Algarve Barrier Islands: A Noncoastal-Plain System in Portugal

Abstract

The small barrier island system of the Algarve (South Portugal) exhibits several unique characteristics that differ from more widely studied systems. The islands exist on a noncoastal-plain coast as a result of a platform on the inner shelf which acted as a cape during the sea level rise. Initial spits became islands as the platform was transgressed, and the islands are transgressive in appearance, although west-to-east lateral processes (inlet migration, spit growth) dominate. The islands exist under moderate to high wave energy and mesotidal conditions. Flood-tidal deltas are dominant and inlet migration is rapid. Aeolian and overwash processes are important to vertical growth of the islands. The largest islands have widened as a result of flood-tidal delta incorporation. After incorporation, aeolian processes modify the delta surface into a dune field before stabilization by vegetation. Backbarrier dissection by spring tide (4 m.) flooding works in opposition to the constructional effects of delta incorporation, over-wash and dune formation. An extensive network of erosional channels has developed on backbarriers as a result of erosion by the ebbing flood Channel location is controlled by antecedent morphology such as incorporated delta channels, interdune areas, and troughs between incorporated recurved spits. Once established, the channels incise, erode laterally and extend into the island, sometimes joining with overwash passes, and forming important conduits of sediment transport to the lagoon/marsh. The lagoonward sediment transport is partially offset by floating sand and shells carried toward the island interior on the incoming flood tide. Spring tide flooding also results in vegetation kills and groundwater contamination.