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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]

Abstract: Appropriate descriptions of turbulence are important in predicting sediment transport and seabed evolution. We use here a three-dimensional model to investigate the impacts of sediment-induced stratification, erosion parameterization, and turbulence closures on sediment transport and morphological simulations. The model is implemented in the Proudman Oceanographic Laboratory Coastal Ocean Modelling System (POLCOMS), which couples a hydrodynamic model to the General Ocean Turbulence Model (GOTM) and to a sediment transport model. This sediment transport model calculates both suspended sediment concentrations and bed evolution. A large number of turbulence closures can be tested and we focus on the methods used to calculate the turbulent length scale and the stability functions. Different modelling scenarios are assessed against experimental data of velocity profiles, suspended sediment concentrations, and trench migration in a laboratory flume. Numerical results show that the processes investigated have little impact on the flow velocity profiles, in spite of some differences on the bed shear stress. The only exception is the one-equation turbulence closure, which cannot reproduce velocity profiles appropriately. Sediment-induced stratification, erosion parameterization, and turbulence closures can all have significant impacts on the suspended sediment concentration and the bed evolution. Model-data comparisons are found to be particularly sensitive to the method used to calculate the turbulent length scale. [Copyright &y& Elsevier]

Abstract: In the future it is believed that extreme coastal flooding events will increase (in frequency and intensity) as a result of climate change. We are investigating the flood risks in the eastern Irish Sea posed by extreme storm events. Here, an 11-year simulation (01/01/1996–01/01/2007) including wave–current interaction has been validated. These data can then be used to investigate the potential for coastal flooding in the study area. To accurately model a storm event in the eastern Irish Sea both wave effects and the influence of the external surge need to be considered. To simulate the waves, we have set up a one-way nested approach from a 1° North Atlantic model, to a 1.85km Irish Sea model, using the state-of-the-art 3rd-generation spectral WAve Model (WAM). This allows the influence of swell to be correctly represented. The Proudman Oceanographic Laboratory Coastal-Ocean Modelling System (POLCOMS) has been used to model the tide–surge interaction. To include the external surge we have set up a one-way nested approach from the 1/9° by 1/6° operational Continental Shelf surge model, to a 1.85km Irish Sea model. For the high resolution Irish Sea model we use a POLCOMS–WAM coupled model, to allow for the effects of wave–current interaction on the prediction of surges at the coast. Using two classification schemes the coupled model is shown to be good and often very good at predicting the surge, total water elevation and wave conditions. We also find the number of low level surge events has increased in the study area over the past decade. However, this time period is too short to determine any long-term trends in the wave and surge levels. [Copyright &y& Elsevier]

Abstract: A numerical study of the effect of the deep-water wave breaking on the simulation of wind-waves in the Irish Sea region during some North Hemisphere winter months (January–February 2003) was carried out. A new formulation that takes into account the nonlinear effect of wave groups on the onset of wave breaking [Alves, J.H.G.M., Banner, M.L., 2003. Performance of a saturated-based dissipation-rate source term in modeling the fetch-limited evolution of wind waves. J. Phys. Oceanogr. 33, 1274–1298.] has been implemented and tested in a third-generation wave model. Its effect is assessed against the standard WAM Cycle 4 formulation and observations in the Irish Sea. The integrated spectral parameters computed by the new implementation of wave breaking tend to be larger (between 10% and 15% for significant wave height and mean period) than those computed by the standard formulation of WAM, especially on the eastern coast of the Irish Sea. It is found that, in the Liverpool Bay area, the only possibility for the waves to resuspend sediment is the occurrence of northwesterly wind conditions. In these situations fetch-limited growth is observed and both standard and new implementations provide similar results. In the southern Irish Sea, stronger mixed sea-swell conditions are predicted by the new formulation, which may have an impact in the description of the areas susceptible to sediment resuspension. [Copyright &y& Elsevier]

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]

Abstract: This study uses a three-dimensional hydrodynamic model to investigate transport timescales in Liverpool Bay, a shallow hyper-tidal Region of Freshwater Influence (ROFI) with a density-driven baroclinic residual circulation. Flushing time, residence time and age are evaluated, providing rigorously defined parameters to describe the rate of offshore freshwater transport and basin replenishment. Additional challenges encountered when assessing these timescales in a tidally mixed regime are highlighted by idealised models. Climatological river gauge data reveals that the numerous local rivers contribute an average of 203m3 s−1 of freshwater to Liverpool Bay. Based upon the mean salinity distribution, this would suggest a flushing time of approximately 136days. The mean residence time of the region is approximately 103days although small concentrations of water are retained over several years due to vigorous tidal mixing. Age in the region is highly variable with regular oscillations caused by tidal advection, whilst long term fluctuations are governed by river flow rates. The mean age gradient is directed offshore, approximately parallel to both the salinity gradient and the major axis of the tidal ellipse, with basin wide average magnitude of 6dayskm−1. It is shown that salinity may be used to estimate the age of freshwater, which is not directly observable in practice. [Copyright &y& Elsevier]

Abstract: Asymmetry in the tide (unequal ebb and flood duration) is a dominant factor in causing residual sediment transport and morphological changes in estuaries. The evolution of estuarine morphology is a process of dynamic equilibrium in the short-term, while these features are ephemeral in the long-term. In this study we investigate the spatial distribution of tidal distortion and asymmetry in the Dee estuary, UK, by 3-dimensional numerical modelling methods. High resolution LIDAR surveys are used to underpin and explain our numerical modelling results in terms of basin hypsometry and areas of recent erosion and deposition. Harmonic analysis of the numerical modelling results showed that the shallower intertidal areas (sand and mud banks) were the most tidally asymmetric, showing flood dominance. The main navigation channels showed some ebb dominance but the tides here were relatively undistorted. This overall flood dominance is likely to induce net sediment import to the Dee, which explains known historical morphological changes (large scale accretion over the last two centuries) and also recent morphological changes as seen from the LIDAR surveys (which show predominantly net accretion between 2003 and 2006). Hypsometrical analysis suggests the Dee may be approaching equilibrium, and that the flood dominance and sedimentation rate may therefore decrease in the future. In an infilling estuary, an increase in the area and elevation of tidal flats can eventually shift an estuary towards ebb dominance, as shown by previous research and by ‘idealised estuary’ modelling results presented in this study. The large tidal amplitude to hydraulic depth ratio of the Dee, however, suggests that the tidal flats would have to be very extensive indeed for this to occur. [Copyright &y& Elsevier]

Abstract: We have implemented new modules of seagrass and macroalgae in the European Regional Seas Ecosystem Model (ERSEM). The modules were tested using a version of ERSEM coupled with the General Ocean Turbulence Model (GOTM) in San Quintin Bay (SQB), a coastal lagoon in Baja California, Mexico. As we are working in a region where horizontal advective transport of nutrients is important, we have included the horizontal nutrient gradients which result in nutrient advection when combined with the local currents. The addition of the Zostera marina and Ulva spp. modules to ERSEM, and the inclusion of advection results in a better simulation of the seasonal and interannual trends in nutrient concentrations and macrophyte biomasses in SQB. The differences between the simulations with and without advection are particularly apparent during the upwelling periods. Therefore, by increasing the horizontal gradients of nitrate in the model during the strong upwelling seasons a stronger advection results in higher nitrate concentrations from May to July in 2004 and 2005. The difference in the seasonal trend in biomasses between both macrophytes, with Ulva spp. reaching its seasonal maximum in June–July and Z. marina reaching it in September–October reflects the different response to the various factors controlling their primary production. Z. marina is particularly sensitive to variations in the photosynthetically active radiation (PAR) and the light limitation factor, while Ulva spp. is more sensitive to changes in the maximum uptake rates of nitrate. The model was forced using field data from the lagoon collected in 2004 and 2005. [Copyright &y& Elsevier]

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]

Abstract: Particles in the marine environment vary in size from sub-micron colloids to flocculated aggregates of the order of millimetres. The LISST-100 (laser in-situ scattering and transmissometer) range of instruments (Sequoia Scientific Inc.) determine the distribution of particle sizes in-situ using laser diffraction, but are limited to specific size ranges governed by the instrument configuration. Using numerical predictions of scattering and a novel observational system to combine digital holography and the LISST-100 type-c, here we examine the response of the LISST to particles larger than the intended size range of the instrument. For spheres greater than the type-c instrument limit of 500μm, both theory and observations indicate that the inversion of the recorded scattering into particle size distributions produces volume distributions that peak at varying sizes between 250 and 400μm. This is caused by the principal peaks in scattering moving off the inside of the ring detectors, leading to the remaining peaks being interpreted as the principal peaks. The aliasing of larger particles as a distribution of smaller particles is also applicable to the type-b configuration of the LISST, only occurring at the 250μm size limit instead of 500μm. When extending the Junge particle size distribution up to varying maximum sizes, numerical tests predict an increase in volume concentration of up to 45%. For power-law distributions with gradients less than that of a Junge distribution, the contamination from large particles becomes increasingly influential over the concentration and shape of the inverted size distribution. [Copyright &y& Elsevier]