Identifying oceanographic conditions conducive to coastal impacts on temperate open coastal beaches and the importance of empirical impact data

Abstract Warnings issued by meteorological or oceanographic agencies are a common means of allowing people to prepare for likely impactful events. Quantifying the relationships between ocean conditions and coastal impacts is crucial for developing operational coastal hazard warnings. Existing studies have largely omitted empirical data, relying on modelling to estimate total water levels and impact potentials. It is well documented that site-specific conditions influence local morphodynamics and as such, detailed data related to the physical environment is a necessary component of these existing approaches. The capacity to collect this data is not always available and there is a need for the inclusion of physical impact events to properly quantify the effects of natural hazards and identify the conditions that they are likely to occur. We propose an alternative empirically-based framework for isolating oceanic conditions that are conducive to impact along open coasts, using two case studies from Victoria, southeast Australia: Port Fairy and Inverloch. Oceanic conditions were defined using data obtained from a WAVEWATCH III (WW3) model hindcast, assessed against newly-installed wave buoys, which evidenced variation in mean conditions between the two sites. We coupled impact-based data derived from citizen-science and social media to modelled and observational data, to identify the oceanic conditions that led to these impacts. We found heterogeneity in the response of the case study locations to deviations from the local mean wave characteristics and still water levels. This paper demonstrates a framework through which impact-based thresholds for erosion could be developed for management applications and early-warning systems.