Assessment of plot scale sediment transport on young moraines in the Swiss Alps using a fluorescent sand tracer

Abstract. Glacial retreat uncovers large bodies of unconsolidated sediment that are prone to erosion. However, our knowledge of over-land flow (OF) generation and sediment transport on moraines that have recently become ice-free is still limited. To investi-gate how surface characteristics affect OF and sediment transport on young moraines, we installed five bounded runoff plots on two moraines of different ages in a proglacial area of the Swiss Alps. On each plot we conducted three sprinkling experiments to determine OF characteristics (i.e., total OF, peak OF flow rate) and measured sediment transport (peak tur-bidity, sediment concentrations, and total sediment yield). To determine sediment transport distances and to visualize where sediment transport takes place, we used a fluorescent sand tracer with an afterglow, together with UV and LED lamps and a high-resolution camera. The results highlight the ability of this field setup to detect sand movement, even for individual fluorescent sand particles (300–500 µm grain size), and to distinguish between the two main mechanisms of sediment transport: OF-driven erosion and splash erosion. The higher rock cover on the younger moraine with higher surface connec-tivity resulted in longer sediment transport distances and a higher sediment yield. In contrast, the higher vegetation cover on the older moraine promoted infiltration and reduced the length of the sediment transport pathways. The study, thus, demonstrates the potential of the use of fluorescent sand with an afterglow to determine sediment transport pathways, and that these observations can help to improve our understanding of OF and sediment transport processes on complex natural hillslopes.