Revista de Gestão Costeira Integrada
Volume 15, Número 1, Março 2015, Páginas 65-80

DOI: 10.5894/rgci515
* Submission: 29 MAY 2014; Peer review: 6 JUL 2014; Revised: 19 SEP 2014; Accepted: 22 SEP 2014; Available on-line: 22 SEP 2014

Evolution of the hydrodynamics of the Tagus estuary (Portugal) in the 21st century *

Martha Guerreiro ^a, André Bustorff Fortunato ^{@, a}, Paula Freire ^a,
Ana Rilo ^a, Rui Taborda ^b, Maria Conceição Freitas ^c, César Andrade ^c, Tiago Silva ^c, Marta Rodrigues ^a, Xavier Bertin ^d, Alberto Azevedo ^a

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@ - Corresponding author: <[email protected]>
a - National Civil Engineering Laboratory, Av. do Brasil, 101, 1700-066 Lisbon, Portugal.
b - University of Lisbon, Faculty of Science, LATTEX, IDL., Bloc C-6, 2nd Floor, Campo Grande, 1749-016 Lisbon, Portugal.
c - University of Lisbon, Faculty of Science, Geology Department, Geology Center, Bloc C-6, Campo Grande, 1749-016 Lisbon, Portugal.
d - CNRS-Université de La Rochelle, UMR 7266 LIENSS, Institut du Littoral et de l’Environnement, 2 rue Olympe de Gouges, 17 000 La Rochelle, France.

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ABSTRACT
The ongoing rise in sea level affects tidal propagation and circulation in estuaries, and these changes can have far reaching consequences on the sediment dynamics, water quality and extreme water levels. This study aims at anticipating the evolution of the tidal dynamics in the Tagus (Portugal) in the 21st century, in particular due to sea level rise (SLR). The existence of a resonance mode of about 8 hours in this estuary, that selectively amplifies both semi-diurnal and quarter-diurnal tidal constituents, makes the response of the Tagus estuary to SLR unique. The study was conducted with a shallow water model, forced by present and future conditions, namely higher mean sea levels and an extrapolated bathymetry based on present sedimentation rates. Model results showed that SLR will significantly affect tidal asymmetry, in particular because the intertidal area can decrease by up to 40% by the end of the 21st century. As a result, the strong ebb-dominance of this estuary will decrease significantly. This evolution of tidal asymmetry will be counteracted by the effect of sedimentation of the saltmarsh areas. Also, SLR will enhance the resonance in the Tagus estuary. As a consequence, extreme water levels will be higher than the sum of present levels with the SLR.

Keywords: tidal asymmetry, resonance, climate change, SELFE, modeling.

Evolução da hidrodinâmica do estuário do Tejo (Portugal) no século XXI

RESUMO
A subida do nível médio do mar afeta a propagação da maré e a circulação em estuários, podendo estas alterações ter consequências significativas na dinâmica sedimentar, na qualidade da água e nos níveis de água extremos. O presente estudo visa antecipar a evolução da dinâmica de maré no estuário do Tejo (Portugal) no século XXI, em particular devido à subida do nível médio do mar (NMM). A existência de um modo de ressonância de cerca de 8 horas neste estuário, que amplifica seletivamente as constituintes semi-diurnas e quarto-diurnas, torna a resposta do estuário do Tejo à subida do NMM única. O estudo foi efetuado com um modelo hidrodinâmico, forçado por condições presentes e futuras, nomeadamente níveis médios do mar mais elevados e uma batimetria extrapolada com base em taxas de assoreamento atuais. Os resultados do modelo mostram que a subida do NMM irá afetar significativamente a assimetria da maré, em particular porque as áreas intertidais podem diminui até 40% até ao final do século. Em consequência, a forte dominância de vazante deste estuário irá diminuir significativamente. Esta evolução da assimetria da maré será parcialmente compensada pelo efeito da sedimentação nas áreas intertidais. A subida do NMM irá também amplificar o efeito de ressonância no estuário do Tejo, o que resultará em níveis extremos superiores à soma dos níveis extremos atuais com a subida do NMM.

Palavras-chave: assimetria de maré, ressonância, alterações climáticas, SELFE, modelação.

 

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