Seismic characterization of bedload temporal variability in a proglacial Alpine stream

In mountainous areas, an increasing sediment delivery from glaciers to the channel network has been argued due to the ongoing global warming. However, quantitative estimations of sediment transport in such harsh environments are particularly challenging. A growing number of studies investigate the use of seismic techniques to perform indirect measurements of bedload transport. Seismic methods are attractive, as they can provide continuous recordings without the need of operators. Hydraulic structures equipped with geophone plates are established methods to monitor bedload in mountain rivers, but have the drawback of being expensive to install and to maintain. Seismometers installed along the channel banks have been successfully tested, but they are quite expensive too. Here, we present the application of a low-cost and easy-to-install geophone network to investigate the temporal variability of bedload transport at the snout of an Alpine debris-covered glacier.

Since 2017, we have been monitoring the upper sector of the Sulden/Solda river basin (South Tyrol, Eastern Italian Alps). The upper Sulden basin ranges in elevation between 2225 and 3905 m a.s.l., has a glacier extent of about 7 km² and is characterized by extended glacier forefields feeding the channel network with sediments. During the summer seasons, we deployed three single-component geophones (4.5 Hz) along the proglacial stream draining the Western Sulden glacier, which is heavily debris-covered. The geophones were installed at a distance of few meters from the channel, immediately downstream of the glacier snout. In 2018 and 2019, we performed monthly sampling campaigns of bedload by portable “Bunte” samplers to calibrate the seismic information. Water stage was measured using a submersible pressure transducer and pictures of the monitored area were taken every hour by an automatic camera. Meteorological data were measured at an automatic weather station located at 2825 m a.s.l., operated by the local hydrographic office. All of these complementary data were used to validate the analysis of the seismic signals.

Here, we analyze geophone data collected in the upper Sulden from 2018 to 2020 and we compare the time-frequency seismic information with air temperature and water discharge. Results show how (i) an array of single component geophones installed close to the flow path can detect both daily and longer period bedload fluctuations; (ii) geophone signal mirrors well the daily melt flow cycles, whereas its relationship with flow rate at a monthly scale varies positively, suggesting that bedload supply progressively increases during the season; (iii) there is a strong control exerted by air temperature on bedload transport, as the seismic energy reach maximum values during warm periods, while large variations of bedload rates cannot be explained in terms of differences in water discharge alone. Field evidence and direct bedload sampling campaigns performed after a glacier front collapse (August 2018) and after a small flood event (26 July 2019) confirmed such conclusions. These results prove how seismic techniques can provide precious insights into the dynamics of bedload export from Alpine glaciers.