Dynamic risk from sediment cascades in the Indian Himalaya

Sediment cascades from the high mountains of the Himalaya are initiated in steep glaciated and fluvial landscapes and transfer downstream through alluvial and bedrock reaches of the river network before exiting at the mountain front. Understanding how the stochastic triggers for processes such as landslides, GLOFS and ‘cloudbursts’ translate into downstream hazards such as sediment-rich floods underpins the changing risk profile for communities in these settings. In a collaboration between the UK Natural Environment Research Council (NERC) and the Indian Ministry of Earth Sciences we analyse the downstream translation of high magnitude sediment transport processes in the Ganga catchment of Uttarakhand. A time series of fast-moving shallow, and slower-moving deep landslides are being mapped through automated remote sensing and field-based monitoring. These are then compared to the distribution of wide alluvial reaches of the channel network where potential ‘sediment bombs’ are accumulating. These accumulations of sediment are mapped using high resolution digital topography and their thicknesses measured using seismic nodes. Based on our understanding of how the locations of ‘sediment bombs’ link to potential landslide sediment sources and/or damming effects, we will then explore triggering mechanisms that translate this material downstream as devastating debris and sediment-rich flows; these will be based on physics-based models for landslide and debris flows (LaharFlow) and sediment-rich flood discharges (Caesar Lisflood). Through the analysis of case studies such as the 2013 Alaknanda floods, and model scenarios, we intend to work with local disaster management authorities in developing evolving hazard forecasts ahead of each monsoon. These forecasts of the changing dynamic risk from year to year will aid in the targeted monitoring of upstream processes.