Incipient transport of silt-sized sediments

Abstract

Laboratory experiments were conducted to determine the influence of stream bed shear stress and water chemistry on the sediment transport rate for silt-sized particles near the critical threshold for motion. Experiments were conducted in two large recirculating laboratory flumes, 40 m and 12 m long, with a small sediment bed 40 cm long. The sediment transport rate was determined from the volume of sediment eroded from this sediment bed per unit time. The smaller flume was filled with deionized water, to which specific electrolytes were added to vary the water chemistry. Dimensional analysis predicted the sediment transport rate of non-cohesive material can be described by two dimensionless groups, one for transport and one for bed shear stress. A new transport model was developed on physical considerations for particles smaller than the thickness of the viscous sublayer, and supported this conclusion. Sediment transport rates were measured in experiments using carefully cleaned glass beads (15 μm to 69 μm) in low ionic strength (10^-4 M) solution by measuring the elevation of the sediment bed along transects with a laser displacement meter every 10 to 30 minutes. The results supported the prediction that the dimensionless transport rate is solely a function of the dimensionless shear stress (Shields parameter) and the water composition, and not of the bed Reynolds number, when the latter is less than one. Experiments were conducted with NaCl and CaCl2 electrolytes at differing concentrations up to 10^-2 M, which reduced the transport rate by up to 2 to 3 orders of magnitude for the finest particles. Calcium was more effective at reducing the sediment transport rate than sodium. These trends were captured by the transport model, but additional work is required in estimating the inter-particle forces. A new criterion for initiation of motion is proposed based on a small dimensionless transport rate qs* = 0.01, corresponding to about 2% of the surface grains in motion. For bed Reynolds numbers u*d/v < 1, the equivalent Shields parameter for critical shear becomes [Greek tau]* = 0.075 for non-cohesive sediment. With cohesion, a new model is used to predict the change in the Shields curve for various dimensionless interparticle forces.

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