Considerations for the design of rock armoured structures on fringing reef coastlines

The wave climate on coastlines of many Pacific Island Countries is mediated by fringing coral reef systems that act to naturally reduce wave conditions impacting the coastline. However, during extreme swell and cyclonic wave conditions it has been demonstrated that larger than expected waves can propagate across reefs/lagoons as a result of deep and steep bathymetric profiles immediately seaward of the reef, as well as complexities of infragravity waves generated within the reef and lagoon systems. Historically these issues have contributed to significant damage of port and coastal infrastructure throughout the Pacific (e.g. Nafanua Harbour 'Eua Tonga, Avatiu Port Cook Islands). While the complexities of wave processes on fringing reef systems is now becoming more widely published, information on the design of coastal protection structures specifically for these conditions is almost nonexistent. For example, guidelines for selecting appropriate design wave conditions when shallow water bore waves and infragravity surge dominate are not available. The applicability of existing methods for designing protection structures when applied to reef type coastlines has yet to be investigated, with practitioners applying methods such as the Hudson and Van Der Meer equations for armour sizing without acknowledging the potential lack of suitability and implications for design risk (through either over or under conservative design estimates). Due to the dependence of Pacific Island nations on sea freight and maritime transport through their ports, as well as protection of their low-lying communities, developing more appropriate design criteria specifically for these coastal environments is therefore of high social and economic importance. This paper presents the results of a physical modelling investigation undertaken at the UNSW Water Research Laboratory which set out to explore the applicability of design methods for protection structures on reef mediated coastlines. The investigation considered a range of rock armoured slopes typical of protection structures found on the coasts of Pacific Islands, and draws comparisons between observed armour damage with that predicted by available design methods. Preliminary guidance is provided on the suitability of available design methods when applied to reef-type situations, with recommendations also presented for further research essential to develop improved design methods.