Flow-type controls on tributary alluvial fan formation along the Andes (18-34°S)

Abstract

Debris flows, locally known as ‘aluviones’ or ‘huaicos’, in Andean tributary alluvial fans form distinctive facies associations that can be classified into High-density Flows (HdF) and Low-Density Flows (LdF) based on sediment-to-water ratios and transitions from highly dense, viscous flows to more diluted ones. This distinction, based on ground evidence and remotely sensed debris flow identification, establishes the first equivalence between field-based and optical satellite imagery observations using Google Earth. By analysing the activity of tributary alluvial fans over the past 20 years with open-access optical satellite imagery, we provide new insights into fan evolution and the extent to which lateral sediment inputs impact sediment transfer along axial river valleys of the Andes.

Our observations reveal that large-volume debris flows, typically associated with HdF, contribute to fan expansion and aggradation. In contrast, more diluted flows -ranging from hyperconcentrated to fluvial flows-promote fan destruction through incision of the feeder channel. Fans dominated by LdF facies associations often exhibit incision and progradation, forming new lobes at the fan toe. These lobes are frequently reworked by the main channel due to the limited sediment supply. Whether tributary fan sedimentation and progradation influence the main channel or if sediments are buffered on the fans largely depends on the characteristics of the flows and on the original topography of the fan. Accordingly, classifying debris flow surges into two distinct facies associations (AF1 and AF2, corresponding to HdF and LdF, respectively) enhances our understanding of fan dynamics and their influence on axial valleys at regional scale. This study underscores the importance of sedimentological flows characteristics in governing fan evolution, influencing both fan development across a broad latitudinal range (18-34°S) and sedimentary signal propagation along the Andes Cordillera. Moreover, the findings have significant implications for national debris flow hazard mitigation efforts and aligns with global strategies outlined in the Sendai Framework for Disaster Risk Reduction, promoting resilience and improved risk management in developing regions.