Anatomy of an Alpine Bedload Transport Event: A Watershed‐Scale Seismic‐Network Perspective.

The way Alpine rivers mobilize, convey and store coarse material during high‐magnitude events is poorly understood, notably because it is difficult to obtain measurements of bedload transport at the watershed scale. Seismic sensor data, evaluated with appropriate seismic physical models, can provide that missing link by yielding time‐varying estimates of bedload transport albeit with non‐negligible uncertainty. Low cost and ease of installation allow for networks of sensors to be deployed, providing continuous, watershed‐scale insights into bedload transport dynamics. Here, we deploy a network of 24 seismic sensors to estimate coarse material fluxes in a 13.4 km2 Alpine watershed during a high‐magnitude transport event. First, we benchmark the seismic inversion routine with an independent time‐series of bedload transport obtained with a calibrated acoustic system. Then, we apply the procedure to the other seismic sensors across the watershed. Propagation velocities derived from cross‐correlation analysis between spatially consecutive bedload transport time‐series were too high with respect to typical bedload transport velocity suggesting that a faster‐moving water wave (re‐)mobilizes local coarse material. Spatially distributed estimates of bedload transport reveal a relative inefficiency of Alpine watersheds in evacuating coarse material, even during a relatively infrequent high‐magnitude bedload transport event. Significant inputs estimated for some tributaries were rapidly attenuated as the main river crossed less hydraulically efficient reaches. Only a small proportion of the total amount of material mobilized in the watershed was exported at the outlet. Multiple periods of competent flows are likely necessary to evacuate coarse material mobilized throughout the watershed during individual bedload transport events. Plain Language Summary: By driving erosion and deposition, bedload transport is a serious challenge for Alpine watershed management. Yet, the way Alpine rivers mobilize, convey, and store coarse material during high‐magnitude events is poorly known, notably due to the difficulty of measuring bedload transport in different locations at the watershed scale. In this contribution, we use a network of 24 seismic sensors to capture the motion of coarse material in a 13.4 km2 Alpine watershed during a high‐magnitude bedload transport event. Collected bedload transport estimates revealed a relative inefficiency of Alpine watersheds in evacuating coarse material, even during a relatively high‐magnitude bedload transport event. Large inputs estimated in some tributaries rapidly deposited as the flow crossed less transport‐efficient reaches, and only a comparatively negligible proportion of the total amount of material mobilized in the watershed was exported at the outlet. Multiple periods of high streamflow are likely necessary to evacuate the coarse material mobilized throughout the watershed during individual bedload transport events. This data set increases knowledge of coarse material motion within Alpine watersheds during high‐magnitude bedload transport events, and may help to improve predictions of bedload transport in the future through a better constraint on changing sediment availability in time and space. Key Points: For the first time, a high‐magnitude bedload transport event was tracked at the watershed scale by a network of 24 seismic sensorsA comparatively low proportion of the material mobilized in the watershed during the event was exported at the outlet (2.5%)Multiple periods of competent flows are likely necessary to evacuate coarse material mobilized during individual bedload transport events [ABSTRACT FROM AUTHOR]