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Understanding Rock Glacier Landform Evolution and Recent Changes from Numerical Flow Modeling

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Understanding Rock Glacier Landform Evolution and Recent Changes from Numerical Flow Modeling The Cryosphere, 10, 2865–2886, 2016 www.the-cryosphere.net/10/2865/2016/ doi:10.5194/tc-10-2865-2016 © Author(s) 2016. CC Attribution 3.0 License. Rock glaciers on the run – understanding rock glacier landform evolution and recent changes from numerical flow modeling Johann Müller, Andreas Vieli, and Isabelle Gärtner-Roer Department of Geography, University of Zurich, Zurich, 8004, Switzerland Correspondence to: Johann Müller ([email protected]) Received: 4 February 2016 – Published in The Cryosphere Discuss.: 29 February 2016 Revised: 8 July 2016 – Accepted: 5 October 2016 – Published: 23 November 2016 Abstract. Rock glaciers are landforms that form as a result 1 Introduction of creeping mountain permafrost which have received con- siderable attention concerning their dynamical and thermal Rock glaciers and their dynamics have received much atten- changes. Observed changes in rock glacier motion on sea- tion in permafrost research and beyond, most prominently sonal to decadal timescales have been linked to ground tem- by the International Panel on Climate Change (IPCC) in the perature variations and related changes in landform geome- context of impacts of a warming climate on high mountain tries interpreted as signs of degradation due to climate warm- permafrost (IPCC, 2014). Time series of rock glacier move- ing. Despite the extensive kinematic and thermal monitoring ment in the European Alps indicate that acceleration in per- of these creeping permafrost landforms, our understanding mafrost creep in recent decades is related to an increase of the controlling factors remains limited and lacks robust in ground temperatures (Delaloye et al., 2010; PERMOS, quantitative models of rock glacier evolution in relation to 2013; Bodin et al., 2015). Furthermore, multitemporal ge- their environmental setting. omorphometric analysis has shown subsidence features and Here, we use a holistic approach to analyze the current structural disintegration of alpine rock glaciers, which are in- and long-term dynamical development of two rock glaciers dicative of landform degradation and destabilization (Kääb in the Swiss Alps. Site-specific sedimentation and ice gener- et al., 2007; Roer et al., 2008b; Bodin et al., 2010; Spring- ation rates are linked with an adapted numerical flow model man et al., 2013; Micheletti et al., 2015). Many studies have for rock glaciers that couples the process chain from mate- addressed the connection between mean annual air temper- rial deposition to rock glacier flow in order to reproduce ob- atures and rock glacier dynamics from a descriptive point served rock glacier geometries and their general dynamics. of view (Ikeda and Matsuoka, 2002; Roer et al., 2005b; De- Modeling experiments exploring the impact of variations in laloye et al., 2010; Springman et al., 2012) or have used mod- rock glacier temperature and sediment–ice supply show that eling approaches to assess rock glacier dynamics (Jansen and these forcing processes are not sufficient to explain the cur- Hergarten, 2006; Kääb et al., 2007; Springman et al., 2012). rently observed short-term geometrical changes derived from Most of these studies focus on the impact of air or ground multitemporal digital terrain models at the two different rock temperature on rock glacier creep. Other authors stressed that glaciers. The modeling also shows that rock glacier thickness rock glacier dynamics cannot solely be explained by temper- is dominantly controlled by slope and rheology while the ad- ature variations and should integrate flow and controlling en- vance rates are mostly constrained by rates of sediment–ice vironmental factors such as sediment supply dynamics and supply. Furthermore, timescales of dynamical adjustment are landform characteristics (Roer et al., 2005b; French, 2007; found to be strongly linked to creep velocity. Overall, we pro- Frauenfelder et al., 2008). Rock glaciers have been defined vide a useful modeling framework for a better understand- as “lobate or tongue-shaped bodies of perennially frozen un- ing of the dynamical response and morphological changes of consolidated material supersaturated with interstitial ice and rock glaciers to changes in external forcing. ice lenses that move down slope by creep as a consequence of the deformation of ice contained in them and which are, thus, features of cohesive flow” (Barsch, 1992, p. 176). Such Published by Copernicus Publications on behalf of the European Geosciences Union. 2866 J. Müller et al.: Understanding rock glacier landform evolution and recent changes a definition includes information on form, material and pro- cess and, therefore, the observable rock glacier characteris- tics are influenced by sediment and ice input, permafrost con- ditions and the geomorphological setting, which in turn con- trol rheology and landform geometry (Barsch, 1996). Very few studies have addressed rock glacier dynamics with such a holistic approach that includes rock wall retreat, sediment and ice dynamics, and climate variations to gain insight into the long-term evolution of rock glaciers (Olyphant, 1983; Frauenfelder et al., 2008) and model validation was ham- pered by restricted observational data. Recent observations show signs of rock glacier destabi- lization such as acceleration, subsidence features and struc- tural disintegration (forming of tension cracks) at several Figure 1. Conceptual model of the dynamic evolution of a rock rock glacier landforms in the Swiss Alps (Kääb et al., 2007; glacier system (adapted from Fig. 1 in Müller et al., 2014a). Black Roer et al., 2008b; Delaloye et al., 2011; Lambiel, 2011; arrows show the sediment transport. t0, t1 and t2 show the rock Springman et al., 2013; PERMOS, 2013; Kenner et al., 2014; glacier surface geometries at different time steps resulting from Bodin et al., 2015). These studies indicate that various factors variations in environmental factors such as warming and a decrease can lead to such degradation but a common triggering for all of sediment–ice input. the cases has not been identified. These potential factors are most likely connected to the complex combination of the lo- cal topography, the thermal state of the permafrost (climate- 2 Conceptual approach to high mountain periglacial induced response), the existence and intrusion of liquid water systems and/or variations in the sedimentation regime affecting the sediment load during long-term landform evolution. The topographic evolution of the rock glacier landform relies Numerous remote sensing techniques are available for ac- on the production, transport and deposition of debris in the quiring data on permafrost creep (see Haeberli et al., 2006, periglacial system and the generation and integration of sub- and Kääb, 2008, for an extensive summary), high moun- surface ice (Wahrhaftig and Cox, 1959; Barsch, 1996). The tain geomorphometry (Bishop et al., 2003) and high moun- development of rock glaciers is therefore dependent upon the tain sediment dynamics (Gärtner-Roer, 2012; Heckmann and supply of debris from the source headwall(s) and the long- Schwanghart, 2013; Müller et al., 2014a). This provides the term preservation of an ice matrix or ice-core-inducing creep necessary foundation for a holistic assessment strategy that (Morris, 1981). Rock glaciers are also dynamic landforms includes the coupling of relevant landforms and processes that are influenced by the warming and melting of ice and and in which sediment supply rates can be quantified, ice vol- changes in sediment input. The variations in environmen- umes estimated and rock glacier rheologies derived (Frauen- tal factors translate into observable changes in geometry and felder et al., 2008; Gärtner-Roer and Nyenhuis, 2010). kinematics, which can be interpreted as a sign of degradation Here we present such a holistic analysis approach to assess and/or destabilization of these periglacial landforms (Roer et long-term rock glacier evolution and the impacts of varia- al., 2008b; Springman et al., 2013). tions in temperature, sediment and ice supply on rock glacier Figure 1 shows the theoretical concept of an idealized geometry and movement. We apply a numerical flow model periglacial mountain slope with a corresponding rock glacier to two rock glaciers in the Swiss Alps with different topo- system and builds the conceptual basis for this study. Two graphic, morphometric and rheological characteristics. The main subsystems contribute to the temporal and topograph- modeling is motivated by observations of topographic and ical development of the rock glacier landforms: the upper kinematic changes for the two rock glaciers revealing signs headwall and talus slope system generate the sediments that of degradation as presented in this study. The aim of the mod- are transported into the lower rock glacier system. Besides eling approach is to relate these changes to the long-term the sediment input, the rock glacier system is also controlled evolution and short-term adaptation of rock glacier systems by the existence, generation and state of subsurface ice and to changing environmental factors and ultimately to a bet- permafrost creep (see Fig. 1). The two subsystems differ ter understanding of the currently observed dominant con- on the basis of several characteristics: topographic features, trols of geomorphological changes. We thereby consider rock typical landform(s) and the dominating mass transport pro- glaciers as an integral part of a coarse-debris cascading sys- cesses. Back-weathering of the exposed rock wall and result-
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