Investigation of landslides triggered by unprecedented rainfall events during 2023 monsoon in Himachal Pradesh, India

Himachal Pradesh, a northern state of India, experienced unprecedented heavy rainfall between July and August 2023 that triggered several devastating and fatal landslides across the state. This paper presents the findings of two key studies that aim to further our understanding of the failure trends and mechanisms that occurred during the 2023 monsoon season. Firstly, the systematic mapping of the landslides using satellite imagery and field mapping techniques to identify trends in the landslide characteristics was conducted. Approximately 6500 landslides were mapped that were triggered during the 2023 monsoon season across an area of approximately 3100 km2 in the Mandi and Kullu Districts. The key failure mechanisms for these landslides are presented and analysed. It was found that the most critical combination for slope failures in the Himachal Pradesh region consists of sandstone lithology. Slides were the most common observed types of failures. Additionally, the rainfall anomaly index clearly highlighted the change in the trend of rainfall in the mountainous region indicating the evident impact of climate change. Secondly, a coupled hydro-mechanical finite element analysis to investigate the detailed mechanisms of failure for an exemplary slope was performed. The numerical modelling revealed that the progression of a wetting front brought about by continued rainfall infiltration caused a reduction in the soil suction and the resulting loss of the shear strength of the soil forming the slope along the potential slip surfaces. The studies presented in this paper significantly contribute to knowledge of rainfall-triggered landslide mechanisms in Himachal Pradesh.