The behaviour of periglacial landforms, such as rock glaciers, or other permafrost related processes can be of high significance for paleoclimatic reconstructions but also a proxy for ongoing climatic changes. In particular, knowledge on surface displacements rates and patterns is an added value for a better understanding of periglacial environment dynamics. However, studying such displacements and their evolution is challenging due to often difficult environmental conditions and remoteness. EO data is of great help in this respect, and the combination of remote sensing approaches with geological and geomorphological information is nowadays more and more applied in such investigations. Among remote sensing techniques, satellite based Synthetic Aperture Radar Differential Interferometry (DInSAR) can be used to measure ground displacements, potentially allowing for millimetric accuracy with current time series techniques and corrections from atmospheric effects. Expected displacements of active rock glaciers are gravitationally driven and irreversible, while active permafrost layers predominantly show seasonal reversible variations of uplift and subsidence. Although atmospheric effects are of lower amplitude than the large linear trends associated with rock glacier creep, atmospheric corrections are essential for the detection of reversible deformation as well as improved accuracy of the linear trends. In this work, we study a 8000 km2 area in northwestern Bhutan, with elevations between 1000 and 5000 m a.s.l.. Our analyses combine information obtained from GoogleEarth optical images and from multitemporal time series analyses. Radar interferograms from 6 different datasets (two Envisat tracks and 4 ALOS-1 frames) were analysed individually for visual interpretations, as well as exploited for the generation of displacement time series based on a small baseline approach (SBAS). We reduced the effect of the atmosphere and refer all cumulative displacements maps to common stable areas by using the GACOS atmospheric correction tool and an by applying an empirical estimation of residual tropospheric delays and long-wavelength ramps. The multi-annual trends over rock glaciers and the amplitudes and the timings of the seasonal displacements over other permafrost areas were then analysed with respect to elevations, aspect, mean annual temperatures and morphological expressions. 167 rock glaciers were identified through available optical images. Of these, 52% and 72% have been classified as active through the analysis of single interferograms and SBAS results respectively. Only 2.4% of the rock glaciers mapped in optical images and 15% of those mapped in the interferograms are not covered by information obtained through SBAS. Our analyses indicate that rock glaciers in the region during our observation period (between February 2007 and February 2011) creep with estimated downslope displacement rates ranging from a few mm/year to 70 cm/year. These velocities are notably lower than the m/years of displacement rates observed for some rock glaciers in other mountain belts (e.g. the Alps) and are likely related to mean annual temperature conditions. Moreover, we were able to single out areas of seasonal reversible deformation lying on low roughness, fine sediment covered valley floors, with gentle slope angles, generally less than 15°. Their spatial extent ranges between 400 m and 3 km in radius and their elevations vary between 4100 and 5100 m a.s.l.. The amplitudes of their seasonal cycles are as high as 15 mm and show a predominant vertical component, with uplift in winter and subsidence in summer, which is compatible with freeze-thaw cycles. These areas show almost no long-term linear trends or increase in the seasonal amplitude, which may indicate that, in the period of observation, no long-term change of permafrost conditions is detected in the region.
Dini, B., Manconi, A., Daout, S., Loew, S. (2019). Rock glacier dynamics and freeze-thaw cycles observed in the Himalaya of Northwestern Bhutan with DInSAR time series analyses. In Living Planet Symposium ESA 2019.
Rock glacier dynamics and freeze-thaw cycles observed in the Himalaya of Northwestern Bhutan with DInSAR time series analyses
Dini, B;
2019
Abstract
The behaviour of periglacial landforms, such as rock glaciers, or other permafrost related processes can be of high significance for paleoclimatic reconstructions but also a proxy for ongoing climatic changes. In particular, knowledge on surface displacements rates and patterns is an added value for a better understanding of periglacial environment dynamics. However, studying such displacements and their evolution is challenging due to often difficult environmental conditions and remoteness. EO data is of great help in this respect, and the combination of remote sensing approaches with geological and geomorphological information is nowadays more and more applied in such investigations. Among remote sensing techniques, satellite based Synthetic Aperture Radar Differential Interferometry (DInSAR) can be used to measure ground displacements, potentially allowing for millimetric accuracy with current time series techniques and corrections from atmospheric effects. Expected displacements of active rock glaciers are gravitationally driven and irreversible, while active permafrost layers predominantly show seasonal reversible variations of uplift and subsidence. Although atmospheric effects are of lower amplitude than the large linear trends associated with rock glacier creep, atmospheric corrections are essential for the detection of reversible deformation as well as improved accuracy of the linear trends. In this work, we study a 8000 km2 area in northwestern Bhutan, with elevations between 1000 and 5000 m a.s.l.. Our analyses combine information obtained from GoogleEarth optical images and from multitemporal time series analyses. Radar interferograms from 6 different datasets (two Envisat tracks and 4 ALOS-1 frames) were analysed individually for visual interpretations, as well as exploited for the generation of displacement time series based on a small baseline approach (SBAS). We reduced the effect of the atmosphere and refer all cumulative displacements maps to common stable areas by using the GACOS atmospheric correction tool and an by applying an empirical estimation of residual tropospheric delays and long-wavelength ramps. The multi-annual trends over rock glaciers and the amplitudes and the timings of the seasonal displacements over other permafrost areas were then analysed with respect to elevations, aspect, mean annual temperatures and morphological expressions. 167 rock glaciers were identified through available optical images. Of these, 52% and 72% have been classified as active through the analysis of single interferograms and SBAS results respectively. Only 2.4% of the rock glaciers mapped in optical images and 15% of those mapped in the interferograms are not covered by information obtained through SBAS. Our analyses indicate that rock glaciers in the region during our observation period (between February 2007 and February 2011) creep with estimated downslope displacement rates ranging from a few mm/year to 70 cm/year. These velocities are notably lower than the m/years of displacement rates observed for some rock glaciers in other mountain belts (e.g. the Alps) and are likely related to mean annual temperature conditions. Moreover, we were able to single out areas of seasonal reversible deformation lying on low roughness, fine sediment covered valley floors, with gentle slope angles, generally less than 15°. Their spatial extent ranges between 400 m and 3 km in radius and their elevations vary between 4100 and 5100 m a.s.l.. The amplitudes of their seasonal cycles are as high as 15 mm and show a predominant vertical component, with uplift in winter and subsidence in summer, which is compatible with freeze-thaw cycles. These areas show almost no long-term linear trends or increase in the seasonal amplitude, which may indicate that, in the period of observation, no long-term change of permafrost conditions is detected in the region.
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