Bedload Fluxes in a Glacier‐Fed River at Multiple Temporal Scales. Issue 10 (29th September 2022) - Coviello, Velio
Abstract: In mountain rivers, long‐term observations of water and sediment fluxes are crucial for understanding the dynamics of bedload fluctuations. We analyze 7 years of continuous data gathered from eight geophone plates at a monitoring station in the glacier‐fed Sulden/Solda River (South Tyrol, Italy) to estimate the bedload flux at 1‐min scale. Sixty‐five bedload samples were used to derive the calibration equations adopted to quantify the transported bedload mass. The signal power is proposed as a more effective metric than threshold‐based impulses for calculating bedload masses. Results show (a) a remarkable variability of bedload rates for the same value of flow discharge, (b) the joint effect of storm‐driven flood events and seasonal changes in sediment supply on bedload rates, and (c) the strong impact of climatic factors (i.e., temperature and snow cover) on bedload fluxes. Moderate bedload rates occurring in late spring/early summer are likely related to the mobilization of riverbed sediments, while sustained bedload transport during melt flows in July–August—corresponding to the effective bedload discharge range—is associated with the activation of glacial and proglacial sediment sources. The data set shows a complex climatic control on bedload transport at the basin scale, where precipitation, air temperature, and snow cover determine flow and glacier melting dynamics. These findings suggest how the effects of climate change in the Alps likely will lead to an increase in peak bedload rates in a context of declining annual bedload yields as melt flows will progressively reduce in the next decades. Plain Language Summary: Water flowing in mountain rivers can transport a large amount of bedload, which is the coarse sediments that jump and roll on the river bed. The knowledge of the bedload flux matters for both improved understanding of the ecosystem and of the effects of human activities. Measuring how much bedload is transported is difficult as particles move below the water level. In this study, we employ seismic sensors installed on the river bed to quantify bedload transported by an Alpine river originating from a glacierized area. Moving particles produce ground vibrations that are more intense when the river transports more material. We found that the intensity of bedload measured by this "seismic noise" is extremely variable during the year and controlled by weather conditions. In particular, the river can rapidly increase the transport of sediments during high runoff peaks induced by glacier melt and summer thunderstorms. These findings are particularly relevant in the context of ongoing global warming that is dramatically impacting mountain areas where newly exposed stream reaches are emerging from the rapidly melting glaciers. Key Points: The power of the geophone‐plate seismic signal better reflects the variability of bedload rate than the number of threshold‐based impulses Highest bedload rates (>10 3 kg m −1 min −1 ) produced by summer rainstorm events occur during snow‐ and glacier‐melt periods Temperature‐controlled glacier melt flows are most effective in transporting bedload, followed by rainfall‐driven 1.5–2 years floods