Avaliação do clima de ondas da praia de Ponta Negra (RN, Brasil) através do uso do SMC-Brasil e sua contribuição à gestão costeira Translated title: Wave Climate Assessment on the Ponta Negra Beach (RN, Brazil) using SMC-Brasil and its contribution to coastal management

Abstract

Para o planejamento e gestão efetivos da orla marítima, visando a implantação de obras de engenharia para contenção da erosão e a manutenção da linha de praia, é imprescindível o conhecimento dos forçantes hidrodinâmicos atuantes, destacando-se o clima de ondas. No Brasil há uma carência de dados de ondas, que pode ser suprida através de bancos de dados gerados através de modelos numéricos (dados de reanálise e com downscaling), tal como o proposto pela Universidade de Cantábria/Espanha, cuja base de dados de ondas foi disponibilizada para o Brasil através do SMC-Brasil, que visa a transferência dessa base de dados, além de ferramentas e metodologia para uma melhor gestão do litoral brasileiro. Este trabalho utilizou o SMC-Brasil para analisar o clima de ondas da região costeira da praia de Ponta Negra localizada em Natal/RN, Nordeste do Brasil, que possui elevada importância turística, com amplo crescimento e ocupação urbana nas últimas décadas. O ponto selecionado como representativo do regime de ondas incidentes, localizado a uma cota batimétrica de 20m, na latitude 5,8775°S/longitude 35,0301°W, apresenta as seguintes características de clima de ondas: predominância de ondas de leste-sudeste (ESE) em mais de 75% dos estados de mar, seguido de ondas de leste (E=20%), sudeste (SE=3%) e leste-nordeste (ENE=2%). Sazonalmente, a predominância de ondulações provenientes de ESE ocorre em todas as estações do ano, porém no verão (Dezembro a Fevereiro) nota-se o aumento na participação de ondas provenientes de E. A altura de onda significativa (Hs) varia entre 0,5m e 2,8m, com 75% dos estados de mar apresentando ondas inferiores a 1,6m. Ondas com Hs superior a 2,6m apresentam um período de retorno probabilístico de aproximadamente 10 anos. Em relação aos períodos de pico (Tp), os valores variam entre 4s e 20s, sendo que 75% dos estados de mar apresentam Tp inferior a 8s. Ondas com Tp superior a 18s apresentam um período de retorno probabilístico de mais de 10 anos. A análise da distribuição conjunta de Hs-Tp e Hs-Dir mostra que no ponto selecionado as ondas mais frequentes são as com Hs entre 1,3 e 1,7m, Tp em torno de 8s, provenientes da direção em torno de 110°. Na propagação dos casos espectrais verificou-se que a região sul da praia de Ponta Negra (área do Morro do Careca) é protegida da ondulação, alcançada por valores máximos de Hs de cerca de 1,5m em eventos de tempestade. Isso ocorre tanto devido aos efeitos de difração e refração, mais evidentes nas ondulações de ESE e SE, quanto também aos efeitos de perda de energia devido ao fundo marinho deste setor, que apresenta uma batimetria mais suave, sendo mais dissipativa. Deste modo, destaca-se o aumento à exposição das ondas de sul para norte na praia de Ponta Negra, em direção à Via Costeira, com ondas de até 2,5m alcançando a linha de costa em eventos de tempestade. Os resultados deste estudo têm implicações relevantes para a gestão da zona costeira, como subsídio à melhoria do planejamento e gestão da orla, nas tomadas de decisões sobre o uso e ocupação dos terrenos próximos à linha de costa. Sugere-se que modelos similares sejam aplicados em outros locais como base para as ações de intervenção humana, em concordância com os princípios de gestão integrada e sustentável da zona costeira.

Translated abstract

For effective planning and management of the coastline, from the implementation of engineering projects to contain erosion to the maintenance of the coastline, it is imperative to understand the hydrodynamic forces that influence coastal morphodynamics, especially wave climate. In Brazil instrumental data collection of waves on a large scale, systematically and in long and continuous time series is still challenging. Aiming fill this gap, some countries have developed methodologies which use numeric models, involve the reanalysis of previous data and perform downscaling in order to generate wave databases. One of these databases was proposed by the University of Cantabria, Spain and was used by SMC-Brasil, which aims to use the database, tools and methodology to improve the management of the Brazilian coast. This study used this database to analyze wave climate of coastal region of Ponta Negra beach located in Natal, northeast Brazil. Ponta Negra beach is important because of its fame for tourism and because it is a driving force behind the urban growth in this area. The wave climate was analyzed here as a basis of the beach erosion dynamics study in the region, as well as to develop a contribution to the management of public policy regarding seafront construction. Ponta Negra beach is a headland-bay beach located in the eastern sector of Rio Grande do Norte state. It is comprised of two sections: Ponta Negra beach itself and the beach along the Via Costeira, which extends to the North. The assessment of the role of wave climate at a point located at 20m depth in the continental shelf (latitude 5.8775°S / longitude 35.0301°W) by evaluating the SMC-Brasil wave database. The wave climate near the beach was determinate by the propagation of spectral wave cases associated with storms and normal conditions, at high and low tide. This representative point exhibited the following wave climate characteristics: predominance of waves from East-Southeast (ESE) and it represented more than 75% of the wave direction, followed by waves from the East (E=20%), Southeast (SE=3%) and East-Northeast (ENE=2%). The seasonal variation in wave direction is slight and the predominant direction is ESE in all seasons; however, in summer (the months of December, January and February), an increase in waves from the East can be observed. The significant wave height (Hs) is between 0.5m and 2.8m and the waves are below 1.6m in 75% of sea states. Waves with an Hs higher than 2.6m showed a probabilistic return period of approximately 10 years. For peak periods (Tp), the values range from 4s to 20s and the Tp is below 8s in 75% of sea states. Waves with a Tp higher than 18s showed a probabilistic return period of more than 10 years. The analysis of the joint distribution of Hs to Tp and Hs to Direction shows that at this point, the most frequent waves are those with an Hs between 1.3 and 1.7m, a Tp around 8s and from an 110° direction. Based on the results obtained from the propagation of spectral wave cases during storms and under normal conditions, it can be seen that one of the characteristics of Ponta Negra beach is a wave height gradient with lower values in the southern region and increasing values along the Via Costeira. This is due to the effects of diffraction and refraction seen mainly in the SE and ESE waves and to the effects of energy loss because of the bathymetry in the southern region of Ponta Negra beach is typically gentler than the bathymetry in the North, with dissipative characteristics. In the protected area near Morro do Careca the maximum Hs values were registered at 1.5m while in the northern region towards the Via Costeira waves up to 2.5m were seen to reach the coastline during storms. By comparing the results from high and low tide wave propagations, it was found that in low tide the waves with the highest significant wave height reached the coastline. Visual assessments of the beach also showed that under these conditions and due to the erosion processes taking place at the coastline and the loss of sediment at the backshore of various sections of Ponta Negra beach, the waves reach the urban structures on the shoreline and are leading to its deterioration. And so, it can be concluded that based on the knowledge of the wave climate in a region such as Ponta Negra beach and based on the database from SMC-Brasil it is possible to plan, manage and take action to protect the coastal area more effectively and sustainably. Currently, the majority of the projects undertaken in the coastal zone do not take studies of marine dynamics into consideration and thus they are inefficient and have caused huge social, environmental and economic long-term damage. Through the use of the tools provided by SMC-Brasil, it has been shown that it is possible to obtain data and put it to use in the planning and management of the coastline. Thus, the results of this study have major implications for coastal zone management, as contribution to improve planning and management of seafront infrastructure, in decisions about the use and occupation of land near the coastline. It is suggested that similar models are implemented in other locations as the basis for the actions of human intervention, in accordance with the principles of integrated and sustainable management of the coastal zone.