Modelling the longevity of beach nourishment and the influence of a detached breakwater

Abstract

This study addresses the longevity of beach nourishment in a high-energy coastal environment. It focuses on a site along the Northern Portuguese Atlantic coast, characterized by intense littoral drift, in the order of 1 million m3 and N–S oriented. The region undergoes an erosion process, and some localities are protected by groynes and frontal defense structures. The longevity of beach nourishment is evaluated through a coastline evolution model (previously calibrated and validated), compared for three scenarios: the absence of nourishment, i.e., the “do nothing” scenario; a (one-shot) 4 million m3 of sand nourishment intervention; and a detached breakwater combined with nourishment. The recent morphological evolution of the study site is evaluated through the results of a four-year (2018–2021) topobathymetric monitoring program implemented by the Portuguese Environment Agency. The lifespan of the nourishment ranges from 4.5 to 7.5 years. While the detached breakwater significantly increases the beach width to the north, it also exacerbates sediment loss to the south, regardless of its position. However, after 10 years, the cumulative sediment balance for the combined scenario closely mirrors that of the nourishment-only approach. The discussion of these results includes reflections on the longer-term impacts of using coarser sediments (relative to native material) on coastline evolution and on the influence of the breakwater’s layout on nourishment longevity.