A shoreface morphodynamic zonation and the equilibrium profile variability on the northern coastline of Rio Grande do Sul, Brazil

Abstract

The sedimentary distribution variation of a 60 km shoreface tract on the northern coastline of Rio Grande do Sul shows a relationship between distinct morphodynamic zones. This relationship was based on 11 beach and shoreface equilibrium profiles of up to 15 m of water depth, at 55 shoreface and 4 beach sedimentary samples. Textural variations displayed contrasting dynamics that were revealed by the distinct designs and shapes of the profiles. These were extended and taken over the closure depth (7.5 m for the area). They revealed a dissipative beach sand systems, with a shape factor of m = 0.74.Ahigh variability was identified, reflecting the erosion tendencies on this coastline segment. Statistical sedimentary parameters for such sedimentary distributions permitted the identification of two main sediment facies: (A) sandy facies: (B) muddy sand with bioclastic gravel facies. Crossshore variations show three morphodynamic zones: high energy (2 to 4 m), transitional (6 to 9 m) and low energy zones (11 to 16 m), and classifying, high, intermediate and low shoreface patterns. The morpho-sedimentary distribution defined three sections: (a) North, having moderate slope profiles (0.010) with less variability in sediment fractions, (b) Center, showing a wide range of profiles (from 0.008 to 0.010) with a finer and coarser sedimentary distribution and (c) Center-South, identifying much steeper slope (0,017) with seaward fining sediments. Morphodynamic shoreface-inner shelf boundaries (toe) were shown at 18 m at the North section, changing to 16 to 25mat the Center and Center-South sections.