Abstract

Coastal ridge plains represent a valuable record of past shoreline deposition. However, there remain questions regarding shoreline behavior on intermediate timescales (sub-centennial), the impact of storms, and process of ridge genesis. We address these questions through high-resolution reconstruction of the sandy-beach progradation at Boydtown Beach in Twofold Bay, southeastern Australia, over the past 1000 years using ground-penetrating radar (GPR) and optically stimulated luminescence (OSL) dating. GPR profiles are dominated by seaward-dipping reflections that result from beach and dune progradation. Prominent reflections with heavy-mineral concentrations are also preserved resulting from storm erosion. OSL ages reveal alternative phases of steady and episodic accretion, rather than a constant progradation. We hypothesize that steady phases may result from moderate storm events where each successive storm only partially erodes the recovery of the previous event. This results in incremental seaward accretion of the active beach. Phases of episodic accretion could be the result of larger storm events or storm clusters when large post-storm recovery rapidly shifts the active shoreline seaward. The two modes of shoreline progradation (steady and episodic) appear broadly associated with a change in ridge-and-swale morphology whereby subdued ridge swale topography is associated with steady or incremental progradation and higher, better-defined ridges with episodic accretion. These results suggest that a single coastal ridge plain experiences variable intermediate-scale shoreline behavior in response to storm events which then lead to multiple modes of ridge genesis.