Wave Motion and Sediment Resuspension Influenced by Aquatic Vegetation With Varying Morphologies

Abstract

Wind-driven sediment resuspension is a common phenomenon and impacts water quality and ecological balance in shallow lake systems. Aquatic vegetation (AV) alters the local hydrodynamics and thus influences the sediment resuspension processes, with its morphology as one of the most important factors. To understand the effect of AV on wave and sediment motion, field experiments were conducted for a year across a complete plant growth cycle in Dongping Lake, China. The vegetation morphology, water velocity, suspended sediment concentration, and wind direction/velocity were monitored within a patch of submerged flexible vegetation (i.e., Potamogeton crispus). Results showed that the existence of AV not only dampened the significant wave height (H_s) within the patch, but also attenuated the in-canopy wave orbital velocity (U_{w_horiz}) compared with the water surface, which indicated a dual reduction for near-bed wave velocity compared with bare-bed conditions. Variations of wave height and velocity reduction were related to vegetation morphological parameters. With vegetation experiencing its flourishing to senescent stages, the decrease of plant roughness density (i.e., from 3.89 to 1.81) weakened the wave velocity attenuation (i.e., from 12.7% to 5.4%). In the present study, the near-bed wave velocity in the centre of the vegetation patch was reduced by 40%–55%, even for the cases with vegetation in the senescent stage. The reduced near-bed wave velocity increased the critical velocity for sediment incipient motion from 3.0 cm/s for bare-bed conditions to 5.0 cm/s in vegetated cases. Besides, relationships between near-bed sediment concentration and hydrodynamics demonstrated wave dominance in resuspension initiation, transitioning to combined wave-current control during sustained suspension events. This study highlighted the effect of vegetation morphology induced by phenological evolution on wave-sediment motion, and its results have great significance for water pollution control and ecological restoration in shallow lakes.