Hydrological drivers of bedload transport in an Alpine watershed

Understanding and predicting bedload transport is an important element of watershed management. Yet, predictions of bedload remain uncertain by up to several order(s) of magnitude. In this paper, we use a five-year continuous time-series of streamflow and bedload transport monitoring in a 13.4 km² snow-dominated Alpine watershed in the Western Swiss Alps to investigate the hydrological drivers of bedload transport. Following a calibration of the bedload sensors, and a quantification of the hydraulic forcing of streamflow upon bedload, a hydrological analysis is performed to identify daily flow hydrographs influenced by different hydrological drivers: rainfall, snow-melt, and mixed rain and snow-melt events. We then quantify their respective contribution to bedload transport. Results emphasize the importance of mixed rainfall and snow-melt events, for both annual bedload volumes (77% in average) and peaks in bedload transport rate. Results further show that a non-negligible amount of bedload transport may occur during late summer and autumn storms, once the snow-melt contribution and baseflow have significantly decreased (9% of the annual volume in average). Although rainfall-driven changes in flow hydrographs are responsible for a large majority of the annual bedload volumes (86% in average), the identified melt-only events also represent a substantial contribution (14 % in average). Through a better understanding of the bedload magnitude-frequency under different hydrological conditions, the results of this study may help to improve current predictions of bedload transport, and we further discuss how bedload could evolve under a changing climate through its effects on Alpine watershed hydrology.