High and Low Frequency Erosive and Constructive Cycles in Estuarine Beaches: an example from Garcez Point, Bahia / Brazil

Abstract

Monitoring of the morphodynamic variations of the beaches associated with an estuary contiguous with Garcez Point, Bahia, Brazil, and the superposition of aerial photographs from the region, show the presence of distinctive erosive and constructive cycles of low and high frequencies. Between 1959 and 1989, one event of shoreline erosion and progradation was recognized on the oceanic beaches just outside the estuary. Inside the estuary, an erosion phase at the southern margin coincides with a constructive phase at the other side, and vice-versa. On the southern estuarine beach, low-frequency cycles of erosion and progradation are also perceived, but with the inverse trend when compared to the contiguous oceanic beach. During the beach monitoring period (February/1991 to July/1992), the oceanic beach showed retreat rates varying from 23.7 m/year, at the channel entrance, to 1.0 m/year, three kilometers away from it. During the same period, the estuarine beach advanced at a rate of 60.3 m/year. The long-term dynamics of the shoreline position in both sides of the estuarine entrance appears to be related to the position of the channel in the ebb-tidal delta.