Sediment bypassing around a headland in a high-energy coastal environment

Abstract

Accurately estimating sediment transport rates is essential for predicting shoreline changes and guiding coastal management strategies. While existing longshore transport models primarily assume alongshore uniform sandy shorelines, the reality is usually more complex. Many coastal environments exhibit natural features and engineered structures that challenge these models by altering sediment transport and morphodynamic processes. This study presents observations from an extensive field campaign at the Cape Dombey headland in Robe, South Australia, incorporating a co-located acoustic current meter and sediment profiler to examine sediment bypassing during summer and winter. Offshore and nearshore wave characteristics and water levels were measured, with nearshore wave heights ranging from $\sim$0.5 m in summer to >2.5 m during winter storms. Statistical analysis revealed wave refraction, diffraction, and breaking over a submerged headland extension as the main drivers for wave transformation around the headland. Three distinct hydrodynamic regimes were identified, characterized by specific current directions and sediment transport rates around the headland. A novel conceptual model for headland bypassing is proposed, describing these regimes and identifying sediment transport patterns over event time scales. Sediment transport rates under swell-dominant (Regime 2) and sea-dominant (Regime 3) conditions were up to 30 and 40 times higher than during calm conditions (Regime 1). Our conceptual model builds upon existing models by providing a detailed description of headland bypassing mechanisms under various hydrodynamic forcing conditions. This study advances understanding of sediment transport around headlands in high-energy environments and provides an adaptable framework for measuring and analyzing headland bypassing in other coastal settings.