A comparative analysis of engineering options for adaptation to sea-level rise: a case study for a vulnerable beach in Shoalhaven NSW

Mollymook, Collingwood and Callala beaches and the communities living near them were identified as especially vulnerable to sea level rise by the Shoalhaven City Council. A number of possible engineering and management solutions have been identified for mitigating or eliminating the effects of expected flooding and erosion (e.g., sea wall, groyne, beach nourishment), based on guidelines developed by Engineers Australia. However, the question remains as to how to assess and compare the benefits (and not just the costs) of each option. While the cost of designing and implementing these options are reasonably easy to estimate, other environmental and aesthetic costs are more difficult to valuate. Even more challenging is quantifying in monetary terms the benefits of each option. Methods are available in the economic literature for estimating some of these parameters, however, their application requires data and resources that are not always available to local government. In this study, we propose a pragmatic approach (relatively simple yet detailed) which combines a monetarybased probabilistic flood-damage estimation technique with an estimate of non-monetary consequences of an adaptation option using local knowledge and stakeholder consultation. These two types of information (monetary and non-monetary) are combined using multi-criteria decision analysis (MCDA) methods in order to generate a ranking of engineering adaptation options and assist in decision-making. We illustrate the method by applying to Callala beach in Shoalhaven. First, we calculate respective cost-benefit ratios of each option by simulating the likely effects of a flood event (with multiple probabilities of occurrence or return period) with and without proposed adaptation options, for different scenarios of sea level rise. Specifically, a flood model of Callala is developed using high resolution LiDAR Digital Elevation Model (DEM) data and tested for impacts under different sea level rise scenarios (using IPCC AR5 projections) and their corresponding exceedance probabilities (using Canute sea level rise calculator). Second, we estimate the potential damage to properties and infrastructures (cumulative over time) through flood damage function curves (quantifying the relationship between flood depth and potential damage cost of private properties and public infrastructure). Third, we estimate the non-monetary benefits of each option using a simplified approach, based on stakeholder consultation. Finally, we use two different multi-criteria decision analysis (MCDA) approaches (simple additive weight and outranking methods) for comparison of a number of engineering adaptation options (both hard and soft measures). Results show that, in general, a combination of beach nourishment & groynes is the most preferred option for Callala beach, across all decision analysis methods. Our analyses also show that hard measures such as sea walls tend to perform better in cost-benefit analyses where non-monetary factors such as community preferences, aesthetics and environmental factors are omitted. On the other hand, including these factors through MCDA methods seems to push sea walls down the rank.