Investigating Wave Damping and Hydrodynamics in Seagrass Meadows under Combined Wave-current Conditions

Abstract:

Seagrass restoration projects aim to establish and spread resilient seagrass meadows, which provide numerous ecosystem services. The abilities of seagrass meadows to separately develop velocity profiles similar to those of mixing layers in unidirectional flow and reduce wave energy are well known. In coastal environments, however, seagrass meadows experience combined wave-current conditions. There is disagreement on how currents affect seagrass-induced wave damping, and on the mutual effects of waves and currents on velocity profile characteristics in a submerged meadow. This study aims to address these questions. Seagrass specimens were harvested from the Annisquam River in Massachusetts to observe blade dynamics under hydrodynamic forcing in a laboratory flume. An artificial seagrass model was designed based on these observations combined with compiled seagrass field measurements. Flume experiments were conducted using a six-meter long model meadow, with a range of densities and multiple submergence ratios. An imposed current caused a mean blade pronation in individual seagrass specimens. The presence of a meadow altered the pronation and blade dynamics of individual members relative to isolated plants. Wave orbital motion caused an asymmetric plant posture biased in the direction of wave propagation. Seagrass-induced wave damping for each tested case will be explored in the context of the ratio of current velocity to wave velocity, meadow density, submergence ratio, blade dynamics, and current direction. We hypothesize that for smaller wave amplitudes, flexible blades contribute enough to the plant-induced drag such that current-induced blade pronation could impact wave damping. For larger wave amplitudes, the rigid sheath contributes most of the drag, such that the plant can be modeled as a rigid cylinder. These parameters may interact with Stokes drift and current-induced shifts in vortex formation to influence wave damping. Results will inform improved seagrass restoration projects and provide insight on fluid-structure interactions in combined wave-current conditions.