Print Email Facebook Twitter Tidal and sediment dynamics in a fine-grained coastal region: A case study of the Jiangsu coast Title Tidal and sediment dynamics in a fine-grained coastal region: A case study of the Jiangsu coast Author Yao, P. Contributor Stive, M.J.F. (promotor) Wang, Z.B. (promotor) Faculty Civil Engineering and Geosciences Department Hydraulic Engineering Date 2016-04-04 Abstract The Jiangsu coast is located in eastern China bordering the South Yellow Sea. It is strongly affected by the semi-diurnal tides. Both the tidal range and the tidal current vary greatly in space due to local tidal wave systems and morphologies. The radial tidal current pattern identified at the central coast is suggested to play a primary role in the evolution of a large-scale radial-shape sand ridge system. Another feature of the Jiangsu coast is the diversity of the bottom sediments with pronounced silt content. Inspired by the characteristics of both the hydrodynamics and sediment dynamics throughout the Jiangsu coast, this thesis focuses on advancing our understanding of the coastal tidal dynamics and the resulting sediment transport. Regarding the radial tidal current pattern at the central Jiangsu coast, there have been plenty of studies exploring relevant formation mechanisms. A generally accepted inference is that the radial tidal current pattern is a consequence of the interaction between the northern rotating tidal wave system and the southern progressive tidal wave. In this study, we examine the emergence of the radial tidal current in a schematized semi-enclosed tidal basin by introducing the tidal Current Amphidromic Point (CAP) and the tidal current inclination angles. After comprehensive numerical experiments, we find that the overall basin scale and the cross-basin phase difference play roles in the emergence of the radial tidal current. The radial tidal current only has an opportunity to emerge in a basin where the basin length (L) is larger than width (B) (i.e. L/B>1), a lateral depth difference exists or the offshore incoming tidal wave has an oblique angle. The Yellow Sea is featured by these aforementioned prerequisites favouring the emergence of the radial tidal current. Furthermore, we discover that the radial tidal current is related to the cross-basin CAP distribution pattern. When the radial tidal current emerges, the focal point is the CAP related to the velocity vectors rotating anti-cyclonically in the Northern Hemisphere. The CAP distribution deserves more attention for the identification of the radial tidal current pattern. To understand the sediment dynamics in a silt-enriched environment in more detail, we have carried out a series of flume experiments under various wave and current conditions with field-collected silt-sand mixtures. According to the experiments, we find that the silt fraction has different features originating from both the sand fraction and the clay fraction. A high concentration layer is observed near the bottom together with ripples under pure wave conditions. Sediment concentrations inside the high concentration layer are quasi-stationary with the bulk Richardson number approaching a constant value. The thickness of the high concentration layer can be scaled with approximately two times the damped wave boundary layer thickness. Thus, the wave motion induced turbulence is considered to be the main reason generating the high concentration layer. Moreover, suspensions inside the high concentration layer have a certain amount of sand content, which is different from the fluid mud in the cohesive muddy bed. For the vertical concentration profile, the silt fraction is also distributed differently from the sand fraction, since the silt concentration decreases logarithmically within high concentration layer, while it is homogeneously distributed outside the high concentration layer. Considering the specific features of the silt fraction, we recalibrated the formulations of van Rijn (2007a, b) based on our experiments and further developed a multi-fraction sediment transport model to predict the vertical concentration profile for silt and sand classes, and then tested the existing sediment formulations. The results show a promising agreement with the measurements, for both wave-only and wave-with-current conditions. Finally, the Jiangsu Regional Model is set up utilizing the aforementioned findings on tides and sediments. The Jiangsu Regional Model is used to examine whether our existing knowledge can be integrated for a relatively long-term (i.e. time scale of years) predictions on the sediment transport and the morphological changes of the Jiangsu coast. To this end, we first reasonably construct the bed composition throughout the model domain. Subsequently, the model is calibrated and validated against two independent measurements on water level, flow velocity and the sediment concentration. The results indicate that the present model can produce good results. The simulated annual-averaged SSCs depict a high value in the coastal region between the Old Yellow River Delta and the northern Radial Sand Ridge Field. The simulated morphological changes show a spatially distributed alternating-erosion-sedimentation pattern in the Old Yellow River Delta rather than pure erosion. Over the Radial Sand Ridge Field, the ridges are continuously growing and the adjacent tidal channels are deepening. The simulated annual-averaged tide-induced sediment budget shows that the northern (i.e. the Old Yellow River Delta) and southern (i.e. the southern Radial Sand Ridge Field) Jiangsu coast are under erosion, while the central coast (i.e. the northern and central Radial Sand Ridge Field) is still in progradation. Furthermore, the simulated sediment bed in the Old Yellow River Delta shows a gradually coarsening trend while an overall fining trend is pronounced in the northern Radial Sand Ridge Field. All these long-term results are in good agreement with observation-based estimations. The present modelling framework indeed has the ability for simulating sediment transport and morphological changes over a relatively long time span (i.e. time scale of years). This thesis addresses series of findings on the radial tidal current pattern, characteristics of the silt-dominated sediments as well as the sediment transport and morphological changes along the Jiangsu coast. The proposed modelling approaches can serve as a basis and provide information on large-scale hydrodynamics and sediment dynamics for the management and planning of the Jiangsu coast. Future studies may be focused on (1) detailed investigation on the influencing factors on the emergence of the radial tidal current by the CAP system distribution; (2) the physics of the layered-bed system (i.e. the hard layer under ripples) for silt dominated mixtures; (3) improving the computational efficiency of the Jiangsu Regional Model for longer time scale (i.e. tens of years). Subject tidestidal wave systemtidal current systemsilt dynamicssediment transportJiangsu coast To reference this document use: https://doi.org/10.4233/uuid:547d2dfd-672e-4b6d-aa96-45ef67b05023 Embargo date 2016-03-21 ISBN 978-90-6464-994-3 Part of collection Institutional Repository Document type doctoral thesis Rights (c) 2016 Yao, P. 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