Revista de Gestão Costeira Integrada
Volume 8, Número 2, Dezembro 2008, Páginas 101-112

DOI: 10.5894/rgci145
* Submissão – 20 Agosto 2008; Avaliação – 19 Outubro 2008; Recepção da versão revista – 30 Outubro 2008; Disponibilização on-line - 14 Novembro 2008

Monitoramento da Linha de Preamar das Praias de Olinda – PE (Brasil) como Ferramenta à Gestão Costeira *

High Water Line Monitoring of Olinda-PE Beaches (Brazil) as Coastal Management Tool

Mirella Borba S. F. Costa ^{1, 2}, Patrícia M. Pontes ²,
Tereza C. M. de Araújo ²

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1 - Autor correspondente - mirella_borba@yahoo.com.br
2 - Laboratório de Oceanografia Geológica do Departamento de Oceanografia UFPE - Universidade Federal de Pernambuco, Av. Arquitetura s/n, Recife, Pernambuco, Brasil, CEP: 50740 550

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RESUMO
Diante do avanço da linha de costa em relação ao continente, em áreas de grande densidade urbana, é observada mundialmente a adoção do método hold the line pelos gestores costeiros. Esse é o caso do litoral do município de Olinda – PE que desde 1958 vem realizando obras de proteção costeira contra o avanço do mar, e hoje conta com a presença de dez quebra-mares paralelos à linha de costa, 34 espigões perpendiculares e vários trechos de enrocamento aderente. O monitoramento da linha de costa nessa região é essencial para definir pontos e períodos críticos de avanço do mar e fornecer subsídios ao planejamento de ações da gestão pública local. Este trabalho analisou o comportamento da linha de costa durante 14 meses (entre agosto/06 e outubro/07) utilizando a linha de preamar máxima (high water line – HWL) como indicador. O método utilizado consistiu na fixação de um referencial de nível (RN), a partir do qual foram realizados perfis perpendiculares em onze localidades, e calculada a largura da pós-praia. Foi observado que em agosto e setembro o avanço do mar em relação ao continente foi mais expressivo devido à ação de marés meteorológicas mais intensas, tendo em vista que nesses meses foram registrados maiores alturas de ondas e velocidade de ventos na costa. O teste de regressão mostrou alto valor preditivo da altura da maré na largura da pós-praia para as praias com presença de quebra-mar e baixo valor, ou até mesmo ausência de correlação para as praias com espigão onde as ondas exercem relativamente maior influência. Os pontos críticos identificados pelo monitoramento, ou seja, áreas onde as estruturas urbanas estão mais comprometidas pelo avanço do mar apresentam largura da pós-praia bastante reduzida e baixa mobilidade da linha de costa. A metodologia utilizada mostrou-se uma ferramenta simples, de baixo custo e bastante eficaz para obter tendências a curto prazo das variações na linha de costa. A implantação de um programa permanente de monitoramento da linha de costa é importante para a obtenção de resultados mais precisos sobre o comportamento da linha de costa e verificação de padrões sazonais na sua posição. Dessa forma, as ações dos gestores costeiros podem ser melhor planejadas.

Palavras-chave: erosão costeira; linha de costa; estruturas de proteção costeira.

ABSTRACT
In densely populated areas, in case of landward migration of the shoreline, the most frequently worldwide used policy option for coastal managers has been hold the line. This is the case of Olinda’s littoral (Brazilian coastal city) where coastal protection structures has been built since 1958 to stabilize the shoreline, and at present, the beach is protected by ten breakwaters parallel to the shoreline, 34 groins perpendicular to it and several parts with seawalls. Olinda’s littoral tides are semidiurnal and in terms of amplitude are classified as mesotidal. When the equinoctial tide coincides with the spring tide it can reach the height of 2.8 m. This can be even more severe during the second semester (August / September) when the winds are more intense. This phenomenon is known locally as storm surge and that periods are the most energy in the local coast causing the greatest changes in beach morphology and destruction of buildings located along the waterfront. Thus, generating concern to authorities and buildings owners located in vulnerable places to erosion. Shoreline monitoring in this region is crucial to set hot spots and critical periods of shoreline erosion, and then subsidize planning activities of local public management. This paper examined the shoreline change for 14 months (between August/06 and October07) using high water line - HWL as indicator. The method used in this study was based on monthly field campaigns, during the spring tide, where profiles alignment perpendicular to the waterline were made from predetermined fixed benchmark (the profile anchor) and backshore (recreational beach) width calculated. The analysis of shoreline mobility was carried out from the calculation of standard deviation of their positions. Tests of simple linear regression for each profile monitored were conducted to ascertain the dependence degree of the backshore width with the tide height. It was observed that in August and September the landward migration of the shoreline was more expressive since the occurrence of intensive meteorological tide in view that in these months higher waves heights and winds velocities were recorded on the coast. The monthly monitoring of high water line position showed that it can vary up to 30m in high tides of same height in different months without any evidence of erosive trend or increased. It was also possible to identify that the urban structures (sidewalk and houses) of two beaches (Bairro Novo and Rio Doce), are most susceptible by high seas. At these beaches lack of backshore was registered during the months of the year where stronger winds and higher waves occurred. The regression analysis showed high predictive value of the tide height in the backshore width for beaches with presence of breakwater and low value, or even lack of relationship for beaches with groins where the waves carry relatively greater influence. The hot spots identified by the monitoring, or urban areas where the structures are further threatened by the sea, have very low backshore width and low shoreline mobility. The methodology proved to be a cost-effective simple tool, and very useful for obtaining short-temporal changing trends and thus can be easily implemented by coastal managers. A beach monitoring program, carried out over several years, could give rise to more accurate results which are useful to characterize shoreline behavior and serve as basis for sound policy decisions. Thus, the actions of coastal managers can be better planned.

Keywords: coastal erosion; shoreline; coastal protection structures.

 

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