Remote Sensing Analyses of Slope Instabilities in Northwestern Bhutan
Slope instabilities adversely affect mountainous regions by posing direct and indirect hazards. Landslides can evolve in catastrophic failures that can directly cause loss of lives, damage to properties and to critical infrastructure. Moreover, indirect effects can cause destruction of arable land a...
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| Other Authors: | , , |
| Format: | Doctoral or Postdoctoral Thesis |
| Language: | English |
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ETH Zurich
2020
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| Subjects: | |
| Online Access: | https://hdl.handle.net/20.500.11850/387619 https://doi.org/10.3929/ethz-b-000387619 |
| Summary: | Slope instabilities adversely affect mountainous regions by posing direct and indirect hazards. Landslides can evolve in catastrophic failures that can directly cause loss of lives, damage to properties and to critical infrastructure. Moreover, indirect effects can cause destruction of arable land and, in case of landslide damming of rivers, subsequent floods that can have an impact a long way downstream of the landslide site. A large-scale overview of unstable slopes in mountainous regions is therefore important in order to understand the spatial distribution, to investigate the activity, to create a basis for landslide hazard assessment and to further the science regarding the predisposing and controlling factors. Bhutan was chosen for this study given its relatively poor regional knowledge of unstable slopes, despite its proneness. The main purpose of our work is to understand the spatial distribution of past large rock slope movements in the region, to quantify the recent rates of displacements and to investigate the structural and geological controls. Given the inaccessibility of the region and the large scale of the study, we used remote sensing techniques through all stages of our investigations. In particular, a combination of optical images and high-resolution digital surface model analysis and synthetic aperture radar differential interferometric (DInSAR) techniques was used. The use of DInSAR allowed also to image, quantify and discern slope displacements that are related to soil slide/creep or rock glaciers creep, moreover, it allowed to illuminate areas with reversible displacements related either to freeze-thaw cycles in the permafrost region, or to groundwater table seasonal fluctuations at lower elevations. We produced some new datasets for the region including: 1) a regional rock slope instabilities inventory based on optical images, 2) a rock glaciers inventory based on optical images, 3) an active slope instabilities inventory based on the analysis of individual interferograms, 4) an inventory of active slope instabilities based on multi-temporal DInSAR analyses which includes gravitational and reversible deformation, 5) a regional structural inventory largely based on remote sensing mapping with the inclusion of some in-situ measurements. We present the analysis of individual interferograms aimed at the identification of actively unstable slopes. We perform a geomorphological classification based on the landslide types and we propose a new method to assess the activity likelihood uniquely based on remote sensing techniques. This methodology is based on an articulated decision tree, which takes into account the number of sensors used for the analysis, the number of interferograms in which displacements are identified, the satellite orbit geometry, vegetation cover, geometrical distortions, estimated velocity and temporal coverage of the data. This method allows lowering or maintaining the confidence regarding the activity of each identified slope instability by analysing what would be the detection expectation. It maximises the III level of information retrievable from standard interferometric analyses and is new and applicable to other mountainous regions. We use multi-temporal analyses based on SAR data to show the potential of the technique in identifying specific instabilities of which no previous knowledge existed. Here we show a methodology to combine a model-based atmospheric correction and an empirical correction to remove stratified, long wavelength signals from the cumulative displacement maps, without an atmospheric correction on the interferograms used to generate them. This method takes advantage of the local nature of slope processes leading to surface displacements to separate them from the long-wavelengths signals related to artifacts. This method allows to identify small-scale reversible displacements related with freeze-thaw cycles in the permafrost region and, moreover, it provides insights into the possibility of observing the dynamics of natural annual ground water table variations on valley flanks, which, had not yet been explored with multi-temporal satellite based InsAR analyses. We show the analysis performed with the combination of our structural dataset and landslides dataset aimed at investigating predisposing factors to large rock slope instabilities in the region. The methodologies developed here involve the generation of a compound structural dataset that makes use of different sources of information. The latter include remotely mapped fault traces and planar valley flanks, but also some field measurements of foliation and faults. The methodology used to compile this dataset addresses the biases deriving from uneven sampling of the different data types across the area and allows to make the dataset more balanced for systematic kinematic analyses. We apply kinematic analyses across the structural domains identified in the study region in a probabilistic way, in order to investigate the regional structural control on large rock slope instabilities formation. These methodologies are also new, and applicable to other mountainous regions where remote sensing analyses are the main source of information. We identify a structural and lithological control on rock slope instabilities across large parts of the study region. A higher predisposition to failure seems to be associated with the presence of specific sets of regional faults and foliation structures. The average orientation of one of such fault sets corresponds to the orientation of the Lingshi fault, a major known structure in the northwest of the region. Moreover, there seems to be a lithological control both on past rock slide activity and on more recent activity in association with shales, quartzite and limestone belonging to the Tethyan sediments and found in the northwest of the region. The analysis by means of DInSAR of rock slope activity in the recent past (our observation window is between 2006 and 2011) shows that the majority of active rock slope instabilities identified have rates which correspond to the slow to very slow categories, with maximum displacement rates of up to 160 mm/year, with only very few exceptions. IV These findings, combined with the field observations of a large number of past rock slide deposits, covered by thick soils, point to the fact that active unstable rock slopes are less abundant in the region in recent times than in the past. We postulate that this may be related to the lack in recent times of major earthquakes that could have previously caused widespread landslide activity at a given point in time. We have also observed that rock glaciers in northwestern Bhutan creep downslope with velocities that are on the whole lower than what has been observed in the Alps in recent years. This could be due to either a less pronounced recent warming trend than at higher latitudes or to less water being available to penetrate into the frozen unconsolidated material supersaturated with interstitial ice and to deeper subsurface shear horizons. |
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