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Evolução morfossedimentar do sapal da Caldeira de Tróia em contexto de subida do nível médio do mar
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As alterações climáticas são uma preocupação global da sociedade e a comunidade científica tem, por isso, o dever de quantificar as suas consequências. Esta dissertação apresenta um estudo sobre o efeito da subida do nível médio do mar no sapal do setor norte da Caldeira de Tróia, localizado no concelho de Grândola, distrito de Setúbal, em Portugal continental. Este trabalho incluiu uma componente de campo, em que se realizaram levantamentos topográficos do sapal e sondagens manuais; e uma componente de laboratório, que incluiu a análise da atividade isotópica de 210Pb e 137Cs (efetuada por Tiago Silva, na Universidade de Genebra, Suíça) com o objetivo de determinar taxas de sedimentação médias relativas às últimas décadas. A abordagem para a resolução da problemática introduzida no primeiro parágrafo passou pelo desenvolvimento de um modelo em Matlab e Arcmap que permite estimar a morfologia futura da área em estudo, tendo em consideração quatro parâmetros fundamentais: as cotas dos limites de maré atuais; a morfologia atual do sapal; as taxas de sedimentação médias e o cenário de subida do nível médio do mar considerado. Para além disso, foi estabelecido um indicador de maturidade do sapal ao qual foi dado o nome de Rácio AS/BS, adaptado da classificação proposta por Davis Jr e Fitzgerald (2004). Foram utilizados quatro cenários de subida do nível médio do mar e os resultados foram organizados de acordo com estes: IPCC RCP2.6, IPCC RCP8.5, Antunes (2016) e NOAA High, sendo o primeiro linear e os restantes com diferentes valores de aceleração da taxa de subida anual. Foi possível aferir que, no que respeita à área total, o sapal estudado apresenta uma elevada resiliência até meados do século XXI (2050). Para 2100, os cenários IPCC RCP2.6 e RCP8.5 permitem antever a conservação de uma fração considerável de área, enquanto concretização das projeções propostas nos cenários Antunes (2016) e NOAA High poderá tornar crítica a manutenção desta área de elevada importância ecológica. Constatou-se ainda que a maturidade deverá sofrer uma redução considerável, independentemente do cenário adotado – efetivamente, esta perda de maturidade deverá estar já, atualmente, a ocorrer. Ter em consideração a variação espacial do sapal é fundamental para compreender a sua evolução e revelou ser uma condicionante importantíssima, pois é a translação deste ambiente para terra que permite a manutenção da maior parte da sua área. Note-se, no entanto, que esta expansão não é ilimitada e é determinada pela morfologia das áreas adjacentes ao sapal, em particular das dunas costeiras. Quando o sapal se aproxima da zona de maior declive das dunas, o seu movimento de translação fica limitado, observando-se, a partir deste momento, uma redução crítica da área total. Por fim, importa acrescentar que a metodologia apresentada para quantificar a resposta do sapal em estudo à subida do nível médio do mar produziu resultados interessantes, mas que devem ser encarados com precaução, dada a sua falta de validação.
Climate changes are a global concern of society and therefore the scientific community ought to quantify its consequences. This dissertation presents a study about the mean sea level rise at the salt marsh of Caldeira de Tróia north sector, located in Grândola, Setúbal district, Portugal continent. This study includes a field component, where several topographic surveys and manual cores of the salt marsh were done together with a laboratorial component, which includes an analysis of the isotopic activity of 210Pb and 137Cs (performed by Tiago Silva at University of Geneva, Switzerland) aiming to get the mean sedimentation rates regarding the last decades. The approach to solve the problem introduced by the first paragraph included the development of a model with Matlab and Arcmap, which enables to estimate the future morphology of the studying area, considering four fundamental parameters: current tidal limits; current morphology of the salt marsh; mean sedimentary rates and the considered scenarios of the mean sea level rise. Furthermore, it was established an indicator of the salt marsh maturity named Ratio AS/BS, adapted from the classification proposed by Davis Jr and Fitzgerald (2004). Four scenarios of the mean sea level rise were used and the results were organized according to these: IPCC RCP2.6, IPCC RCP8.5, Antunes (2016) and NOAA High. The first one is linear and the others have different acceleration values of the increase annual rate. It was possible to verify that, concerning the total area, the studied salt marsh presents a high resilience until the middle of the 21st century. By 2100, the IPCC RCP2.6 e RCP8.5 scenarios allow to foresee the conservation of a considerable fraction of the marsh area, while the concretization of the forecasts proposed by Antunes (2016) and NOAA High scenarios may be critical for the maintenance of this highly important ecological area. It was also possible to observe that the maturity will suffer a considerable reduction, no matters the adopted scenario – this loss of maturity may effectively be already occurring. The spatial variation of the salt marsh is fundamental to understand its evolution and it revealed to be a very important constraint, as only the translation of this environment to land allows the maintenance of most of the area. However, this expansion is not unlimited and it is constrained by the morphology of the salt marsh surrounding areas, mainly by the coastal dunes. When the salt marsh is near higher slopes of the dunes, its translation movement is limited and from that moment on, a critical reduction of the total area is observed. Finally, it is important to add that the presented methodology to quantify the Caldeira de Tróia salt marsh response to the mean sea level rise produced interesting results, but they should be looked carefully, regarding their lack of validation.
Climate changes are a global concern of society and therefore the scientific community ought to quantify its consequences. This dissertation presents a study about the mean sea level rise at the salt marsh of Caldeira de Tróia north sector, located in Grândola, Setúbal district, Portugal continent. This study includes a field component, where several topographic surveys and manual cores of the salt marsh were done together with a laboratorial component, which includes an analysis of the isotopic activity of 210Pb and 137Cs (performed by Tiago Silva at University of Geneva, Switzerland) aiming to get the mean sedimentation rates regarding the last decades. The approach to solve the problem introduced by the first paragraph included the development of a model with Matlab and Arcmap, which enables to estimate the future morphology of the studying area, considering four fundamental parameters: current tidal limits; current morphology of the salt marsh; mean sedimentary rates and the considered scenarios of the mean sea level rise. Furthermore, it was established an indicator of the salt marsh maturity named Ratio AS/BS, adapted from the classification proposed by Davis Jr and Fitzgerald (2004). Four scenarios of the mean sea level rise were used and the results were organized according to these: IPCC RCP2.6, IPCC RCP8.5, Antunes (2016) and NOAA High. The first one is linear and the others have different acceleration values of the increase annual rate. It was possible to verify that, concerning the total area, the studied salt marsh presents a high resilience until the middle of the 21st century. By 2100, the IPCC RCP2.6 e RCP8.5 scenarios allow to foresee the conservation of a considerable fraction of the marsh area, while the concretization of the forecasts proposed by Antunes (2016) and NOAA High scenarios may be critical for the maintenance of this highly important ecological area. It was also possible to observe that the maturity will suffer a considerable reduction, no matters the adopted scenario – this loss of maturity may effectively be already occurring. The spatial variation of the salt marsh is fundamental to understand its evolution and it revealed to be a very important constraint, as only the translation of this environment to land allows the maintenance of most of the area. However, this expansion is not unlimited and it is constrained by the morphology of the salt marsh surrounding areas, mainly by the coastal dunes. When the salt marsh is near higher slopes of the dunes, its translation movement is limited and from that moment on, a critical reduction of the total area is observed. Finally, it is important to add that the presented methodology to quantify the Caldeira de Tróia salt marsh response to the mean sea level rise produced interesting results, but they should be looked carefully, regarding their lack of validation.
Descrição
Tese de mestrado em Geologia do Ambiente, Riscos Geológicos e Ordenamento do Território, apresentada à Universidade de Lisboa, através da Faculdade de Ciências, 2017
Palavras-chave
Sapal Subida do nível médio do mar Modelação Alterações climáticas Estuário do Sado Teses de mestrado - 2017