Modelling and Analysis of Fine Sediment Transport in Wave-Current Bottom Boundary Layer

Abstract

Although all care is taken to ensure integrity and the quality of this publication and the information herein, no responsibility is assumed by the publishers, the author nor IHE Delft for any damage to the property or persons as a result of operation or use of this publication and/or the information contained herein. A pdf version of this work will be made available as Open Access via http://repository.tudelft.nl/ihe. This version is licensed under the Creative Commons To my family AbstractThe evolution and utilization of estuarine and coastal regions are greatly restricted by sediment problems. Inspired by the Caofeidian sea area in Bohai Bay, China, this study aims to better understand silty sediment transport under combined action of waves and currents, especially in the wave-current bottom boundary layer (BBL), and to improve our modelling approaches in predicting estuarine and coastal sediment transport.Field observations were carried out in northwestern Caofeidian sea area of Bohai Bay and field data were collected on several other silt-dominated coasts. Analysis shows that silt-dominated sediments are sensitive to flow dynamics: the suspended sediment concentrations (SSCs) increase rapidly under strong flow dynamics (i.e., waves or strong tidal currents which can stir up sediments), and high concentrations cause heavy sudden back siltation in navigation channels. In the following, details of silty sediment transport are studied, focusing on the BBL and high concentration layer (HCL).From laboratory experiments and theoretical analysis, an expression for sediment incipient motion is proposed for silt-sand sediment under combined wave and current conditions. The Shields number was revised by adding the cohesive force and additional static pressure, leading to an extended Shields curve.To study the HCL, a process based 1DV model was developed for flow-sediment dynamics near the bed in combined wave-current conditions. Based on the physical processes, special approaches for sediment movement were introduced, including approaches for different bed forms (rippled bed and 'flat-bed'), hindered settling, stratification effects, mobile bed effects, reference concentration and critical shear stress. The HCL was simulated and sensitivity analysis was carried out by the 1DV model on factors that impact the sediment concentration in the HCL. The results show that the HCL is affected by both flow dynamics and bed forms; the thickness of the HCL is about twice the height of the wave boundary layer; bed forms determine the shape of the concentration profile near the bottom, and flow dynamics determine the magnitude. For finer sediment, stratification effects and mobile bed effects impact the sediment concentration greatly.Finally, based on the 1DV model, the formulations of the mean sediment concentration profile of silty sediments were studied. By solving the time-averaged diffusion equation for SSC and considering the effects of bed forms, stratification and hindered settling, expressions for time-averaged SSC profile...

Mean Current Profile over Rippled-Beds in the Presence of Non-Breaking Waves and Analysis of Its Influencing Factors

“…where K c is the current eddy viscosity. By substituting Equation (26) into Equation (27), Equation ( 27) can be rewritten as…”

“…One-equation models, also called mixing length models, use the turbulence kinetic energy and dynamic mixing length to represent the eddy viscosity, as in Davies et al [22], and Soulsby et al [23]. Two-equation models, including k-ε models and k-ω models, use the turbulent kinetic energy and turbulent kinetic energy dissipation rate to represent the eddy viscosity, as in [24][25][26][27]. Among these, [27] used the k-ε model to study the characteristics of hydrodynamics and sediments in flat beds and rippled-beds, and proposed toe-shaped wave eddy viscosities based on the results of the 1DV model.…”

“…Two-equation models, including k-ε models and k-ω models, use the turbulent kinetic energy and turbulent kinetic energy dissipation rate to represent the eddy viscosity, as in [24][25][26][27]. Among these, [27] used the k-ε model to study the characteristics of hydrodynamics and sediments in flat beds and rippled-beds, and proposed toe-shaped wave eddy viscosities based on the results of the 1DV model.…”