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On the dynamics of rock glaciers


Cicoira, Alessandro. On the dynamics of rock glaciers. 2020, University of Zurich, Faculty of Science.

Abstract

Mountain permafrost is currently enduring substantial modifications due to climate change.A drastic increase in the creep rates of rock glaciers and the onset of rock glacier destabiliza-tion have been observed at the regional scale since the 1990s, concurrent with ground tem-peratures warming and widespread ice loss. These observations rise several questions on thefundamental mechanisms controlling rock glacier dynamics and its coupling to the climate,with implications in the fields of paleo-climatology, natural hazard management and plan-etary sciences. As a result, large research efforts have been undertaken in order to answerthese questions. The statistical analysis of kinematic time series highlighted a strong correla-tion between creep rates and air temperature at seasonal-, inter-annual, and decennial tem-poral scales. Detailed field investigations have demonstrated the importance of rock glacierhydrology (through pore water pressures) in determining the short-term velocity variations.At the state of the art however, an accurate and coherent description of the evolution of rockglacier dynamics at the regional scale is hampered by its multi-disciplinary nature and bythe scarcity and the limitations of the available database. In fact, on the one hand the currentdynamical and geomorphological state of rock glaciers is at present largely unknown at aregional scale, and on the other hand our understanding of rock glacier dynamics and itscomplex interactions with thermal and hydrological processes at the local scale remains lim-ited and primarily qualitative. With the aim of contributing to fill these knowledge gaps, thisthesis investigates the processes controlling the evolution of rock glacier dynamics at multispatio-temporal scales by means of process-based numerical modelling, data collection andanalysis, and remote sensing techniques.At a local scale, I investigated the dynamic response of rock glacier dynamics to varia-tions in external temperature and water input at seasonal and inter-annual temporal scalesby designing a novel process-based modelling approach. The modelling accounts for heattransfer into the ground, the catchment hydrology, the hydro-mechanics of the rock glacierand its rheology. By these means, I critically discussed the hypotheses that (i) external tem-perature forcing through heat conduction and (ii) water input through pore water pressureat the shear horizon can explain seasonal and inter-annual variations in rock glacier dy-namics. For five well documented study cases in the Swiss Alps, I showed that the directinfluence of external temperature forcing on rock glacier rheology can explain only up to25%of the observed seasonal and inter-annual variations in surface velocities so that themagnitude of the variations is underestimated at least by one order of magnitude and thephase of the seasonal peaks is delayed by 2-3 months. On the contrary, when including the influence of pore water pressure at the shear horizon depth, our model could reproduce thevelocity variations both in magnitude and phase at seasonal and inter-annual time scalesand could be used to derive indirect information on the hydro-mechanical regime of rockglaciers. The results corroborated the hypothesis that the rhythm of rock glacier dynamicscan be explained by water and external temperature, with a preponderant influence of waterat the shear horizon depth, also indirectly controlled by temperature. Temperature changesover the entire thickness of the rock glacier can cause substantial variations in creep rates,but require changes in climate over decades or centuries.In order to extend the investigation at a regional scale, I investigated a large databasecomprising information about rock glacier geometry (thickness and slope angle), geomor-phological and permafrost conditions, and surface velocities. The analysis of a restricteddataset, for which detailed information about the thickness of the rock glaciers is available,showed that the typical driving stress is92±13kPa, similar to ice glaciers and therefore,rock glacier thickness can be efficiently estimated with the inversion of simple models (e.g.perfectly plastic model). Thus, I developed a general theory of rock glacier creep by couplingthe thickness model with a creep model for ice-rich permafrost. I introduced the Bulk CreepFactor BCF, a dimensionless parameter which allows the disentanglement of the two contri-butions to the surface velocity from (i) material properties and (ii) geometry. The proposedapproach only requires remote sensing observations on creep velocities and surface slope an-gle, hence can be applied operationally over large areas. The application at a regional scaleshowed that most alpine rock glaciers are characterized by low valuesBCF <5, whereasonly rock glaciers currently experiencing destabilization or set in conditions unfavourableto permafrost occurrence show larger valuesBCF <20. At a local scale, I found that for dy-namically stable rock glaciers the geometry can explain the spatial variability in creep rateswith almost constant rheological properties (BCF), while destabilized rock glaciers showcontrasting and discontinuous values. Thus, the evaluation of the dynamical state of a rockglacier should account for its geometry, material properties and the processes controlling itsmovement rather than solely on geomorphological and kinematical observations.The synthesis of the regional- and local scale investigations, based on in-situ and re-mote sensing observations analysed through the lens of process-based modelling, advancedour understanding of the processes controlling rock glacier dynamics at different spatio-temporal scales. Air and ground temperatures appear to be the major drivers of rock glacierdynamics, determining the rheological properties of the rock glacier material, but also con-trolling mass and energy fluxes through multiple non-linear processes. Eventually, ap-proaching isotherm conditions at0° C, the onset of permafrost degradation leads to impor-tant changes in the structure of the rock glacier itself. Hence, thermal mechanisms becomeless important and hydro-mechanical processes (also through the onset of rock glacier desta-bilization) take over the control of the short-term dynamic variations of the rock glacier.In a nutshell, this dissertation provides the first quantitative framework for the assessmentof the influence of climatic forcing on rock glacier dynamics and, with the development ofa new theory, discloses great potential for long-term regional-scale analysis of rock glacierevolution.

Abstract

Mountain permafrost is currently enduring substantial modifications due to climate change.A drastic increase in the creep rates of rock glaciers and the onset of rock glacier destabiliza-tion have been observed at the regional scale since the 1990s, concurrent with ground tem-peratures warming and widespread ice loss. These observations rise several questions on thefundamental mechanisms controlling rock glacier dynamics and its coupling to the climate,with implications in the fields of paleo-climatology, natural hazard management and plan-etary sciences. As a result, large research efforts have been undertaken in order to answerthese questions. The statistical analysis of kinematic time series highlighted a strong correla-tion between creep rates and air temperature at seasonal-, inter-annual, and decennial tem-poral scales. Detailed field investigations have demonstrated the importance of rock glacierhydrology (through pore water pressures) in determining the short-term velocity variations.At the state of the art however, an accurate and coherent description of the evolution of rockglacier dynamics at the regional scale is hampered by its multi-disciplinary nature and bythe scarcity and the limitations of the available database. In fact, on the one hand the currentdynamical and geomorphological state of rock glaciers is at present largely unknown at aregional scale, and on the other hand our understanding of rock glacier dynamics and itscomplex interactions with thermal and hydrological processes at the local scale remains lim-ited and primarily qualitative. With the aim of contributing to fill these knowledge gaps, thisthesis investigates the processes controlling the evolution of rock glacier dynamics at multispatio-temporal scales by means of process-based numerical modelling, data collection andanalysis, and remote sensing techniques.At a local scale, I investigated the dynamic response of rock glacier dynamics to varia-tions in external temperature and water input at seasonal and inter-annual temporal scalesby designing a novel process-based modelling approach. The modelling accounts for heattransfer into the ground, the catchment hydrology, the hydro-mechanics of the rock glacierand its rheology. By these means, I critically discussed the hypotheses that (i) external tem-perature forcing through heat conduction and (ii) water input through pore water pressureat the shear horizon can explain seasonal and inter-annual variations in rock glacier dy-namics. For five well documented study cases in the Swiss Alps, I showed that the directinfluence of external temperature forcing on rock glacier rheology can explain only up to25%of the observed seasonal and inter-annual variations in surface velocities so that themagnitude of the variations is underestimated at least by one order of magnitude and thephase of the seasonal peaks is delayed by 2-3 months. On the contrary, when including the influence of pore water pressure at the shear horizon depth, our model could reproduce thevelocity variations both in magnitude and phase at seasonal and inter-annual time scalesand could be used to derive indirect information on the hydro-mechanical regime of rockglaciers. The results corroborated the hypothesis that the rhythm of rock glacier dynamicscan be explained by water and external temperature, with a preponderant influence of waterat the shear horizon depth, also indirectly controlled by temperature. Temperature changesover the entire thickness of the rock glacier can cause substantial variations in creep rates,but require changes in climate over decades or centuries.In order to extend the investigation at a regional scale, I investigated a large databasecomprising information about rock glacier geometry (thickness and slope angle), geomor-phological and permafrost conditions, and surface velocities. The analysis of a restricteddataset, for which detailed information about the thickness of the rock glaciers is available,showed that the typical driving stress is92±13kPa, similar to ice glaciers and therefore,rock glacier thickness can be efficiently estimated with the inversion of simple models (e.g.perfectly plastic model). Thus, I developed a general theory of rock glacier creep by couplingthe thickness model with a creep model for ice-rich permafrost. I introduced the Bulk CreepFactor BCF, a dimensionless parameter which allows the disentanglement of the two contri-butions to the surface velocity from (i) material properties and (ii) geometry. The proposedapproach only requires remote sensing observations on creep velocities and surface slope an-gle, hence can be applied operationally over large areas. The application at a regional scaleshowed that most alpine rock glaciers are characterized by low valuesBCF <5, whereasonly rock glaciers currently experiencing destabilization or set in conditions unfavourableto permafrost occurrence show larger valuesBCF <20. At a local scale, I found that for dy-namically stable rock glaciers the geometry can explain the spatial variability in creep rateswith almost constant rheological properties (BCF), while destabilized rock glaciers showcontrasting and discontinuous values. Thus, the evaluation of the dynamical state of a rockglacier should account for its geometry, material properties and the processes controlling itsmovement rather than solely on geomorphological and kinematical observations.The synthesis of the regional- and local scale investigations, based on in-situ and re-mote sensing observations analysed through the lens of process-based modelling, advancedour understanding of the processes controlling rock glacier dynamics at different spatio-temporal scales. Air and ground temperatures appear to be the major drivers of rock glacierdynamics, determining the rheological properties of the rock glacier material, but also con-trolling mass and energy fluxes through multiple non-linear processes. Eventually, ap-proaching isotherm conditions at0° C, the onset of permafrost degradation leads to impor-tant changes in the structure of the rock glacier itself. Hence, thermal mechanisms becomeless important and hydro-mechanical processes (also through the onset of rock glacier desta-bilization) take over the control of the short-term dynamic variations of the rock glacier.In a nutshell, this dissertation provides the first quantitative framework for the assessmentof the influence of climatic forcing on rock glacier dynamics and, with the development ofa new theory, discloses great potential for long-term regional-scale analysis of rock glacierevolution.

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Additional indexing

Item Type:Dissertation (monographical)
Referees:Vieli Andreas
Communities & Collections:07 Faculty of Science > Institute of Geography
UZH Dissertations
Dewey Decimal Classification:910 Geography & travel
Language:English
Place of Publication:Zürich
Date:2020
Deposited On:10 Feb 2021 10:09
Last Modified:10 Feb 2021 10:09
Series Name:Schriftenreihe Physische Geographie Glaziologie und Geomorphodynamik
Number of Pages:143
ISBN:978-3-906894-17-1
OA Status:Closed

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