The Dynamics of Coarse Sediment Transfer in an Upland Bedrock River

Abstract

Bedrock channels in UK upland environments have received relatively little attention despite their importance within upland river systems and their influence on controlling the conveyance of sediment downstream. This thesis aims to quantify and model the transfer of coarse sediment through Trout Beck, an upland bedrock reach in the North Pennines, UK. The transport of coarse sediment has been quantified through field monitoring of the sediment characteristics, repeat magnetic tracer surveys and in situ bed load impact sensors. This was carried out in conjunction with surveys of channel morphology, using terrestrial laser scanning and repeat dGPS surveys and continuous flow monitoring. This has enabled sediment transport dynamics to be related to the hydraulic conditions throughout the reach.
Differences between channel types have been conceptualised using the continuum of the ‘fluvial trinities’. This model demonstrates that the interaction of sediment and channel morphology is partly disconnected in bedrock channels. Conversely, in partially alluvial and alluvial channels there are important feedbacks between sediment stored locally in the channel, channel form and sediment transport. It has been shown that bedrock, partially alluvial and alluvial sections of the river channel have a considerable and varied influence on conveyance of sediment through these types of reaches.
Sediment storage defines the partially alluvial and alluvial sections of the channel, with very little sediment storage in bedrock reaches, except in hydraulically sheltered sites. More efficient sediment transfer through bedrock channels is the result of the local hydraulics. The low resistance to flow and stable channel boundaries cause little sediment storage and a downstream conveyance of the full grain-size distribution during periods when flow is competent and sediment is supplied from external sources.
The detailed morphological survey has provided the necessary boundary conditions, along with the flow data, to apply a one-dimensional hydraulic model (HEC-RAS) of the bedrock channel. The modelling results have quantified the hydraulic regime of the channel. Furthermore, using local shear stress as a proxy for sediment transport, sediment transport potential for the dominant grain-size distribution of the reach (16-256 mm) has been assessed for different locations in the channel. There are significant differences in the critical threshold of shear stress for sediment transport down reach. Sediment which is transported through the bedrock reach will be deposited and stored, in the partially alluvial and alluvial sections of the reach, at the same flow conditions. As the flow magnitude increases above the critical threshold, the sediment transport potential increases throughout the whole channel until the conditions in the whole reach have the potential to transport sediment. The sediment transport potential in the bedrock sections of the channel is always greater than in the partially alluvial and alluvial sections of the channel.
By combining the field and modeling approaches an improved understanding of the flow thresholds and spatial variations in sediment transport, in an upland bedrock channel, has been achieved.