Revista de Gestão Costeira Integrada
Volume 19, Issue 4, November 2019, Pages 209-220
DOI: 10.5894/rgci-n148
* Submission: 9 OCT 2017; Peer review: 11 OCT 2017; Revised: 2 DEC 2019; Accepted: 2 DEC 2019; Available on-line: 27 MAR 2020
Nearshore currents and safety to swimmers in Xai-Xai Beach
Correntes costeiras e segurança de banhistas na Praia de Xai-Xai
Antonio Mubango Hoguane1, @, Tor Gammelsrød2, Kai H. Christensen3, Noca Bernardo Furaca1, Bilardo António da Silva Nharreluga1, Manuel Victor Poio4
@ Corresponding author, hoguane@yahoo.com.br, Tel: +258823152860
1 Centre for Marine Research and Technology, Eduardo Mondlane University, Mozambique
2 Geophysical Institute, University of Bergen, Bergen, Norway, torg@gfi.uib.no
3 Research and Development Department, Norwegian Meteorological Institute, Oslo, Norway, kaihc@met.no
4 Centre for Sustainable Development for Coastal Zone, Xai-Xai, Mozambique, mvictorpoyo@yahoo.fr, Tel: +258843110000
ABSTRACT
Xai-Xai Beach is a shallow semi-enclosed lagoon, about 2,000m long, 200m wide and 3m average depth, protected from ocean swell by a reef about 0.75m above the Mean Sea Level, with small gaps along its extension. Despite being protected from ocean waves, the lagoon, which is popular with tourists, is a dangerous place to swim, with an average of 8-9 drownings each year. The present paper examines the longshore and rip currents in the lagoon as the potential cause for these fatalities. Drifters were deployed for measuring the magnitude and direction of the nearshore currents. Unidirectional, northwards, longshore currents, with velocity up to 1.4ms-1 and strong rip currents, with velocity up to 3.4ms-1, 5-10m width and duration of less than 5 minutes, were observed. The observed rip currents pose a real danger even to experienced swimmers, hence it is concluded that they are the major cause of the deaths. The areas of major incidence of rip currents were identified. The result allows us to recommend a zoning of the beach into three areas as follows. The southern part, with weak currents, is classified as a "safe area", flagged green, and is recommended for swimming. The northern part, with strong rip currents, is classified as a "hazardous area", flagged red, and prohibited for swimming. In the intermediate zone, classified as a "moderate area" and flagged yellow, swimmers should proceed with caution. Regular patrols by lifeguards and beach assistants are highly recommended during the summer, the peak tourism season.
Keywords: Longshore currents, Channel rip current, swimming, drowning, beach zoning, beach safety.
RESUMO
A Praia de Xai-Xai é uma lagoa semifechada pouco profunda, com cerca de 2.000m de comprimento, 200 m de largura e 3 m de profundidade média, protegida por um grés de cerca de 0,75m de altura acima do Nível Médio do Mar, e com aberturas e fissuras ao longo da sua extensão. Apesar de estar protegida das ondas do oceano tem havido relatos de mortes, em média 8 a 9 casos por ano, devido a afogamentos. Neste trabalho investigam-se as correntes de deriva litorânea e de retorno como as potenciais causadoras dos afogamentos observados. Boías de deriva foram lançadas na praia para medir a magnitude e direção das correntes costeiras. Observou-se uma corrente unidirecional para o norte com intensidade média de 1,4ms-1 e correntes forte de retorno com intensidade média de 3,4ms-1, e com 5-10m de largura e um período de cerca de 5 minutos. As correntes de retorno observadas constituem perigo mesmo para nadadores experientes, daí que são consideradas as principais causadoras dos afogamentos observados na Praia de Xai-Xai. Os locais de incidências destas correntes foram identificados. Os resultados permitiram o zoneamento da praia em: parte sul, com correntes fracas, classificada como "área segura" e sinalizada com bandeirolas de cor verde, recomendada para natação; parte norte, com fortes correntes de retorno, classificadas como "área de risco" e sinalizada com bandeirolas de cor vermelha, onde a natação deve ser proibida; e a zona intermediária, classificada como "área moderada" e sinalizada com bandeirolas de cor amarela, onde a natação deve ser realizada com cautela. Recomenda-se a patrulha periódica por autoridades do corpo de salvação pública e monitoria permanente por unidades de salva-vidas durante o verão, o período do pico de turismo.
Palavras-chave: deriva litorâneos, corrente de retorno fixa, nado, afogamento, zoneamento da praia, segurança na praia.
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