Connectivity between hillslope and channel sediment stores and links to cascading hazards in the Indian Himalaya

Flood frequency and magnitude in the Himalaya are projected to increase over the next century due to accelerating glacier melt and more intense monsoons. Most research has focused on the upstream triggers of floods, such as glacial lake outbursts, avalanches, or intense rainfall. Entrainment of sediment stored within mountain valleys as they travel downstream can transform these floods into debris or hyperconcentrated flows, increasing both their travel distances and downstream impacts. This hazard cascade means the risk to downstream communities is conditioned not just by the trigger events but also the distribution of sediment fill along the flow path. However, controls on the spatial and temporal dynamics of sediment accumulation in mountain valleys are poorly constrained due to a lack of systematic, large-scale datasets that capture both channel sediment and landsliding in steep mountain catchments. Inventories of slow moving landslides, which can also supply sediment to channels, are even more scarce. Therefore, our understanding of how shallow and slow landslides contribute to stores of channel sediment in these catchments is limited.

Here, we apply (semi-) automated approaches to generate inventories of channel and hillslope sediment stores for the Alaknanda Basin systematically through time, using a combination of deep learning and remote sensing data. Using these inventories, we develop a framework that explores connectivity between hillslope and channel sediment. We assess whether certain topographic and/or channel characteristics control the spatial and temporal dynamics of sediment supply and fill. We will use these findings to gain a better understanding of the role sediment stores in mountain landscapes that are highly susceptible to sediment-rich flood events.