Flow patterns around groynes in the coastal zone.
Lucca, PierLuigi Antonio.
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Semi-permeable groynes are used to modulate the long-shore transport for beach management purposes, but the optimal design of these structures remains unresolved. This in part is due to their ability to behave as both a permeable and an impermeable structure. A 2D depth-averaged flow model for an isolated groyne was developed to undertake a parametric study to investigate the characteristic flow patterns and how these flow patterns change under different conditions. These include various constrictions to the alongshore flow; changing water levels, groyne permeability, and geometric parameters. The flow model was coupled with a spectral wave model to simulate wave-driven flow near the groyne. The alongshore tip velocities, cross-shore rip currents and the flow separation length were analysed for different wave conditions and groyne lengths. A method was developed to model the permeability of the groyne by manipulating friction that impeded the through-flow. This was compared with an alternative model using a permeable pile screen. Parametric studies using the models show how flow patterns are sensitive to changes in permeability and the geometry of the groyne but also sensitive to the level of constriction of cross-sectional flow. The level of constriction is the ratio of the groyne length to the width of the breaker zone. The change in constriction affects the predominant flow pattern at the tip of the groyne, where either the cross-shore or alongshore current velocity is larger. Alongshore current tip velocities increase and rip current velocities decrease when the constriction is low and vice versa when the constriction is high. Changes in the permeability of the structure also significantly affect the circulation patterns - the influence of the groyne’s constriction reduces as permeability increases. Field measurements were carried out in Durban, South Africa. An array of current meters was deployed to map the flow field around an existing groyne for comparison with the numerical simulations from the parametric studies. Groyne tip velocities, rip currents, separation length and velocity vectors were consistent with the flow patterns from the simulations. Severe scour has been observed near groyne tips at the case study site. This can be explained in terms of the above-mentioned flow patterns and how they change for different wave and tide conditions. The observed flows are shown to be sufficient to cause bedload sediment transport around the groyne.