Journal of Coastal Research, Special Issue No. 65, 2013 Influence of subtidal sand banks on tidal modulation of waves -macrotidal beaches 1821 Influence of subtidal sand banks on tidal modulation of waves and beach morphology in Amazon macrotidal beaches Luci Cajueiro Carneiro Pereira†, Ana Vila-Concejo‡ and Andrew D Short∞ †Instituto de Estudos Costeiros, Alameda Leandro Ribeiro, s/n, Aldeia, Bragança, 68600-000, Brazil cajueiro@ufpa.br ‡School of Geosciences F09. The University of Sydney, Sydney, NSW 2006, Australia ana.vilaconcejo@sydney.edu.au ∞ School of Geosciences F09. The University of Sydney, Sydney, NSW 2006, Australia andrew.short@sydney.edu.au INTRODUCTION More than three decades ago, several models were elaborated to predict beach state with respect to wave and sediment parameters in microtidal beaches, and these do not consider the tide effect (Wright and Short, 1984; others). The state of the art about the morphodynamics in macrotidal beaches was firstly showed in an overview descript by Short (1991). In fact, tidal effects on beach processes in meso-and macrotidal beaches have received less attention than those dominated by swell and-or higher wind-wave energy in open-coastal beaches (Masselink and Hegge, 1995; Jackson et al., 2005; others). One of the conceptual beach model applied in macrotidal beaches around the word is that proposed by Masselink and Short (1993). This model was generated using macrotidal beaches in Australia and it is based in two parameters (Ω and RTR). The dimensionless fall velocity (or Dean´s number, Ω) was proposed firstly by Gourlay (1968) and followed by Dean (1973). This parameter was used by Wright et al. (1985) that described a simple model classifying the beaches in reflective, intermediate and dissipative. In addition, the relative influence of tides and waves were firstly proposed by Davis and Hayes (1984). Based on this, Masselink and Short (1993) considered an additional parameter, the Relative Tidal Range (RTR) used to quantify tidal effects on beaches. The basis of this model are wave, tide and sediment characteristics (i.e., modal breaking wave height, modal breaking wave period, upper beach face sediment characteristics and mean spring tidal range). Thus, the model proposed by Masselink and Short (1993) suggest that increasing Ω and RTR, the beach morphology can be classified in wave-dominated beach, mixed wave-tide beach and tide-dominated beach. For the first time, this model has been applied in Equatorial beaches located under influence of high riverine discharges, macrotides and moderate wave energy. In these beaches, the wave energy is modulated during low tides on subtidal sand banks (Monteiro et al., 2009; Pereira et al., 2012a; Pereira et al., 2012b). This means that these beaches are partially sheltered during low tides receiving lower wave heights and shorter wave periods. An opposite pattern is observed during high tide elevations, when the waves of longer periods and higher heights propagate along the shoreline. To understand the influence of subtidal sand banks on tidal modulation of waves and beach morphology in Amazon coastal area, this study aims to apply the morphodynamic beach model proposed by Masselink and Short (1993) under nearshore (ebb and flood tide periods) and offshore wave conditions. The study was undertaken in three macrotidal beaches under the influence of different wave energy conditions. STUDY AREA Climate is equatorial humid with annual average temperature between 26 and 27°C and rainfall normally from 2000 to 3000 mm, with approximately 75 to 85% of the rainfall levels falling in the wet season, between January and May (INMET, 2009). ABSTRACT 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. In: Conley, D.C., Masselink, G., Russell, P.E. and O’Hare, T. J. (eds.), Proceedings 12 th International Coastal Symposium (Plymouth, England), Journal of Coastal Research, Special Issue No. 65, pp. 1821-1826, ISSN 0749-0208. 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 tide- dominated 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. ADDITIONAL INDEX WORDS: Macrotides, beach profiles, moderate energy, Amazon littoral. www.JCRonline.org www.cerf-jcr.org ____________________ DOI: 10.2112/SI65-308.1 received 07 December 2012; accepted 06 March 2013. © Coastal Education & Research Foundation 2013