Publicação
As cheias na construção e evolução da Leziria do Tejo: sintese de dois projetos
| Resumo: | This article presents a summary of the final results of two projects funded by the Foundation for Science and Technology (FCT) in the field of fluvial dynamics, specifically, the floods of the greatest Portuguese rivers and the results of which are scattered in chapters of books, articles, abstracts, conferences or have never been disclosed as a whole: IDAFRE (An Interdisciplinary Approach to Flood Risk Evaluation - Hydrology, Geomorphology and Sedimentology of the Douro, Tagus and Sado Basins) (1997-2001) and GEOTARIF (Geology and Archaeology of the Tagus Floodplain Over Time) (2002-2006), where the Tagus river was mainly studied. The methods used can be divided into three categories: 1) data research in libraries, periodical archives, topographic and geological maps, history books, newspapers and magazines and old and recent bibliography, reconstruction of the history of the Tagus through the various sources consulted, with special relevance to the changes of the river channel throughout history, elaboration of flood and degree hazard maps; 2) field work for in situ observation of flood marks and their cataloguing, carrying out both manual and mechanical cores, sampling in present-day geomorphological elements of the alluvial plain, field observation of old abandoned channels and other morphological elements, monitoring and photographing flood episodes that took place between 1997 and 2006; 3) laboratory analyses of samples from all the cores, including sedimentological, pollen, organic matter, geochemical, 137Cs and 210Pb dating analyses, and analyses of radionuclide concentrations in the sediments of the alluvial plain. This set of techniques and methodologies made possible the development of a Flood Hazard map, calculation of the return period, the development of a table of the impact indices for progressive floods, variations in channel positioning, channel bars, natural dikes and flood plain and channel infilling, the variation of sedimentation rates throughout the twentieth century, the concentration of the floodplain by the contamination by heavy metals and radionuclides. The available data also allowed evaluate the evolution of the Tagus since the last glacial maximum, where it was probably an anastomosed river that carried sandy and pebbly sediments regarding a lower sea level than the present-day one. During the Holocene, the river built an alluvial plain composed of fine sediments with large and relatively stable isles between the channels, in an anastomosed pattern. However, successive human interventions in the alluvial plain are responsible for its transformation into a single channel river with alternating bars. The building up of the Tagus alluvial plain, throughout the Holocene, depended on natural causes affecting the entire drainage basin (climatic fluctuations and sea level rise) and anthropogenic causes, manifested either in the basin or in the floodplain, markedly obvious during the Bronze Age (from 3500 cal BP), with large-scale deforestation and consequent soil erosion, leading to a huge increase in the sedimentation rates of the floodplain. The direct human intervention on the Tagus alluvial plain was felt mainly from the Roman period on and intensified at the beginning of Portuguese nationality; XIII-XIV centuries (reign of D. Dinis). These interventions were aimed at draining alluvial plains with poor drainage, protecting agricultural lands from flood destruction and making the Tagus navigable, by avoiding its silting. The results show the importance of a multidisciplinary approach, in which the combination of different data allows a clear evolution in the research of sedimentary environments as complex as the alluvial plains. |
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| Autores principais: | Azevêdo, Teresa Mira de |
| Outros Autores: | Ramos Pereira, A.; Nunes, Elisabete |
| Assunto: | Floods Paleofloods Holocene evolution Dating Anthropogenic forcing factor Contamination Climatic fluctuations |
| Ano: | 2019 |
| País: | Portugal |
| Tipo de documento: | artigo |
| Tipo de acesso: | acesso aberto |
| Instituição associada: | Universidade de Lisboa |
| Idioma: | português |
| Origem: | Repositório da Universidade de Lisboa |
| Resumo: | This article presents a summary of the final results of two projects funded by the Foundation for Science and Technology (FCT) in the field of fluvial dynamics, specifically, the floods of the greatest Portuguese rivers and the results of which are scattered in chapters of books, articles, abstracts, conferences or have never been disclosed as a whole: IDAFRE (An Interdisciplinary Approach to Flood Risk Evaluation - Hydrology, Geomorphology and Sedimentology of the Douro, Tagus and Sado Basins) (1997-2001) and GEOTARIF (Geology and Archaeology of the Tagus Floodplain Over Time) (2002-2006), where the Tagus river was mainly studied. The methods used can be divided into three categories: 1) data research in libraries, periodical archives, topographic and geological maps, history books, newspapers and magazines and old and recent bibliography, reconstruction of the history of the Tagus through the various sources consulted, with special relevance to the changes of the river channel throughout history, elaboration of flood and degree hazard maps; 2) field work for in situ observation of flood marks and their cataloguing, carrying out both manual and mechanical cores, sampling in present-day geomorphological elements of the alluvial plain, field observation of old abandoned channels and other morphological elements, monitoring and photographing flood episodes that took place between 1997 and 2006; 3) laboratory analyses of samples from all the cores, including sedimentological, pollen, organic matter, geochemical, 137Cs and 210Pb dating analyses, and analyses of radionuclide concentrations in the sediments of the alluvial plain. This set of techniques and methodologies made possible the development of a Flood Hazard map, calculation of the return period, the development of a table of the impact indices for progressive floods, variations in channel positioning, channel bars, natural dikes and flood plain and channel infilling, the variation of sedimentation rates throughout the twentieth century, the concentration of the floodplain by the contamination by heavy metals and radionuclides. The available data also allowed evaluate the evolution of the Tagus since the last glacial maximum, where it was probably an anastomosed river that carried sandy and pebbly sediments regarding a lower sea level than the present-day one. During the Holocene, the river built an alluvial plain composed of fine sediments with large and relatively stable isles between the channels, in an anastomosed pattern. However, successive human interventions in the alluvial plain are responsible for its transformation into a single channel river with alternating bars. The building up of the Tagus alluvial plain, throughout the Holocene, depended on natural causes affecting the entire drainage basin (climatic fluctuations and sea level rise) and anthropogenic causes, manifested either in the basin or in the floodplain, markedly obvious during the Bronze Age (from 3500 cal BP), with large-scale deforestation and consequent soil erosion, leading to a huge increase in the sedimentation rates of the floodplain. The direct human intervention on the Tagus alluvial plain was felt mainly from the Roman period on and intensified at the beginning of Portuguese nationality; XIII-XIV centuries (reign of D. Dinis). These interventions were aimed at draining alluvial plains with poor drainage, protecting agricultural lands from flood destruction and making the Tagus navigable, by avoiding its silting. The results show the importance of a multidisciplinary approach, in which the combination of different data allows a clear evolution in the research of sedimentary environments as complex as the alluvial plains. |
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