On the influence of coastal structures on shoreline variability and beach interconnection

Abstract

Approximately 24 % of sandy beaches worldwide are experiencing erosional processes often driven by natural forces that are intensified by human activities. Erosion becomes a significant issue, particularly when it affects areas of socioeconomic and ecological importance. Thus, this study aimed to utilize coastline data from a city in southern Brazil, extracted from satellite images using free computational tools, with three main objectives: (1) identify spatial and temporal trends and cycles; (2) assess the impact of coastal structures; and (3) identify beach interconnection indicating bypassing processes in sandy beaches. For the first two objectives, shoreline data from 2015 to 2023, as well as statistical data, were extracted using the CASSIE tool. These data were filtered to minimize tidal effects and remove outliers. Subsequently, they were visualized using a Hovmöller diagram to provide a systemic overview. Through this diagram, it was possible to identify sediment transport toward the north and a sub-annual cycle of seasonality, with erosion occurring during the southern hemisphere's spring months, a period characterized by high-energy events. The relationship between erosional/accretional stretches and anthropogenic structures highlighted the negative impact of longitudinal structures, which exacerbated erosional processes, and the limited effectiveness of transverse structures in retaining necessary sediment. Moreover, the detrimental effect of building a jetty on its adjacent beaches was identified due to the alteration of the refraction control point and consequently, the equilibrium state of the beaches, which are unable to adapt due to coastal urbanization. For the third objective, shoreline data detected by the CoastSat tool from 1984 to 2023 were used. These data were subjected to Pearson correlation analysis. Perpendicular barriers to the shoreline that exhibited low correlation indicate limits of circulation cells that prevent the continuous bypassing process. Conversely, high correlation values suggest that the barrier allows for the occurrence of this process.