Mechanisms for the Asymmetric Motion of Submerged Aquatic Vegetation in Waves: A Consistent-Mass Cable Model

Abstract

Submerged aquatic vegetation (SAV) provides primary products for the food web, as well as shelter and nursery for many juvenile species. SAV can also attenuate waves, stabilize the seabed, and improve water quality. These environmental services are influenced by the dynamic motion of SAV. In this paper, a consistent-mass cable model was developed to investigate flow interaction with a flexible vegetation blade. Compared with previous vegetation models, the cable model showed improvements in simulating blade motions in waves with and without currents, especially for “second-normal-mode-like” blade motion. Wave asymmetry would cause blade motion to be asymmetric. However, asymmetric blade motion may also occur in symmetric waves. Results indicate that the asymmetric blade motion in symmetric waves is induced by two major mechanisms: (i) the spatial asymmetry of the encountered wave orbital velocities (wave motion relative to blade) due to blade displacements and (ii) the asymmetric action on the blade by vertical wave orbital velocities. Consequently, the blade motion is asymmetric even underneath symmetric waves unless (i) blade length ((Formula presented.)) is much smaller than the wavelength ((Formula presented.)), (ii) blade length is much smaller than the water depth ((Formula presented.)) in finite water depth waves, or (iii) water depth is much smaller than the wavelength ((Formula presented.)). Peak asymmetric blade motion occurs as (Formula presented.) increases to a critical value. The peak asymmetry increases with wave height and blade length but decreases with increasing blade flexural rigidity. Blade motion characteristics play an important role in wave-vegetation interaction, wave-driven currents, wave-attenuation capacity, breakage of vegetation and ecosystem services.