Flow Structure in an Artificial Seagrass Meadow in Combined Wave-Current Conditions

Rachel B. Schaefer,Heidi M. Nepf

doi:10.3389/fmars.2022.836901

Rachel B. Schaefer, Heidi M. Nepf

Open Access

https://doi.org/10.3389/fmars.2022.836901

Copy DOI

Abstract

Experiments were conducted in a laboratory flume using an artificial seagrass meadow, modeled after Zostera marina, to examine the impact of waves on the vertical structure of time-averaged current, Reynolds stress, and turbulent kinetic energy (TKE) under combined wave-current conditions. With the addition of smaller waves, defined by a ratio of wave velocity to current velocity Uw/Uc &lt; 2.5, the time-averaged velocity peaked above the meadow, which was similar to pure current conditions. When Uw/Uc &gt; 2.5, the presence of waves caused the time-averaged velocity to peak near the top of the meadow. For Uw/Uc &gt; 1 the presence of waves reduced the magnitude of peak Reynolds stress. For all conditions considered, the wake production of turbulence dominated the shear production of turbulence in the meadow. However, the wave velocity was less efficient than the current velocity in generating TKE in the meadow because the movement of the blades forced by the oscillatory fluid motion reduced the relative velocity between the blades and the wave. A modified hybrid model for wake production of TKE in a flexible canopy under combined wave-current conditions was proposed to account for the relative contributions of waves and currents. Wake production of TKE was dominated by waves when Uw/Uc &gt; 1 and dominated by currents when Uw/Uc &lt; 1. The models and observations proposed in this study contribute to an enhanced understanding of the relative influences of waves and currents on seagrass meadow flow structure in realistic combined wave-current conditions.

Highlights

Meadows of submerged aquatic vegetation, such as seagrass, can damp wave energy (e.g., Knutson et al, 1982; Fonseca and Cahalan, 1992), reduce erosion, and improve water quality (e.g., Ginsburg and Lowenstam, 1958; Ward et al, 1984; Moore, 2004)
Laboratory experiments with an artificial seagrass meadow described the impact of waves on the time-averaged velocity, Reynolds stress, and turbulent kinetic energy (TKE) within a flexible submerged meadow
One major result of this study is that for a flexible meadow, wave velocity is less efficient in generating TKE, because wave-induced motion of the blades reduces the relative velocity between the blade and the waves

Summary

Introduction

Meadows of submerged aquatic vegetation, such as seagrass, can damp wave energy (e.g., Knutson et al, 1982; Fonseca and Cahalan, 1992), reduce erosion, and improve water quality (e.g., Ginsburg and Lowenstam, 1958; Ward et al, 1984; Moore, 2004). The reduction of current velocity within the meadow enhances the creation of near-bed habitat relative to unvegetated regions (e.g., Fonseca et al, 1982; Homziak et al, 1982). These ecosystem services depend on the interactions between the meadow, waves, and current. This paper describes mean and turbulent flow within submerged aquatic vegetation under combined wavecurrent conditions, which can provide a deeper understanding of meadow hydrodynamics and enable a more accurate assessment of ecosystem services

Objectives

Methods

Results

Conclusion