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Nearshore sediment transport: coupling sand tracer dynamics with oceanographic forcing

Publication . Bosnic, I.; Cascalho, J.; Taborda, R.; Drago, T.; Hermínio, J.; Rosa, M.; Dias, J.; Garel, Erwan

The understanding of the sedimentary links between the beach and the continental shelf is crucial for the evaluation of the coastal sediment budget. However, the comprehension of this dynamics is still poorly understood owing greatly to the lack of direct sediment transport measurement at seasonal and longer time scales. This work aims at evaluating sediment transport just seaward of the closure depth through a sand tracer experiment coupled with wave–current monitoring and modelling. Observations were carried out over 1 year at 14 m depth over a sandy continental shelf offshore Tavira (southern Portugal). The sand tracer experiment was carried out by injecting 400 kg of fluorescent tracer followed by four sediment sampling surveys. Tracer results show a high dispersion of the tracer cloud with a net transport of low magnitude. Time-averaged alongshelf sediment transport rate was estimated in 0.61 m3/m/yr (southwestward) while the cross-shelf transport rate was estimated in 0.31 m3/m/yr (onshore). During the observational period nearbed currents were dominated by the northeastern component, thus flowing in the opposite direction of the tracer displacement. However, when wave-current bed shear stress exceeded the threshold of particle motion, nearbed currents were dominated by a southwestern component which is compatible with tracer displacement. Overall this study showed that seaward the closure depth bottom sediment dynamics is characterized by frequent remobilization but with very low net transport rates.

2017Journal article

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Longshore and cross-shore morphological variability of a berm-bar system under low to moderate wave energy

Publication . Lopez-Doriga, Uxia; Ferreira, Oscar

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.

2017-09Journal article

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Characterisation of coastal counter-currents on the inner shelf of the Gulf of Cadiz

Publication . Garel, Erwan; Laiz, I.; Drago, T.; Relvas, Paulo

At the Gulf of Cadiz (GoC), poleward currents leaning along the coast alternate with coastal upwelling jets of opposite direction. Here the patterns of these coastal countercurrents (CCCs) are derived from ADCP data collected during 7 deployments at a single location on the inner shelf. The multiyear (2008–2014) time-series, constituting ~ 18 months of hourly records, are further analysed together with wind data from several sources representing local and basin-scale conditions. During one deployment, temperature sensors were also installed near the mooring site to examine the vertical thermal stratification associated with periods of poleward flow. These observations indicate that the coastal circulation is mainly alongshore and barotropic. However, a baroclinic flow is often observed shortly at the time of flow inversion to poleward. CCCs develop all year-round and exclusively control the occurrence of warm coastal water during the upwelling season. On average, one poleward flow lasting 3 days was observed every week, corresponding to CCCs during ~ 40% of the time without seasonal variability. Thus, the studied region is distinct from typical upwelling systems where equatorward coastal upwelling jets largely predominate. CCCs often start to develop near the bed and are frequently associated with 2-layer cross-shore flows characteristic of downwelling conditions (offshore near the bed). In general, the action of alongshore wind stress alone does not justify the development of CCCs. The coastal circulation is best correlated and shows the highest coherence with south-eastward wind in the basin that proceeds from the rotation of southward wind at the West coast of Portugal, hence suggesting a dominant control of large-scale wind conditions. In agreement, wavelet analyses indicate that CCCs are best correlated with alongshore wind occurring in a band period characteristic of the upwelling system (8–32 days). Furthermore, in the absence of wind coastal currents tend to be poleward during summer. This set of observations supports that CCCs develop in response to the unbalance of an alongshore pressure gradient during the relaxation of (system-scale) upwelling-favourable winds, oriented south-eastward in the basin. The relaxation periods defined based on this wind direction show a good correspondence with the periods of poleward flow.

2016Journal article

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