Sediment transport and provenance in the rías de vigo and muros with especial emphasis in the bottom layer and beach morphodynamics driving processes

Resumo

In this thesis the sediment transport dynamics in the Rías de Muros and Vigo (NW Iberian Peninsula) have been studied. The hydrodynamic forcements affecting the bottom sediment dynamics in the marine inner sector of the Ría de Muros and in the marine outer-mid sector of the Ría de Vigo under summer conditions (upwelling conditions, low wave energy and low fluvial input) have been analysed. The physical processes and factors controlling the morphodynamics of the Ría de Vigo beaches have been also studied. The morphodynamics and transport conditions of one of these beach systems, Samil, have been characterized and modelled in detail for practical purposes. Under summer conditions, the shelf wind produced subtidal circulation was the main driving process of the bottom SPM dynamics subtidal area of the Rías de Muros and Vigo marine sector. In the inner marine sector of the Ría de Muros the SPM measured in the bottom was mainly of fluvial origin transferred from superficial waters under downwelling condition (South shelf wind). The oceanic waters arriving under upwelling conditions (North shelf wind) did not transport SPM producing the lowest turbidity conditions. On the contrary, in the Ría de Vigo the upwelling events produced high SPM concentration, suggesting the advection of the SPM from the shelf to the outer marine sector. In both embayments the tidal currents did not produce bottom sediment resuspension events, they moved locally the SPM clouds when the subtidal circulation made it available. The wave action in the Ría de Muros could be punctually considered as local SPM source in the shallowest areas. In the Ría de Vigo the wave action did not produce resuspension events. However, it controlled the average bottom SPM concentration in the outer-mid sector, avoiding the SPM settling in the margins (high energy level) and allowing it in the central channel (low energy level). The study of the Ría de Vigo beaches revealed that three different areas can be defined in base to their profiles and sedimentary texture and composition: innermost zone (estuarine), middle part (combination of estuarine and marine), and the outer area (marine). Three factors controlling the beach morphodynamics were identified; the wave climate, the sediment provenance (from East to West), and the geomorphological set-up of the ría that drives north-south variation in the beaches. The study of the interaction between the planned underwater outfall pipe and Samil beach sediment dynamics showed that the future structure alternatives would not affect the global sediment dynamics of the beach in any of the simulated cases. The modelled and observational data showed that the minimum jacking depth is 8 m, indicating that the seabed surrounding the submerged structure will suffer erosion above this depth.