Deposition of Fine and Coarse Sand in an Open-Work Gravel Bed

Abstract

The siltation of an experimental gravel bed, with three grades of sands moving in suspension and as bedload, was examined in a flume. For low suspended sediment concentrations (<300 mg∙L−1) the mean deposition rate was 1.34% of the initial gravel volume filled per hour. Deposition rates for sands 0.15–1.4 mm in diameter with suspended sediment concentrations of 38–9110 mg∙L−1 and Froude numbers in the range 0.008–1.21 were constant with respect to Froude number. For all concentrations the deposition rate was strongly linearly correlated with the suspended sediment concentration. The downstream decrease in siltation rate from a point source was a negative exponential function of distance from that source. Turbulent resuspension of sediment prevented deposition in a surface layer of gravel of thickness approximately equal to the mean grain size of the gravel. Mean flow data, especially where derived from velocity profile data measured in the outer boundary layer, have limited value for siltation investigations concerned with processes occurring very close to the bed. Practically, results indicate that open-work gravels will rapidly become silted in flows with low concentrations of suspended solids. Consequently, mechanical cleansing of spawning beds has limited value where sediment pollution is likely to be a recurrent problem. A simple equation describing the downstream extent of silting from a point pollution source is presented that requires verification in natural streams.