Influence of coastal processes on grain-size features of sediments

Abstract

In this work, field observations of chanes on the beach sediment characteristics (considering the whole grain size distribution) of Fortaleza City, Brazil are presented. The studied area is a sandy-rich environment with very well developed dune fields (heights up to 50 m) and long, straight beaches. It is a mesotidal environment (tidal range up to 3.2 m) and it is subjected to the persistent action of easterrn waves. This dominant wave climate generates a very intense longshore sediment transport directed towards the NW (potential transport capacity of about 800,00 m³. yr^-1). Field data obteined show a time variation in the sediment grain size parameters (e.g. mean, standard deviation) as well as in the distribution, i.e. cyclic changes from unimodal to bimodal distributions and vice versa. These changes are related not only to the winnowing of finer fractions but also to a surplus of coaser fractions not previously detected. Moreover, the described changes are not constant across the profile neither in the alonshore direction. Samples taken at diferente locations (vertically with respect to the MWL as well as alongshore) show a specific behaviour that can be related to beach profile changes (generation of swash bars under high energy waves and subsequente welding during mild conditions), which are of great interest to understand and to model coastal morphodynamics. Thus, most of the time when a morphodynamic model is used, uniform sediment characteristics are assumed. In some cases, spatial variations in the sediment characteristics are considered but as a general rule, the sediment distribution is assumed stead. Due to this, the analysed processes can be used as a field validation case for models considering the concepto f “sediment mixtures”, i.e. non-uniform sediment.