Detached breakwaters, yes or not? A modelling approach to evaluate and plan their removal

Abstract

Detached breakwaters have been widely employed as a measure against beach erosion and for coastal protection. However, this type of structures may cause different critical issues, and the advantages are not always evident. Water stagnation, rip currents development in the gaps between multiple structures, and obstruction of the natural panorama are well-recognised issues. However, their removal is always problematic because it is generally associated with rapid erosion of the beach and high costs. This work proposes a modelling approach to obtain a preliminary evaluation of a theoretical removal intervention and discusses its possible consequences. The study case is the Moneglia beach, a tourist beach located on the eastern Ligurian coast, NW Italy. The beach is characterised by the presence of three detached breakwaters, arranged over the entire length of the beach. A dataset consisting of LiDAR data, multibeam data, single-beam data and DGPS data was used to obtain an accurate digital elevation model (DEM) of the study area. Subsequently, the XBeach model was used to simulate three scenarios: i) state-of-the-art scenario (S0), ii) detached breakwaters removal scenario (S1), iii) detached breakwaters removal scenario + nourishment (S1N). The S1 scenario was implemented to evaluate the beach morphodynamic response in the case of a removal intervention. S1N was implemented to simulate a beach recovery plan through a beach nourishment intervention. Once the S1N final scenario was obtained, XBeach was implemented to get a comparison with the sea flooding hazard in the S0 and S1 scenarios. The sea flooding hazard assessment was performed considering storm events with a return time of 50 and 100 years, respectively. The results showed both the potential of the XBeach model as a tool and the sustainability of a theoretical detached breakwater removal intervention.