Morphodynamics of Ponta da Areia Beach: Considerations on the Construction of a Coastal Groin in a Macrotidal Environment

Abstract

The Ponta da Areia beach, located on the oceanic shore of São Luís - Maranhão, is dominated by a macrotidal regime whose height reaches 7.5 m and waves of 1.5 m. This beach, whose name is due to its origin and evolution on a sandy spit that extends in the W-SW direction carried by the coastal drift of sediments, is subject, like so many other beaches on the northern Brazilian coast, to erosive processes. In 2011 the construction of a coastal groin began in Ponta da Areia beach with the objective of containing coastal erosion in that location. The present study is an evaluation of the post-groin beach morphodynamic behavior during 12 mouths in the years 2015 and 2016. For this purpose, four transects were monitored monthly (from January to December) through the performance of topographic profiles in the main coastal compartments surrounding the spit (Sectors 1, 2, 3 and 4). The results showed that the greatest sediment retention (accretion) occurred in Sector 3 immediately upstream of the spit. In this sector, the coastline is prograding, that is, at each tidal cycle the beach advances towards the center of São Marcos bay. In this sector, there is also the development of a free and vegetated dunes field, demonstrating that the increase in the beach is providing a greater wind fetch, expanding wind transport punctually in this sector. The sectors that are farthest from the action area of spit have alternated between periods of accretion and erosion, still demonstrating a certain instability of the coast, showing that the coastal spit has only softly mitigated the erosive processes acting on the Ponta da Areia beach.