Numerical investigations of the turbulent kinetic energy dissipation rate in the Rhine region of freshwater influence
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The turbulent kinetic energy dissipation rate, ε, in tidal seas is maximum at the bottom during full flood and during full ebb, i.e. when tidal currents are strongest. In coastal regions with tides similar to a Kelvin wave, this coincides with high water and low water. If there is a freshwater source at the coast, stratification in such a region will be most stable at high water and least at low water. Measurements of ε in the Rhine region of freshwater influence performed by previous studies have revealed bottom maxima at both high and low water. In addition, a maximum in the upper half of the water column was found around high water, which cannot be explained by tidal shear at the bottom, convective instabilities or wind mixing. This study investigates the dissipation rate and relevant physical properties in the Rhine region of freshwater influence by means of three-dimensional numerical simulations using the General Estuarine Transport Model and idealised conditions. The measurements are well reproduced; two distinct peaks of ε are evident in the upper layer shortly before and after high water. These maxima turn out to be due to strong peaks in the alongshore shear occurring when the fore- and the back-front of the plume transit the water column.
KeywordsDissipation rate maximum Rhine ROFI 3D numerical modelling
This work has been supported by the EU-funded project European Coastal Sea Operational Observing and Forecasting System (ECOOP, Contract No. 36355). The authors are grateful to Alejandro Souza (Liverpool, UK) for providing the measurement data (Fig. 2) and support in data analysis, to Gerben de Boer (Delft, The Netherlands) for delivering the grid data, to Lars Umlauf (Warnemünde, Germany) for helpful discussions and to two anonymous reviewers whose comments helped to improve this paper. Furthermore, the first author wishes to thank Eefke van der Lee (Warnemünde, Germany) for assistance with MATLAB and for proofreading, as well as The Challenger Society for Marine Science for granting her a Student Travel Award to the Physics of Estuaries and Coastal Seas (PECS) conference 2008 in Liverpool, where the first results of this work were presented.
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