Morfodinâmica recente da planície de maré externa e litoral adjacente em Icapuí – CE

Abstract

In the Icapuí city, Ceará State (Brazil), the existence of an intriguing external tidal flat (ETF), referred as an ebb-tidal delta in the literature, captures the attention. It is also part of an Environmental Protection Area with a rich ecosystem for migratory birds. With approximately 12 km2, the Icapuí ETF is one of the largest in the State, and it is about 60% larger than its internal counterpart (lagoon), being also directly attached to the beach on both sides of its inlet. Its progradation, however, is not associated with an embayed region, in addition to the absence of a significant river course, either at its inlet or on the internal tidal flat. The region is also marked by a relatively strong coastal drift current, up to 0.31 m/s, oriented SE-NW. In this context, the main objective of this study was to estimate and calculate sediment transport vectors to understand the dynamics of the Icapuí ETF, reverberating on the interpretation of its origin and classification. For this purpose, 188 sediment samples of the ETF were collected and analyzed to calculate transport vectors based on the statistics of the first three Pearsons’ moments of the grain size. Thus, for the two-dimensional analysis, 26 transport trend vectors (14%) were accepted with an average direction towards ESE (120o AZ, L95=0.91), by significance testing in the GSTAST program. However, all vectors with greater significance (n=9, L95=1.19) point towards NNE (28o AZ). In the first case, it is interpreted the general effect of the ebb tide current over the ETF oppositely to the expected longshore drift, and in the second, its more specific effect through subtidal channels in front of the inlet. In isolation, the one-dimensional granulometric analysis of the beach, from a heavy mineral index, favors transport in the SE direction on both sides of the inlet, but especially on the side of the same quadrant that, in such direction, would be dominated by the ebb tide as opposed to longshore drift. This one-dimensional interpretation for the beach is supported by the heavy mineral index for transport (sillimanite-garnet, SGi), whose value is high (65% on average) and increases towards SE (up to >85%). This set of characteristics allows assuming that the local transport of the Icapuí PME is dominated by the ebb tide, corroborating the classification of ebb-tidal delta for the Icapuí ETF and, at the same time, implying an atypical condition for its origin and maintenance over time