Morfodynâmica do Cabedelo da Foz dor Rio Douro

Abstract

The dissertation we proposed to develop comprises, besides an intense bibliographic search, a strong fieldwork component. For the Douro mouth sand spit actual evolution knowledge we had resorted to GPS, technology that allows to obtain precise facts, whose analysis and interpretation have permitted us to infer about the behaviour of this sandy structure during the observation period. The morphodynamic evolution of the Douro river mouth sand spit is related with several factors, which interfere on the Portuguese coast line, detaching natural and anthropic actions. The Douro mouth region where, after a sinous and long course, the river contacts with the ocean, is endowed with self-proper dynamic, morphology and evolution. The Douro river sand spit located on the left margin of this estuary is close to N/S direction, being genetically considered as a result of a fluvial-marine commitment. Through the years, the Douro mouth problems, in general, and the sand spit problems, in particular, have showed difficult resolution or even unavoidable. The analyse of some historical details, namely the floods, the shipwrecks, the navigation difficulties, the implementation of several works and the countless proposals for improvement of the approach to Douro bar are the proof of that difficulty. The necessity of making security commercial changes culminated in the Leixões artificial harbour building, which seems to practise, cumulatively with other anthropic conditionants, some influence on sand spits morphodynamics. The deposition on this place is due to the decrease of solid material transport, related, obviously, with a flow velocity reduction, which results, by its turn, from the reciprocal and counterwork between the fluvial current, the tidal flow and the swell, without despising the wind action. The alluviums carried by the river, in time floods basically, and deposited in the terminal part of its course carries to sandbanks formation which, through the year, the marine action drags away to the open sea or to the continent, thickening or thinning the sand spit. The terminal part of this sandy structure, by the action of fluvial flow, as well the tide and wave, bends then, to upstream or downstream, acquiring rebellious and capricious shapes. The sand spit morphology reveals oneself, by consequence, unsteady and its variability constitutes a conditioning of the major or minor facility of the bar transposition. The sand spit was mapped since the last XVI century. Till our days, when it's possible to use scientific and technologically advanced methods, many cartographic works were made, whose elaboration obeyed, basically, to the demands from the socio-historic context of each epoch.