Observations in the Rhine region of freshwater influence (ROFI) system in the North Sea show evidence of large semidiurnal oscillations in stability, superimposed on a mean stratification, occurring throughout the stratified region at times of reduced mixing. The amplitude of this semidiurnal variation is of the same order as the mean stability and frequently results in conditions being mixed or nearly mixed once per tide. It is deduced that this semidiurnal variation results primarily from cross-shore tidal straining which interacts with the density gradient to induce stratification. This conceptual picture of the contributing processes has been tested in a one-dimensional point model forced by the observed slopes and the local density gradients. The model exhibits the same qualitative behavior as the observations, produces oscillations in stratification of the amplitude observed, and confirms the critical role of cross-shore tidal straining. The large cross-shore shear under stratified conditions is identified with the changes in ellipse configuration which are observed between mixed and stratified conditions. The occurrence of semidiurnal variations in stability in the Rhine ROFI is thus inferred to be a consequence of the development of mean stability whenever the horizontal density gradients relax in conditions of low stirring.

[1] Modern coastal ocean modeling systems are now capable of numerically simulating a variety of coastal and estuarine problems and can thus provide useful information for managing coastal zones. Here we review state-of-the-art Eulerian implementations of bottom-up sediment transport and morphological change in coastal ocean hydrodynamic models. In order to investigate the fate of suspended sediment in coastal and estuarine waters as well as the evolution of sea or river beds, sediment dynamics need to be represented at a scale relevant to the numerical discretized solution, and significant effort is devoted to parameterize sediment processes. We discuss boundary layer hydrodynamics and the computation of the bed shear stress. We also focus on approaches used to represent near-bed processes such as bed load transport and sediment erosion and deposition. Sediment diffusivities, settling velocities, and cohesive processes such as flocculation all have an impact on suspended sediment throughout the water column. We then describe the implementation of process parameterizations in coastal hydrodynamic models, explicitly reviewing five widely used systems. The approaches implemented in these coastal models may present distinct strengths and shortcomings with regard to some important issues for coastal zones, both numerical and physical. While these detailed limitations need to be considered as part of model assessment, more general issues also hinder present state-of-the-art models. In particular, sediment transport is inherently highly empirical, which is further compounded by issues arising from turbulence closure schemes. We conclude by suggesting some possible directions toward improving sediment dynamics understanding and coastal-scale predictive ability.Citation: Amoudry, L. O., and A. J. Souza (2011), Deterministic coastal morphological and sediment transport modeling: A review and discussion, Rev. Geophys., 49, RG2002,

Abstract--We report on recent observations of the vertical structure of density and velocity profiles, using a bottom mounted ADCP, in the Rhine ROFI system in the North Sea which confirms previous indications that the presence of stratification modifies the vertical structure of the tidal ellipse characteristics. During periods of stratification, the ellipses change from degenerate to a more circular pattern, with the surface ellipse rotating clockwise and the bottom ellipse rotating anticlockwise. The surface to bottom ellipticity difference ae is found to be closely related to a bulk Richardson number which incorporates both the stratification and a measure of the tidal shear. An explanation of the observed dependency of elliptieity on the density structure is offered in terms of the different thickness of the frictional layers for clockwise and anticlockwise motion in a rotating system. The changes in polarization of the flow are large enough to introduce a significant cross-shore velocity component which enhances the vertical shear in the tidal flow and is responsible for the strong semi-diurnal variation of stratification observed in this ROFI system.

− The intensity, structure and variability of the slope current have been determined from 16 months of observations with Acoustic Doppler Current Profilers (ADCP) and conventional current meters on a cross-slope section at the Hebridean shelf edge during the Shelf Edge Study (SES) programme. After removal of the tidal signals, the mean flow over the upper slope is found to be closely parallel to the topography with speeds of ≈ 20 cm·s -1 . The flow extends down to a depth of 500 m and is predominantly barotropic, especially in winter when the flow is practically uniform between 350 m and the surface. In summer, there is a significant baroclinic component with a pronounced maximum in current at a depth of about 200 m but more than 80% of the kinetic energy is in the barotropic component. Flow in the core of the current is highly persistent with the Neumann's steadiness St > 0.8 in summer. In winter the flow is generally more energetic and variable and extends onto the adjacent shelf. The cross-slope profile of sea surface elevation, computed from the mean barotropic currents, shows a consistent relation to seabed topography through the seasonal cycle. Long-term averages of the cross-slope components are generally small (≈ 2 cm·s -1 ) with some indication of persistent down-slope flow in the bottom Ekman layer. Measurements with shipboard ADCP on sections at intervals along the slope show a high degree of continuity in the structure of the flow. The core of the flow appears to be related to a weak positive salinity anomaly and a depression of the 9.5°C isotherm near the shelf, but there is no strong correlation between the core of the slope-current and the core of the salinity anomaly. It is proposed that this may be due to differences in the cross-stream diffusion of salt and momentum which have different boundary conditions at the slope. The observed cross-stream structure of the current supports the hypothesis that JEBAR is the principal forcing mechanism but the result cannot be regarded as conclusive since a uniform potential vorticity model of the flow produces a similar cross-sectional structure of the current. OCEANOLOGICA ACTA ⋅ VOL. 24 -Supplement long du cycle saisonnier. Les moyennes à long terme des composantes du courant transversal au talus sont généralement faibles (≈ 2 cm·s -1 ) et rèvèlent un flux descendant persistant au bas de la couche d'Ekman. La courantométrie sur des sections transversales au talus continental indique que la structure du flux est fortement continue. La veine centrale du courant semble associée à une légère anomalie positive de la salinité et à une dépression de l'isotherme de 9,5°C près du plateau continental, mais sans corrélation marquée entre les positions du courant et de l'anomalie en sels. Cela pourrait être du aux différences entre le moment et la diffusion des sels à travers les courants, tous deux présentant des conditions aux limites distinctes sur la talus continental. La structure transversale du courant confirme l'hypothèse selon laquelle JEBAR est le ...

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 shortterm, while these features are ephemeral in the long-term. In this study we investigate the spatial distribution of tidal distortion and asymmetry of 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 2 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. 1

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 tideinducing 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.

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.

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 1º North Atlantic model, to a 1.85kmIrish Sea model, using the state-of-the-art 3 rd -generation spectral Wave Model 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. This time period is too short to determine any long-term trends in the wave and surge conditions.