Assessing the contribution of slow moving landslides to erosion in the Himalayas – SLIDE

In mountain ranges, landslides play a dominant role in the erosion of hillslopes and the evolution of landscapes. They vary greatly in size, geometry and rate of deformation, from catastrophic landslides to extremely slow deformation. Despite this diversity, most studies on the contribution of landslides to denudation have often been limited to counting rapid landslides, ignoring the contribution of slow moving landslides (SML).
However, in the Himalayas, recent studies by the proposing team and other scientific groups suggest that the latter contribution could be very significant. Those studies indicate the existence of numerous hillslopes slowly moving at speeds of a few centimetres to metres per year, with a significant associated erosive flux. In this project, we hypothesise that these slow moving landslides are a major contributor to the denudation of Himalayan slopes.
To test this hypothesis and identify the respective roles of the various forcings on these dynamics, the SLIDE project, which brings together 3 laboratories (ECGS in Luxembourg, CRPG and ISTerre in France) with complementary expertise, proposes to tackle the problem in three stages: WP1. Analysing and understanding the detailed dynamics of a few selected slow moving landslides; WP2. On the basis of this understanding, producing a large-scale map of all the slow moving landslides in central Nepal and the rates of deformation; WP3. Integrating these velocities at depth using models, in order to produce an average flux of material exported from hillslope to rivers.
In detail, it is planned to:
WP1: Measure and understand the dynamics of a selection of slow moving landslides, using : (a) Satellite methods by merging information from correlation of optical and radar images, digital surface models and InSAR time-series measurements (L-, C- and X-band radar images) to ensure high temporal and spatial resolution in 3D; (b) In-situ seismic and GNSS data to document phases of rapid displacement and estimate landslide depth and structure, while hydroclimatic forcing will be measured simultaneously. The satellite part will be carried out by a PhD student co-supervised by the CRPG and the ECGS.
WP2: Map at large-scale the slow moving landslides using multi-temporal InSAR (MT-InSAR) and pixel tracking approaches using freely available high-frequency Sentinel-1 and Sentinel-2 images. The task involves: 1. to assess the quality of the signal obtained by comparing it with the results obtained for the sites studied in WP1 with maximum precision; 2. to implement, with the support of a post-doc at ECGS and co-supervised by ISTerre, a machine learning approach to detect and map slow moving landslides based on interferograms and morphometry.
WP3 : Calculate the average erosion flux associated with slow moving landslides: 1. by integrating over depth using the 3D surface velocity measured remotely and/or scaling laws; 2. by integrating over time the accelerated phases due to extreme events using a numerical model to identify the sensitivity of deformation to hydroclimatic or seismic forcing. The modelling will be carried out by a post-doc co-supervised by the CRPG and the LMV.

If the project's hypothesis is verified, it will represent a significant paradigm shift in our vision of mountain slope erosion. The project will also have a societal contribution by improving our knowledge of the exposure of populations and infrastructures to the hazards associated with slow moving landslides in the Himalayas.
Finally, this collaborative PRCI project will strengthen expertise and encourage its transfer between the 3 international teams. Software developments and scientific publications will be made freely available, and a special effort will be made to disseminate and train our local partners in Nepal.