Drivers of seasonal and decadal change on an estuarine beach in a fetch-limited temperate embayment

Abstract

Beaches in estuaries and bays (BEBs) are critical landforms along many coastlines, providing vital protection to human infrastructure and ecosystem services. Like open-ocean beaches, their morphodynamics are driven by the interaction of waves and tides with the boundary conditions determined by sediment supply and geology. However, as estuaries and bays tend to be lower energy than the open ocean, and are often fetch limited, BEB behaviour is typically controlled by high magnitude events rather than modal conditions. In this study, we assess the dynamics of a BEB in a semi-enclosed bay (Port Phillip Bay, Victoria, Australia). We analyse shoreline and subaerial cross sectional beach change across a range of time scales, including in response to storm events, using a combination of aerial photography, satellite imagery and UAV-based surveys spanning multiple decades. The study assesses the impact of geological controls on beach dynamics in this estuarine/embayed setting, investigating the impact of an intertidal rocky reef and the role of sediment budgets. The rocky outcrops throughout the compartment appear to exert little long-term control on beach morphology. We find that the location of the beach with respect to the sediment compartment boundaries is fundamental to determining long-term shoreline evolution. St Leonards Beach is located near the boundary between two longshore transport systems flowing in opposite directions. This results in low sediment flux across the compartment and therefore a paucity of sand supply for beach and dune development. As a consequence, the beach is relatively stable on a seasonal scale, but has a net erosional trend over the 75-year aerial photographic record. Erosion appears to be event driven as sand is transported out of the sediment compartment during storms. This work shows the importance of understanding local boundary conditions especially local sediment budgets in estuarine environments when analysing beach dynamics.