Initial motion and pivoting characteristics of sand particles in uniform and heterogeneous beds: experiments and modelling

Abstract

Experiments are described in which the threshold conditions for sediment entrainment are measured for uniform and mixed sand beds beneath both steady and combined steady/oscillatory¯ows. Derived critical shear stresses are compared with the mixed bed entrainment model of Wiberg & Smith (1987). As predicted by the model, coarser grains within a sand mixture are entrained at lower bed shear stresses than progressively ®ner grains. Entrainment occurs generally at lower shear stresses than predicted by the model, especially under unidirectional¯ows. This may be the result of grains resting in unusually unstable positions during the experiments because the beds are`unworked' at the start of the experiments.The model of Wiberg and Smith predicts threshold conditions more accurately for the mixed beds if the bed pivoting angle is correctly de®ned. The pivoting angles of the beds used here are measured using a new technique designed speci®cally for comparison with the threshold data. The measured angles repeat the ®nding that the coarse grains are more mobile than the ®ner fractions of a mixture. The results are poorly described by the pivoting angle model presented by Wiberg & Smith (1987) and are better represented by a model of the form F aD c (D i /D 50 ) b (after Li & Komar, 1986), where a, c and b are empirical constants. The threshold model is found to be more effective using the improved pivoting relationship.The entrainment of grains is found to be easier beneath unidirectional¯ows than combined¯ows, in accordance with previous authors' ®ndings. A suggestion that this result is caused by a change in the erosion mechanism beneath wave¯ows is made. Wave boundary layers may act as an extended laminar sublayer over bed grains and reduce the erosive ef®ciency of the overlying current¯ow. The results of the experiment have implications for the natural sorting mechanisms of sediment beds being deposited in near-threshold¯ows.

On the suspension of graded sediment by waves above ripples: Inferences of convective and diffusive processes

“…This increases the critical stress for finer particles and decreases it for more exposed coarser particles (Wiberg and Smith, 1987;Van Rijn, 1993;Wallbridge et al, 1999;Hassan 2003). In a multi-fraction approach Van Rijn (2007b) included an additional correction to the shear stress itself due to Day (1980).…”

Sediment mobility in response to tidal and wind-driven flows along the Belgian inner shelf, southern North Sea

“…The mud/sand ratio influences the transition between cohesive and non-cohesive sediments and has a major influence on the erosion behaviour, on suspended particulate matter (SPM) concentration and on the benthic ecological properties (Williamson and Torfs 1996;Torfs et al 1996;Panagiotopoulos et al 1997;Wallbridge et al 1999;Flemming and Delafontaine 2000;van Ledden et al 2004;Waeles et al 2007;Van Hoey et al 2007). Frequently, mixed sediments occur as an alternation of sand and mud layers.…”

“…Understanding of sediment distribution and mobility in such areas requires the use of multi-parametric observations. These should account for the mutual interaction of sand-mud mixtures as a function of bed armouring, size fraction, sheltering and exposure effects (Wiberg et al 1994;Wallbridge et al 1999;Wu et al 2003).…”

Hydro-meteorological influences and multimodal suspended particle size distributions in the Belgian nearshore area (southern North Sea)

“…Understanding of sediment mobility in such environments requires the use of multi-parametric observations evaluated by mathematical methods. The observations should account for the mutual interaction of sand and mud in suspension as a function of bed armouring, sheltering and exposure effects, erosion and resuspension, and flocculation dynamics (Wiberg et al 1994;Wallbridge et al 1999;Wu et al 2003). The fragility of flocs makes them difficult to sample, and measurements of floc properties should rely on high-resolution in situ techniques employing, for example, LISST, floc cameras and holographic cameras (van Leussen 1994;Agrawal and Pottsmith 2000;Manning et al 2006;Mikkelsen et al 2006;Benson and French 2007;Graham and Nimmo-Smith 2010).…”