Sediment grain size variation along a cross-shore profile – representative d50

Abstract

The coastal zones management not only relies on a profound knowledge of the shoreline but also on a good assessment of morphodynamics, sediments transport and coastal interventions required. Many coastal studies, such as beach slope estimates, equilibrium beach profile definition or sediment transport analysis, depend on the sediment grain size at the site. This paper aims to gain a better understanding of grain size distribution along the cross-shore profile and variation of d50, allowing to define a representative d50 to characterize the beaches. On the study site chosen, Barra beach, Aveiro, an extensive field campaign was performed, from October 2010 to May 2011. In a weekly basis, samples from 5 points along a cross-shore profile were collected. During this period, data related to wave climate was collected from the Portuguese Hydrographic Institute in temporal series of 10 min. Tidal projections from Hydrographic Institute were also identified for the period of the field campaigns. The sediment grain size distributions showed that, although the mass-median-diameter does not usually pass the 1 mm, d50 presented a great variation during winter, in the intertidal zone. On the other side, the first and last points of the cross-shore profile, located far from the intertidal zone, presented the smaller d50 and variation through time. The significant wave height presented a mean value of 2.16 m. The most energetic wave climates happened in November, January and February. During those months, an increase of d50 in the wave breaking zone was noticed. Generally, the expected behavior of the cross-shore sediment grain size distribution is in line with the data collected in this paper and it was verified that the grain size is coarser on the intertidal zone and after storm periods. Also, despite the complexity of defining a characteristic d50 due to its great temporal and spatial variation, sensitivity analysis on the data obtained helped the identification of the upper foreshore limit, at high tide, as the better location for a sediment sample collection representative of a cross-shore profile.