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Title: Long-term morphological modelling of tidal basins
Author: McCann, David
ISNI:       0000 0004 2717 6233
Awarding Body: Prifysgol Bangor University
Current Institution: Bangor University
Date of Award: 2011
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Shallow coastal environments, particularly sandy tidal basins, are coming under increasing pressures due to human development and a changing climate. This has led to a demand for long-term predictions of sediment transport using deterministic, process-based coastal area models. Due to continually growing model complexity this has involved an increased computational cost, resulting in the development of a variety of methods to speed up computation time while having a minimal impact on model accuracy. The present body of work introduces one of the most popular speed-up methods, the Morphological Factor (MF) method, to the TELEMAC hydro-informatics modelling system. Using the depth-averaged, shallow water hydrodynamic and sediment transport models, TELEMAC 2D and SISYPHE respectively, a long-term simulation of a schematised tidal basin system is conducted. From infinitesimal perturbations to an initially flat bed a highly regular morphological pattern is seen to evolve spontaneously through the processes of self-organisation, subsequently forming the basis of a dynamic and intricate tidal channel network. Due to the time-stepping nature of the model, the initial pattern is responsible for all future simulated morphologies, yet this linkage has received little attention in the literature. Further simulations identify the first stages of the bed pattern as a dissipative structure, requiring more energy from the flow to create and maintain than the initial flat bed itself, which is contradictory to a linear, mechanistic treatment of the system. Dunes, sedimentary bed forms ubiquitous to the coastal zone, have a significant impact on flow speed (and therefore sediment transport and morphological change) through the roughness created by their geometry. Although the effects of dune-scale roughness on the flow and sediment transport are discussed in the literature the impact of a time-evolving roughness feedback between the bed and the flow in a coupled morphological model is poorly understood. In the present work a roughness feedback mechanism is implemented in TELEMAC 2D and SISYPHE and the predicted roughness is validated in a model of the Dyfi Estuary, Wales, using the results of a swathe sonar bathymetric survey. The model results show very promising agreement with the data given the simplifying assumptions made. The tidal residual sediment transport rate in a simplified model case is shown to not only change in magnitude, but also in sign, between cases with and without bed roughness feedback, showing that the method has significant implications for long-term morphological change. Deterministic models are frequently used to predict changes in the long-term, yet their mechanistic nature forces the outcome to be sensitive to initial conditions. Natural systems, however, contain both deterministic and probabilistic elements that create structures and patterns at a variety of different scales, many of which can be captured by process based models such as TELEMAC 2D and SISYPHE. In the present work a stochastic, infinitesimal perturbation is introduced to three otherwise identical long-term model simulations, resulting in system divergence through three independent morphological system parameters. The sensitivity to initial conditions and the important role of self-organisation are seen to produce dynamic morphological behaviour that is inherent to the theory of open, dissipative systems.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available