Nearshore morphological changes and their relation to wave-induced forcing at Isipingo embayment, KwaZulu-Natal.
The nearshore zone is one of the most active sedimentary environments on the continental shelf, frequently impacted by energetic wave conditions and storm–generated waves and flows. Despite its proximity to the beach and shoreline, the nearshore is a difficult domain to study and little is understood with regards to its response to storm waves and the agents responsible for the morphological evolution of the region. Previous research has provided insight into the nearshore, however high-resolution approaches to gain a three dimensional understanding of these environments are still lacking. This is especially true in geologically constrained environments as coastal embayments. Advances in the acquisition and analysis of high- resolution multibeam bathymetry of the nearshore, coupled with wave modelling techniques, can provide an improved understanding of coastal response to high energetic wave conditions and the subsequent morphological changes that result from the impact of storm events. Detailed nearshore multibeam bathymetric surveying conducted at the Isipingo embayment before and after the 2017 winter season provided a framework to analyse the seasonal morphological changes that were driven by winter storms along the KwaZulu-Natal coastline. High-resolution bathymetric grids were implemented in the hydrodynamic model SWAN (Simulating Waves Nearshore) to simulate the nearshore wave field of Isipingo embayment for a variety of wave conditions. Morphological changes were evaluated in conjunction with wave-induced forcing to determine the potential for sediment mobilisation. Spectral wave modelling results of the wave field and bed shear stresses agree with the observed morphological changes. Significant erosion and deposition occurred in the shallower regions (5 m to 14 m) of the study area and along the northeast and southwest sections of the embayment. Modelling results presented in this study indicate that the spatial variation and distribution of orbital bottom velocities and bed shear stresses are strongly dependent upon the bathymetric configuration of large-scale bedforms and the magnitude of the nearshore wave field. Consequently, the greater the energy of the wave conditions (i.e. as a result of major storm events) the greater the wave-induced forcing, which causes the change of the bedforms and the morphological evolution of the nearshore. Large-scale features such as shoreface-connected ridges, rippled scour depressions and large subaqueous dunes tend to increase the significant wave height and the bed shear stresses acting on the seabed. Thus, waves are focussed towards the NE headland resulting in an extensive zone of erosion in the embayment. The main geological constraints at the embayed region of Isipingo are imposed by the NE and SW headlands and the shoreface-connected ridges. Such constraints drive the development of topographically controlled rips against the NE headland, set up by the prevalent south-north longshore current, and the shadowing effect of the SW headland also contributes to the observed erosional (NE region) and accretion (SW region) patterns on the nearshore. The hydrodynamic forcing of rippled scour depressions and shoreface connected ridges control the persistence of the non-stationary rip currents of the NE section of the embayment. As a consequence, the absence of rip activity at the centre of the embayment creates a relatively stable region. Additionally, the study reveals that the patterns of morphological change in the nearshore mimic those of the apparent rotation of the beach, explained by the breathing mode described from other embayments. Model simulations of extreme storm events depict that the morphological change is driven by an energetic wave field, and the wave induced orbital motions and bed shear stresses have a strong bathymetric imprint. This study demonstrates the usefulness of high-resolution bathymetry in understanding nearshore morphological change. In providing a complete three dimensional image of changes at incredibly high-resolutions, and with high (2-3 month) temporal resolution, an increased level of understanding of the nearshore changes seaward of bars can be garnered. This includes rip-dynamics, erosion patterns, geological control on wave-forcing and seasonal accretion/erosion monitoring. These are areas which have not yet received attention in this level of detail and in which it is demonstrated that good results can be obtained.
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