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In this work we present a methodology based on remote sensing data to map and classify unstable slopes in high alpine areas and apply this methodology to the Himalaya of northwestern Bhutan. We leverage on radar images acquired by the ESA Envisat and the JAXA ALOS-1 satellites between 2007 and 2011. Synthetic aperture radar differential interferometry (DInSAR) is used to identify and quantify recent ground surface displacements associated with potentially unstable slopes. Optical satellite images and a high-resolution digital surface model is used for the geomorphological classification of individual landforms and deposits associated with displacements. The analysis of the geomorphological characteristics reveals that displacements are associated with: rock slides, mountain slope deformation and rock slope deformation (77.5%), soil creep (4%), soil slides (1.9%) and rock glaciers (12.3%); 4.3% of the detected displacements remain of unknown origin. The method applied here for the assessment of activity likelihood uses a very large number of interferograms obtained with images acquired by different satellites. The method, which is new and replicable, takes advantage of the combination of different acquisition geometries and then combines indices of vegetation cover, layover, velocity estimate and temporal sampling associated with each detected active area to assign a weight. The analysis shows that a cluster of unstable slopes exists in the northwest, potentially related to the presence of an important regional lineament and the distribution of permafrost. Comparatively little ongoing activity is detected in the south, but field validation documents substantial landslide activity in the recent geological past. The dataset here generated, containing 700 potentially unstable areas with an associated activity likelihood, is a novel dataset that makes an essential basis of future analysis to investigate rock slope instability distribution and predisposing factors. Moreover, this dataset represents a new important step towards future landslide hazard assessment in the Himalaya of Bhutan. • 693 previously unknown unstable slopes in NW Bhutan were found with DInSAR • Geomorphological classification revealed that almost 78% are rock slope instabilities • The spatial distribution of potentially unstable slopes in the region is clearly non uniform • Activity of unstable slopes is likely influenced by regional tectonic structures • Unstable slopes seem to be less abundant in the region in recent times than in the past [ABSTRACT FROM AUTHOR]

Recently, advances in the evaluation of river channels have shown that transient erosional signals can preserve detailed information on the formation of topography and morphology of mountain ranges (e.g., Perron and Royden, 2012). Records of temporal variations in erosional activity within a landscape have also been demonstrated to be retained in the morphology of affected hillslopes (e.g., Colman and Watson, 1983). Here, we present a new method bringing together both the record of erosional activity stored in the hillslopes and the transient erosional signals stored along the river channel.The limit equilibrium relationship between topography and erosional processes has been described in the work of both Selby (1982) and Blöthe et al. (2015), the former showing a direct dependence of stable slope angle to rock mass strength, while the latter study discusses the erosional potential of slopes exceeding the critical angle of the material (excess topography) through landsliding. We use these mechanical concepts in addition to geometrical criteria to identify hillslope surfaces related to former erosional stages in two large (~5000 km2) catchments in the Bhutan Himalaya. These can then be tied to knickpoints extracted along the river profiles, thus allowing for a side by side analysis of hillslope and fluvial processes related to the recent tectonic and erosional activity of the region within and across catchments.ReferencesBlöthe, J. H., Korup, O., & Schwanghart, W. (2015). Large landslides lie low : Excess topography in the Himalaya- Karakoram ranges, 43(6), 523–526. https://doi.org/10.1130/G36527.1Colman, S. M., & Watson, K. (1983). Ages Estimated from a Diffusion Equation Model for Scarp Degradation. Science, 221(4607), 263 LP-265. https://doi.org/10.1126/science.221.4607.263Perron, J. T., & Royden, L. (2012). An integral approach to bedrock river profile analysis. Earth Surface Processes and Landforms, 3302. https://doi.org/10.1002/esp.3302Selby, M. J. (1982). Controls on the stability and inclinations of hillslopes formed on hard rock. Earth Surface Processes and Landforms, 7, 449–467. https://doi.org/10.1002/esp.3290070506 [ABSTRACT FROM AUTHOR]

Slope deformation in mountainous terrain can be driven by different processes, the nature of which is either gravitational and irreversible or seasonal and reversible, the latter induced by permafrost variations or by hydromechanical or thermomechanical effects. The importance of identifying such deformations is not only related to the hazard they can pose, but also to the understanding of changes that permafrost or local hydrological conditions undergo. Here, we carry out a multi-temporal InSAR analysis over a mountainous area 8000 km2 large, straddling north-western Bhutan and southern Tibet. We propose a methodology to separate locally deforming areas from the effects of long-wavelength signals through the analysis of the spatio-temporal behaviour of 4-years long time series of surface displacements. We present the mapping of hundreds of small-scale features that appear to be actively deforming, as well as several examples of reversible deformation rarely detected at this scale in such a challenging and vast region. The analysis of the multi-annual trend of ground deformation shows a relatively small number of irreversible gravitational movements clearly related to rock slides which attests a low level of recent activity of large rock slope instabilities in the region. In the southernmost, lower elevation parts of the study area, we quantify reversible surface displacements with amplitudes ranging between 5 and 17 mm, and showing maximum displacements towards the satellite in summer months, thus compatible with hydro-mechanical effects related to groundwater table variations. In addition, the ground movement induced by the active layer's response to thawing and freezing over the gentler slopes and high-elevation permafrost regions of Bhutan and southern Tibet is on average around 10 mm, with maxima up to 28 mm. The localised displacements appear to be largely associated to braided stream plains, glacier outwash plains or low angle, fine sediment covered slopes. • Inventory of gravitational and reversible surface displacements for NW Bhutan • Classification of slope processes made on the basis of time series trends • Reversible, mostly vertical deformation, observed in the permafrost region • Reversible deformation with horizontal component observed at lower elevations • Rock glaciers appear to have lower creep rates than in the Alps. [ABSTRACT FROM AUTHOR]