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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]

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]