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Revista de Gestão Costeira Integrada

versão On-line ISSN 1646-8872

Resumo

ROSA, T. L.; BARATA, A.; CABACO, J. Geadas  e  TELES, M.. Interventions to Dredge the Aveiro Bar (Portugal) and Improve Protection of the Coastal Area to the South. RGCI [online]. 2012, vol.12, n.1, pp.56-76. ISSN 1646-8872.

The Aveiro coastal lagoon is located on the NW coast of Portugal. It is the largest Portuguese lagoon system which is permanently open to the sea. It is centred within a sand coastal section about 150km long which has regular bathymetry, with bathymetric lines almost parallel along the coast. This regular pattern is only disturbed in the vicinity of the Aveiro Lagoon, which is connected to the ocean through a channel that provides shipping access to the port of Aveiro, located inside the lagoon. The NW waves generated in the North Atlantic, by predominant N and NW winds, prevail near the NW Portuguese coast during almost 80% of the year. In the channel connecting the lagoon to the ocean, and in contrast to most of the Portuguese coast, the currents induced by the tide are very strong. Sometimes they can reach about 4ms-1 at the entrance to the lagoon. Along the shoreline, off Aveiro bar, the currents are mainly southward during most of the year, with velocities about 0,5ms-1, and tide reaches of about 2,85m during spring tide, and 1,25m in neap tide conditions. The ocean near the lagoon is characterised by an intense transport of mainly non-cohesive sediments, southwards along the shoreline. Since the 1950’s the area has been subject to several interventions intended to improve shipping access to the port, including the construction of maritime protection structures. The main north breakwater at the entrance of the lagoon improved access by reducing sediment deposition in the navigation channel, but also interrupted the littoral drift, promoting sand deposition to the north of the north breakwater, and increasing erosion to the south of the entrance. After the northern zone became saturated, silting of the channel commenced. Several coastal protection works, along the coastline, have attempted to minimize this problem, including seawalls, groins, breakwaters, and dredging of the channel to guarantee the depths required for shipping. The present study evaluated the most suitable sites for the dredging of 1 000 000 m3 of sand from the channel or surrounding area, as well as the deposit of the dredged material to the south of the lagoon entrance, in order to: guarantee safety and operability of the port of Aveiro; avoid silting-up of the coastal zone adjacent to the port entrance; and minimise erosion of the coastal areas to the south. The study was performed in 2005, for the three authorities with jurisdiction over the coastal area of Aveiro Lagoon (IPTM - Institute of Ports and Maritime Transport; APA - Aveiro Port Administration and INAG - Portuguese Water Institute) and took into account the local hydrodynamic conditions and the characteristics and constraints of the project. It contributed significantly to coastal management in the area, since supports decision-making, conducting to the definition of an integrated solution for the situation referred to above. This paper describes the methodology used to define the possible sites for dredging and deposition of the sand, as well as the criteria used to select the preferred solution. Firstly, the available hydrographic information was analysed, in order to characterize the area and the bathymetric evolution of the sea bed. This information was then used to study the morphodynamic behaviour in the area, including estimating the sediment transport. Alternative sand dredging and deposition solutions were tested using a 2D hydrodynamic numerical model, including waves and tide propagation, currents induced by the waves and sedimentary transport. Finally, the preferred solution was chosen using multi-criteria analysis for a comparative analysis of seven dredging and deposition solutions, considering technical, environmental and economic criteria. The final solution consists of dredging of 1 000 000m3 of sand in the Aveiro bar entrance, between 9,0m and 12,5m depth, and the deposition of the dredged material in the sea to the south, in the submersed sector of the beach, between 2,0m and 5,0m depth, over an area between the third and fifth groins of Costa Nova. Finally, the study recommended the development of a regular monitoring programme to take place after the project implementation, in order to objectively evaluate sedimentary and bed evolution over the dredged and deposition areas (beach and bed evolution). The chosen solution was implemented by the competent authorities after 2008.

Palavras-chave : Numerical modelling; hydrodynamics; littoral drift; dredging; integrated coastal zone management.

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