Evolução recente do Portinho da Arrábida: Contributos da geologia para uma correta gestão ambiental

Resumo

The Portinho da Arrábida beach has long been recognized as one of the most beautiful Portuguese seashores. Geological evolution, with recent significant changes in beach size and sediment quality, led to a decline of beach environment. In parallel with geological changes, significant modifications occurred in relation to the land use and occupation density, in accordance with an absent or inadequate land management. In order to improve the understanding of the coastal processes in this area, a review of the short-term geological and geographic changes that occurred in the bay were made, and major problematic issues were identified. The analysis of the evolution of Portinho da Arrábida beach shows that the amount of sand has dropped significantly since the beginning of the last century. Presently, there is a retreat of sand with damaging effects on the beach quality. The beach size decreased significantly, remaining only 40% of its area and 37% of its length. In addition, the relative percentage of pebbles on the beach face has increased, and a significant part of the remaining 37% of the beach is often covered by coarse material in the intertidal zone. The erosive trend has been maintained, and the retreat of the coastline, although slow, already started to reach the dune at the western part of the Creiro beach. Geological indicators suggest that the sand volume variations in Praia do Portinho are related to a natural trend. However, due to the close relationship of the beach with the Sado delta, the maintenance of the navigation channel and the associated dredging, promote changes in the delta dynamics that may play a negative role on the beach size. This work intends to be a contribution to improve the knowledge of Portinho da Arrabida beach dynamics, in order to help to find solutions, so that upcoming plans in the area can, desirably in an integrated way, stop and/or reduce the effects of natural changes and anthropic pressures undertaken in the bay.