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Transport of sediments is a critical process in the coastal zone because of its relation with coastal erosion, productivity and pollution. Of particular interest are the dynamics of suspended cohesive sediments, known as flocs, which can aggregate and break-up during the flocculation process. This changes their size, density, settling velocity and overall transport. Even though turbulence is widely accepted to be an important control on floc aggregation and break-up, specific and detailed floc behaviour is still not fully understood. The present study seeks to help in the understanding of the intra-tidal turbulence-induced flocculation under different current-wave regimes. Observations of floc size and currents at high sample rates are used to investigate the changes throughout a fortnightly cycle. The occurrence of waves at different stages during the sampling period enabled determination of three regimes of currents dominant, combined waves and currents, and wave dominant. The first two regimes showed quarter-diurnal floc size variability with aggregation during low turbulence (slack waters) and higher floc aggregation magnitude on low water slack. Break-up occurred with high turbulence (flood and ebb) with higher magnitude after ebb. During the “currents-waves” regime, waves were tidally modulated and led to enhanced aggregation and break-up, with larger floc size range than during the “current dominant” regime. Wave tidal modulation and quarter-diurnal variability of floc size were lost when waves were dominant. Flocs sizes exhibited a low range related to wave height. Inverse relationships between turbulent properties and median floc size were found for the three regimes, with higher scatter of data for the Kolmogorov microscale and shear rate due to different floc behaviour during flood and ebb phases. Effective kinetic energy obtained from the combined effect of both currents and waves seems to have a better relationship with floc size, which suggests its use as a floc size predictor instead of shear stress. [ABSTRACT FROM AUTHOR]

Meirelles, S.; Henriquez, M.; Souza, A.J.; Horner-Devine, A.R.; Pietrzak, J.D.; Rijnsburg, S., and Stive, M.J.F., 2016. Small scale bedform types off the South Holland coast. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 423-426. Coconut Creek (Florida), ISSN 0749-0208. This study presents the small scale bedform states found off the South-Holland coast during a 31+ days field observation of seabed acoustic imagery and near the bed velocities. Six main bed states were encountered: current ripples (C), wave ripples (W), combined wave-current ripples (WC), current ripples with subordinate wave ripples (Cw), wave ripples with subordinate current ripples (Wc) and poorly developed ripples (P). Direct visual detection of the bed state from the images showed good agreement with a simple predictor based on the mobility number. The most frequent type of bedform was C which is governed by the tidal currents. Wave ripple prevailed only during a storm with waves higher than 2 m. The combined Cw, WC and Wc types comprised 22% of the occurrences. Poorly developed ripples were associated with the neap tide during fair weather conditions. [ABSTRACT FROM AUTHOR]

Most of the world's megacities are located in estuarine regions supporting commercial ports. Such locations are subject to sedimentation and require dredging to maintain activities. Liverpool Bay, northwest UK, is a region of freshwater influence and hypertidal conditions used to demonstrate the impact of baroclinicity when considering sediment disposal. Although tidal currents dominate the time-varying current and onshore sediment movement, baroclinic processes cause a 2-layer residual circulation that influences the longer-term sediment transport. A nested modelling system is applied to accurately simulate the circulation during a three month period. The hydrodynamic model is validated using coastal observations, and a Lagrangian particle tracking model is used to determine the pathways of 2 sediment mixtures representative of locally dredged material: a mix of 70% silt and 30% medium sand and a mix of 50% fine sand and 50% medium sand. Sediments are introduced at 3 active disposal sites within the Mersey Estuary in 2 different quantities (500 and 1500 Tonnes). Following release the majority (83% or more) of the particles remain within the estuary due to baroclinic influence. However, particles able to leave follow 2 distinct pathways, which primarily depend on the sediment grain size. Typically the finer sediment moves north and the coarser sediment west. Under solely barotropic conditions larger sediment volumes (up to 5 times more) can leave the estuary in a diffuse plume moving north. This demonstrates the necessity of considering baroclinic influence even within a hypertidal region with low freshwater inflow for accurate particle tracking studies. [ABSTRACT FROM AUTHOR]

There is a need to understand the interactions of waves and currents in the nearshore and estuarine areas. By using observational data and an advanced model an assessment of the wave–current interactions was performed in a hypertidal estuary. The circulation model includes both barotropic and baroclinic processes arising from tides, rivers and atmospheric forcing. It is coupled to a spectral wave model and a turbulence model. Waves within the estuary are strongly modulated by the tide. Significant wave height and period are mainly controlled by time-varying water depth, but wave periods are also affected by a Doppler shift produced by the current. The major-axis depth-averaged current component is tidally dominated and wave-induced processes do not have a significant effect on it. However, the inclusion of wave effects, in particular 3D radiation stress, improves the depth-averaged minor-axis (transverse) current component. The residual currents show a clear two-layer system, indicating that the baroclinic river influence is the dominant process. The wave effects are second order, but their consideration improves the long-term modelled residual circulation profile, specially the along estuary component. The main improvement appears when a 3-dimensional radiation stress coupling is considered. The 3D version of radiation stress produced better results than the 2D version. Within the estuary, wave setup has little effect on the storm surge, while 2-way wave–current interaction improved the wave simulation. Using a 3D Doppler shift further improved the model compared with using a 2D version. [ABSTRACT FROM AUTHOR]

This study investigates the influence of tidal straining in the generation of turbidity maximum zones (TMZ), which are observed to extend for tens of kilometers along some shallow, open coastal seas. Idealized numerical simulations are conducted to reproduce the cross-shore dynamics and tidal straining in regions of freshwater influence (ROFIs), where elliptical current patterns are generated by the interaction between stratification and a tidal Kelvin wave. Model results show that tidal straining leads to cross-shore sediment convergence and the formation of a nearshore TMZ that is detached from the coastline. The subtidal landward sediment fluxes are created by asymmetries in vertical mixing between the stratifying and destratifying phases of the tidal cycle. This process is similar to the tidal straining mechanism that is observed in estuaries, except that in this case the convergence zone and TMZ are parallel to the shoreline and perpendicular to both the direction of the freshwater flux and the major axis of the tidal flow. We introduce the term minor axis tidal straining (MITS) to describe the tidal straining in these systems and to differentiate it from the tidal straining that occurs when the major axis of the tidal ellipse is aligned with the density gradient. The occurrence of tidal straining and the coastal TMZ is predicted in terms of the Simpson (Si) and Stokes (Stk) numbers, and top–bottom tidal ellipticity difference (Δε). Based on our results, we find that SiStk2 > 3 and Δε > 0.5 provide a limiting condition for the required density gradients and latitudes for the occurrence of MITS and the generation of a TMZ. [ABSTRACT FROM AUTHOR]

Abstract: Wave‐supported gravity flows (WSGFs) generate rates of sediment flux far exceeding other cross‐shelf transport processes, contributing disproportionately to shelf morphology and net cross‐shelf fluxes of sediment in many regions worldwide. However, the conditions deemed necessary for the formation of WSGF limit them to a narrow set of shelf conditions; they have been observed exclusively in regions where the seabed consists of very fine‐grained sediment and typically co‐occur with nearby river flood events. Here we document the occurrence of a WSGF event on a predominantly sandy seabed and in the absence of a preceding river flood. Our measurements confirm that the dynamics are governed by the friction‐buoyancy balance observed in other WSGF and that WSGF can form in mixed grain‐size environments and transport high concentrations of sand. The occurrence of WSGF on a predominantly sandy seabed suggests that they may occur under a much wider range of conditions and, given the global prevalence of sandy shelves, they may be a more frequent and more ubiquitous feature of shelf dynamics than previously thought. [ABSTRACT FROM AUTHOR]

The Sand Engine is a 21.5 million m 3 experimental mega-nourishment project that was built in 2011 along the Dutch coast. This intervention created a discontinuity in the previous straight sandy coastline, altering the local hydrodynamics in a region that is influenced by the buoyant plume generated by the Rhine River. This work investigates the response of the cross-shore stratified tidal flow to the coastal protrusion created by the Sand Engine emplacement by using a 13 h velocity and density survey. Observations document the development of strong baroclinic-induced cross-shore exchange currents dictated by the intrusion of the river plume fronts as well as the classic tidal straining which are found to extend further into the nearshore (from 12 to 6 m depth), otherwise believed to be a mixed zone. Estimates of the centrifugal acceleration directly after construction of the Sand Engine showed that the curvature effects were approximately 2 times stronger, suggesting that the Sand Engine might have played a role in controlling the cross-shore exchange currents during the first three years after the completion of the nourishment. Presently, the curvature effects are minute. [ABSTRACT FROM AUTHOR]

A region of freshwater influence (ROFI) under hypertidal conditions is used to demonstrate inherent problems for nested operational modelling systems. Such problems can impact the accurate simulation of freshwater export within shelf seas, so must be considered in coastal ocean modelling studies. In Liverpool Bay (our UK study site), freshwater inflow from 3 large estuaries forms a coastal front that moves in response to tides and winds. The cyclic occurrence of stratification and remixing is important for the biogeochemical cycles, as nutrient and pollutant loaded freshwater is introduced into the coastal system. Validation methods, using coastal observations from fixed moorings and cruise transects, are used to assess the simulation of the ROFI, through improved spatial structure and temporal variability of the front, as guidance for best practise model setup. A structured modelling system using a 180 m grid nested within a 1.8 km grid demonstrates how compensation for error at the coarser resolution can have an adverse impact on the nested, high resolution application. Using 2008, a year of typical calm and stormy periods with variable river influence, the sensitivities of the ROFI dynamics to initial and boundary conditions are investigated. It is shown that accurate representation of the initial water column structure is important at the regional scale and that the boundary conditions are most important at the coastal scale. Although increased grid resolution captures the frontal structure, the accuracy in frontal position is determined by the offshore boundary conditions and therefore the accuracy of the coarser regional model. [ABSTRACT FROM AUTHOR]

Observations from a field experiment along the south-Holland coast, the Netherlands, were carried out in order to obtain new insights about the impacts of the Rhine ROFI (Region of Freshwater Influence) on the sand transport patterns. The net alongshore sand transport is generally governed by tides. The sediment concentration in the middle shoreface increased significantly with approaching waves of higher than ~1 m and Tm0 > 5 s. The southward net transport of the alongshore component does not agree with the literature. Modulations of the net cross-shore transport direction were observed. The net transport is more relevant during the neap tides where the velocity magnitudes are smaller. [ABSTRACT FROM AUTHOR]