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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]

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]

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]

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]

Meirelles, S., Horner-Devine, A.R., Henriquez, M., Stive, M., Pietrzak, J., Souza, A.J., 2014. Middle shoreface sand transport under stratified regimes. In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 182-186, ISSN 0749-0208. 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]

The effect of tides, river, wind and Earth's rotation on the three-dimensional circulation in the Dee, a macrotidal estuary, are investigated using a fine-resolution model. The interactions of the large tidal amplitude, currents, river, and wind-generated circulation require baroclinic and unsteady studies to properly understand the estuarine dynamics. Assessment of the model skill has been carried out by model-observation comparisons for salinity, which is the main control for density, surface elevation, current, and turbulence. Stationary nondimensional numbers were only partially able to characterize the dynamics in this (real) complex macrotidal estuary. At low water, tidal straining and constrained river flow cause stratification. Large spatial variability occurs in the current and residual patterns, with flood-dominated maximum values occurring within the tidal channels. The tides control residual circulation by modulating stratification through tidal straining and bathymetric constraint on river flow. Tide-stratification-river interaction causes an unsteady pattern of residual circulation and tidal pulses. River-induced pulses are enhanced near low tide-inducing density-driven circulation. Wind effects are concentrated near the surface, mainly occurring at high tide because of increased fetch. Even though Coriolis has, overall, a small contribution it produces tidal pulses modifying the current and salinity distribution. [ABSTRACT FROM AUTHOR]