Preliminary assessment of the Shiv Bawdi landslide in Shimla, Himachal Pradesh, India

Rainfall-induced landslide (RFIL) disasters have become a major concern within the Indian Himalayan region. On August 14, 2023, at around 7:20 AM (India Standard Time), an excessive rainfall event triggered a landslide in the Summer Hill area of Shimla, Himachal Pradesh, India, named locally as the Shiv Bawdi landslide. The abrupt occurrence of this landslide claimed 20 lives and caused damage to the Upper and Lower Summer Hill Roads and the Kalka-Shimla Railway track. Therefore, a preliminary analysis was carried out to understand the landslide characteristics and possible failure mechanism. This includes field investigation, Interferometric Synthetic Aperture Radar (InSAR) analysis, unmanned aerial vehicle (UAV)-based mapping, and pore water pressure (PWP) simulations. The obtained results indicate that the landslide affected an area of approximately 7.52 \(\times\) $\times$ 10³ m² and had a volume of about 3.71 \(\times\) $\times$ 10⁴ m³. Similarly, the estimated maximum velocity of this landslide was approximately 9.27 m/s, with a maximum momentum of 3.44 \(\times\) $\times$ 10⁵ kg m/s. The time-series InSAR analysis also revealed pre-failure deformation signatures at the landslide site. Further, using the recorded rainfall, transient seepage analysis was performed to assess the pre-failure PWP condition. The analysis suggests that continuous antecedent rainfall resulted in the formation of positive PWP, leading to the development of localized perched aquifers by saturating the slope material. Moreover, the high-intensity rainfall on the day of failure ultimately triggered the landslide in the form of a debris flow.