Small harbours risks: lowering for fishery and increasing erosion. The case of Portinho da Aguda (NW Portugal)

Abstract

The harbour Portinho da Aguda was established between October 2001 and July 2002, at Aguda village. The area presented some rocky outcrops that, in some way, protected the coastal border against direct wave impact, on a coastal stretch where erosion has been a continuing problem. In an attempt to create a protected area for the entrance and way out of the community’s small fishing boats a detached curved breakwater was built. After some months, still during the works, a wedge beach was created against the temporary groin, promoting the enlargement of the beach located updrift and the erosion of the downdrift coastal segment (Granja Beach). To remediate this situation, sand transfer from the updrift area was done, to compensate the accentuated loss of sediments of Granja Beach, a famous old seaside resort. But, due to the very high energetic conditions, sands did not remain on the beach, and new spring to early summer recharges became necessary. Based on results from a comprehensive monitoring program of the coastal border north and south of Aguda between 2001 and 2009 with DGPS, sedimentary budgets were calculated and morphological changes identified, comparing situations before and after breakwater construction. Breakwater impacts on neighbouring areas of the harbour are presented, as are the evolution trends of the harbour itself. Some possible mitigation measures are pointed out and the efficiency and relevance of this kind of coastal structures is discussed.