Anatomy of an Alpine bedload transport event: a watershed-scale
seismic-network perspective
Abstract
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 absolute time-series
of bedload transport. Low cost and ease of installation allows 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 capture the motion of coarse
material in a 13.4 km2 Alpine watershed during a
high-magnitude bedload transport event. First, we benchmark the seismic
inversion routine with an independent time-series obtained with a
calibrated acoustic system. Then, we apply the procedure to the other
seismic sensors across the watershed. Spatially-distributed time-series
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 measured for
some tributaries were rapidly attenuated as the main river crossed less
hydraulically-efficient reaches, and only a comparatively negligible
proportion of the total amount of material mobilized in the watershed
was exported at the outlet. Cross-correlation analysis of the
time-series suggests that a faster moving water wave (re-)mobilizes
local material and bedload is expected to move slower, and over shorter
distances. Multiple periods of competent flows are likely to be
necessary to evacuate the coarse material produced throughout the
watershed during individual source-mobilizing bedload transport events.