Pereira, L.C.C., Vila-Concejo, A.and Short, A.D., 2013. Influence of subtidal sand banks on tidal modulation of waves and beach morphology in macrotidal beaches.
Amazon beaches are controlled by large tidal ranges, strong tidal currents and an ample supply of sediments, which create subtidal sand banks that modulate wave energy on the shoreline. Given this, the objective this study was to study the effects of tidal modulation of waves on spatial and temporal beach morphodynamics. This paper presents morphological and hydrodynamic data from three beaches on the Amazon coast of Brazil. Data was taken between November, 2008 and October, 2009. This study has two time scales: short term intensive campaigns with hydrodynamic measurements and topographic surveys and medium term measurements that refer to climatologic and offshore wave data obtained from National Institutes. For the analyses considering the ebb tide and flood tide conditions we have found higher RTR modal values and lower Ω modal values during the ebb tide periods, mainly in the most sheltered beach (higher influence of sand banks), showing the influence of tidal modulation. On the more exposed beach (lower influence of sand banks), a greater influence of wind-wave contributed to a similar pattern among the ebb and flood tide phases, prevailing low tide bar and non-barred dissipative characteristics. On the most sheltered area, the tidal processes dominate maintaining ultradissipative beaches and tidedominated tidal flat. On the macrotidal Amazon beaches tidal elevation and subtidal sand banks modulate the wave energy – during low tide, the waves break offshore on the sand banks and they only start to propagate over the banks when tide is increasing, reaching the maximum wave energy during high tides.