The shape and residual flow interaction of tidal oscillations

• Published in: Estuarine, coastal and shelf science Vol. 276; p. 108023
• Main Authors: Reeve, D.E., Horrillo-Caraballo, J.M., Karunarathna, H.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 05.10.2022
• Subjects: Anisotropy; Computational model; Tidal residual currents; Vorticity; Tidal residual currents; Computational model; Anisotropy; Vorticity
• ISSN: 0272-7714; 1096-0015
• DOI: 10.1016/j.ecss.2022.108023

Tidal flows are seldom exactly sinusoidal, leading to a small discrepancy, or residual, over each tidal cycle. Although residuals are generally small in comparison with instantaneous currents, their cumulative effect is important for sediment transport and dispersion of contaminants. The meso-scale characteristics of tidal residual currents are investigated with a computational model of the Irish Sea. The role of the tidal oscillations, or eddies, in forcing the residual flow is considered first through theoretical considerations and secondly by calculating time mean flow quantities directly from the computational model. The tidal eddies contribute to the time mean vorticity balance through the tidal stresses which can be written in the form of a divergence of an eddy vorticity flux. In regions where the tidal flows are approximately horizontally non-divergent the anisotropy of the tidal eddies is strongly linked to their contribution to driving the residual flow. A measure of eddy anisotropy is proposed and this mirrors the shape and orientation of the tidal ellipses of the main tidal constituent. The vorticity balance of the residual flow is dominated by the frictional torque and the eddy vorticity flux divergence, with vorticity advection and vortex stretching by the residual flow generally being of secondary importance. •Tidal residual currents are forced by tidal stresses.•Tidal stresses can be written in the form of the divergence of a vorticity flux.•We show that tidal stresses arise from the anisotropy of tidal eddies.•An anisotropy factor, based on the velocity correlation tensor, is proposed.•Main vorticity balance is between eddy vorticity flux divergence and friction.