Cascading rainfall-induced sediment disaster in Mandi District, Himachal Pradesh (2025): multi-hazard characterization and microstructural insights

Abstract

This study presents a detailed analysis of a multi-hazard cascading event that unfolded across Mandi district, Himachal Pradesh, India, between June and July, 2025. The sequence was initiated by extreme rainfall events which triggered widespread landslides, debris flows, hyperconcentrated flows, river channel widening, and infrastructure damage, resulting in three confirmed fatalities and one missing person. Field investigations at seven locations revealed distinct failure mechanisms ranging from dry granular slides and boulder-laden debris flows to saturated, slurry-rich runouts. The event ultimately converged in the Thunag valley, where sediment-laden flows from several gullies merged, dramatically widening channel sections and inundating nearby built-up areas. To better understand material behavior within this cascading system, debris samples from four depositional zones were analyzed using X-ray diffraction (XRD), scanning electron microscopy with energy-dispersive spectroscopy (SEM/EDS), and rheological testing. The samples were predominantly quartz-rich with varying proportions of feldspar, illite, and secondary oxides, while SEM imaging confirmed strong textural contrasts between proximal and distal deposits. Rheological measurements showed predominantly shear-thinning behavior, indicating that fine-grained, partially saturated mixtures significantly enhanced flow mobility and contributed to river damming and high-impact deposition. Overall, this event exemplifies how intense rainfall, heterogeneous sediment properties, and rapid geomorphic coupling between hillslopes and channels can produce destructive cascading hazards in the Western Himalayas. The findings underscore the urgent need for stricter regulation of construction along gullies and floodplains to reduce future risk in rapidly urbanizing Himalayan districts such as Mandi. © Springer-Verlag GmbH Germany, part of Springer Nature 2026.