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The paper describes the recent devastating landslide of Kotrupi, Himachal Pradesh in India which had claimed many human lives and properties along its course on 13 August 2017. This event was the third reactivation of the slide as per data available. The study for this landslide involves a preliminary investigation of the site and comparison of pre- and post-event slope stability analysis. Other than rainfall, the combined effect of differential weathering rate of the debris and soil layers, low material strength, and the presence of a fault zone was found to be the primary cause of instability. The analysis was carried out with the geotechnical investigation and numerical modeling with the help of finite element model-based Phase 2 simulator. The significant findings of this paper include numerical simulation of pre- and post-event slide as stated under unsaturated and saturated condition. The study finds a low factor of safety of the present slope. The unstable condition of the slope may trigger another event of the same intensity shortly. [ABSTRACT FROM AUTHOR]

Landslide risk assessment is based on spatially integrating landslide hazard with exposed elements-at-risk to determine their vulnerability and to express the expected direct and indirect losses. There are three components that are relevant for expressing landslide hazard: spatial, temporal, and magnitude probabilities. At a medium-scale analysis, this is often done by first deriving a landslide susceptibility map, and to determine the three types of probabilities on the basis of landslide inventories linked to particular triggering events. The determination of spatial, temporal, and magnitude probabilities depend mainly on the availability of sufficiently complete historical records of past landslides, which in general are rare in most countries (e.g., India, etc.). In this paper, we presented an approach to use available historical information on landslide inventories for landslide hazard and risk analysis on a medium scale (1:25,000) in a perennially typical data-scarce environment in Darjeeling Himalayas (India). We demonstrate how the incompleteness in the resulting landslide database influences the various components in the calculation of specific risk of elements-at-risk (e.g., buildings, population, roads, etc.). We incorporate the uncertainties involved in the risk estimation and illustrate the range of expected losses in the form of maximum and minimum loss curves. The study demonstrates that even in data-scarce environments, quantitative landslide risk assessment is a viable option, as long as the uncertainties involved are expressed. [ABSTRACT FROM AUTHOR]

In the present study, the Sataun landslide was investigated along the National Highway-72 (NH-72) between Sirmauri Tal and Sataun towns of the Sirmaur district of Himachal Pradesh, Northwestern Himalaya, India. Geologically, the Sataun landslide is composed of Middle Siwalik sandstones, which are highly fractured and faulted due to the interaction of the active Sirmauri Tal Fault zone and the Malgi Fault at the site of the Sataun landslide. Through the geological field investigations, three unstable zones were identified, which are prone to landslides. A major failure along one of the unstable zones (Sataun landslide, Zone 1) occurred on the 9th of October 2019. The investigation of the Sataun landslide was carried out using geological, geomorphological, geotechnical, and kinematic analysis. A pre-landslide topography was used to analyse the slope stability using FEM (finite element modelling) technique. Geomorphological studies and field investigations suggest the presence of an active fault zone (Sirmauri Tal Fault zone) passing through the Sataun landslide zone. Grain size analysis demonstrates that the soil is non-cohesive poorly graded sandy soil and geotechnical parameters indicate low cohesion (0.17 kPa) and low angle of internal friction (33.09°). Pre-landslide slope modelling suggests a low factor of safety (0.76). Toe cutting by the Giri River, the presence of radial, transverse cracks on the landslide body, and tension cracks on the crown propose the active and progressive nature of the landslide. Although the role of active fault is the most crucial factor for the Sataun landslide failure, rainfall played a critical triggering role in the Sataun landslide. This study shows that the unstable condition of the Sataun landslide may trigger another event of similar or greater intensity in the near future; therefore, proper monitoring is required to mitigate this landslide. [ABSTRACT FROM AUTHOR]