Deposition and erosion behaviour of cohesive sediments in the upper River Taw observatory, southwest UK: Implications for management and modelling

Fine sediment is a key driver of water quality degradation in rivers globally and improved knowledge of its source and transport is required to assess the efficacy of sediment control options and pollution abatement strategies. Here the transport characteristics (deposition, erosion and flocculation) of fine sediment were studied experimentally in an annular flume to provide parameters necessary to model fine sediment transport in the upper River Taw in southwest England. Deposition experiments were carried out using a sediment–water mixture in the flume and in-situ size distributions of sediment flocs were measured to assess flocculation and settling behavior of sediment. The influence of bed shear stress and bed consolidation were investigated in a series of erosion experiments. These data were used to calibrate and test a mathematical model of fine sediment transport in annular flumes (FLUME model) and the measured and modelled data were in good agreement. Transport parameters for cohesive sediments, such as the critical shear stresses for deposition and erosion as well as the particle density, porosity and settling velocity as a function of floc size were established. The density and settling velocity of the cohesive sediments were sensitive to the biological activity within the floc structure. Data from the flume experiments are necessary as input parameters to models such as RIVFLOC or SHETRAN that simulate fine sediment transport in the River Taw at the catchment scale and assess the efficacy of sediment control options and catchment management scenarios.