 2004, a rock avalanche detached from the southeastern slope of Punta Thurwieser at an altitude between 3250 and 3600 m a.s.l. (Cola, 2005; Sosio et al., 2008). The

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initial kinematics was typical of a large rockslide controlled by the weak limestone layers

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dipping 30–40° to the S–SW, and by sub-vertical, highly persistent joint sets. The rockslide evolved into a rock avalanche mainly due to the slope steepness and the large volume involved.

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The rock avalanche travelled over the Zebrù Glacier, and successively funneled into the Marè

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Valley, reaching almost the confluence with the Zebrù River Valley. There it stopped after a

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2900 m long runout and a 1400 m height drop (Fig. 1b,c). Velocities attained from a video record reached a mean value of 38 m s−1 with peaks of 70–80 m s−1 along the Zebrù Glacier surface (Sosio et al., 2008). The thickness of the deposit ranges from sub-metric, on the Zebrù Glacier, to 35 m in the lower part of the Marè Valley that was almost completely filled (Fig. 2). The deposit grain size ranges from very fine particles, associated with the enormous dust cloud generated during the runout, to large blocks with volumes of several cubic meters, up to a maximum of about 5,000 m3. For the gravelly-sand matrix, the grain size distribution of 8 samples has been presented in Crosta et al. (2007) and Supplementary Materials 1 and 2. A notable secondary event occurred on September 5, 2010, when some 100,000 m3 collapsed from the eastern flank of the 2004 scar (Bonardi et al., 2012). In addition to the 2004 and 2010

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ACCEPTED MANUSCRIPT events, an intense rockfall activity was observed throughout the following decade, mainly

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during summer and early autumn, progressively decreasing after the first two years.

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3. Materials and methods

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Since 2005, yearly surveys were performed during summer and early autumn in order to detect changes in the rock avalanche deposit and in the surrounding areas that were indirectly

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impacted. From 2005 to 2011, we collected ground points with two Leica 1200 differential

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phase GNSS receivers over the entire rock avalanche deposit. GNSS points have been collected with higher spatial density (i.e., one point per square meter) in areas characterized by evident

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morphological change. From 2011 to 2014, the use of a Terrestrial Laser Scanner (TLS, Riegl VZ-

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1000) allowed acquiring high-resolution point clouds with an accuracy of 8 mm and a precision

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of 5 mm at 100 m distance. Newly formed channels and debris fans were scanned at higher spatial resolution (0.01 to 0.05 m) to better reproduce topographic variations due to deposition and/or erosion. In order to properly geo-locate point clouds during post-processing, we included GNSS targets in an ad hoc reference scan. Due to limited performance of TLS on snow and ice surfaces, the Zebrù Glacier survey was carried out with differential phase GNSS also for the period 2012–2014. The GNSS data were post-processed with data from the Malles and Bormio fixed GNSS stations located respectively 12 and 23 km away. This allowed minimizing bias and positioning errors in GNSS points as well as geo-referencing TLS point clouds to the WGS 84 geographical coordinate system. Raw points were corrected based on RINEX data. Obtained uncertainty associated with GNSS points was estimated, depending on satellite cover and quality of acquired data, between 0.02 and 2.3 m (Table 1). 8

ACCEPTED MANUSCRIPT TLS point clouds were georeferenced by means of GNSS targets, coarse manual registration and the Iterative Closest Point (ICP) algorithm (Besl and McKay, 1992) within the RiSCAN PROTM

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software (Riegl LMS GmbH). Successively, TLS datasets were filtered to remove vegetation

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points and noise signals and finally spatially standardized through application of a subsampling

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algorithm (CloudCompareTM, http://www.danielgm.net/cc/). This procedure considerably reduced the point cloud density in areas of little interest (e.g., bedrock stable areas), without

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quantified in less than 0.05 m for each scene.

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introducing additional geodetic error. The uncertainty associated with TLS point clouds was

In addition to data derived from fieldwork, three high-resolution airborne LiDAR DEMs were

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acquired in 2005, 2007 and 2010 (Table 1). The 2005 DEM covers only the upper half of the

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Thurwieser landslide deposit. Finally, a 2 m gridded DEM of the Marè Valley before 2004 was

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obtained through photogrammetric techniques from 1:30,000 aerial photographs taken in September 2003 (Compagnia Generale Riprese Aeree srl).

Table 1. Summary of available topographic data and imagery. The GNSS accuracy is expressed as 3D CQ (Leica, 2011). The accuracy for TLS is the sum of laser accuracy and the accuracy of GNSS data used for referencing. For LiDAR, the accuracy is the 1 standard deviation as declared by the provider. For the photogrammetric DEM, the accuracy is RMSE. Date

Data Source

9/2003 9/2003 9/2005

Photogrammetric DEM Orthophoto Lidar

2-4/8/2005 5-7/9/2006

GNSS GNSS

Grid-size Number of points (m) 2

Accuracy (m)

Source

1.8

This study

1 2.5

n.a. Altimetric: 0.55 0.02 0.29

1,236 1,501

Regione Lombardia < Provincia Autonoma di Bolzano Riva (2009) Riva (2009)

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Orthophoto GNSS TLS

21/7/2014 21/7/2014

GNSS TLS

0.5 6,274 17,979 27,629 25,548 1 28,203 29,360,014 a (1,026.8 pts m−2) 0.3 4,264 28,960,218 a (293.9 pts m−2) −2 b (39.6 pts m ) 10,707 85,150,721 a (208.1 pts m−2) b (64.2 pts m−2)

Average point density for debris fan F3 (see Fig. 3)

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Average point density for the rock avalanche deposit

Regione Lombardia Riva (2009) Riva (2009) This study This study ISPRA This study This study

n.a. 0.07 0.08

Regione Lombardia This study This study

2.3 0.08

This study This study

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0.08 0.21 0.65 0.55 Altimetric: 0.15 Planimetric 0.3 0.54 0.07

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2012 8-10/7/2013 8-10/7/2013

Regione Lombardia

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GNSS TLS

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5/7/2011 4-5/7/2012

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2007 22-23/9/2008 22-23/6/2009 25-26/9/2009 23/9/2010 11-13/9/2010

Photogrammetric DEM Orthophoto GNSS GNSS GNSS GNSS LiDAR

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2007

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By comparing multitemporal topographic data sets, we performed quantitative estimates of the topographic changes occurred along the Marè Valley after the Thurwieser rock avalanche. Within the GIS environment, we performed analysis of the variation in elevation between the GNSS points (every year from 2005 to 2014) and the raster-based LiDAR DEMs (2005, 2007, and 2010). Similarly, we analyzed the elevation difference between sequential LiDAR DEMs (2005, 2007, and 2010) and between LiDAR and TLS point clouds (2012, 2013, and 2014). A point cloud comparison of TLS datasets acquired from 2012 to 2014 was performed using the M3C2 algorithm (Lague et al., 2013) implemented in CloudCompare. This procedure allowed obtaining an accurate estimation of both vertical and horizontal topographic variations, just above the level of detection (instrumental error 0.05 m).

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ACCEPTED MANUSCRIPT 4. Results Different volume estimates have been proposed by different authors, for either the detached

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rockslide (1.2 Mm3 by Dei Cas et al., 2004; 2.7 Mm3 by Godone et al., 2005), or the rock-

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avalanche deposit (2.9 Mm3 by Bellingeri and Zini, 2005; 2.5 Mm3 by Sosio et al., 2008,

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considering a 25% bulking of the detached volume).

The comparison of the pre-event topography in 2003 and the post-event topography in 2005 in

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the upper part and 2010 in the lower part allowed reassessing the rock-avalanche volume. The

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detached volume from the main scarp amounts to 2.99 ± 0.27 Mm3 (± 1 standard deviation). The uncertainty was calculated as the standard deviation of the elevation difference between

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the pre- and post-event topography in steep areas located nearby the main scarp, where

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morphological changes were considered negligible during the period of interest. The volume of

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the accumulated debris amounts to 4.00 ± 0.45 Mm3 (± 1 standard deviation). This figure was assessed by comparing 2003 and 2010 topography, because the 2005 LiDAR DEM does not cover the southernmost portion of the study area, and the 2007 Photogrammetric DEM is considered less reliable in comparison to the 2010 LiDAR DEM (Fig. 2). This volume is probably underestimated, due to the compaction of the deposit occurred between 2004 and 2010 (details are explained in Section 4.1), and because it does not include the material deposited on the glacier. A rough estimate of the latter, assuming an average depth of 0.5 m, yields a value of about 100,000 m3.Comparison of scar and deposit volumes gives a bulking coefficient of about 30% with final porosities ranging between 18% and 35% (Sherard et al., 1963; Hungr, 1981; Chen et al., 2006; Crosta et al., 2011).

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Fig. 2. Change in elevation between the pre-event topography in 2003 and the post-event topography in 2005 in the upper part and 2010 in the lower part, as separated by the dashed black line. The solid blue line bounds the main footprint of the rock avalanche. The dotted blue lines in the upper part bound the maximum lateral spreading of material during the initial collapse. The position of samples used for grain size characterization is reported (Supplementary Material 1).

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Fig. 3. Post-event geomorphic changes: deposit compaction (A), a new channel within the rock avalanche deposit (C1), new intra-basin debris-flow fans (F1 and F2), a new channel (C2) with an associated alluvial fan (F3) outside the rock avalanche deposit, and differential glacier ablation (G).

4.1. Evolution of the rock avalanche deposit The comparison of the GNSS data with the more recent LiDAR DEMs revealed a topographic lowering ranging from a few decimeters up to 1.5 m (Fig. 4) in the middle and lower portions of

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ACCEPTED MANUSCRIPT the avalanche deposit (A in Fig. 3). This lowering, which in the case of the Thurwieser decreases with time (Fig. 3), is associated with the compaction of the deposit, fine suffusion and

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subsurface erosion (Dunning et al., 2007; Korchevskiy et al., 2011). Compaction and fine

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suffusion seem to be stronger in the distal portion of the deposit, possibly due to the

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concentration of groundwater flow (Fig. 4 a,b).

Fig. 4. Evolution of rock avalanche deposit. a) From 2005 (GNSS data) to 2007 (photogrammetric DEM). b) From 2007 (photogrammetric DEM) to 2010 (Lidar DEM). c) From 2010 (Lidar DEM) to 2014 (TLS DEM).

In 2005, we observed the formation of a new channel in the mid portion of the avalanche deposit (C1 in Figs. 3 and 4a), which grew progressively over the years up to 5 m deep and 10 m wide in 2014. Local annual erosion and deposition along the channel during the 2013–2014

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ACCEPTED MANUSCRIPT seasonal cycle was estimated (i.e., by TLS cloud comparison) to vary between −0.50 m and + 0.40 m, depending on site location within the avalanche deposit.

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From the elevation difference of the reconstructed post-event DEM and the 2014 TLS survey,

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the total volume of the material remobilized and eroded by the channel between 2004 and

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2014 was estimated to be 12,701 ± 734 m3 (± 1 standard deviation) (C1 in Figs. 3 and 5).

Fig. 5. Net erosion (negative sign) and deposition (positive sign) along the newly-formed stream channel in the period 2005–2014. Values are expressed in m3.

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The channel C1 extends from about 2540 to 2390 m a.s.l., where a gentler sector within the

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rock avalanche deposit causes an abrupt change in slope and the consequent deposition of

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material. Here, two distinct fans F1 and F2 developed (Figs. 3, 5 and 6). The central fan (F1) is

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fed by channel C1 that transports, almost exclusively, avalanche-derived material. The eastern fan (F2) is built with sediment derived from the Little Ice Age (LIA) moraine located on the left-

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hand side of the Marè Valley, hence unaffected by the rock avalanche. Differences in sediment

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texture and color (i.e. rock avalanche deposit vs. lateral moraine) are visible from photos and LiDAR shaded relief (Fig. 6). The deposition of the two debris fans started in the spring of 2005,

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when the sediment, transported mainly by debris-flow events, has been trapped by the rock

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avalanche deposit. During fieldwork, we observed that almost the entire sediment flux

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originating from the lateral valley accumulated in the eastern fan (F2) that remained dammed by the central fan (F1) for the entire period 2005–2014. The central fan (F1) was able to evacuate part of the sediment coming from the rock avalanche deposit during high-flow periods. The growth rate of F2 was found to be roughly linear (Fig. 7), with a mean annual increase of 196.9 m3 over the last ten years, and a total cumulative volume recorded in the summer of 2014 equal to 2,148 ± 191 m3 (± 1 standard deviation).

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Fig. 6. Central (F1 in Fig. 3) and eastern (F2 in Fig. 3) debris fans in the middle sector of the accumulation (photo 2011, Lidar 2013). The white arrow in the upper photo points to two men for scale reference.

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Fig. 7. Sediment volume in the eastern fan (F2 in Fig. 3 and 6). Uncertainty (± 1 standard deviation) is shown. The regression line was obtained by fixing the volume in 2004 to 0,

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resulting in a growth rate of 196.9 m3 a-1 (Adjusted R2 = 0.98). The 95% confidence bands are

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shown as dashed red lines. Note significantly lower volume accumulation in 2013.

4.2. Impacts outside of the avalanche deposit Along the eastern side of the intermediate portion of the avalanche deposit, a new channel appeared in 2006 (C2 in Figs. 3 and 5). The upper part of this channel, between 2390 and 2140 m a.s.l., runs along the eastern margin of the rock avalanche and cuts through the deposit with a high slope gradient. The intermediate part of the channel extends for about 500 m in a small topographic depression originally occupied by mountain pastures down to a slope break located at 2200 m a.s.l. Both erosion and deposition occurred here, showing braided channel planform with bar formation and generalized channel widening. Occasionally, particularly in the upper part, the channel flows on bare bedrock. Changes in elevation range between −2.5 and +2.0 m over the whole monitoring period, while for the seasonal cycle, 2013–2014 values range 18

ACCEPTED MANUSCRIPT from −0.5 to +0.4 m. The lower part of the channel extends from the slope break (2200 m a.s.l.) to the Zebrù River Valley. In this steep part, intense erosion and stripping of soil and regolith

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occurred. In the summer of 2009, after five seasonal cycles, the incision reached a peak value of

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about 5 m, cutting through the underlying bedrock.

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At the bottom of the Zebrù River Valley, the new channel aggraded forming an alluvial fan (F3 in Figs. 3 and 5, Supplementary Material S2). By the summer of 2014, the fan had a volume of

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2,037 ± 597 m3 (± 1 standard deviation), with its distal part being extensively eroded by the

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Zebrù River during flood events. The fan is composed mainly of debris originating from the rock avalanche deposit, with a minor contribution from the LIA moraines. The temporal evolution of

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this debris fan indicates the occurrence of large depositional events during 2006 and 2007, with

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a subsequent reduction in sediment supply, as also documented by the development of

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pioneering vegetation on the fan (Fig. 8).

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ACCEPTED MANUSCRIPT Fig. 8. Evolution of Zebrù fan (F3 in Fig. 3). a) Pre-event situation, showing that fan F3 and channel C2 were absent (2003 orthophoto as background). b) 2007–2009, grid cell size 2 m

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(2007 orthophoto as background). c) 2009–2012, grid cell size 2 m (2013 Bing aerial photo as

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background). c) 2012–2014, grid cell size 0.1 m (2013 Bing aerial photo as background). 2007

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data come from Lidar, 2009 data from GNSS survey, 2012 and 2014 from TLS.

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When comparing topographic changes across 2007–2009, 2009–2012, and 2012–2014 (Fig. 8),

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it is apparent how the deposition in the first period was almost an order of magnitude larger. Subsequently, channel C2 was actively eroded in the upper part (Fig. 8) and the debris fan

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aggraded, causing the filling of the lower part of C2 channel, albeit with a notable decrease in

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volume of material deposited in comparison to previous years. Most topographic changes on

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the debris fan are observed on the eastern part, with a mean increase in elevation of 0.45 ± 0.50 m, while the central sector was slightly eroded.

4.3. Impacts on the Zebrù Glacier

The 2004 rock avalanche affected both the Zebrù and the East Zebrù glaciers (Figs. 2 and 3). Photos taken the day after the event show the terminal portion of the Zebrù Glacier covered by an heterogeneously distributed layer of debris over an area of about 200,600 m2 (Fig. 9).

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Fig. 9. The Zebrù Glacier and the East Zebrù Glacier covered by debris on the day after the rock avalanche (Figs. 2 and 3). Variations in the thickness of debris cover can be recognized. a)

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Supraglacial deposits on September 19, 2004 (photo by Massimo Ceriani). b) and c) Debris cover on September 5, 2006. d) Debris cover on August 21, 2015. 1: lateral scarp edge; 2:

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rockfall and snow-avalanche accumulation area; 3: thin discontinuous debris cover; 4: dismantled portion of pre-event supraglacial debris; 5: glacier terminus of the Zebrù Glacier

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(thick debris cover); 6: debris accumulation on the East Zebrù Glacier; and 7: glacier terminus of the East Zebrù Glacier (thick debris cover).

The effect of thermal insulation on the glacier due to sediment cover was quantified for the 2003–2005 and 2005–2010 periods by evaluating elevation changes via the DoD (DEM of Difference) of the relevant DEMs both in the debris-covered and in the debris-free portions of the glacier (Fig. 10). In the covered portion of the Zebrù Glacier, we attribute the elevation change observed between 2003 and 2005 to the glacier ablation and the attenuation effect due to the sediment cover. In consideration of a field-based estimate of debris thickness ranging between 0.1 and 2 m, we calculated a mean glacier elevation change rate of −2.16 m a-1 (Fig. 10

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ACCEPTED MANUSCRIPT a,c). We argue that most of the ablation has probably occurred before the 2004 avalanche event, i.e., the 2003 DEM was acquired in September 2003.

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This value was then compared to the mean elevation change rate of the clean-ice portions of

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the glacier at the same elevation in the period 2003–2005. This amounts to −2.91 m a-1,

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indicating that the overall effect of thermal insulation after the event was a reduction of the ablation by 26%. This value averages different ablation rates within the debris-covered area of

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the glacier. The ablation reduction was limited or negligible in areas with shallow debris cover

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(3–4 in Fig. 10a), and reached 55% where the debris thickness was higher (1–5 in Fig. 10a). The elevation changes between 2005 and 2010 display larger differences between the debris-

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covered and the clean-ice portions of the glacier (Fig. 10b, d). The mean elevation change rate

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over the covered area amounts to −0.68 m a-1, against a clean-ice rate of −2.76 m a-1, resulting

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in a mean ablation reduction of about 75%.

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Fig. 10. Elevation changes on the Zebrù and East Zebrù glaciers. a) and b) Ice thickness variations (in m) for the period (a) 2003–2005 and (b) 2005–2010. c) and d) Elevation change rate versus elevation of random points within the clean glacier (blue points) and the debris covered glacier (red points). Random points are visible in (a) and (b). 1: lateral scarp edge; 2: rockfall and snow-avalanche accumulation area; 3: thin discontinuous debris cover; 4: dismantled portion of pre-event supraglacial debris; 5: glacier terminus of the Zebrù Glacier (thick debris cover); 6: debris accumulation on the East Zebrù Glacier; 7: glacier terminus of the East Zebrù Glacier (thick debris cover); and 8: accumulation of 2010 rockfall. 23

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The development of a steep debris scarp along the eastern margin of the rock-avalanche

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deposit (#1 in Figs. 9 and 10; Fig. 11) represents a clear evidence of differential ablation on the

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glacier modulated by debris cover. The temporal evolution of this scarp was reconstructed

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through comparison of Lidar- and GNSS-derived DEMs (Fig. 12). Figs. 12 and 13 show scarp heights up to 10 m in 2014, with a constant increase throughout the decade. The cross profiles

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A and B (Fig. 12) exhibit a progressive downslope migration of the scarp, due to the glacier’s ice

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flow.

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ACCEPTED MANUSCRIPT Fig 11. NE lateral scarp forming the left hand side of the deposit on the Zebrù Glacier. The scarp

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Photos (a), (b) and (c) were kindly provided by Giuseppe Cola.

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position at different times is shown: a) 2005, b) 2006, c) 2011, d) 2012, e) 2014, and f) 2015.

Fig. 12. NE scarp forming the left hand margin of the deposit on the Zebrù Glacier. a) Scarp position at different times, together with displacement vectors (blue arrows) along the assumed glacier flow direction (cf. Fig. 13). The notched box whiskers plot shows the annual horizontal surface displacement of the scarp (25 points equally distributed along the scarp profile). Whiskers indicate the 5/95 percentiles, diamonds stands for the outliers. b) and c)

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ACCEPTED MANUSCRIPT Multitemporal elevation profiles along two different alignments (A–A’ and B–B’ in (a) oriented

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along the flow direction.

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The horizontal surface displacement of the scarp along the flow lines is investigated in four

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different periods from 2005 to 2014. We consider it as a proxy of the glacier surface speed, and it shows an overall speed reduction from 2005–2007 (~5–8 m a-1) to 2010–2012 (~3–5 m a-1)

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with a minor recent increase in 2012–2014 (~4–5 m a-1 ) (Fig. 12).

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The spatially distributed surface velocity of the glacier’s lower portion was estimated by tracking visible boulders in sequential orthophotos (2007 and 2012) (Fig. 13). Velocities are

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highest, up to 10 m a-1, in the northern portion of the glacier, and decrease toward the glacier

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terminus (Fig. 13). The large dataset (n = 399) and the overall consistent displacements

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recorded in the different areas of the glacier surface minimize the possible bias introduced by some of the boulders that could have slid independently. In this context, the comparable magnitudes of the scarp displacements corroborate the reliability of the methodology adopted to estimate the motion of the glacier’s surface (Figs. 12 and 13).

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Fig 13. Surface glacier flow rate (2007–2012) as obtained by tracking boulders within the rock avalanche deposit. (a) Displacement rate map based on spatial interpolation of the nearest neighbour method. The pre-event glacier extent (black line) refers to 2003 orthophoto updated with field surveys conducted in august 2004, few weeks before the event. b) Displacement rate measured along the scarp (Fig. 12) and from the interpolated values along the A–A’ transect.

5. Discussion Three main geomorphological and hydrological impacts have been recognized during the 10 year post-event monitoring of the Thurwieser rock avalanche. These include an increase in sediment yield within the host watershed, a reduction in the glacier ablation rate of the portion

27

ACCEPTED MANUSCRIPT covered by the rock avalanche deposit, and a change in both surficial hydrology and

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groundwater circulation.

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5.1. Effects on sediment dynamics

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Immediately after the rock avalanche event, the changes in landscape, the creation of new landforms and the erosion in the vicinity and within the deposit proceeded at an extremely high

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rate. Then, a progressive decrease in deposition rate has been observed, mainly due to a

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reduction in debris availability. As illustrated in Section 4.2, this behavior has been recognized by measuring the changes of the alluvial fan at the Zebrù River Valley bottom (F3 in Fig. 3),

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2012–2014 (Fig. 8).

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where the sediment deposition largely decreased from the period 2007–2009 to the period

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To assess the impact of the rock avalanche on sediment dynamics, the ordinary pre-event sediment yield must be compared with the post-event sediment yield. The ordinary sediment yield has been estimated by monitoring the evolution of the eastern debris fan (F2 in Figs. 6 and 7) which is fed by a lateral catchment extending 2.6 km2 (Fig. S3a in Supplementary Material 3). This catchment, unaffected by the rock avalanche, is characterized by an undisturbed yield, which we could measure by the sediment volume trapped by the rock avalanche deposit and the nearby debris fan F1 (see Fig. 3). As reported in Section 4.1, the F2 fan grew up to 2,148 m3 from 2005 to 2014, corresponding to a sediment yield of about 138 t km-2 a-1 and a denudation rate of about 0.091 mm a-1, assuming a bulk density of 1,500 kg m-3 (Hinderer et al., 2013). Considering that the sediment trap could have been only partially efficient in the whole period,

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ACCEPTED MANUSCRIPT especially for finer sediments, we should regard this value as a lower bound of the actual sediment yield.

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The sediment dynamics of the rock avalanche deposit has been reconstructed by calculating the

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amount of erosion within channels and the amount of sediment progressively stored as debris

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fan deposits F1 and F3 (Fig. 5). The net difference corresponds to sediments delivered to the main fluvial system of the Zebrù Valley, i.e., the post-event sediment yield of the last 10 years.

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In the middle sector of the rock avalanche, a channel up to 8 m deep was incised (C1 in Figs. 3

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and 5), resulting in about 12,700 m3 of debris reworked (Fig. 5). This material moved downstream and was partially deposited in the central fan F1 (2,425 m3). In the downstream

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area, the channel along the eastern boundary of the rock avalanche presents a negative budget

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of about 6,430 m3 due to soil and bedrock erosion from the bed and banks of the channel.

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Finally, the newly-formed fan at Zebrù River Valley bottom recorded a net accumulation of 2,037 m3. The sediment yield from the upper-to-medium rock avalanche sector amounts to about 680 t km-2 a-1. The sediment yield for the entire rock avalanche system is about 460 t km-2 a-1, and the amount delivered to the fluvial system equals 410 t km-2 a-1. These values have been compared with the ordinary sediment yield calculated for the eastern fan F2, observing that the rock avalanche caused an increase of sediment yield in the order of 3 to 6 times. This comparison is meaningful because the two basins share similar morphology and lithology (Supplementary Material 3). The slope gradient is 31.6° and 32.5° for the rock avalanche basin and the lateral basin, respectively. Lithology consists of dolostones (41.3% vs 42.2%), and unconsolidated glacial/scree deposits (18.3% vs 14.9%), with glaciers covering 38.8% of the rock avalanche basin, and 42.5% of the lateral basin. The sediment yield values calculated in the

29

ACCEPTED MANUSCRIPT study area are in the range typical for small, partially glacierized basins in alpine areas (Fig. 14, Hinderer et al., 2013). However, such a comparison should be made with caution due to the

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short period of observation of the sediment dynamics, possibly affected by short-term events,

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making the comparison with long-term sediment yield difficult (Kirchner et al., 2001).

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Fig. 14. Sediment yield (SY) calculated from 2005 to 2014 for the rock-avalanche deposit (post-

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event SY) and the unaffected eastern watershed (ordinary SY) in comparison with literature data for alpine catchments from Hinderer et al. (2013).

Table 2. Sediment volumes calculated in the study area for the period 2005–2014 (see Fig. 3 for labels). Sediment yield is computed assuming a sediment bulk density of 1,500 kg m -3.

Ordinary sediment yield

Rock-avalanche affected sediment yield

Eastern fan F2

Volume

Basin area

Denudation rate

Sediment yield

m3

km2

mm a-1

t km-2 a-1

2,148

2.6

0.082

123.9

Delivered to central fan F1

12,701

2.79

0.455

682.8

Delivered to Zebrù fan F3

16,704

5.4

0.309

464.0

Delivered to river

14,667

5.4

0.272

407.4

5.2. Glacier modifications

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ACCEPTED MANUSCRIPT Many studies report effects of rock avalanche on glacier dynamics (e.g., Tarr and Martin, 1914; Bull and Marangunic, 1968, Gardner and Hewitt, 1990; Deline, 2005; Vacco et al., 2010). Usually

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the most important effect is the increase of glacier flow velocity due to both an instantaneous

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gain of mass (Shulmeister et al., 2009) and a relative “ice thickening” consequence of the

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reduction in ablation beneath the debris (e.g., Reid, 1969; McSaveney, 1975; Reznichenko et al., 2010; Reznichenko et al., 2011; Shugar and Clague, 2011). For instance, Shugar et al. (2012)

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reported an immediate increase in surface velocity of up to 44% for the Black Rapids Glacier,

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Alaska, in the zone of three rock avalanche deposits.

A possible acceleration of the Zebrù Glacier flow was not assessed because of the lack of

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reliable pre-event flow velocity measurements. However, the modest thickness of the debris

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deposited on the glacier, and its position located mainly at the glacier terminus, suggest a

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minor effect of the rock avalanche on the glacier flow velocity. On the other hand, the observed post-event decrease in surface ice flow velocity (Fig. 12) might be related to the high (> 10 m) clean-ice glacier thinning (Figs. 10 and 12) associated with the intense climatic driven glacier retreat that has been affecting the European Alps since 1990 (Paul et al., 2011, Scotti et al., 2014), which is particularly evident in the Ortles/Cevedale massif (Carturan et al., 2013). In terms of the glacier mass balance (Table 3), rock avalanches are usually very effective in reducing the ablation, usually when the thickness of the avalanche-derived debris is > 1 m (Shulmeister et al., 2009). In this respect, the Thurwieser rock avalanche represents an unusual example, as it travelled over the glacier leaving a discontinuous deposit with only a minor portion exceeding 50 cm (Fig. 9). In addition, the rock avalanche cleaned a glacier portion that was previously covered by debris, hence increasing the local ablation rate (zone 4 in Figs. 9 and

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ACCEPTED MANUSCRIPT 10). In spite of this, a 75% post-event ablation reduction was measured between 2005 and 2010 (Fig. 10). This is slightly lower than, but comparable to ablation reductions observed in other

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case studies where rock avalanches stopped on glacier surfaces, with deposition of significantly

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thicker debris covers (e.g., 80%, McSaveney, 1975; up to 90%, Purdie and Fitzharris, 1999 and

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Reznichenko et al., 2011).

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Table 3. Mass balance of five glaciers in the southern Central Italian Alps. Distance from the

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Zebrù Glacier is reported. Zebrù Glacier mass balance is computed with geodetic method (Difference of DEMs) whereas for the other sites the superficial mass balance was measured by

Zebrù (clean ice)

Aspect

SW

Distance

Elevation

Period

Surface mass balance

Km

m.a.s.l.

-

2925-3000

2005-2010

-1200

-10.9

-2.0

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Glacier

TE

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ablation stakes.

year

cm

m w.eq

m w.eq a

This study

Zebrù (debris covered)

SW

-

2900-3000

2005-2010

-340

-3.1

-0.6

Dosegù

SW

12.2

2975

2007-2011

-1188

-10.8

-2.2

NE

6.6

2925-3025

2006-2010

n.d.

-9.2

-1.8

NW

35.1

2970

2007-2010

-807

-7.3

-1.8

E

65.7

2910

2007-2010

-431

-3.9

-1.0

Vedretta Lunga Campo Nord Vazzeda

Reference -1

Bonardi et al., 2014 Provincia di Bolzano, 2008-2011 Scotti et al., 2014

5.3. Impact on surface hydrology and groundwater circulation In spite of its importance for the stability of landslide dams and the hydraulic regime of mountain areas, analysis of water circulation in rock-avalanche deposits has been poorly documented in the literature (Meyer et al., 1986; Crosta et al., 2011; Ischuk, 2011; Petitta et al., 2010). Following the Thurwieser rock avalanche, the drainage system and the hydrologic response of the Marè Valley were strongly modified (Fig. 14). Field observations and personal 32

ACCEPTED MANUSCRIPT communications from the personnel managing the hydropower water diversion at the outlet of the Marè valley suggest that the discharge at the Marè Valley outlet is nearly constant and

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within the range of pre-rock avalanche base flow values (about 30–50 l s−1). Therefore, water

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that infiltrates the deposit during rainfall events and most of the icemelt/snowmelt water still

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flows toward the Marè Valley outlet. The difference with respect to pre-event conditions consists in the limited increase in response to rainfall or icemelt/snowmelt events. During these

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events, the new lateral channel is activated, redirecting large part of the water and sediment

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downstream toward the new debris fan F3 with the thick rock-avalanche deposit partially buffering the excessive discharge (Fig. 14). Geomorphic evidences, such as the shape of the

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channel, the mobilized sediment calibers and the lobe-shaped deposits on the debris fan,

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suggest that episodic debris-flow and slush-flow events may be important in controlling the

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evolution of the newly-formed drainage channel inside and outside the rock avalanche deposit. Hence, the rock avalanche deposit created a composite hydrological system (Fig. 15) with a response changing according to the flow regime. The first hydrologic system corresponds to the groundwater flow within the rock-avalanche deposit. The permanent drainage occurring during most of the year at the bottom of the rock avalanche suggests a relatively low hydraulic conductivity due to the presence of an important amount of finer materials below the coarse highly-permeable carapace. Similar materials have been reported in the literature (Cruden and Hungr, 1986; Hewitt, 1988; Strom, 2004; Dunning et al, 2006; Crosta et al., 2007) and could derive either from glacial/alluvial deposits entrained by the rock avalanche (Dufresne et al., 2010) or from dynamic fragmentation during propagation (Davies et al ,1999; Crosta et al., 2007, De Blasio and Crosta, 2015). The moderate to low hydraulic conductivity determines a

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ACCEPTED MANUSCRIPT high water table level, which eventually reaches the surface during high-flow periods, saturating the deposit in the mid-portion of the rock avalanche where the cross-section area

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available to seepage is reduced by local morphology (fans F1 and F2). This activates the second

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hydrologic system that efficiently discharges large quantities of water through the newly-

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formed ephemeral channel C2, transporting large amounts of fine and coarse sediments to the alluvial fan F3. Here the material is temporarily stored and finally delivered to the fluvial

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system. A similar behavior has been observed for talus deposits (Pierson, 1982; Clow et al.,

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2003; McClymont et al., 2010). To support this conceptual model, we calculated the discharge through the cross section E–E’ (Fig. 15), under the hypothesis that the deposit is saturated up to

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the level of the fan F1 outflow (2388 m a.s.l.). By estimating the hydraulic conductivity through

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the Kozeny-Carmen’s empirical relationship (Bear, 1972; details are in Supplementary Material

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4) and assuming the hydraulic gradient to be equal to the slope gradient of the pre-event basal surface (about 17°), groundwater discharges between 23.6 and 142 l s−1 were obtained, depending on the assumed deposit porosity (0.15 to 0.25) (Supplementary Material 5). These values embrace the baseflow field estimates (30 to 50 l s−1).

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Fig. 15. Schematic representation of the hydrologic and sedimentary pathways in the Thurwieser rock avalanche area. Black circles indicate the location of sediment samples collected for grain-size characterization. Section E–E’ is used for discharge calculation (Supplementary Material 5).

6. Conclusions Ten years after the occurrence of the Thurwieser rock avalanche it is possible to delineate a balance of the short-term effect of the event on geomorphological processes and landforms

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ACCEPTED MANUSCRIPT both within and outside the deposit. The rock avalanche deposit has been progressively eroded, thus resulting in a significant increase of the sediment yield toward the main river system, up to

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6 times the undisturbed pre-event value. This effect on sediment yield has been observed by

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several authors (Ohmori, 1992; Ries, 2000; Sutherland et al., 2002, Korup et al., 2004; Hancox et

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al., 2005; Chen et al., 2006), and was not only confirmed but also quantified by this study. An interesting effect of the rock avalanche is the formation of a composite hydrologic system,

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acting differently according to the flow regime. Under ordinary flow regime, the water

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percolates through the rock avalanche deposit, flowing as subsurface flow along the old valley axis. During storm or intense icemelt/snowmelt conditions, a new drainage channel is activated,

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and this mechanism has generated a new alluvial fan on the main valley floor. The future

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evolution of the hydrologic system is unclear. Clogging of the deposit could decrease its

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permeability reducing the infiltration and favouring surface flow along the lateral valley or incision of the deposit. This could lead to a further increase in volume of the newly deposited debris fan or the cutting of the rock avalanche deposit. Alternatively, if the groundwater flow will cause subsurface erosion by piping and fine suffusion, the rate of infiltration could increase causing a progressive abandonment of the lateral channel, triggering local instabilities within the deposit or the retrogressive erosion from its tip. This scenario, even if characterized by a low probability of occurrence, could induce large debris-flows along the main valley, as occurred in other rock-avalanche events (Crosta, 2001; Mikos et al., 2006). The effect of the rock avalanche on the differential ablation of the glacier caused the formation of a lateral scarp up to 20 m high between covered and non-covered ice. Different trends were recognized on the glacier, ranging from null ablation under thick debris to enhanced ablation where the rock

36

ACCEPTED MANUSCRIPT avalanche contributed to the removal of the pre-existing supraglacial cover. Overall, the rock

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avalanche insulated the glacier, with a reduction of ablation of 75% in the 2005–2010 period.

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Acknowledgements

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The authors are grateful to Giuseppe Cola for providing the Thurwieser peak photos, a detailed report of 2010 event and other personal communications about rock avalanche effects and

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evolution of Marè Valley. Alberto Villa helped in processing raw GNSS and TLS data. Elena

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Marinoni and Michele Bariselli at 5th Alpini hut (Fig. 1) has offered great hospitality and support during field surveys, which were conducted with the help of Paolo Florean, Xueliang Wang.

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Massimo Ceriani from Lombardy Civil Protection provided helicopter photos taken the day after

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the rock avalanche. We acknowledge Tim Davies and an anonymous reviewer for their

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