Shoreline variability and coastal vulnerability: Mossel Bay, South Africa

Abstract

Coastal erosion may cause significant damage to property and infrastructure with far reaching socio-economic consequences. Assessing the site-specific shoreline dynamics is fundamental to understand the morphodynamic behaviour of a particular coastal area, as well as the associated coastal hazards. However, changes in shoreline position, even when significant, are not necessarily associated with increased coastal hazards. In this contribution we investigate the impact of short-term changes in shoreline position within a crenulated embayment of Mossel Bay. The 30 km-long embayment, located in the Western Cape region of South Africa, lies in a high-energy wave-dominated, micro-tidal setting. Mossel Bay is heavily populated and experiences an influx of tourists year-round. Much of the coastal community and infrastructure lies within 25–40 m of the foredune toe. Georeferenced Landsat 7/8 and Sentinel 2A scenes are used to manually digitise shoreline position in ArcMap, using the “wet/dry” line as a shoreline position proxy. The Digital Shoreline Analysis System was then used to generate shoreline change statistical metrics. Wave conditions were modelled using SWAN wave model, implemented using a nested grid approach with a high-resolution (10 m) inshore grid, and a lower resolution (50 m) offshore regional grid. The nearshore wave field during mean and storm conditions was obtained along the 15 m isobaths along the entire embayment. The embayment’s orientation in relation to the prevailing swell direction results in significant alongshore variability in nearshore wave conditions; wave heights increase towards the east along the embayment. This variability in wave forcing is reflected by the changes in shoreline position in both long and short-term, computed using the end-point rate method. However, the areas of higher shoreline change are not those experiencing the worst detrimental effects. Over the long-term, the present-day Mossel Bay embayment is relatively stable, with no significant signs of extensive accretion or erosion. However, rapid migration the shoreline is documented on a seasonal scale (short-term) with significant change proximal to river mouths, areas influenced by megacusps, and regions where the highly dynamic shoreline behaviour is constrained by rocky platforms and unable to freely adjust to variations in forcing. Thus, Mossel Bay is divided into three sub-cells in terms of coastal processes and coastal vulnerability with hazards associated with the location of such infrastructure rather than the specific patterns of shoreline change.