Sounding out the river: Seismic and hydroacoustic monitoring of bedload transport

Abstract

Seismological observations provide a non-invasive and continuous means for indirectly measuring fluvial bedload transport. A significant challenge remains in independently characterising the seismic signature of bedload transport from other sources such as turbulence. We present a unique dataset from an alluvial Scottish river, combining seismic data and hydroacoustic measurements, to analyse bedload transport during three high-flow events occurring within the same year. By studying three successive events, we assess the consistency of bedload transport thresholds in response to changing flow conditions and explore the presence of hysteresis in seismic data versus water level as an indicator of coarse bedload transport. Through the use of hydroacoustic data to independently characterise bedload transport, our findings reveal that bedload transport occurred during all three events but that the threshold for entrainment varied. These entrainment thresholds were influenced by antecedent events, with a drop of 15%–20% of the threshold flow depth following the largest of the three events. In agreement with recent studies, we also found that hysteresis in the seismic versus water level data is not sufficient for identifying and analysing bedload transport: Distinct hysteresis was only observed during the largest of the three events despite all events experiencing bedload transport as observed through the independent hydroacoustic data. Our work shows the value in combining independent datasets for long-term monitoring of bedload transport to understand the evolution in the thresholds of bedload motion, providing crucial information for effective river and land-use management in a changing climate with potentially impacted high-flow events.