An investigation into the threshold of sediment motion, in two non-uniform English gravel-bed rivers

The sediment dynamics of river systems are of significant importance from an ecological, social and engineering viewpoint and are defined by complex relationships of numerous variables. It is widely accepted amongst authors that as flow increases as does the amount and size of sediment entrained. Threshold theories surrounding this have been proposed by many authors including Hjulstrom, Van Rijn, Du Boys and Shields. The aim of this research is to investigate thresholds of motion for sediments within bi-modal riverbeds, including examining incipient motion for various particle sizes. Previous research methods which have investigated this have focused on either field methods using small sample populations from sediment traps and sediment tracers or laboratory studies (flume models). In this paper an alternative method is presented utilising repeat UAV imagery of two gravel-bed river reaches in England to identify individual sediment clasts and monitor movement. By determining whether a sediment has moved or stayed and comparing this to a maximum flow rate during the interval between surveys, derived from hydraulic modelling, a maximum local shear stress value can be established for when each grain is mobilised. Initial findings have shown that as flow increases so does the amount of sediment moved. However, the size of mobilised sediment does not necessarily increase. Sediments over a certain size (0.3 m B-Axis length) display little movement despite rising shear stress values. Shielding and exposure are also investigated to examine why these larger sediments remain stable despite increasing local shear stress values. This has shown that in non-uniform gravel-bed rivers bed armouring and sediment sorting can heavily impact transportation. Armouring of coarser sediments consolidates a bed leaving finer sediments exposed to the flow and constantly moving through the system. 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 is turn makes movement of grains more difficult. The conclusions established within this research are of significant importance to understanding the complex relationships and various parameters for sediment dynamics within these systems.