LAUSR

Abstract In Alpine environment, studying the evolution of sediment mobility from hillslopes to lower landforms and channel network is a challenging task, mainly due to the complex interplay of erosion and colluvial processes, topographic features and climatic conditions. The combination of direct field surveys and digital terrain models can provide a good opportunity to accurately investigate the sediment delivery processes and improve the understanding of geomorphic sediment cascade. The main aim of this work is to analyze sediment dynamics in sediment source areas of an Alpine basin (Rio Cordon catchment, 5 km2), integrating techniques that are usually applied separately: sediment tracing with Passive Integrated Transponders (PITs) and DEM of Difference (DoD). Additionally, the relationship between rainfall characteristics and sediment source behavior was analyzed. Sediment transfer through source areas and the geomorphic evolution of those areas were analyzed by field investigations between October 2017 and September 2018. Specifically, eleven source areas with five different typologies were investigated by installing 268 tracers to analyze sediment mobility along hillslopes. The source areas and the tracers’ displacements monitoring was carried out after snowmelt (late May 2018) and periodically during Summer 2018 (July, August and September. To investigate the geomorphic changes in source areas, high-resolution DEMs (0.05 m cell) were derived with a Structure from Motion (SfM) technique in two different periods, after snowmelt in early June 2018 and in the following September; the collection of these data permitted computation of DoDs. To analyze the rainfall characteristics, five rain gauging stations were used. Rainfall analysis revealed that three different events exceeded the threshold of a two-year recurrence interval for different durations, which in turn were considered to be triggering events for each monitored period. The tracers exhibited an interesting mobility rate (37.7%) with different dynamics for source typologies, while geomorphic evolution computed with DoDs showed consistent changes mainly in debris-flow channels and debris-flow deposits. The rainfall intensity resulted as a variable controlling the sediment mobility through the source areas, while the integration of the two methodologies highlighted how mobilized tracers originate mainly from erosional surfaces. The research provides a detailed analysis of hillslope dynamics in the surveyed area, providing increased knowledge regarding the delivery processes that act on sediment sources and better interpretations of sediment mobility in Alpine environments.