Deforestation and coastal erosion: A case from east Brazil

Many beaches from the eastern Brazilian coast are in erosive conditions. A coastal reach around the town of Alcobaca, located at the estuary of the ho monymous river, was monitored through daily observations of meteorologic, o ceanographic and sedimentologic parameters during 10 months. Two points are of special interest in the Alcobaca dynamics: 1) the erosion phenomenon is restricted to the last 30 years, when 30 to 40 meters of the beach have be en lost; and 2) beaches located on both sides of the estuary are subjected to erosion. During most of the observation time, littoral drift was southwa rd as a consequence of the NE wave field. Erosion occurred when a NE-wind i ntensification or a wind change to SE, associated with the arrival of a col d front, coincided with the high tide, generating a beach scarp. During spr ing tides, the beach scarp reaches the permanent vegetation zone. The destr uction of these previously stabilized areas induces an irreversible migrati on of the shoreline. The coastal erosion at Alcobaca is considered to be re lated to a disequilibrium in the Alcobaca River outlet. Beach erosion on bo th outlet sides indicates that both drift directions are sediment-starved a nd that the retention capacity of the Alcobaca River outlet has been amplif ied, supplanting the ability of both drift directions to bypass the sedimen t across it. The fact that the ebb-tidal delta in the river outlet has grow n over the last few decades, concurrent with the erosive phenomenon, also c orroborates this hypothesis. Patterns of heavy minerals content and the gra nulometric distribution of the sands around the estuarine spit also support the enhanced retention model, providing evidence for sand trapping at the ebb-tidal delta of the estuary outlet. The increase of the hydraulic retent ion effect, termed here hydraulic mole, which causes the beach erosion, is here interpreted as due to changes in the Alcobaca River characteristics, m ainly the increase of its suspended-load concentration, as a result of inte nse deforestation of the Atlantic rain forest. Therefore, we consider that the ultimate responsibility for the erosion at the Alcobaca area lies with the deforestation and its direct consequences, and not the wind-waves-litto ral drift dynamics, which act merely as reequilibration agents.