Sediment dynamics in the Roman Rock area of False Bay, South Africa

Abstract

This study investigated sediment dynamics in the Roman Rock area of False Bay, South Africa, by integrating sediment grain size data with hydrodynamic and wave modelling. We examined two significant events: a long-period southwesterly swell in 2008, and a prolonged southeasterly gale in 2009. Using two-way coupled Delft3D FLOW and SWAN models, we assess wave conditions and their ability to mobilise sandy sediments based on established threshold velocities. The 2008 event was linked to a deep low-pressure system located far south of the subcontinent, generating long-period waves with heights exceeding 10 m offshore. These waves entered False Bay maintaining sufficient energy to produce near-bed orbital velocities that surpassed the sediment movement thresholds over much of the study area. In contrast, the 2009 event created higher, short-period waves that were locally generated and produced lower near-bed velocities, which were not enough to initiate widespread sediment transport. Our findings challenge earlier assumptions that summer southeasterly winds are the primary drivers of sediment mobility around Roman Rock. The results emphasise the significance of episodic long-period swell events in shaping the seabed and highlight the limitations of applying quartz-based threshold models to bioclastic carbonate sediments. This study provides a foundation for future studies exploring the sediment dynamics in False Bay.