Lisboa, 24, 25 e 26 de junho de 2014 297 Sediment textural selection during sub-aerial headland bypassing. An example from the Nazaré coastal system (Portugal) J. Cascalho (1) , J. Duarte (2) , R. Taborda (3) , M. Ribeiro (2,3) , A. Silva (3) , I. Bosnic (2,3) , M. Carapuço (3) , C. Lira (3,4) and A. Rodrigues (2) (1) Centro de Geologia, Faculdade de Ciências, Universidade de Lisboa. Museu Nacional de História Natural e da Ciência, Universidade de Lisboa. jpcascalho@fc.ul.pt (2) Instituto Hidrográfico. (3) Instituto Dom Luiz, Faculdade de Ciências, Universidade de Lisboa. (4) Centro de Geologia, Faculdade de Ciências, Universidade de Lisboa. Abstract: This work aims to characterize and understand the textural selection process related to headland sediment bypassing at three adjacent beaches (Norte, Prainha and Nazaré) located at the west coast of Portugal. To characterize sediment median grain size variability across the study site, 72 samples were collected and photographed with a digital single-lens reflex camera. This procedure generates a set of 261 digital images that were processed using the image autocorrelation method in order to compute the median sediment grain size. Preliminary results show that the Nazaré headland (between Norte beach and Nazaré bay) can act as a filter to the coarser sediment particles transported by the southward longshore drift, during the observed field conditions. The coarser particles (of coarse sand) were often found updrift of the Nazaré headland (at Norte beach), while at the downdrift side (Prainha beach) these particles are scarcer. Key words: littoral, sand, grain-size, selection, transport 1. INTRODUCTION Grain size is the most widely used textural parameter to describe the sedimentary environments and to deduce the sediment transport dynamics (e.g. Folk and Ward, 1957; Mason and Folk, 1958; Friedman, 1961; Visher, 1969; McLaren and Bowles, 1985; Flemming, 1988). Over the last decades the development of the digital photography associated to the enhancement of computer processing capabilities has enabled the development of new techniques to determine several grain size parameters. Of these, the median grain size (D 50 ) can be easily determined by image analysis, using the spatial autocorrelation, thus making this technique ideal to characterize a high number of samples (Rubin, 2004; Barnard et al., 2007; Bosnic et al., 2012). The main objective of this work is to characterize the D 50 variability of the Nazaré coastal system (Norte-Guilhim-Prainha) and understand the sediment bypassing processes at Nazaré headland. This work, supported on sediment image analysis, is based on data obtained during a large scale fluorescent tracer experiment that took place in this coastal system, during September of 2013 (Duarte et al. 2014). 2. FIELD SITE The studied area is located at the Portuguese west coast, near Nazaré village, approximately 120 km north of Lisbon (Fig. 1 e 2). This coastal stretch is fully exposed to the north Atlantic swell and has a semi-diurnal mesotidal regime. Fig. 1. Nazaré coastal area location (Source: Esri basemap, 2011-03-18). Fig. 2. General view of the studied beaches: Norte (upper left), Guilhim (right) and Prainha (below left).