Morphodynamic Equilibria of Embayed Beach Systems

Embayed beaches are well-known for the prominent curvature of their shorelines and are often observed in states of dynamic or static equilibrium. These equilibrium states are typically assumed to be influenced by headland geometry, cellular circulation patterns, wave obliquity at the shoreline and diffraction in and around the shadow zone. One of the main aims of this study is to gain a comprehensive understanding of the role of (i) wave forcing, (ii) environmental conditions and (iii) the geological setting in long-term embayed beach evolution. In doing so, a state-of-the-art morphodynamic model was used to simulate the evolution of a schematic embayment under idealized wave forcing conditions. Wave forcing is varied between a mixture of time-invariant and time-varying cases. Environmental conditions are varied by changing sediment size, tidal amplitude and mean wave height. The geological setting is varied by changing the angle of obliquity of the waves and the bay width. Several wave climate variables influence the distribution of wave energy throughout the bay and in the shadow zone: wave direction, directional spreading and wave height. Diffraction is shown to be dominant only when the incoming wave conditions are both directionally narrow-banded and highly oblique. Nevertheless, time varying wave directions (as little as 6%) can account for shoreline curvature in the shadow zone. Changes in environmental conditions and geological setting generally affect the rate of development of the bay as well as the equilibrium size of the bay. For example: increased tidal amplitude enhances the size of the shadow zone due to modulation of the wave energy in this area, and wider bays require an exponentially larger period of time to attain equilibrium. Progressive weakening of the residual long-shore current and sediment transport as the bay develops is shown to be consistently related to long-term, non-uniform shoreline cutback (beach rotation). Hence, the curvature of the shoreline planform is primarily due to weakened shoreline erosion processes resulting from beach rotation. The research aims are extended to investigate seasonal and event-driven changes based on real-world cases. In doing so, the model has been used to reconstruct the medium-term, quasi-equilibrium morphology of a bay by discretising the measured wave climate variability in terms of wave heights and directions into several representative wave conditions. The effect of extreme events appeared to be balanced by average forcing conditions occurring over a longer period of time. Additionally, the equilibrium bathymetry is largely determined by wave direction variability; therefore it is necessary to have a high level of directional resolution in order to obtain accurate results. A nine-month field campaign was conducted at embayed beaches of Tairua and Pauanui, Coromandel Peninsula, New Zealand. During this period the beaches exhibited highly dynamic behaviour in response to storm events characterised by non-uniform cross-shore sediment movement within the bay. The residual sediment transport pathways between the surf zone and the adjacent beach and shoaling zones will be determined in future