Characterisation and Assessment of a Flash Flood in the Himalaya: Understanding the Significance of High Magnitude Events in Sediment Mobilisation

Abstract

An increase in the frequency of high magnitude events (climatological, geomorphic and meteorological) has been observed in the recent times. The observation has been closely linked to anthropogenic activities and climate change. The events have been studied on the basis of their trends, frequency, geographical distribution and impacts. A flash flood from 2018 in a small catchment in Uttarakhand, North West Himalayas is reported here. The event has been characterized based on the possible causes and observed effects. The runoff generation tendency of major subcatchments within the study area was assessed and the area was mapped based on sediment connectivity. The hydrological characteristics of the event was calculated using Manning's equations. The event generated a peak discharge of 9953 m³/s flowing at 15 m/s with a stream power of 6.4 MW. Based on the hydrological assessment, the thresholds of bedload movement, sediment transport and redistribution was analysed. The peak flow had a capacity of mobilising boulders of 6m diameter. Putting together the field and modelled data, the difference in the nature of the of the topography was analysed that caused only a particular sub-catchment to respond to the same climatic forcing significantly more than the others and also highlighted the significance of such high magnitude stochastic events in controlling the rates of long term geomorphic processes that shape the landscape.