Numerical modeling of hydro-morphodynamics in fluvial/tide-dominated coastal environmentsFrom the tidal channel to the inner shelf

Resumen

It is well known that coastal environments are of enormous relevance from a social and environmental perspective. These are complex transitional environments that have attracted large settlements of human population and their valuable natural resources have allowed the development of considerable socioeconomic activities. At the same time, coastal systems provide unique ecosystems for a wide range of animal and vegetal species, favoring biodiversity. Their exposure to climate hazards and global warming and the continuos threat of human-induced activities makes them highly vulnerable and new efforts to develop efficient strategies are required to their proper management. This vulnerability combined with the socio-economical pressure has led to a significant increase of the scientific interest in the behaviour of these ecosystems. However, despite their critical role for society and biodiversity, numerous processes at the heart of coastal environments are still poorly understood. Sediment transport and hydrodynamic processes are the major mechanisms responsible for the variety of morphologies observed in these coastal environments. There exists a vast literature concerning the physical processes involved in their mutual interaction, which ultimately dictates coastal environments dynamics. However, questions regarding coastal processes encompass such a large variety of coastal units and environmental conditions (geology, sediment properties, external forcing). Such coastal units can be defined conform the spatial boundaries that limits coastal processes during the Quaternary period extended. These boundaries organize the coastal zone in three morphological units that comprise the coastal plain, the shoreface and the continental shelf. These units constitute the spatial erodible scenario over which the outflow of the rivers, waves generated by wind and tidal currents interact and shape the landscape, being ultimately responsible for the evolution of coastal environments. Therefore, the relative degree of dominance of the three main hydrodynamic processes (waves, tides and river outflow) alongside the variety of inherent geological and sedimentary features that characterize the different morphological units generate a wide range of coastal scenarios that evolve at different spatial and temporal scales, producing a richness of landforms and a number of complexities that hinder the knowledge of their phenomenology. The main objective of this Thesis is to numerically explore and understand the quantitative testing of specific and unclear hydro-morphodynamic processes that take place at different spatial- and temporal-scales on fluvial/tide-dominated coastal environments, encompassing different coastal units: (i) coastal plain (the inner tidal channels dissecting the shelf surface and a shallow coastal bays system), (ii) shoreface (the area in front of the mouth of rivers) and (iii) continental shelf (sedimentary morphologies in the relatively shallow waters of the inner shelf). The subdivision of this Thesis in different morphological units and related efforts to model their dynamics provides an insight into the way in which modelers deal with the complexities of spatial- and temporal-scale and process integration. To accomplish the main objective, this Thesis tries to shed light on some specific questions that remain unanswered on coastal environments: (i) Could the offshore plume appreciably affect the morphodynamic equilibrium of a tidal channel? (ii) How do storm events affect the resilience and stability of a coastal bays system? (iii) What are the effects of the river mouth geometry on the river outflow hydrodynamics and related river mouth bar formation, a key geomorphological process on delta evolution? (iv) How do the ocean basin characteristics determine the formation of sedimentary wedges and seafloor undulated structures associated to a turbidity current event? Clarifying these issues may thus help in making correct interpretations of hydro-morphodynamic processes in coastal environments which knowledge is still limited. The assessment of the following specific objectives, which were defined to accomplish the main objective, constitutes the main body of the Thesis. A brief summary of the main results and conclusions collected after approaching these specific objectives is also presented below. -To develop a coupled onshore and offshore numerical model in order to analyze the hydro-morphodynamics feedbacks between tidal channels and related discharge plumes in semienclosed tidal embayments: We discuss the existence of a long-term equilibrium configuration of a tidal channel dynamically coupled to its offshore plumes. This is accomplished adopting a one/two-dimensional numerical model that resolves the fully nonlinear unsteady shallow water, sediment bed load transport and suspended sediment advection-diffusion equations along with the Exner equation for the bathymetric changes in the tidal channel. These equation are dynamically coupled to a simple model of the plume hydrodynamics to estimate the net exchange of sediment associated with the two-phase sequence of tidal events. Whereas the ebb-flow is modeled as a bounded plane turbulent jet, a Scharwz-Christoffel conformal transformation is implemented to map a plane, irrotational, sink flood-flood in the physical plane. Results reveal that the offshore plume plays a crucial role on shaping the tidal channel. As a consequence of the offshore plume, the seaward concavity of the profile reduces and smaller inlet depths arise. The model results are also in good agreement with nature in terms of bathymetric profile for the Punta Umbr\'ia r\'ia (Southwestern Spain). Numerical experiments with different channel, oceanic and tidal characteristics are performed, and the hydro-morphodynamic consequences are also examined. Finally, the feedback between channel and plume also alters the plume dynamics and progressively reduces the ability for tidal channel to generate mouth bar deposits. -To evaluate how intense storm events affect the resilience of a coastal bays system: The stability of a shallow coastal bays system in the face of tropical cyclones and other storms is evaluated and quantified. This has major implications for coastal inundation risk to lives and property, as well as the resilience of these coastal wetlands to a changing climate. Developing a sediment budget for these coastal bays is imperative to understanding how storm events affect the resilience of the system. Using high resolution numerical simulations, we show that intense storms import sediment into a system of coastal bays within the Virginia Coast Reserve (VCR), located on the Atlantic side of the Delmarva Peninsula, USA. Duration and magnitude of storm surge are among the most important factors in sediment import, suggesting that intense storms increase the stability of coastal bays by providing the sediment necessary to counteract sea level rise. Thus, results suggest that increased storminess may continue to increase the resilience of coastal bays, providing the material necessary to counteract rising sea levels and increasing the systems resilience. Indeed, yearly mean accumulation of sediment within the domain from the modeled storms during the 5 years of simulations ranged from 2.01 mm to 5.30 mm. -To analyze the effects of basin bottom slope at the river mouth on the discharge dynamics and on the related sedimentary processes: A theoretical model of jet-theory and the high-resolution hydro-morphodynamic model Delft3D were employed to explore how the receiving basins slope alter the hydrodynamic structure of an entering sediment-laden turbulent jet and its consequences on related sedimentary patterns. An updated turbulent jet theory is proposed with a slope-dependent entrainment coefficient. Numerical results suggested that the entrainment coefficient displays a power law increase as the slope of the receiving basin increases. Besides, the basin slope alters jet dynamics and favors the formation of an unstable meandering jet. Consequently, hydrodynamic implications of the basin slope determine sedimentary processes responsible for bar formation such as river mouth bar geometry, relative importance of bar aggradation and progradation, hydrodynamic requirements for flow bifurcation, and river mouth bar time formation. To establish a framework of mouth bar morphologies as a function of the basin slope and the river discharge angle: We study the role of the receiving basin slope and the river discharge angle using a high resolution coupled hydrodynamic- sediment transport model. This analysis shows that the river mouth geometry can deflect jet direction and alter the jet spreading rate. As a consequence of these hydrodynamic alterations, the conditions encountered by the sedimentary processes are modified, resulting in five different river mouth morphologies: central river mouth bar with or without lateral levees, side bar with or without lateral levees, and sand-spits. We also establish a theoretical framework for the specific morphology of the mouth bar as a function of the river mouth geometry. Finally, predicted mouth bar morphologies display similarities to natural systems. -To determine the role of the inner shelf characteristics on the sedimentary processes of turbidity currents: The role of the slope and friction of the inner shelf on the formation of sedimentary wedges and seafloor undulations over the latter by turbidity currents usually linked to short, mountainous, and seasonal fluvial systems is addressed using a numerical model of hyperpycnal plumes. Results suggest that the receiving basin slope highly dominates the formation and subsequent development of deltaic wedges, whereas the bottom friction dictates the evolution of their wavy structure.