Stability of artificial beaches in Port Phillip Bay: patterns and processes driving beach change along an urbanised coastline

Beach morphodynamics in fetch-limited, highly modified estuarine systems such as Port Phillip Bay are poorly understood. Beaches are significant in such settings as they protect coastal infrastructure and property from wave attack, and provide recreational value. Many beaches that are located in urban centres are periodically renourished to maintain wide profiles, and shore protection structures such as sea walls and groynes are common. Understanding estuarine beach morphodynamics, and how they change over the short to medium-term, is central to understanding likely future changes to these important systems, including their response to sea level rise. Estuarine beaches are dominated by short period, locally generated waves. This energy is dependent on the local climate, estuarine size and fetch. Wave energy is often highly variable, fluctuating in response to changing wind conditions. Comprehensive models of beach morphodynamics have largely been developed in higher energy open-ocean environments, and have limited applicability to fetch-limited systems. Models specific to estuarine beaches remain limited and largely qualitative in nature. Despite differences in boundary conditions some similarities can be drawn between estuarine morphologies and those described by open-ocean models. This indicates that aspects of open-ocean models may still be applicable in estuarine settings, particularly under high energy conditions such as storms. This investigation into estuarine beach morphodynamics focuses on the northeast coastline of Port Phillip Bay. From Middle Park Beach in the northwest to Sandringham Beach in the southeast, an area spanning 15 km and six distinct beach systems was considered. At each location across-shore beach profiles were repeatedly surveyed from March to August 2012 to quantify seasonal changes. Aerial photo analysis enabled identification of longer-term trends in planform beach morphology. Sediment analysis of samples from each beach was also undertaken. Patterns of beach morphology and sedimentology varied across the study sites. Beaches in this study ranged in width from 11.0 m to 84.8 m, and contained features such as berms, steps and bars. Morphology was similar to that described in open-ocean models, including reflective unbarred beaches and intermediate beaches with low-tide terraces or transverse bar-rip systems. Sediment character varied both across-shore and alongshore, coarsening from the back of the beach to the lower swash limit, then fining towards the bar (if present). Northwestern beaches (Middle Park and St Kilda) had significantly finer sediment than those in the south. Throughout the study period a pattern of southward winter littoral drift was observed, with beaches accreting and eroding depending on proximity to headlands and groynes. Aerial photo analysis showed that all beaches have eroded over the past decade, except at locations that have been artificially widened through beach renourishment. On renourished beaches erosion rates rapidly increase immediately after the addition of sand, and then decrease through time. The consistency of wave energy across the study sites suggests that sediment is the primary determinant of the variations in beach morphology, with finer grained beaches adopting morphologies similar to higher energy open-ocean beach states. Due to the relative low energy nature of Port Phillip Bay it appears however that only storm conditions are energetic enough to mobilise sediment and alter beach morphology. During subsequent calm periods beaches generally remain stable. Additionally, longshore sediment transport is a major driver of seasonal beach change. Over the longer-term it appears that these beaches are eroding towards a landward position of equilibrium. This change has not been driven by an increase in sea level, which has remained relatively stable in Port Phillip Bay over the past 10 years. With current projections of sea level rise it is expected that rates of beach erosion and sediment loss will accelerate over the coming decades, leading to an increased necessity for beach renourishment or other management interventions to maintain wide beach profiles