Interactions between aeolian dune fields and debris flows in alluvial fans.

The relationships between aeolian sediments in dune fields and adjacent sedimentary environments are critical for understanding arid landscapes. They provide valuable proxies for paleoclimatic reconstructions, as shown in various desert regions worldwide. Studies have highlighted how aeolian environments modulate sediment transport in fluvial systems, acting as buffers (e.g., East et al., 2015), an essential consideration for comprehensive source-to-sink sediment budgets. While research has primarily focused on fluvial-aeolian interactions, studies on alluvial fan-aeolian interactions are limited. Alluvial fans, when not bypassed, are excellent sedimentary archives for reconstructing paleoclimates in arid regions. It is known that increased aridity tends to expand aeolian coverage over fan surfaces, whereas increased runoff activity restricts aeolian environments to distal fan areas, which then serve as sediment sources for sand dune fields. However, there is a gap in understanding how fans and aeolian sediments interact when both operate simultaneously, independent of climatic variability. To address this, we studied alluvial fans in the Atacama Desert, where prolonged aridity provides a natural laboratory to explore interactions between aeolian and alluvial fan processes, with exceptional preservation of surface morphologies. Rare episodic storms generate runoff that transports sediments from catchments to alluvial fans, which may be partially or fully covered by aeolian sands. The selected fans exhibit debris flow lobes across all fan segments, not just at the apex.

Our study investigates how fan morphology (e.g., roughness and relief) (Cook and Pelletier, 2007) controls saltation transport processes and pathways, and examines the interactions between dune formation and debris flow lobes. By analyzing surface grain size and topography and leveraging Synthetic Aperture Radar (SAR) backscatter intensity data from C and L Bands, calibrated with field grain size distributions and laboratory analyses, we automated the mapping of fan sediment cover. Our findings reveal that aeolian covers, including barchan dunes, do not prevent debris flows from reaching mid and distal fan areas on fans with gradients of ~10°. This contrasts with observations from the southwestern US fans, where star dunes can obstruct debris flow pathways (Anderson and Anderson, 1990). The interactions we have identified are relevant for improving debris flow runout modelling, interpreting past fan sedimentary arrangements, and understanding fan evolution and sediment fluxes in arid environments. These insights have broader implications for the evolution of arid landscapes, sheeding light on the dynamic interplay between aeolian and alluvial fan processes.