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Nearshore sedimentary dynamics in a wave-dominated coast
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Coastal management issues are mainly associated to imbalances in the coastal sediment budget and concerns regarding this topic have significantly increased in recent years. One of the main consequences of these imbalances is reflected in coastal erosion, that in medium to long-term compromises the stability of the coastline. Thus, understanding the movement of sediments in and out of the beach system is a key to understand how the coast evolves and how it responds to coastal engineering interventions. However, the existence of the depth of closure (DoC) concept casts some doubts regarding the importance of the sedimentary exchanges between the beach and the continental shelf. The lack of information concerning the nature and magnitude of sediment transport across the whole nearshore environment, especially seaward of the DoC, is likely to be the main cause of the uncertainties associated to the sedimentary activity on this environment. This work aims to advance the understanding of the nearshore sedimentary dynamics and to recognize its relationship with forcing mechanisms, especially seaward of the depth of closure. To reach the objective of this work, a pluridisciplinary approach was carried out involving sediment transport, oceanographic, sedimentological and morphological data across two distinct nearshore environments at the Portuguese continental shelf. Nearshore sediment transport was estimated through in situ measurements and numerical modelling. Local measurements were performed at the mid shelf through a sand tracer experiment by injecting 400 kg of fluorescent tracer and monitoring its displacement during one year. The modelling approach was conducted by the application of a wave-current bottom boundary layer (bbl) model developed in the scope of this work and validated by the in situ measurements. Oceanographic data was based on in situ measurements of waves and currents complemented by wave propagation modelling, and also served as input to the bbl model. In situ data included upward-looking ADCP measurements and wave buoy data, and was taken as representative of typical annual conditions. The sedimentological dataset was based on both in situ measurements as well as pre-existing textural distribution maps. Bottom morphology relied essentially on field data from single beam and multibeam surveys, and to a lesser extent on pre-existing morphodynamic studies. The multiproxy approach was replicated at two energetically contrasting nearshore environments (except for the tracer experiment that was only held in one study area). The first case study is representative of a moderate-energy environment in southern Portugal, offshore Tavira barrier island. The second one is representative of a high-energy environment in the Portuguese west coast, offshore Almagreira beach, Peniche. Modal wave conditions are characterized by offshore significant wave heights of 0.92 and 2.2 m, and peak periods of 8.2 and 10.4 s, for the moderate and high-energy environments, respectively. Despite the energetically distinct character of the nearshore zones addressed in this work, the integrated analysis of multidisciplinary data put in evidence common features between them. These similarities, mainly represented by sediment texture and related dynamics, led to the definition of depth limits that define domains with characteristic dynamics, being them: upper and lower inner shelf, mid shelf and outer shelf. Also, the integrated analysis of the multidisciplinary data allowed the development of a conceptual sediment dynamic model for a wave-dominated nearshore zone with low sediment supply. In this model the sedimentary dynamics within each nearshore domain is described and supported mainly by results from this work, complemented with external data. In the present conceptual model, the inner shelf domain is characterized by littoral deposits with a seaward fining trend - maintained by reworking and redistribution processes induced by the present hydrodynamic regime. Overall, the bedload mode of transport is considered the mechanism driving coarser sands onshore, while the seaward transport of finer sands is made essentially as suspended load. This domain can be segmented in two sections: 1) the upper inner shelf; and 2) the lower inner shelf. The first is marked by intense remobilization and frequent sediment exchange with subaerial beach, translating into measurable morphological changes and limited offshore by the morphological depth of closure. The lower inner shelf extends down to the maximum depth where littoral sediments are found (that can be regarded as the sedimentological depth of closure), and is characterized by finer sands frequently mobilized but only transported under high energy wave events. At the mid shelf, whereas the bottom energy induced by waves decreases, the grain size of bottom sediments increases, consisting of medium to coarse sand. Sediment remobilization within this domain is significant, but net sediment transport is very low and dominated by an alongshelf component. Finally, the abrupt contact with muddy sediments (mudline) marks the beginning of the outer shelf where sediments are in equilibrium with the milder hydrodynamic regime. The proposed conceptual model constitutes a step forward in the harmonization of the sedimentological and oceanographic settings affecting the inner, mid and outer continental shelf domains. The coupled behaviour between sediment distribution and hydrodynamic processes allowed the prediction of the depth limits between nearshore domains through theoretical calculations solely based on wave conditions. Still, and despite the valuable contribution to the understanding of nearshore sediment dynamics made by this work, further efforts are needed regarding this subject. The incorporation of wave-related sediment transport due to wave streaming into numerical models, as well as more sediment transport measurements are improvements to be prioritized. These improvements, associated with the predictive capacity of numerical models, can constitute a powerful tool for the coastal management of natural resources, especially with regard to the current sea level rise scenario and the consequent potential coastal sediment budget imbalances.
Descrição
Tese de doutoramento, Geologia (Geodinâmica Externa), Universidade de Lisboa, Faculdade de Ciências, 2017
Palavras-chave
Teses de doutoramento - 2017