The effects of wind on tidal asymmetry in shallow tidal basins with varying hypsometries and consequences for sediment transport

Geomorphological characteristics of tidal basins have been shown to control the shape of the tidal wave and hence, sediment transport potential in coastal regions. In this study we examine links between the entrance geometry, hypsometry and hydrodynamics within shallow estuaries. We use a calibrated numerical model to examine how tidal distortion changes in six sub-basins of Tauranga Harbour, New Zealand. Results demonstrate that those basins with a constricted entrance geometry are characterised by deep entrance channels and convex hypsometric profiles with elevated intertidal regions. These basins have high rates of flood-directed tidal velocity asymmetry, but ebb-directed slack water asymmetries, indicating potential for deposition of larger particles but export of fine sediments. Conversely, un-constricted geometries are associated with less convex hypsometric intertidal profiles. The tidal velocity asymmetry within basin centres is ebb-directed, whereas the slack tide duration asymmetries are weakly flood-dominant, indicating that, although there is limited potential for overall sediment deposition, some limited input of fine sediment into the basins is also possible.

We subsequently investigate how wind-generated currents modulate these patterns of horizontal tidal velocity asymmetry, using simulations in a series of idealised shallow basins. We find that an increase in wind speed promotes more extreme tidal asymmetry ratios without substantially changing overall tidal asymmetry patterns. However, the changes to the tidal asymmetry are strongly depth-dependent and most evident for wind directions parallel to the main axes of the tidal channel. Additionally, the timing and duration of a wind event are found to influence the asymmetry patterns. Wind events coinciding with flooding tide result in the most noteworthy differences between peak flood- and ebb-dominant asymmetry ratios inside the basins, for wind event durations of both three and six hours.