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 similar to 0) dominated when the largest and longest storms occurred and is associated with storm erosion and poststorm recovery. Longshore dominance (LvC similar to 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.