López-Dóriga, U. and Ferreira, Ó., 2017. Longshore and cross-shore morphological variability of a berm–bar system under low to moderate wave energy.
The morphological variability of a berm–bar system is highly dependent on wave conditions with direct consequences for the evolution of the nearby coast; however, an understanding of this variability based on field measurements is still lacking, in particular with respect to the assessment of both longshore and cross-shore components. In this study, the berm–bar morphological changes were assessed by simultaneously measuring longshore and cross-shore variability as well as the forcing mechanisms (i.e. wave conditions). A sandy beach in southern Portugal (Tavira Barrier Island, Ria Formosa) was surveyed from the dune crest to 20-m depth along six profiles over a period of 16 months. Wave time series between surveys were analysed to obtain relationships between wave conditions, the depth of closure, and berm–bar variations. For the surveyed period, waves approached mainly from the W–SW (78% of the time) and E–SE (18%), and eight moderate storms were recorded. Results show that the active zone of the profiles descends to 6 m below mean sea level (MSL), the maximum depth of closure registered. On the basis of the pattern of vertical variability, the profiles can be divided into three cross-shore sectors: A (emerged beach and berm), B (subtidal terrace), and C (submerged longshore bar). A new index (longshore vs. cross-shore [LvC]) is proposed, relating the dominance of longshore or cross-shore processes to volumetric changes resulting from sediment transport. In the study area, cross-shore sediment transfer (LvC ∼ 0) dominated when the largest and longest storms occurred and is associated with storm erosion and poststorm recovery. Longshore dominance (LvC ∼ −1 and 1 for erosion and accretion, respectively) occurred during periods with smaller and shorter storms. The proposed index allows the relative roles of cross-shore vs. longshore processes in coastal areas with berm–bar interaction to be distinguished.
