On the Dependency of Bottom Drag and the Eddy Viscosity upon Flow Structure in the Coastal Boundary Layer

Yao-Zhao Zhong,Meng-Yu Lin,Anna Wargula,Hwa Chien,Jia-Lin Chen

doi:10.3390/jmse10030324

Abstract

The physical processes governing coastal exchange between the surf zone, the inner shelf, and the open ocean is critical for estimating mass exchange and its impact on ecological processes. The present study combined field measurements and theoretical approaches to explore the hydrodynamics in the coastal boundary layer (CBL) in which both bottom drag and shore friction affect the transport and mixing processes. Observed drifter-cluster trajectories in a nearly alongshore-uniform coastal area showed that the occurrence of current reversal varies with cross-shore distance, which confirmed the tidal phase difference between different cross-shore distances predicted by the proposed CBL model. According to the CBL model, tidal phase difference is affected by the bottom drag coefficient and horizontal eddy viscosity coefficient. With the results of three experiments under different wave conditions, this study also discusses the effects of waves on the CBL. Data analysis based on observations indicates that the bottom drag term is closely related to the bottom shear stress induced by the interactions of waves and currents. The bottom drag coefficient under the more energetic wave condition was much greater than that under milder wave conditions during the experiment. The study also suggests that in addition to pressure gradient and bottom drag, flow structure is subject to lateral stress, which reflects the impact of shoreline roughness in the nearshore region and that the estimated eddy viscosity coefficient decreases linearly with distance from the shoreline.

Highlights

IntroductionAs the boundary area between land and sea, the coastal zone is the primary channel for terrestrial materials transferring into the ocean
This study aims to understand the dependency of bottom drag and horizontal eddy viscosity coefficient to the flow structure in the coastal boundary layer (CBL)
The time of tidal current reversal varying with cross-shore distance is described well by a theoretical momentum balance of local acceleration, barotropic pressure gradient, lateral stress, and bottom drag

Summary

Introduction

As the boundary area between land and sea, the coastal zone is the primary channel for terrestrial materials transferring into the ocean. The physical processes governing coastal exchange between the surf zone, the inner shelf, and the open ocean is critical for estimating mass exchange and its impact on ecological processes. This boundary area with attenuated flow adjusted to the presence of the shoreline is mainly referred as the ‘coastal boundary layer’ (CBL) [1–9]. Understanding the dynamic response in this region is challenging, owing to the non-linear interactions among wind, waves, barotropic tides, wave-generated currents, and shoreline geometry in the coastal ocean [10–14]. Cross-shore transects of velocity-profile observations along the California coast have shown that the depth-averaged alongshore

Objectives

Discussion

Conclusion