O efeito dos arenitos de praia no comportamento da linha de costa da praia de Arembepe, Camaçari, Litoral Norte do estado da Bahia

Abstract

Although beachrocks are considered coastal protection agents, the different ways in which they are distributed longitudinally and transversely on the beach does not allow this statement to be generalized. This work aims to evaluate the effect of beachrocks on the behavior of the coastline of Arembepe (North Coast of Bahia, Brazil), since in this locality there are two or more generations of beachrocks, located either in the nearshore, or on the beach face or in both positions, making this place a laboratory for studies of the influence of beachrocks in different locational situations. The methodology of the work consisted of 13 campaigns to survey beach profiles based on the Emery’s Method (1961), in addition to the mapping of beachrocks and coastlines from 1976 and 2022. 21 transects were also plotted to measure the displacement of the coastline between these years and calculated the respective rates and errors. The profiles were located in different situations: i) in circulation cells limited at the ends by beachrocks (profiles 1 to 3), corresponding to the southwestern section of the study area; ii) in front of a sandstone body attached to the beach face and another on the nearshore, without and with the presence of a channel between them (profiles 4 and 5, respectively), pattern that occurs in the central part of the study area; iii) on two faces of an salient with a discontinuous sandstone present in the nearshore (profiles 6 and 7), corresponding to the northeastern portion of the study area. The analysis of beach profiles 1 to 3 contributed to the understanding of sedimentary dynamics, showing seasonal changes in the direction of longshore drift and demonstrating a transport for NE between April and September, and for SO between October and March. This dynamic can be extrapolated to other coastal stretches in the North Coast of Bahia, which also have NE-SW coastline orientation and are subjected to the same atmospheric conditions. In addition, the results allowed to identify the influence of beachrocks in long (Holocene), medium (about 50 years) and short term. In the long run the coastline of Arembepe, seen in plan, is stepped, which resulted from the successive formation of asymmetrical tombolos, which were gradually calcified. In the medium term, in the southwestern part of the studied area, where the beachrocks have only the role of closing a circulation cell (profiles 1 and 3), a progradation was observed in the last 46 years, possibly associated with changes in wave regime. On the other hand, the central part of the study area, where beachrocks are present both on the beach face and in the nearshore (profiles 4 and 5), is in equilibrium, showing that in 1976 the tombolos were already consolidated. In the northeast section, in turn, there is a situation of progradation and stability. This pattern can be attributed to discontinuous nearshore beachrocks, which produces a coastline with intercalations of salients and coves, with different behaviors. In the short term, it was observed a great sedimentary dynamics and variability of the position of the coastline in the southwest section, where the beachrocks present at the ends of the circulation cell allow a beach rotation throughout the year. In the central part, the presence of beachrocks on the beach face was responsible for a little seasonal variation in the beach profiles and little variability in the position of the shoreline, reinforcing the idea that the sandstones on the beach face hinder the sediment exchange on the beach. In the northeastern section, the variability of coastline position was intermediate to the other two sections. Finally, it is concluded that the protection provided by beachrocks was not homogeneous on the beach of Arembepe, depending on the different situations in which they are: closing circulation cells, attached to the beach face or presenting themselves discontinuously on the nearshore.