Morphodynamic variations of a macrotidal beach (Atalaia) on the Brazilian Amazon Coast.

Beach morphology, waves, tides, and current speeds were measured in a moderate-energy macrotidal environment of the Amazon coast (Atalaia beach, Brazil). The study was based on two complementary sets of data: (i) hydrodynamic and topographic data collected in four 25-hour campaigns during equinoctial and non-equinoctial periods, and (ii) climatological data (wind speed and direction, and rainfall) obtained from the Brazilian Institute of Meteorology. The results indicate that tidal currents run predominantly to the northwest during the flood tide and southeast during the ebb. The highest current speeds were recorded during the equinoctial period, reaching a maximum of 0.5 m/s in the flood tide. In March and June (rainiest months), ebb tide currents reached a maximum speed of 0.4 m/s. The tidal cycle was weakly asymmetric with the ebb tide lasting up to 6 hours and 40 minutes. High tides ranged from 4 m (nonequinoctial) to 5.7 m (equinoctial). Wave energy was modulated slightly during the low tide due to wave attenuation on the sandbanks, but substantial changes were not observed during ebb and flood tides. Modal wave height (Hb) varied from 1.1 to 1.43 m, associated with wave periods of 5.0-7.5s, with the highest values being recorded in November (intense winds). This beach can be classified as tide-modified, presenting primarily dissipative (O > 5) characteristic. Wave-dominated characteristics were observed in November, when Hs values were around 1.5 m and tidal elevation was approximately 4 m (non-equinoctial period). [ABSTRACT FROM AUTHOR]