Hydrological Characteristics of 7th February 2021 Rishi Ganga Flood: Implication towards Understanding Flood Hazards in Higher Himalaya

A flash flood that originated from Raunthi Gad-a tributary of the Rishi Ganga river, in Garhwal Himalaya, caused unprecedented loss to lives and damaged two hydropower projects on 7th Februray 2021. In order to asses the flood magnitude, the flow parameters of the flood were calculated using the super-elevation of the flood marks preserved in the flood affected valleys. The textural characteristics of the flood deposits in the upper reaches of the valleys indicate dominance of debris flows. The peak discharge upstream of the confluence of Rishi Ganga and Dhauli Ganga was around 1.1×105 m3/s, which was four order of magnitiude higher than the normal peak discharge (∼ 3 m3/s). The flow achieved a velocity of 30±3 m/s. An exponential reduction in the flow velocity (from ∼37 to 2 m/s) with distance is observed. For which the river gradient and increase in sediment load is implied flow that along its entrained way downstream between Raini and Tapovan. Considering the sensitivity of paraglacial zones to climate change, the paper calls for detailed studies pertaining to the response of paraglacial zones to extreme weather events. Importantly, it is necessary to have more hydrological data covering multiple valleys for predictive model simulation of the nature and magnitude of such disasters in future.