Physical and Economic Feasibility of Rubble Mound Breakwater Upgrades for Sea Level Rise

Sea level rise with climate change poses a threat to the stability of rock armoured breakwaters in which thearmour size varies with design wave height (and water depth) cubed. In this paper, the physical andeconomic feasibility of upgrading typical rock and concrete armoured breakwaters is assessed with casestudies representative of typical Australian east coast sites.The physical feasibility was investigated in two dimensional wave flume testing of armour stability. Recentresults show that a single layer of larger upgraded armour stone can be placed on top of a conventional twolayer rock breakwater and thus provide protection for sea level rise. These results for rock are promising andcontrast with previous testing of concrete armour unit CAU breakwaters.Economic feasibility based on Net Present Value analyses incorporating storm probabilities and breakwaterperformance criteria was developed to evaluate different planned upgrade sequences and determine a costeffective strategy. The model showed that breakwater upgrade decisions are most sensitive to the discountrate. For a case study with rock breakwater and design life of 100 years, with real interest rates below 4.5%the best economic decision in 2015 is to design and build the breakwater for the 2115 projected conditions.For interest rates between 4.5 and 8.5% structures should be designed for 2075 conditions and upgraded forthe 2115 conditions in 2065