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

Abstract

The relationship between the grain size distribution of the sediment on the bed and that found in suspension due to wave action above ripples is assessed here using detailed, pumped sample, measurements obtained at full-scale and also at laboratory scale. The waves were regular and weakly asymmetrical in most tests, and irregular in a minority of tests. The beds comprised fine and medium sand and were rippled in all tests. The cycle-mean sediment concentrations (C) from the pumped samples were split into multiple grain size fractions and then represented by exponential Cprofile shapes. The analysis of these profiles was carried out in two stages to determine: i) the relationship between the size distribution of the sediment on the bed and that found in the reference concentration, and ii) the behaviour of the exponential decay scale of the C-profiles. From this analysis inferences are made about the relative roles of diffusion and convection in the upward sediment flux linked to the process of vortex shedding from the ripple crests. The Transfer Function (Tr) defined to relate the bed sediment size distribution to that of the reference concentration indicates that, while finer fractions are relatively easily entrained, the suspension of some coarser fractions is caused by an additional convective effect that supplements diffusion. The evidence for this becomes pronounced above steep ripples, and the Transfer function suggests further that irregular waves increase the occurrence of coarser fractions in suspension. A functional form for Tr is suggested incorporating these principles. The exponential decay scale LS arising from the fractional C-profiles is also examined to assess the mechanisms responsible for the upward transfer of grains and a parameterisation of LS related to ripple size is suggested. The separate findings for Tr and LS present supporting evidence of diffusion affecting the finer fractions in suspension and combined diffusion + convection affecting the coarser fractions. The methodology developed allows the vertical profile of suspended median grain size to be predicted given knowledge of both the bed grain size distribution and also the flow conditions.

Developments in acoustics for studying wave-driven boundary layer flow and sediment dynamics over rippled sand-beds

“…These bedform images are mainly used to categorise bed features, though quantitative estimates of bedform wavelength can obtained, and limited information on ripple height extracted (Hay and Wilson, 1994;Traykovski et al, 1999b;Williams et al, 2000;Hay, 2011). ARP's usually use a downward pointing narrow pencil beam pulse of sound around 1-2 MHz, mounted 1-2 m above the bed and radially rotates vertically through about 150° to provide a digitised profile of the bed along a transect of around 3-5 m. Both bedform height and wavelength can be obtained quantitatively (Bell and Thorne, 1997a, b;Bell et al, 1998;Thorne et al, 2003;William et al, 2005;O'Hara Murray et al, 2012;Larsen et al, 2015;Davies and Thorne, 2016). More recently combining the aerial coverage of the SSS with the quantitative measurements of the ARP the development of 3D-ARP's has taken place (Traykovski, 2007;Bell and Thorne, 2007;Thorne et al, 2013;Kramer and Winter, 2016;Moate et al, 2016).…”

“…f(xo) and χ(xo) represent the ensemble mean scattering values obtained by integrating the intrinsic scattering characteristics over the particle size probability density function, n(a), of the particles in suspension and xo=kao. To obtain values for ao, f(xo) and χ(xo) the size probability density function for the bed sediments, nb(a), was measured and this was translated to n(a) followingDavies and Thorne (2016). f(xo) and χ(xo) were evaluated at the three BASSI frequencies and used in an inversion(Thorne and Hurther, 2014) to calculate the suspended sediment concentration.The main objective for developing the BASSI was to be able to image suspended sediment structures over bedforms and thereby contribute to a better understanding of boundary layer sediment dynamics.…”

Sand settling through bedform‐generated turbulence in rivers

“…Murray, 1970; Stout et al ., 1995) suggest that isotrophic (non‐directional) turbulence, on the scale that occurs in rivers, may generate a nonlinear drag effect that reduces the settling rate of sand‐sized sediment particles. There has been less study of how anisotropic (directional) turbulence, such as occurs in bedform wakes, explicitly impacts particle settling; however, processes linked to anisotropic turbulence have been shown to play a dominant role in maintaining suspension of bed material (Zedler and Street, 2001; Marchioli et al ., 2006; Chang and Park, 2016; Davies and Thorne, 2016). Further, the influence of anisotropic turbulence structures, in the form of ‘sweeps’ and ‘bursts’ (Grass, 1971), on promoting the entrainment and near‐bed transport of bed sediment has been well documented (McLean et al ., 1994; Nelson et al ., 1995; Schmeeckle, 2015).…”

Sand transport processes in the surf and swash zones

“…At the shoaling and inner surf zone, turbulent vortices are primarily bed generated and have a relatively small time and length scale. These small vortices lead to size selective pick up and also to vertical segregation of suspended sediment due to differences in vertical mixing and settling for each sediment size fraction, as shown for sheet flow conditions in an oscillatory flow tunnel (Hassan, 2003) and for rippled beds under uniform non breaking waves (Sistermans, 2002;Davies and Thorne, 2016). In the wave breaking region, turbulent vortices are of larger scale and are more energetic (van der A et al, Submitted), i.e.…”

“…Observations of graded sediment transport under oscillatory sheet flow conditions have shown that coarser grains are transported more easily than finer grains because they are more exposed (de Meijer et al, 2002;Hassan and Ribberink, 2005). The suspended load transport generally contains a relatively high fraction of fine grained particles which are more easily entrained and mixed than coarse grains (Nielsen, 1992;Wiberg et al, 1994;Davies and Thorne, 2016) and which are advected by the mean current (Sistermans, 2002). In time, the removal of fine grained particles from the bed may lead to coarsening of the seabed's top layer.…”