Seismic and hydroacoustic monitoring of bedload transport in an alluvial river

Abstract

Monitoring the mobilisation and transport of bedload in rivers is key to understanding landscape evolution and sediment transfer, as well as providing valuable information for problems in the fields of ecology, river and landuse management, and civil engineering. When rivers transport bedload their erosive capacity increases which not only affects infrastructure such as rivers and dams, but it also influences channel change by eroding or aggrading bars and banks altering the channel capacity and subsequently flood risk. Bedload transport has traditionally been measured through the use of geomorphic measurements, such as manual sediment sampling, geomorphic mapping, and empirical equations. However, advances in measurement methods and technologies such as drone surveys, numerical modelling, and acoustic monitoring have been at the forefront of bedload transport studies over the last couple of decades. These methods extend the spatial and temporal resolution of monitoring efforts, allowing better understanding of the characteristics of sediment mobilisation and the effects of bedload transport over longer timescales. In particular, seismic monitoring has emerged as a valuable tool for monitoring river processes such as propagating flood waves and the movement of bedload. This provides an opportunity to indirectly monitor river processes over greater spatial and temporal scales to previous methods. A significant challenge remains in independently characterising the seismic signature of bedload transport from other sources such as turbulence. Additionally, there remains some uncertainty in the interpretation of seismic bedload transport signals in complex alluvial channels. This thesis examines seismic signals recorded adjacent to an alluvial mountain river in Scotland (the River Feshie) and presents a unique dataset combining three-component seismic data with complementary hydroacoustic measurements, to analyse bedload transport during high river flows.

In the first part of this thesis, I examine the characteristics of the recorded river-induced seismic signals during high river discharge events. Using data from eight seismic sensors located along the study reach along with local stream gauge data, I compare the site and event variabilities and find that there is greater variability in the signals recorded at different sites than there are over different hydrological events. Additionally, I observe that the sensor locations have a significant effect on the signals recorded with those on flatter ground and/or closer to the river, located in open areas, recording clearer and stronger seismic signals with more distinct bedload transport signals. This can inform on siting of seismic sensors in future seismic studies.

Through the analysis of seismic signals recorded over high flow events combined with independent interpretation of the occurrence of bedload mobilisation through hydroacoustic measurements, I then assess the consistency of bedload transport thresholds in response to changing flow conditions. Results indicate that bedload entrainment thresholds vary from event to event and may show a link with event duration and magnitude, as well as being influenced by the occurrence of successive events due to armouring effects. This analysis also revealed that the relationship between bedload-induced seismic power and water level varied between events with events exceeding ∼ 1.40 m water level exhibiting distinct variations in this relationship between the rising and falling limb of the hydrograph with increased seismic power prolonged on the falling limb. Previous studies have used these hysteretic relationships as an indicator of bedload transport; however, my work reveals that bedload transport can occur without these relationships present in the data.

In the third part of this thesis, I explore the directional-component dominance of the seismic signals recorded adjacent to the river to interpret whether river-induced seismic signals, and specifically bedload signals, exhibit stronger signals in a single direction. Analysis of the three orthogonal components of the recorded seismic signals revealed that river-induced signals are strongest in the two horizontal directions, consistent with Love-wave propagation. This suggests that existing fluvial seismic models and studies may be incorrectly using the Rayleigh wave interpretation under the assumption that the dominant component of bedload-induced seismic signals is in the vertical direction. Additionally, the signals from the movement of bedload particles generally induce a stronger stream-parallel signal as a result of downstream particle motion.

This work adds to the growing body of research using indirect bedload transport monitoring methods, such as seismic and hydroacoustic measurements, to understand complex fluvial dynamics in alluvial rivers. By advancing the use of seismic monitoring, it improves understanding of sediment movement and its effects on river dynamics. Using a unique dataset from the River Feshie in Scotland, this research reveals: (1) the importance of sensor placement for clear seismic signals produced by bedload transport; (2) variability in bedload entrainment thresholds, which are influenced by event duration and magnitude, as well as the occurrence of successive events; and (3) a strong horizontal directional component to the seismic signals generated by bedload transport, challenging assumptions about vertical dominance. These findings demonstrate the value of combining independent datasets for longt-erm monitoring of bedload transport, offering insights into the spatial and temporal evolution of sediment mobilisation. This provides crucial information for effective river and land-use management, particularly as climate change alters the frequency and intensity of high-flow events.