Evolução da vulnerabilidade à erosão costeira na Praia de Massaguaçú (SP), Brasil

Abstract

Coastal erosion can cause major environmental and social damage to coastal areas, and disorderly and inadequate human occupation at these sites can lead to erosion. Massaguaçú Beach, on the northern coast of São Paulo State, has suffered severe erosion in recent years. In the most severe cases, erosion had already begun to affect the road way Dr. Manuel Hyppolito Rego (SP-55) that runs next to the beach. Several efforts have been implemented to contain this problem, but have proved to be unsuccessful. The cause of these erosions remain unclear, however, studies suggest that these changes in the coast line may be related to the increasing urbanization around the beach. Because of this phenomenon, this study aimed to analyze the coastal erosion vulnerability at the beach over a period of 40 years, with the objective of evaluating if human occupation had a significant impact in the local erosion. Data was gathered using nine vulnerability indicators: beach width, shoreline position, offshore settings, presence of rivers and/or inlets, terrain elevation, vegetation, coastal engineering structures, occupation rate and soil permeability. The methodology consisted of analysis of several aerial pictures of the beach (from 1962, 1973, 1977, 1987 e 2001) and data from earlier works. To facilitate the analysis of Massaguaçú beach, its 7.5 km stretch was divided into three distinct sectors: north, center, and south sector. The division was made based on the different features present in each sector. The north sector is characterized by the presence of the Cocanha Islands, which serve as a natural barrier for the beach, and the Bracuí river. The center sector contains the largest stretch of the road way SP-55 that runs next to the entirety of the center sector, and has absence of rivers and offshore barriers. The south sector is characterized by the absence of the road way and by the presence of the Capricórnio River. The results show that coastal erosion vulnerability has increased in a non-constant way over the analyzed period. The indicators beach width and shoreline position demonstrated oscillating behavior, which in turn made the final vulnerability result oscillate as well. The other indicators demonstrated stable or increasing vulnerability results. The central part of Massaguaçú—where erosion is intense—presented the highest rates of coastal erosion vulnerability over time. Although human occupation seemingly has been influential in the results, the study concluded that it might not be the direct cause of the current erosion in Massaguaçú. This holds true because there was a predominance of low vulnerability even in years where the occupation was growing. Recent studies suggested that the largest contribution to the recording of the erosion in the studied area came from the longshore current that transports sediment from the center sector to the neighboring sectors. The method used in this study has proven effective in answering the questions posed. It is a quick, inexpensive method to evaluate the environment, making it accessible for coastal managers to carry out rapid environmental assessments. As a result, better understanding of areas vulnerable to erosion may lead to more responsible city planning along the coastal area.