Coarse sediment transfer and geomorphic changes in an alpine headwater stream

Headwater sediment dynamics is very important for fluvial processes in downstream reaches, but have been poorly studied until now, and existing sediment processes are not well understood. In fact, quantifying bedload transport and assessing river changes is challenging, in mountain environments, and traditional bedload formulas tend to overestimate sediment fluxes. This work aims to analyze the coarse sediment transfer processes acting in the high-altitude headwater reach of Rio Cordon (eastern Italian Alps). Pebbles provided by Passive Integrated Transponders (PITs) were deployed in the study site and their position was periodically monitored over 20 months, in order to quantify sediment mobility (i.e., travel distances and virtual velocities) and, the bedload volumes. The geomorphic changes due to flood events were analyzed through high resolution DEMs (0.05 m cell), derived with Structure from Motion (SfM) in three different periods (October 2017, September 2018 and June 2019), allowing to compute the DEMs of Difference (DoDs). The study period was characterized by two distinct flood events, a short and intense summer rainstorm in August 2018 (recurrence interval RI > 2 years) and a severe cyclonic rainfall event (i.e. Vaia storm, RI > 50 years) in October 2018. The combination of tracer and DoDs analysis shows that ordinary flood maintained the reach in an apparent equilibrium state, with balanced volumes of erosion and deposition. Differently, the high-magnitude flood was characterized by bedload fluxes an order of magnitude higher than ordinary event and an evident net erosion. However, in both events the volumes mobilized in the headwater did not contribute significantly to the sediment fluxes delivered at the Rio Cordon basin outlet. This highlighted the reduced efficiency in sediment propagation mainly due to a marked dis-connectivity associated with the presence of a hanging valley in the middle part of the basin. This study discusses the complex dynamism of headwater streams, stressing the importance of understanding sediment fluxes and connectivity conditions that can strongly affect human activities and infrastructures even in lowland reaches.