Approaches to risk assessment on Australian coasts: a model framework for assessing risk and adaptation to climate change on Australian coasts: final report
The Australian coast is particularly diverse comprising a range of landforms which will be increasingly influenced by the impacts of sea-level rise. Although many beach-dune systems will continue to be subject to storm-related beach and dune instability, the longer-term imbalances in sediment transport which in most cases result in gradual, largely imperceptible, shoreline recession will account for a larger proportion of overall shoreline retreat through the 21st century. The decisions made by coastal managers and policy-makers need to be based on realistic projections of future shoreline positions that incorporate the short-term recurrence of storm erosion, the longer-term geomorphologically-determined net sediment imbalance that drives recession (or in some cases accretion), and a component that accounts for sea-level rise. A framework is outlined that incorporates many elements of these coastal hazard assessment procedures that are already adopted in several states. It involves subdivision of the coast into compartments and cells on the basis of sediment budget in order to estimate net sediment loss or gain, following a methodology termed the Coastal Tract (CT). Within a compartment or cell, processes of storm cut need to be modelled using a probabilistic approach. Storms cause acute erosion of beach and foredunes, and wave conditions should be modelled recognising that the most extensive erosion is often associated with several storms in succession. A procedure called the Joint Probability Method (JPM) is described which incorporates such storm sequencing to estimate beach erosion. This wave modelling can be used to develop probabilistic approaches to determining estimates of coastal recession. One such model, the Probabilistic Coastline Recession (PCR) model is outlined, and it is shown how this can be incorporated into economic considerations to derive rational setback lines; this is called the Probabilistic Coastal Setback Line (PCSL) model. There are uncertainties which compound each of these components, and the framework we propose incorporates these through a probabilistic modelling process, adopting the range of potential values for key variables and Monte-Carlo modelling. An example of this modelling approach is described which applies the JPM, PCR, PCSL, within the context of the CT, to Narrabeen Beach in northern Sydney. Repeated surveys of this beach over more than 30 years have provided field evidence that enables calibration and testing of the models. The framework, using these or other independently verified models, can be extended from the relatively closed compartments of the heavily-embayed coast of Southeastern Australia to other coasts around the nation. A sediment budget model, the Shoreface Translation Model (STM), has been used to extend the approach to coasts dominated by longshore sand transport; it has been trialled in southwestern Australia, where production on the shelf appears to deliver carbonate sediment to the shore, and could be modified to apply to entirely biogenically-derived sand cays on the Great Barrier Reef. There remain challenges to the wider adoption of this framework; it is necessary to incorporate the role of estuaries, as sediment sinks or sources, into the modelling, and it will also be necessary to enable modelling of adaptation measures, such as beach nourishment.