Revista de Gestão Costeira Integrada
Volume 8, Número 2, Dezembro 2008, Páginas 113-126

DOI: 10.5894/rgci143
* Submissão – 25 Julho 2008; Avaliação – 29 Agosto 2008; Recepção da versão revista –
12 Novembro 2008; Disponibilização on-line - 23 Dezembro 2008

Morfodinâmica de um Canal de Maré Tropical: Estudo de Caso na Costa Norte Rio Grandense, Nordeste do Brasil*

Morphodynamic of a Tropical Tidal Inlet: Case Study on the Rio Grande do Norte Coast, Northeast Brazil

Helenice Vital ^{1, 2}, Francisco dos Santos Neto ²,
José Saraiva Plácido Junior ²

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1 - Autor correspondente: [email protected]
2 - Departamento de Geologia, Laboratório de Geologia e Geofísica Marinha e Monitoramento Ambiental-GGEMMA, Universidade Federal do Rio Grande do Norte, Campus Universitário, Caixa Postal 1596, Natal-RN, 59072-970, Brasil.

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RESUMO
Este artigo trata da morfodinâmica de um canal de maré em barreira costeira tropical, localizado na Reserva de Desenvolvimento Sustentável Ponta do Tubarão-RDSPT, nordeste do Brasil. A região esta sujeita à alta energia sendo classificada como costa mista, dominada por ondas e marés. A influência significativa das marés sobre o sistema barreiras/canais de marés provoca a formação de proeminentes deltas de maré vazante.
A compreensão dos mecanismos de crescimento da barreira costeira e canais associados da Ponta do Tubarão auxiliarão na reconstrução da dinâmica e evolução costeira desta região. Nesta área a barreira costeira tem se modificado de barreira pontal para pontal e ilha barreiras a partir de aberturas de canais de marés no pontal inicial. O primeiro canal aberto naturalmente em 1977, originou a ilha Barreira Ponta do Tubarão, e desde então este canal de maré denominado Barra do Fernandez, tem sido usado como acesso de navegação pelos habitantes das comunidades locais de Barreiras e Diogo Lopes. Entretanto, como a distância entre o canal e Diogo Lopes é maior, persistiram as pressões publicas para abertura de acessos adicionais a navegação. A abertura natural de um novo canal em 2006 proporcionou a oportunidade para acompanhar sistematicamente as mudanças ocorridas em um canal de maré tropical. Desta forma, objetivando uma melhor compreensão dos processos costeiros atuantes na região o segundo canal tem sido monitorado e os dados aqui apresentados incluem a integração de sensoriamento remoto, levantamento batimétrico, sedimentologia, monitoramento da morfodinâmica e forçantes oceanográficas.
As correntes costeiras são responsáveis pelo transporte de sedimentos ao longo da costa, predominantemente para oeste-noroeste da ordem de 1,0m/s. No interior do canal correntes de maré vazante são as mais intensas atingindo até 0,85m/s. O monitoramento morfodinâmico atesta as constantes modificações nos canais de marés. As características geológicas e hidrodinâmicas do Canal da Barra Nova permitem classificá-lo como canal de maré litoral de energia mista dominado por vazante.
O registro de mudanças morfológicas significativas em curto espaço temporal indicam a necessidade do monitoramento continuo dos processos costeiros para entender a evolução de sistemas barreiras-pontal em costas tropicais. Adicionalmente este tipo de dado irá permitir aos tomadores de decisão um manejo racional da zona costeira.

ABSTRACT
This paper examines the tidal inlet morphodynamics on a tropical sandy coast, located on the “Reserva de Desenvolvimento Sustentável Ponta do Tubarão-RDSPT”, northeastern Brazil. . The region experiences high-energy been classified as a mixed coast, tide and wave dominated. The significant tidal influence to the barrier/inlet system causes formation of a substantial ebb-tidal delta. The mechanisms of growth and elongation of the coastal spit and tidal inlets from Ponta do Tubarão are underway to reconstruct the historical evolution and dynamics. The barrier on this area was completely joined to the mainland as a spit; around 1977 an inlet was naturally created and the barrier island was called Ponta do Tubarão. Since then this inlet, called Barra do Fernandez, is used as navigational access to lagoon by the habitants from Barreiras and Diogo Lopes villages. The distance between this inlet and Diogo Lopes is quite large, creating public pressure for additional navigation access. A new tidal inlet opened in 2006 give the oportunity to monitoring the changes in a tidal inlet. This way, we discuss the type of data that are being collected and analyzed to understand the environmental forcing affecting the inlet’s morphology. In a novel approach the integrated data set presented here includes remote sensing, bathymetric, althymetric, oceanographic, and sedimentologic data. The longshore currents flow toward west-northwest (oblique to the coast). They are by far the dominant contributor to the net sediment transport along the coast of Rio Grande do Norte. Currents measured inside a new inlet show maximum speed of 0.85m/s during ebb phase, while nearshore it can reaches 1.0m/s. The littoral environments of sedimentation are influenced by tidal, waves and wave-generated (longshore) currents. Aeolian deposits are also observed to migrate from the shoreline landward over tidal flat and lagoon sediments. A composite vertical sedimentary sequence in this region is composed of barrier spit/beach and aeolian dune sediments, over tidal inlet and shoreface units. The Reserva de Desenvolvimento Sustentável Ponta do Tubarão coast has nowadays two active inlets, naturally occurring. The net drift is to the West-Northwest along this coast. Natural inlets were created by overwash processes during more energetic periods. Our results indicated that, Barra Nova can be classified as a littoral origin, mixed energy, ebb-dominated tidal inlet.
If the hydrodynamic conditions persist, to maintain navigational interests, the Barra Nova inlet needs to be stabilized. This requires that this barrier spit coast be viewed as a system of structural features and transient morphology that interact with the environmental forcing at the inlet to attain an ever changing “dynamic equilibrium”. The imperative for sustainability at this area is managing the inlet in terms of extended time-space scales as defined by natural sediment processes, which means continuous monitoring. This data set is important to develop a detailed understanding of this kind of barrier-spit system evolution, to provide good architectural analogues for the nearby subsurface reservoir, and will enable planners to make the right decisions for a rational management of the coastal zone.

 

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