Análise da estabilidade em planta e dinâmica de praias do Rio de Janeiro: um estudo de caso nas praias de Copacabana, Macumba e Grumari (Rio de Janeiro – Brasil)

Abstract

The beaches of Copacabana, Macumba, and Grumari (Rio de Janeiro – Brazil) exhibit distinct characteristics regarding the level of human intervention, coastline orientation, and geomorphology. The objective of this study is to analyze the coastal stability of these beaches in Rio de Janeiro, using a methodology that assesses the stability of their planform and aspects related to coastal dynamics. The following variables were evaluated: wave climate; the energy flux of breaking waves on the beach (EF); longitudinal sediment transport (LST); variation in the coastline (VLCO); and how close the average coastline angle is to the angle of the static equilibrium planform (costa). The analysis of compiled information revealed the identification of four stability zones in Copacabana beach: zone 1 representing a stable region, relatively protected from wave energy; zone 2, a more unstable region, showing an increase in VLCO and EF and a decrease in average LST; zone 3, considered more stable, indicating a region with less variation in the width of the beach; and zone 4, an unstable region with high EF values. In Macumba beach, four stability zones were identified based on the stability classification: zone 1, where instability is mainly attributed to high EF and VLCO values; zone 2, a more stable region, showing cells of convergence and divergence of LST; in zone 3, the presence of a convergence cell of LST is observed, but the presence of the Macumba Rock outcrop in this location acts at times trapping sediments, resulting in erosion; zone 4 is a region of high instability caused mainly by costa, VLCO, and average LST. In Grumari, three zones were identified: zone 1 is generally stable, displaying a convergence cell of LST; zone 2 covering a relatively unstable region; and zone 3 is a highly unstable region, where VLCO plays a crucial role in this classification. This study demonstrated that along the same beach, different factors can be responsible for observed coastal instability. Additionally, beaches located in nearby regions may be subject to different conditions contributing to the instability observed in each of them. Overall, in the analyzed beaches, the planform orientation, combined with other parameters, proved to be an effective indicator of stability, highlighting a trend of higher costa values in regions considered unstable. Furthermore, evaluations of coastal stability should consider not only the dynamics of the coastal zone but also the geological inheritance observed at each location.