Influence of Blade Flexibility on the Drag Coefficient of Aquatic Vegetation

Abstract

Estimates of energy dissipation due to vegetation depend on the selection of an appropriate drag coefficient. The drag afforded by submerged and emergent vegetation depends, to varying degrees, on the incident forcing, the characteristics of the vegetation field, and the morphology of the individual blades. Recent field and laboratory evidence suggests that blade rigidity is an important, but poorly understood, control on wave attenuation, in which greater flexibility reduces drag and limits wave attenuation. The purpose of this study is to quantify the influence of blade rigidity on the drag coefficient through a laboratory experiment in which wave attenuation is measured through artificial vegetation of varying rigidity over a range of water depths and wave forcing. In general, greater flexibility caused a reduction in the drag coefficient (C D), independent of changes in water depth, although the drag would be expected to decrease as the canopy was submerged further. However, the drag coefficient of semi-rigid vegetation that pivots from the base exhibits a more complex relationship with the Reynolds number, with a significant decrease in drag as the vegetation transitions from submergent to emergent in response to the increase in oscillatory velocities at a lower point on the stem. The variation in the drag coefficient among the artificial vegetation data and previously published data from a seagrass meadow exhibits a statistically significant variation with rigidity, calculated as the ratio of Young’s modulus and wave forcing. The dependency of the drag coefficient on rigidity provides a means to estimate energy dissipation by vegetation of a given blade morphology using simple design variables.