Preliminary Assessment: 2021 Debris Flow Impact on River Incision and Land Subsidence in Joshimath Town, Garhwal Himalayas, India
Authors: Lakhera Shobhana , Jaboyedoff Michel , Derron Marc-Henri , Goswami Ajanta , Kumar Maletha Anil 
Published: February 2023

Joshimath lies in the Chamoli district of Indian Himalayas, at an elevation of around 2000 m. It is reportedly located on old landslide debris with a very high slope of 60&#186;, which marginally stabilized during the geologic past. Positioned in the vicinity of active Vaikrita thrust, Joshimath has been intermittently sinking for quite many decades and records incidences of slope failures even during the dry seasons (Valdiya et al., 2014). The lithology of the area is characterized by banded gneiss with interbedded schists and localized calc-silicate gneiss lenses with plunging folds and many minor faults (Valdiya et al., 1983). Topographically it is influenced by long-term river incision with Dhauliganga and Alaknanda, sharply incising the toes of the hill slopes in Joshimath, draining along NW direction, and cutting sharply around the curvatures mainly near Vishnuprayag and Maarwadi village. The 2021 debris flow and 2013 flash floods, resulted in noticeable increase in river undercutting and sediment load with changes in the river flow regimes resulting from deposition of mid-channel and side-channel bars, from the flow sediments and landslides along the toe region, respectively. Thus, this increase in fluvial undercutting may cause the large-scale destabilization of mountain slopes and induce deep-seated gravitational slope deformation (DGSD) that can eventually lead to catastrophic failures (Tsou et al., 2015).A study on the impacts of 2013 flash flood by the Geological Survey of India (GSI) revealed, reactivated and newly activated landslides mainly occurred, either in river terraces or in slope debris material on steep slopes. The main causative factor responsible for triggering these landslides was toe-erosion by the river, accompanied by increased overburden due to heavy rainfall, during the 2013 flash flood (Sharma et al., 2014). Similarly, reactivated landslides can be spotted on google earth images post 2021 event, these are in and around the Joshimath town at the hill toe, thus implying high incision. Currently, land subsidence is being reported in almost 561 buildings across the Joshimath town, with severe cracks along with some muck water seepage. Many news reports have pointed on the breached and blocked natural drainage systems, from rapid urbanization which may be a reason for water seepage in the subsiding buildings. The maximum damage has been so far reported at the JP colony which lies in the rim area of the hill slope, thus being more susceptible to undercutting (Tsou et al., 2015). Many speculations on probable reasons, from the impact of tunneling for the Tapovan-Vishnugad project, blasting and mountain cutting for road construction, increased urbanization, improper drainage, and increased toe erosion have been listed. In this study, we try to analyze if any, the impacts of increased toe-incision induced by the 2021 flash floods, using geological and remote sensing techniques under the changing climate regimes, on the subsidence in Joshimath town.&#160;Keywords: Deep-Seated Gravitational Slope Deformation (DGSD), Subsidence, Incision, Undercutting, Landslides