Sediment transport in rough bed rivers

Sediment entrainment thresholds in rivers have been widely studied, informing the prediction of bedload transport rates that underpin morphodynamic modelling. However, quantification of sediment entrainment in complex channels with varying degrees of bedrock exposure and sediment cover remains limited. Here, physical modelling was conducted to simulate a 1:10 scaled model of a bedrock reach from the River Garry in Scotland. Sediment entrainment thresholds were measured for isolated grains corresponding to selected percentiles of the bed grainsize distribution (D₁₀, D₅₀,  D₉₀) over a bedrock surface with five sediment cover percentages (0, 25, 50, 75, 100%). Bed roughness metrics (e.g. standard deviation of bed elevation) of each bed were calculated. Sediment entrainment was found to be modulated by the bed roughness, resulting from the bedrock topography and infilling of depressions with sediments. Particle tracking revealed the spatial transport of sediment grains, along with the influence of localised bed roughness features on sediment dynamics.