Probabilistic coastal hazard lines for risk based coastal assessment

As part of a recent NCCARF funded project "Approaches to Risk Assessment on Australian Coasts", a modelling framework was developed which integrated geological, engineering and economic approaches for assessing the risk of climate change along the Australian Coast. This paper aims to demonstrate the working of the framework in deriving probabilistic coastal hazard lines. Within the framework, means for combining results from models that focus on the decadal to century time scale (geomorphic), and those that focus on the short term and seasonal time scales (storm bite and recovery) have been developed. This combination is necessary for the derivation of probabilistic hazard lines. The Narrabeen - Collaroy embayment on the northern beaches of Sydney was chosen as an appropriate study site due to its data rich nature, with directional wave records extending back 20 years, and ongoing repeated beach survey available since the mid 1970's. The site has been subject to extensive study over recent decades. To demonstrate operation of the framework two models with stochastic capabilities were adapted for use in the study. These are the Shoreface Translation Model (STM), for century scale geomorphic evolution, and the Joint Probability Method - Probabilistic Coastline Recession (JPM-PCR) for shorter term beach erosion and recovery. Both models are introduced and discussed. When projecting forward to future scenarios involving sea level rise, the framework also enables sea level rise over time to be input as a probabilistic variable. Recent research has also provided some guidance as to how this can be achieved using outputs from the most recent IPCC estimates. Overall, the research efforts have aimed to point a way forward that enables the quantitative assessment of coastal hazard likelihood for use in robust coastal risk assessment. This contrasts with present practice which typically adopts a more qualitative approach to risk assessment.