O processo erosivo e as variações morfodinâmicas praiais na Ilha de Mel (Baia de Paranagua - PR)

Abstract

Resumo: O principal objetivo deste trabalho e apresentar os dados obtidos sobre o processo erosivo existente hoje sobre a ilha do Mel em especial sobre seu istmo. A erosão na área critica foi acompanhada desde 1991. Foram cartografadas, em escala 1:1.000, as linhas de costa do istmo nos anos de 1953,1985, 1991, 1992 e 1995. Alem disso foram efetuados 40 perfis de praia durante o verao de 1991/92 e repetidos durante o período de inverno de 1992, resultando em 499 amostras sedimentologicas, as quais permitiram a caracterização sedimentologica das praias da ilha, Apesar da homogeneidade dos sedimentos da ilha, foi possível agrupa-los em compartimentos diferentes. Constatou-se que a erosão no istmo e episódica, ocorrendo preferencialmente durante as conjunções de mares de sizígia e mares meteorológicas originadas pela passagem de frentes frias, fenômeno conhecido regionalmente como "ressaca". O recuo da linha de costa, na parte mais estreita do istmo, ocorrido entre 1953 e 1991 foi estimado em, no mínimo, de 120 metros. Entre maio de 1991 e maio de 1992 foram erodidos 20 metros e entre julho de 1992 e setembro de 1992 12 metros. Durante algumas mares de sizigia e tempestades uma porção do istmo fica submersa.

Abstract: The aim of this work is to present the data reached on the study of the erosional process installed over Mel Island isthmus and its other beaches. This island is located on Paranagua Bay's mouth, and on plant, looks like a badly drawn eight, with a northwestern part bigger than the southeastern one, connected by a narrow isthmus. The island coastline has a perimeter of 35 Km. The isthmus is, nowadays, under a strong erosional process, and during spring tides a small belt is putted under the sea waters, remaining few time before completing the separation process. There is the possibility that the separation could modify completely the dynamics of the bay mouth. Protected bay areas will be submitted to an open sea dynamics. These modifications could affect Galheta Channel, which, nowadays, is the access to Paranagua Harbor, the third most important harbor in Brazil and the biggest one in the grain sector in Latin America. The erosional dynamics was monitored during 1991 and 1992 and map-made on 1:1.000 scale. The new coast line has been compared to the 1985 one, (from ITCF chart, on the same scale), and to the 1953 aerial photographs (1:25.000). It also have been done 80 beach profiles around the island, 40 on summer time and 40 on winter, resulting on 499 sediment samples. It has been observed that the erosional process is episodic and it is associated to sea storms. The phenomenon occurs when there is the association of spring tides and storm tides, both are relationed with climatic cold fronts. The coastline, at the isthmus, has retreated, between 1953 and 1991, at least 120 meters, and only between may of 1991 and may of 1992 20 meters were eroded. This work also brings the complete sedimentological characterization of Mel's island beaches, which have been grouped on compartments according to their similarities.