Análise da dinâmica de praias na borda continental do litoral Sul Fluminense

Abstract

The present study aims to analyze the dynamics of beaches located on the continental portion of the southern coast of Rio de Janeiro between 2022 and 2023. This coastline has numerous islands, small coves and narrow coastal plains, influenced by the presence of the crystalline escarpment of Serra do Mar. The beaches from Reserva do Sahy (Mangaratiba), Biscaia, Grande e Brava (Angra dos Reis) and Mambucaba (Paraty) were selected for monitoring, as they are relatively sheltered beaches within the bays and due to their importance for the region. The methodology consisted of carrying out a total of 5 seasonal fieldworks, carried out in all seasons of 2022 and in the summer of 2023. A total of 51 beach topographic profiles were acquired; 133 sediment samples were collected for particle size analysis and morphoscopy; in addition to records about sea conditions and geoindicators related to coastal erosion; the volume of sediment emerging from the beaches was calculated in order to verify gains or losses between monitorings; and mapping the coastline between 2005 and 2022, to identify areas susceptible to erosion. The results obtained show that the beaches studied exhibit a distinct morphological dynamic, as a reflection of their location in relation to the entry of waves into the bays, which hit Brava and Mambucaba beaches with greater energy, facing directly towards the western entrance of Ilha Bay. Big. The beaches of the Sahy, Biscaia and Grande Reserves are the least dynamic, with homogeneous profiles between the seasons. Brava beach showed variations in width between a maximum of 88 m (summer 2022) and a minimum of 50 m (summer 2023). Mambucaba was the most dynamic among the beaches studied, with variations in width between a maximum of 63 m (spring) and a minimum of 25 m (winter), both in 2022. The calculation of the volume of emerged sediments shows that the beaches present a sedimentary balance, in accordance with the beach topographic profiles acquired in 2022 and 2023, with few fluctuations in sediment gain and loss resulting from variations in sea conditions. The mapping of the coastline between 2005 and 2022 showed that the Biscaia and Grande beaches presented small variations resulting from the dynamics of the beach environment itself; Brava and Mambucaba beaches showed more significant variations due to changes in sea conditions. On the Reserva do Sahy beach, variations point to a retreat of 0.5 meters/year (southern sector), which may indicate an erosion process during this period. The sediments of these beaches are predominantly made up of medium quartz sand, subangular to subrounded, with a glassy shine; the degree of selection varies from very good to moderate. The size of the grains gradually increases towards the underwater portion on the beaches of Sahy, Biscaia and Grande, which does not occur on Brava and Mambucaba. The beaches studied generally exhibit a medium level of susceptibility to hangovers, due to the absence of dunes, scarcity of vegetation and the presence of urban structures close to the sand strip. It is expected that the data presented here will be used by local managers to mitigate and resolve coastal problems.