Deterministic approach for susceptibility assessment of shallow debris slide in the Darjeeling Himalayas, India

• The present study assesses slope stability in three different scenarios.
• Stream head areas were identified as unstable zones in SHALSTAB.
• Vegetation root cohesion plays a significant role in slope stability.
• Probability of slope failure is a function of topography, soil and land use properties.
• SHALSTAB could explain up to 64.50% spatial variability of slope stability.

High magnitude rainfall triggers numerous shallow debris slides in the Darjeeling Himalayas causing widespread damage to the environment, loss of life and property. Thin soil cover and steep topography make the region vulnerable to debris slides. The objective of the present study is to assess the susceptibility of the eastern part of Darjeeling Himalayas (covering about 330 km2) to shallow debris slides through the functional relationship of hillslope hydrology and mechanical properties of slope materials. Deterministic approach-based shallow landsliding stability (SHALSTAB) model following Mohr–Coulomb failure law was adopted to assess landslide susceptibility. Topographical parameters were derived from 8-m resolution Cartosat-1 digital elevation model (DEM) and mechanical properties of soil were obtained from an analysis of 15 soil samples. For slope stability assessment, the topographical and soil parameters were put into three different scenarios — (i) assuming the surface entirely free from vegetation (Model-1), (ii) involving the role of vegetation root cohesion (Model-2) and (iii) surcharge of vegetation, buildings and other structures along with root cohesion (Model-3). These predictive models were used to classify the area into various susceptibility classes with specific amounts of critical rainfall (Qc). The result shows that 28%, 9% and 10% of the study area come under unconditionally unstable class in the three models, respectively. About 22% land in Model-1 and 42% in each Model-2 and Model-3 come under unconditionally stable class. Protective capacity of roots against debris slide played a significant role in Model-2 and Model-3. Performance of models was validated by comparison of observed–predicted landslide areas and the area under the receiver operating characteristic (ROC) curve. It is found that the overall success rate of all the three models is relatively low (56.60% to 64.50%). Thus, it may be concluded that the SHALSTAB model in assessing landslide should either not be used at all at a regional level in the Himalayas or be used only with great caution along with additional field and lab data.

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