Rock avalanche induced flash flood on 07 February 2021 in Uttarakhand, India—a photogeological reconstruction of the event

A large debris flow triggered by a rock avalanche in the Raunthi glaciated valley resulted in flash floods in the Rishiganga and Dhauliganga rivers on 07 February 2021 in Uttarakhand, India. Hydel projects, houses, roads and bridges in the path of debris flow were damaged resulting in many deaths. We have used high-resolution satellite data (e.g. Pleiades, WorldView, Kompsat, Cartosat, Resourcesat, Planet) to study the source of flash floods and cause of the slope failure. Our detailed geological assessment, carried out using stereoscopic Pleiades images (50 cm), revealed rock avalanche as the main source of slope failure. The slope failure has exposed a ~197-m-high head scarp near the crown and is controlled by two sets of joints and a foliation that helped in the wedge type failure. The volume of failed mass (rock and ice) estimated by cut and fill method using digital elevation models (DEMs) is ~ 29.3 million m3. The rock and ice descended from a height of ~5474 m and then crashed onto the moraine and ice bridges present in the valley at ~3732 m after travelling ~2.9 km along a steep slope. The heat generated by friction during run out and conversion of potential energy to kinetic energy due to the crashing on valley floor melted snow and ice. The ice melt water along with enhanced snow melting due to high ambient temperature on that day got intermixed with debris and created a slush, which was mobilised as a channelised flash flood. Multi-temporal high-resolution satellite data analysis showed that the debris flow was initiated at ~10:08:45 h (IST), and it travelled with a velocity of ~10.6 m/s. The rock avalanche event lasted for ~50 min, and the crash impact created a severe air blast in the valley. The rock avalanche has also resulted in debris blocking the Raunthi gad valley. Estimated Morphological Obstruction Index (MOI) and Hydro-morphological Dam Stability Index (HDSI) indicate the debris dam to be in an unstable domain.