Turbulence-resolving numerical investigations of coastal bottom boundary layer and fine sediment transport
Date
2020
Authors
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Publisher
University of Delaware
Abstract
The bottom boundary layer in coastal ocean is of intrinsic scientific importance in many disciplines, serving as the near-bed passage where many benthic transport processes occur and it is a vital bridge connecting the wave propagation, currents and upper ocean processes with the seafloor. The present work focuses on the fluid dynamics in coastal bottom boundary layer (CBBL) and the associated sediment transport process using the technique of turbulence-resolving numerical simulation (TRNS). Particularly, we are interested in the entrainment, suspension and transport of fine sediment in the wave bottom boundary layer (WBBL) driven by surface gravity waves. ☐ Wave-supported gravity flows (WSGFs) have been identified as a key process in CBBL delivering fine sediment across continental shelves in the offshore direction. Though the formation mechanism of WSGF is well-documented, our understanding on the various factors controlling the maximum sediment load and the resulting gravity current speed remains incomplete. In the first part of this study, the role of bed erodibility and wave direction in WSGFs over a flat bed are addressed using TRNSs. Under the energetic wave condition reported on Northern California Coast with a shelf slope of 0.005, simulation results show that WSGFs are transitionally turbulent and that the sediment concentration cannot exceed 30 g/L due to the attenuation of turbulence by the sediment-induced stable density stratification. Wave direction is found to be important in the resulting intensities of gravity current. When waves are in cross-shelf direction, the downslope current has a maximum velocity of 1.2 cm/s, which is increased to 2.1 cm/s when waves propagate in the along-shelf direction. Further analysis on the wave-averaged momentum balance confirms that when waves are parallel to the slope (cross-shelf) direction, the more intense wave-current interaction results in larger wave- averaged Reynolds shear stress and thus in a smaller current speed. Findings from this study suggest that the more intense cross-shelf gravity current observed in field may be caused by additional processes, which may enhance the sediment-carrying capacity of flow, such as the bedforms. ☐ Recognizing the importance of bedforms in CBBL, in the second part of this study, we conduct direct numerical simulations of sinusoidal oscillatory flow over out- of-equilibrium vortex ripples to study the fluid dynamical controls in determining their distinctive equilibrium dimensions. In comparison with the equilibrium case, the coherent vortex and the magnitude of total shear stress on the ripple surface (or bottom shear stress experienced by the overlying flow) are identified as the key fluid dynamical controls. Based on these two factors, we propose a mechanism in the selection of vortex ripple dimensions, such that the interaction between overlying flow and ripples tends to generate stronger coherent vortices while in the meantime, the ripple surface prefers a smaller shear stress. Through the triple-decomposition of flow, the component of ripple-induced fluctuation is found to be the most important in controlling the resulting flow features. ☐ To close the loop in identifying the degree of bedforms in controlling the sediment carrying capacity of flow, the last part of this study evaluates fluid dynamical controls over sub-orbital ripples and the resulting suspended fine sediment transport. In order to match field observations of high near-bed sediment concentration exceeding 50 to 100g/L, we test the hypothesis that the presence of suborbital ripples on seabed can significantly enhance the sediment-carrying capacity due to the ripple- induced fluctuations. Simulation results for turbulent flow over sub-orbital ripples are first presented to understand the vortex dynamics, shear stresses on ripple surface, streamwise momentum balance and vertical kinetic energy budgets. These results are also contrasted with the orbital ripple condition. Three additional simulations of flow over sub-orbital ripples laden with difference amount of suspended fine sediment prove our hypothesis that, the presence of ripples can significantly enhance the capability of flow in carrying sediment in the WBBL to a concentration about 100g/L without laminarizing the flow. This implies stronger WSGFs observed in the field may be due to the presence of bedforms. We also provide a physical explanation on this finding through examining the vertical budget of sediment flux.
Description
Keywords
Coastal engineering, Direct numerical simulation, Turbulent boundary layers, Vortex flows