Sediment Entrainment In Two Large Upland Gravel Bed Rivers

Remers, Amelia

Abstract

The sediment dynamics of river systems are of importance from an ecological, social and engineering viewpoint and are defined by complex relationships of various variables. Numerous threshold theories (such as Bagnolds stream power equation, Hjulstrom’s curve and Shield’s shear stress formula) for sediment entrainment have been proposed considering grain size, shear stress and shielding factors as influencing parameters. The aim of this research is to investigate thresholds of motion for sediments within two large upland gravel bed rivers, including examining incipient motion for various particle sizes. In this dissertation an alternative method to traditional tracer, sediment traps and flume studies for estimating entrainment is presented utilising repeat UAV (unmanned aerial vehicles) imagery to identify individual sediments and monitor movement over an 18-month period. By determining whether a particle has moved or stayed at a point and comparing this to a maximum flow rate during the interval between surveys a maximum local shear stress value can be established for when each grain is mobilised. Using this approach it was possible to establish a threshold for mobilisation for a sample of approximately 10000 particles. Results have shown that as flow increases so does the amount of sediment moved. However, sediments over a certain size (0.3 m B-Axis length) display little movement despite rising shear stress values. Shielding and exposure are shown to have a substantial impact upon entrainment demonstrating that in non-uniform gravel-bed rivers bed armouring and sediment sorting can heavily impact transportation. Furthermore, this bi-modal sediment impacts roughness within the channel. A smooth channel with cohesive material decreases roughness, whereas in a non-uniform channel with larger grains roughness increases; this makes movement of grains more difficult. The conclusions established within this research are of importance to understanding the complex relationships between shear stress, hiding/exposure factors and grain size for entrainment within river systems.

• Thesis Type: Thesis
• Deposit Date: Jul 21, 2023
• Publicly Available Date: Sep 30, 2025
• Award Date: Sep 29, 2023