Wave-induced coherent turbulence structures and sediment resuspension in the nearshore of a prototype-scale sandy barrier beach

Abstract

Abstract:The suspension of sediments by oscillatory flows is a complex case of fluid-particle interaction. The aim of this study is to provide insight into the spatial (time) and scale (frequency) relationships between wave-generated boundary layer turbulence and eventdriven sediment transport beneath irregular shoaling and breaking waves in the nearshore of a prototype sandy barrier beach, using data collected through the Barrier Dynamics Experiment II (BARDEX II). Statistical, quadrant and spectral analyses reveal the anisotropic and intermittent nature of Reynolds' stresses (momentum exchange) in the wave boundary layer, in all three orthogonal planes of motion. The fractional contribution of coherent turbulence structures appears to be dictated by the structural form of eddies beneath plunging and spilling breakers, which in turn define the net sediment mobilisation towards or away from the barrier, and hence ensuing erosion and accretion trends. A standing transverse wave is also observed in the flume, contributing to the substantial skewness of spanwise turbulence. Observed low frequency suspensions are closely linked to the mean flow (wave) properties. Wavelet analysis reveals that the entrainment and maintenance of sediment in suspension through a cluster of bursting sequence is associated with the passage of intermittent slowly-evolving large structures, which can modulate the frequency of smaller motions. Outside the boundary layer, small scale, higher frequency turbulence drives the suspension. The extent to which these spatially varied perturbation clusters persist is associated with suspension events in the high frequency scales, decaying as the turbulent motion ceases to supply momentum, with an observed hysteresis effect. The suspension of sediment in turbulent flows is a complex case of fluid-particle interaction, governed by shear stresses (momentum exchanges) at the bed and within the benthic boundary layer (BBL). Defining the physical processes which dictate the resuspension of sediments in coastal and estuarine settings is fundamental for accurate predictions of bed morphology evolution (Van Rijn et al., 2007), and has profound implications for the biogeochemical processes that shape their local ecology (Thompson et al., 2011). It is also a prerequisite to quantifying erosion and deposition trends, and hence guiding engineering applications such as beach nourishment, defence schemes against erosion and flooding, maintenance of marine infrastructure and waterways, and aggregate dredging. There is a genuine need for better, robust models of suspended sediment transport in the coastal zone (Aagaard and Jensen, 2013). In a vision paper on future research needs in coastal dynamics, Van Rijn et al. (2013) highlighted the pressing need for research to support such models, focusing in particular on sand transport in the shoreface (non-breaking waves), surf and swash zones; employing field and controlled laboratory experiments.The mobilisation of sediments in the nearshore and shoreface is dominated b...

Hydrodynamic Controls on the Particle Size of Resuspended Sediment from Sandy and Muddy Substrates in British Columbia, Canada

“…in Kassem et al (2015) to estimate the turbulent kinetic energy, shear stress, and spectral properties of the flow. Quality control entails removing raw measurements with correlations ,80% (conservatively exceeding the recommendation for the sampling frequency suggested by Elgar, Raubenheimer, and Guza [2005]) and interpolating these using a moving average algorithm.…”

Intra-Annual Patterns in the Benthic-Pelagic Fluxes of Dissolved and Particulate Matter

“…Our current understanding of B/P exchange processes is largely based on short-term investigations of specific B/P coupling pathways, often within a single period or focused on the more biologically active half of the year (although there are some exceptions such as Chipman et al, 2012). From this we know that in winter, wave action and current flow can lead to resuspension and advective flushing events (Lou et al, 2000;Tengberg et al, 2003;Kassem et al, 2015), especially during storm events (Jago et al, 1993;Madsen et al, 1993;Corte et al, 2017). It is also known that pelagic primary productivity and specifically the deposition of organic matter from phytoplankton blooms is a major driver of benthic biological activity (Gooday and Turley, 1990;Zhang et al, 2015).…”

Observations of Nearbed Turbulence over Mobile Bedforms in Combined, Collinear Wave-Current Flows

“…This comprises time series of the streamwise (U, defined as the mean direction of wave propagation along the flume); the cross-wise (V, perpendicular horizontally to U, transverse across the flume); and the vertical (W, perpendicular to the (xy) plane, positive upwards) velocity components of the flow. Turbulence is extracted from these time series by Reynolds decomposition, following the approach outlined in Kassem et al [24]. This encompasses a quality check such that no more than 20% of the record in a given ensemble falls below the correlation threshold, set at 70% following Elgar et al [36] to account for noise arising from signal aliasing [37][38][39].…”

“…In addition, such models also impose a zero-flux condition limiting turbulent quantities; and by consequence sediment concentrations, at the interface of the wave boundary layer with the core flow [22], despite documented evidence of vertical momentum exchanges extending beyond the oscillating layer in the nearshore [23][24][25]. Holmedal et al contend that such simplification is problematic given production and dissipation of turbulent kinetic energy are taking place within the current boundary layer at the interface with the wave boundary layer [22].…”