Initiation of Sediment Resuspension by Combined Wave-Current Conditions in an Artificial Seagrass Meadow

Abstract

Laboratory experiments examined the impact of current on ripple formation and the onset of wave-driven resuspension within an artificial seagrass meadow modeled after Zostera marina. Within the meadow, the current was less than or equal to the wave velocity. Meadows were constructed with three shoot densities: 247, 455 and 962 stems/m², and each shoot had six flexible blades. The sediment bed, consisting of $65\mu m$ spherical grains, was initially 1.4 cm thick, allowing ripple and scour hole formation. The formation of wave-orbital ripples was dependent on meadow density and current magnitude. Over bare beds and sparse meadows, ripples were present and not impacted by the addition of current, such that the wave velocity resuspension threshold with current was the same as that in pure wave conditions. In medium-density meadows, the addition of current reduced ripple height due to plant-generated turbulence. As current increased, ripple size and ripple-generated turbulence decreased, requiring a higher wave velocity to resuspend sediment. That is, for medium density meadows, the critical wave velocity increased as the current velocity increased. Finally, in dense meadows, no ripples formed and resuspension was driven by a critical value of plant-induced turbulence, which was proportional to the total velocity (current plus wave velocity), such that as the current velocity increased, the critical wave velocity decreased. A model predicting the critical wave velocity for the dense meadow was derived based on the assumption that resuspension was driven by a critical level of stem-generated turbulence.