Evaluation of cliff retreat and beach nourishment in southern Portugal using photogrammetric techniques

This paper develops an evolutionary model of shoreline retreat along the cliffed coast of Forte Novo-Garrao (Algarve, Portugal) and investigates the relationship of the retreat pattern to coastal protection works, to the Quarteira fishing harbor, and to artificial beach nourishment at Vale do Lobo. The amounts and rates of cliff retreat for six coastal cliff sectors were measured using high-precision digital photogrammetric techniques utilizing data from three aerial photograph surveys conducted in 1991, 1999, and 2001. A geographic information system was used to integrate photogrammetric data and to quantify the temporal and spatial patterns of retreat. Forte Novo was the cliff sector with the highest amount of retreat, 22.7 m between 1991 and 2001. The other coastal sectors recorded retreats of less than 10 m for the same period. Retreat rates ranged from 2.27 m/y(-1) at Forte Novo to a minimum retreat rate of 0.14 m/y(-1) at Garrao. The replenishment measures at the Vale do Lobo beachfront have been largely successful, as there was negligible erosion between 1999 and 2001 along the cliff sectors protected by beach enlargement. These coastal management measures have been significant in reducing the coastal hazard to people and property located near, and downdrift of, the replenishment.