Remobilisation of fine sediment from chalk stream gravel beds under flushing flows: A flume experiment

Groundwater dominated chalk streams regularly exhibit higher quantities of accumulated fine sediment (inorganic and organic particles <2 mm) within their gravel beds compared with other UK systems due to their natural flow conditions, notably low bed mobilising flows. This characteristic, in combination with their fine sediment-sensitive species, creates a high propensity for lethal/sub-lethal ecological impacts. Current approaches to management targets and targeted interventions have failed in chalk streams due to a lack of scientific knowledge underpinning them. Although research has quantified fine sediment infiltration and accumulation in chalk stream gravel beds, little is understood regarding remobilisation that leads to the ‘cleanout’ of fine sediment. To address this gap, flume experiments were carried out to investigate the remobilisation depths of fine sediment (especially cohesive sediment <62.5 μm) from the ecologically sensitive surface layer (0–10 cm) of a typical chalk stream gravel bed, across a range of flow conditions. Bed shear stresses in the flume experiments ranged from 0.6 to 8.1 Pa; increases in bed shear stress corresponded to increases in fine sediment cleanout depth. Fine sediment remaining after experiment runs indicated two processes of remobilisation important in keeping the surface layer of gravel beds clean of excessive fine sediment: flushing from the bed framework and hydraulic winnowing within the bed framework. The data were used to evaluate the validity of established models for predicting fine sediment remobilisation from gravel beds. Comparisons between observed and predicted cleanout depths demonstrated that established models tend to overpredict cleanout depths. Existing models appear unsuitable for use in chalk streams due to assumptions within these models and their failure to represent the natural characteristics of chalk stream gravel beds. The novel data generated by this study can be applied to direct revised fine sediment targets, management and restoration activities