Construction, destruction and fan-river coupling dynamics during two consecutive debris flow events in an arid fluvial system

This study investigates the shifts in fan-river coupling during two major consecutive debris flow events (March 2015 and May 2017) on tributary-junction alluvial fans situated in the Atacama Desert in the Andes. Studying the geomorphological consequences of debris flow events in these fans provides a unique opportunity to understand the mechanisms of fan formation and destruction in arid landscapes. To do so, we integrate repeated high-resolution topography of two end-member fans representative of fans built in confined fluvial settings in the Andes, along with detailed geomorphological and sedimentological mapping.

Here, we identify how fan-river interactions shift after flood events, influenced by: (i) sediment volumes supplied and buffered in the fan, (ii) changes in feeder channel gradient and (iii) the previous position of the main river channel in the trunk valley. The primary geomorphic consequences observed in the fans include deep incisions and entrenchment of the abandoned alluvial surfaces by the feeder channels, leading to the formation of new lobes at the fan toes. In many cases, debris flow partially block the main river, diverting the main channel. We interpret that the sedimentological differences within and between fans during both debris flow events result from varying water-to-sediment ratios, which are influenced by the sediment storage characteristics of the catchments (e.g., net sediment storage, internal buffers in drainage networks, transport capacity).

Our findings provide valuable insights into the mechanisms responsible for the formation of tributary-junction alluvial fans and their interactions with axial valleys in arid, confined valley settings. This approach can be applied to tributary-junction fans elsewhere, contributing to sediment budget estimates, downstream sediment propagation signals and the development of hazard mitigation strategies in regions expected to experience an increase in the frequency and intensity of rainstorm events.