Resumen de Transporte y distribución de residuos plásticos en estuarios debido a la asimetría mareal
- español
Los residuos-plásticos suponen una importante amenaza para el medio marino. Los ríos constituyen uno de los principales aportes de residuos y la hidrodinámica costera contribuye a atrapar estos residuos en el interior de los estuarios. La mayor parte de la investigación existente sobre acumulaciones de residuos analiza las escalas global y regional. Sin embargo, en la escala de estuario es mucho más escasa y las técnicas utilizadas presentan limitaciones importaciones. Esta Tesis busca mejorar la comprensión sobre los procesos que afectan el transporte y dispersión de los residuos-plásticos en los estuarios. Concretamente, investiga la relación entre la hidrodinámica estuarina, profundizando en las corrientes debidas a la marea-astronómica y su asimetría, y la distribución de residuos-plásticos en estuarios mareales. También contribuye con el desarrollo de una metodología que puede aplicarse con unos recursos mínimos en cualquier estuario del mundo para proporcionar información sobre acumulaciones de residuos en un horizonte temporal específico.
- English
Marine litter, mainly plastics (80%), represents a significant threat to marine environments, causing damage at ecological, economic, and social levels. Rivers are one of the main sources of plastic debris and coastal hydrodynamics contribute to trap the plastics within estuaries, which become significant environmental hazards.
The transport and fate of plastic debris within estuaries are largely unexplored and most of the research has focused on open oceans or regional areas. Two different approaches have traditionally been used to assess the marine-litter distribution within estuaries: field surveys and numerical models. Both approaches have some limitations, such as the dependence between the obtained results and the sampling method selected for field surveys or the high computational cost due to the use of numerical models at these spatial scales. Therefore, greater efforts are needed to overcome these limitations to address plastic debris research on an estuary-scale. To this aim, the first step is to improve understanding of the processes affecting the transport and fate of plastic debris within estuaries.
Astronomical tide plays a key role in the behaviour of tide-dominated estuaries. Tidal deformation generates sea-level and current-speed asymmetries that can lead to a net transport and cause an estuary trend to import or export materials, respectively. Tidal asymmetry arises from both the interaction among the main tidal constituents and the harmonics generated when tide propagates through shallow waters. Most previous research focuses on the tide deformation within estuaries; however, ocean tide may show asymmetry at the estuarine entrances, which implies that the boundary condition is already deformed. This fact has important implications for tide propagation, estuarine transport processes, and flow exchanges between estuaries and open oceans. Therefore, a detailed characterization of tidal asymmetry at the mouth of estuaries is a key point to start the study. Regarding the transport processes, most of the published theories and studies relate tidal asymmetry with sediment transport and morphological trends in estuaries. However, no analysis on its direct effect on transport and fate of plastics debris is found.
The general objective of this Thesis is to study the effect of hydrodynamics on the transport and fate of plastic debris in tide-dominated estuaries, emphasizing hydrodynamics due to tidal asymmetry. This objective is divided into the following specific objectives: (1) characterization of tidal asymmetry on a global scale to know its effect, as a boundary condition, in the estuarine transport processes, (2) study of the spatial evolution of tidal asymmetry when tide propagates through different estuarine geometries and its effect in the fate of plastic debris, and (3) development of a methodology to assess the probability of plastic debris accumulation in estuaries.
As the first step in this Thesis, the astronomical tide is classified on a global scale according to its asymmetry and periodicity. The purpose is to provide a guiding framework of representative astronomical tide types (ATtypes) on a worldwide scale to be used as a reference for further research on the transport of substances in general and plastic debris in particular in estuaries. The applied methodology is based on the use of the TPXO9-atlas global barotropic tidal solution and detailed statistical analysis. Probability density functions of the tidal elevation time derivative and the tidal form factor (parameters related to the transport of substances in estuaries) are extracted from TPXO9-atlas with a spatial resolution ranging from 1/6º, enough for open oceans, to 1/30º, more suitable for coastal areas. The K-means algorithm is applied to these parameters, and 25 representative ATtypes are identified. The classification is validated with 757 worldwide tide gauge records, showing that 97% of the clustered parameters characterized well or very well the world's coastal areas and only 3% exhibit poor or unacceptable representativeness. The results show that 11.3% of coastal areas show negative asymmetries, 11.3% positive asymmetries, while symmetric tides dominate 77.4% of coastal areas. In coastal areas showing external tidal asymmetries, the propagation of the tide through estuaries may be initially conditioned by the asymmetry imposed on the mouth. Nevertheless, in symmetrical tidal areas, tidal asymmetries arise exclusively from overtides and compound tides generated during inland tidal propagation, without being externally conditioned.
Second, the effect of estuary morphology on the propagation of different (a)symmetric tidal types and on the fate of plastic debris within estuaries is analysed. The applied methodology is based on hydrodynamic-numerical modelling and Lagrangian-transport modelling. The steps consist of propagating three tidal types (positive-asymmetric, symmetric, and negative-asymmetric) through four study estuaries characterized with different morphologies, where cross-sections and tidal-flat areas change, and using the resulting hydrodynamics to move a set of representative particles of plastic debris. The skewness, representative of the difference between the flood/ebb-current intensities; kurtosis, showing the frequency of the strongest tidal currents; and the average probability of plastic-debris accumulation in a neap-spring tidal cycle are the analysed parameters. The results show that the skewness of estuary mouths conditions the flood/ebb dominance and plastic debris accumulation in estuarine areas characterized by the main channel that favours tidal circulation. Positive tidal asymmetries show an import capacity 50% higher than symmetric or negative asymmetric tides. Conversely, when there are areas with the significant opposing flow or intertidal storage, the estuarine geometry defines a clear flood dominance. The probability of plastic debris accumulation is 90% for positive asymmetric tides and varies from 70-80% for symmetric and negative asymmetric tides. A novel aspect derived from this study is the corrective and regulatory behaviour of kurtosis that counteracts the transport induced by skewness.
Finally, with the knowledge acquired on tidal asymmetry and transport of plastic debris, a general methodology, based on numerical models and statistical analysis, is developed to identify the most probable plastic debris accumulation areas (hotspots) on an estuary scale. The purpose is to assist in the definition of clean up and mitigation strategies in estuaries. The methodology includes four main steps: K-means clustering to identify representative metocean scenarios, dynamic downscaling to obtain high-resolution drivers with which to force a transport model, numerical transport modelling to generate a database of potential plastic debris trajectories, and statistical analysis of this database to obtain probabilities of debris accumulation. The efficacy of this methodology is demonstrated by its application to Santoña Bay (Spain), an estuary along the northern coast of Spain, by comparing the numerical results with field data. Furthermore, the necessary criteria to ensure its applicability to any other estuary are provided. As the main conclusion, the developed methodology successfully assesses the plastic debris distribution in estuaries with a minimum computational effort.
