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

Collinear wave-current shear interactions are often assumed to be the same for currents following or opposing the direction of regular wave propagation; with momentum and mass exchanges restricted to the thin oscillating boundary layer (zero-flux condition) and enhanced but equal wave-averaged bed shear stresses. To examine these assumptions, a prototype-scale experiment investigated the nature of turbulent exchanges in flows with currents aligned to, and opposing, wave propagation over a mobile sandy bed. Estimated mean and maximum stresses from measurements above the bed exceeded predictions by models of bed shear stress subscribing to the assumptions above, suggesting the combined boundary layer is larger than predicted by theory. The core flow experiences upward turbulent fluxes in aligned flows, coupled with sediment entrainment by vortex shedding at flow reversal, whilst downward fluxes of eddies generated by the core flow, and strong adverse shear can enhance near-bed mass transport, in opposing currents. Current-aligned coherent structures contribute significantly to the stress and energy dissipation, and display characteristics of wall-attached eddies formed by the pairing of counter-rotating vortices. These preliminary findings suggest a notable difference in wave-following and wave-opposing wave-current interactions, and highlight the need to account for intermittent momentum-exchanges in predicting stress, boundary layer thickness and sediment transport

Coherent turbulence structures and sediment resuspension in the coastal benthic boundary layer

This work addresses the complex interactions between bed‐generated coherent turbulence structures and suspended non‐cohesive sediment in wave‐dominated environments and under combined wave‐current flows. Coherent structures are intermittent, connected fluid masses with a defined spatial extent and life‐span and characterised by vorticity; and define entrainment and momentum exchanges in turbulent flows. Prototype‐scale experimental observations of flow, bedforms and suspensions are presented; focusing on the structural, spatial and temporal dynamics of these motions and their role in sediment entrainment. Two scenarios are considered: (a) shoaling (and breaking) erosive and accretive waves in the nearshore of a sandy barrier beach (spilling and plunging breakers); and (b) collinear steady currents aligned with, and opposing the direction of wave propagation over a rippled, sandy bed.Measurements collected in the nearshore of a prototype‐scale sandy barrier beach show intermittent momentum exchanges characterised by large wave induced coherent structures within the benthic boundary layer, corresponding to the mean flow properties. These structures can be described by a 3D bursting sequence which plays a significant role in moving and maintaining sediment in suspension. The temporal variability of these events dictates the net onshore and offshore transport. Periods associated with a succession of powerful turbulent events cause powerful suspension clouds across multiple frequency scales. The bulk of suspension is attributed to wave‐induced fluctuations of low frequencies. These modulate smaller, rapidly decaying high‐frequency turbulence extending outside the boundary layer. As larger structures persist for a considerable amount of time, suspensions near the bed are amplified before decaying as the supply of momentum ceases. Outside the boundary layer, momentum transfer via turbulent fluctuations maintains the suspensions until their energy is depleted.In combined wave‐current flows, the current plays a significant role in dictating the prevalence of specific turbulent motions within a bursting sequence. The current‐aligned structures contribute significantly to the stress, and display characteristics of wall‐attached eddies formed by the pairing of counter‐rotating vortices. In aligned wave‐current flows, the flow is characterised by a local balance between turbulent production and dissipation, and displays the ‘universal’ inertial cascade of energy in the outer flow, while just outside the combined boundary layer a superposition of eddies is observed, often linked to intermittent coherent turbulent structures at an intermediate range between energy production and dissipation. When the current opposes the direction of wave propagation, stress‐bearing coherent structures break rapidly as they are ejected higher in the water column. Suspended clouds through vortex shedding thus cease rapidly in opposing flows, compared to more continuous sediment transport in aligned flow.Both studies indicate that bed‐induced coherent turbulence structures play a significant role in the entrainment and transport of suspended sediment in flows typically encountered in the coastal environment. Suspension clouds induced be vortex shedding are maintained by the continuous supply of momentum through vortex clusters; and transport is described by the advection of these particles through net currents. A complex feedback is then imparted by the suspended particles on these structures. This merits a reevaluation of coastal sediment transport models, and a shift towards a stochastic description of the problem.<br/

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

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 event-driven 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 influence of bed roughness on turbulence: Cabras Lagoon, Sardinia, Italy

Estimates of bed roughness used for predictions of sediment transport are usually derived either from simple scalars of the physical roughness (i.e., ripple height or grain size) or from the hydrodynamic roughness length (Zo) based upon velocity gradient estimates in the benthic boundary layer. Neither parameter accounts for irregular bed features. This study re-evaluates the relation between hydrodynamic roughness and physical bed roughness using high-resolution seabed scanning in the inlet of a shallow lagoon. The statistically-robust relationship, based on a 1D statistical analysis of the seabed elevation at different locations of the Cabras lagoon. Sardinia, has been obtained between Zo and the topographical bed roughness Ks by defining Ks = 2*STD + skin friction, with STD the standard deviation of the seabed elevation variations. This correlation between Ks and Zo demonstrates that the roughness length is directly influenced by irregular bed features, and that the Reynolds number accounts for the total drag of the bed: the data points collapse on the Law of the Wall curves with a fitting factor x = 0.5. Further testing must be done in other locations and in the fully-rough domain in order to test how widely those new parameters can be applied

Sea surface temperature trends in the coastal zone of southern England

Sea surface temperature (SST) trends along the south coast of England (northern English Channel) were examined based on data from systematic buoy measurements deployed by the National Network of Regional Coastal Monitoring Programmes of England (NNRCMP) since 2003. These data were supplemented with the following: 1) long-term, coastal SST measurements by the Centre for Environment, Fisheries and Aquaculture Science (CEFAS); 2) global data sets compiled by the Hadley Centre since 1900, and 3) satellite-derived observations from Moderate Resolution Imaging Spectroradiometer (MODIS Aqua) since 2002. These data sets were used to evaluate de-seasoned nearshore trends in SST along the south coast of England, and examine links to regional ocean-atmosphere teleconnections. The analyses of long-term, CEFAS data (collected at five sites along the Southern English Coast) support the proposal that prior to the mid-1980s there were no de-seasoned trends in SST and conditions from year to year were relatively stable. Subsequently, inter-annual fluctuations appear to have increased and are , associated with a period of warming between 1985 and 2003 (0.28oC/decade). Post 20052003, interannual fluctuations in SST monitored by the NNRCMP data buoys continued, and the warming trend appears to be greater (0.42 oC/decade). This trend in SST is greatest in the nearshore and decreases with distance offshore in a systematic fashion. The warming in SST also varied greatly from month to month. The greatest warming took place from December to March, whilst the least heating (and sometimes cooling) occurred between September and November. Analysis of Hadley (HadSST1.1) and MODIS data sets substantiated these trends. The greatest warming (post 2003) was found west of Portland Bill (up to 0.76 oC/decade) and decreased towards the Strait of Dover. Despite this west-to-east trend, all 12 NNRCMP stations between Penzance and Folkestone showed remarkably similar results, suggesting regional and global sources of heat rather than local sources. This is further corroborated through wavelet coherence analysis linking SST anomalies to regional/global ocean-atmosphere teleconnection indices at seasonal scales

BARDEX II: Nearshore sediment resuspension and bed morphology

Sediment resuspension in the region outside the surf zone is known to contribute to the morphological response of barrier beaches to wave forcing, such as onshore bar migration processes. However, few measurements in this region exist, limiting our ability to quantify its contribution. These processes are complicated by the presence of bedforms in the nearshore, which alter the sand transport processes while modifying bed roughness in a complex feedback mechanism. The Hydralab IV funded BARDEX II experiments, which took place in the Delta Flume in 2012, were used to provide measurements of these processes in the nearshore of a sandy barrier beach (D50 = 0.42mm) under a range of wave conditions (Hs = 0.3 - 0.8 m; Tp = 4 – 2 s) and water levels, through deployment of a suite of acoustic instruments measuring flow velocity, near-bed turbulence, sediment resuspension profiles and bed morphology in cross-section and plan view. Initial findings indicate that sediment suspension in the nearshore appears to be controlled by a combination of near-bed turbulent bursting processes which results in near-instantaneous low concentration suspensions restricted to the bottom boundary layer, and vortex shedding from bedforms which results in higher concentration suspensions which are larger in scale than vertical eddy sizes, and perpetuate outside of the bottom boundary layer

Hydrodynamic controls on the particle size of resuspended sediment from sandy and muddy substrates in British Columbia, Canada

A benthic annular flume, Sea Carousel, was deployed at both sand-dominated (Baynes Sound) and mud-dominated (Carrie Bay) stations in British Columbia, Canada, to examine the character of near-bed flow over these contrasting bottom types and its control on particle size of resuspended sediment. An assessment has also been made of the turbidity-induced drag reduction due to suspension of bottom sediments. The median sizes of suspended material from the sandy sites have been compared with the well-known Rouse theory, whereas, the aggregates resuspended from muddy stations were scaled with the energy dissipation rate (ε) determined from high-frequency 3D flow measures in the flume. There was no evidence in the turbulence spectra in the Sea Carousel of energy inputs in the paddle and lid rotational frequencies, and a f-5/3 slope for f &gt; 2 Hz in turbulent transitional flows was evident. The bed roughness length of sandy sites was Reynolds-number dependent but was asymptotic to a constant value of 2 mm at high flows. This equated to a dimensionless drag coefficient at 1 m above bed of a constant, 3 x 10-3 (also at high Reynolds numbers), which agrees well with values reported in the literature. The median size of suspended sand (from the sandy sites) and equivalent still water settling rate (ws) scaled with the friction velocity (u^*) in the form: ws/u*=D*/8 . The median size of resuspended aggregates (d_f) scaled inversely with dissipation (ε) in the form: df=5×10-6 ε-0.24 m which is close to the relationship found in the literature