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 destabilization have been observed at the regional scale since the 1990s, concurrent with ground temperatures warming and widespread ice loss. These observations rise several questions on the fundamental mechanisms controlling rock glacier dynamics and its coupling to the climate, with implications in the fields of paleo-climatology, natural hazard management and planetary sciences. As a result, large research efforts have been undertaken in order to answer these questions. The statistical analysis of kinematic time series highlighted a strong correlation between creep rates and air temperature at seasonal-, inter-annual, and decennial temporal scales. Detailed field investigations have demonstrated the importance of rock glacier hydrology (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 rock glacier dynamics at the regional scale is hampered by its multidisciplinary nature and by the scarcity and the limitations of the available database. In fact, on the one hand the current dynamical and geomorphological state of rock glaciers is at present largely unknown at a regional scale, and on the other hand our understanding of rock glacier dynamics and its complex interactions with thermal and hydrological processes at the local scale remains limited and primarily qualitative. With the aim of contributing to fill these knowledge gaps, this thesis investigates the processes controlling the evolution of rock glacier dynamics at multispatio-temporal scales by means of process-based numerical modelling, data collection and analysis, and remote sensing techniques. At a local scale, I investigated the dynamic response of rock glacier dynamics to variations in external temperature and water input at seasonal and inter-annual temporal scales by designing a novel process-based modelling approach. The modelling accounts for heat transfer into the ground, the catchment hydrology, the hydromechanics of the rock glacier and its rheology. By these means, I critically discussed the hypotheses that (i) external temperature forcing through heat conduction and (ii) water input through pore water pressure at the shear horizon can explain seasonal and inter-annual variations in rock glacier dynamics. For five well documented study cases in the Swiss Alps, I showed that the direct influence of external temperature forcing on rock glacier rheology can explain only up to 25% of the observed seasonal and inter-annual variations in surface velocities so that the magnitude of the variations is underestimated at least by one order of magnitude and the phase 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 the velocity variations both in magnitude and phase at seasonal and inter-annual time scales and could be used to derive indirect information on the hydromechanical regime of rock glaciers. The results corroborated the hypothesis that the rhythm of rock glacier dynamics can be explained by water and external temperature, with a preponderant influence of water at the shear horizon depth, also indirectly controlled by temperature. Temperature changes over 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 database comprising information about rock glacier geometry (thickness and slope angle), geomorphological and permafrost conditions, and surface velocities. The analysis of a restricted dataset, 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 coupling the thickness model with a creep model for ice-rich permafrost. I introduced the Bulk Creep Factor BCF, a dimensionless parameter which allows the disentanglement of the two contributions to the surface velocity from (i) material properties and (ii) geometry. The proposed approach only requires remote sensing observations on creep velocities and surface slope angle, hence can be applied operationally over large areas. The application at a regional scale showed that most alpine rock glaciers are characterized by low values BCF <5, whereas only rock glaciers currently experiencing destabilization or set in conditions unfavourable to permafrost occurrence show larger values BCF <20. At a local scale, I found that for dynamically stable rock glaciers the geometry can explain the spatial variability in creep rates with almost constant rheological properties (BCF), while destabilized rock glaciers show contrasting and discontinuous values. Thus, the evaluation of the dynamical state of a rock glacier should account for its geometry, material properties and the processes controlling its movement rather than solely on geomorphological and kinematical observations. The synthesis of the regional- and local scale investigations, based on in-situ and remote sensing observations analysed through the lens of process-based modelling, advanced our 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 glacier dynamics, determining the rheological properties of the rock glacier material, but also controlling mass and energy fluxes through multiple non-linear processes. Eventually, approaching isotherm conditions at 0° C, the onset of permafrost degradation leads to important changes in the structure of the rock glacier itself. Hence, thermal mechanisms become less important and hydromechanical processes (also through the onset of rock glacier destabilization) 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 of a new theory, discloses great potential for long-term regional-scale analysis of rock glacier evolution.
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 destabilization have been observed at the regional scale since the 1990s, concurrent with ground temperatures warming and widespread ice loss. These observations rise several questions on the fundamental mechanisms controlling rock glacier dynamics and its coupling to the climate, with implications in the fields of paleo-climatology, natural hazard management and planetary sciences. As a result, large research efforts have been undertaken in order to answer these questions. The statistical analysis of kinematic time series highlighted a strong correlation between creep rates and air temperature at seasonal-, inter-annual, and decennial temporal scales. Detailed field investigations have demonstrated the importance of rock glacier hydrology (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 rock glacier dynamics at the regional scale is hampered by its multidisciplinary nature and by the scarcity and the limitations of the available database. In fact, on the one hand the current dynamical and geomorphological state of rock glaciers is at present largely unknown at a regional scale, and on the other hand our understanding of rock glacier dynamics and its complex interactions with thermal and hydrological processes at the local scale remains limited and primarily qualitative. With the aim of contributing to fill these knowledge gaps, this thesis investigates the processes controlling the evolution of rock glacier dynamics at multispatio-temporal scales by means of process-based numerical modelling, data collection and analysis, and remote sensing techniques. At a local scale, I investigated the dynamic response of rock glacier dynamics to variations in external temperature and water input at seasonal and inter-annual temporal scales by designing a novel process-based modelling approach. The modelling accounts for heat transfer into the ground, the catchment hydrology, the hydromechanics of the rock glacier and its rheology. By these means, I critically discussed the hypotheses that (i) external temperature forcing through heat conduction and (ii) water input through pore water pressure at the shear horizon can explain seasonal and inter-annual variations in rock glacier dynamics. For five well documented study cases in the Swiss Alps, I showed that the direct influence of external temperature forcing on rock glacier rheology can explain only up to 25% of the observed seasonal and inter-annual variations in surface velocities so that the magnitude of the variations is underestimated at least by one order of magnitude and the phase 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 the velocity variations both in magnitude and phase at seasonal and inter-annual time scales and could be used to derive indirect information on the hydromechanical regime of rock glaciers. The results corroborated the hypothesis that the rhythm of rock glacier dynamics can be explained by water and external temperature, with a preponderant influence of water at the shear horizon depth, also indirectly controlled by temperature. Temperature changes over 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 database comprising information about rock glacier geometry (thickness and slope angle), geomorphological and permafrost conditions, and surface velocities. The analysis of a restricted dataset, 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 coupling the thickness model with a creep model for ice-rich permafrost. I introduced the Bulk Creep Factor BCF, a dimensionless parameter which allows the disentanglement of the two contributions to the surface velocity from (i) material properties and (ii) geometry. The proposed approach only requires remote sensing observations on creep velocities and surface slope angle, hence can be applied operationally over large areas. The application at a regional scale showed that most alpine rock glaciers are characterized by low values BCF <5, whereas only rock glaciers currently experiencing destabilization or set in conditions unfavourable to permafrost occurrence show larger values BCF <20. At a local scale, I found that for dynamically stable rock glaciers the geometry can explain the spatial variability in creep rates with almost constant rheological properties (BCF), while destabilized rock glaciers show contrasting and discontinuous values. Thus, the evaluation of the dynamical state of a rock glacier should account for its geometry, material properties and the processes controlling its movement rather than solely on geomorphological and kinematical observations. The synthesis of the regional- and local scale investigations, based on in-situ and remote sensing observations analysed through the lens of process-based modelling, advanced our 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 glacier dynamics, determining the rheological properties of the rock glacier material, but also controlling mass and energy fluxes through multiple non-linear processes. Eventually, approaching isotherm conditions at 0° C, the onset of permafrost degradation leads to important changes in the structure of the rock glacier itself. Hence, thermal mechanisms become less important and hydromechanical processes (also through the onset of rock glacier destabilization) 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 of a new theory, discloses great potential for long-term regional-scale analysis of rock glacier evolution.
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Cicoira, Alessandro