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The article discusses the highlights of the 14th International Biennial Conference on Physics of Estuaries and Coastal Seas (PECS 2008) which was held in Liverpool, England from August 15-29. The conference is chaired by Charitha Pattiaratchi and adopted the theme on process studies in the pre-operational era. The sponsors of the conference, including the Proudman Oceanographic Laboratory, the UK Environment Agency and the Explorocean Ltd., are mentioned.

This article reflects on the 12th International Biennial Conference on Physics of Estuaries and Coastal Seas (PECS) held in Mérida, México from October 19 to 22, 2004. PECS philosophy has been to stimulate the cooperation between the coastal engineers and coastal oceanographers. The topic of PECS 2004 was Hydrodynamics and Morphodynamics in Estuaries and Coastal Seas, including estuarine hydrodynamics, coastal hydrodynamics, coastal transport and morphodynamics.

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: 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]

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]

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]

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]