Canopy resistance on oscillatory flows

Abstract

Aquatic vegetation in the coastal zone dissipates wave energy and provides a form of natural coastal protection against storm waves. The capacity of aquatic vegetation to attenuate incident waves depends on the extent to which wave energy is dissipated by small-scale hydrodynamic interactions within a canopy, which in turn depends on the work done by drag forces exerted by the canopy. Canopy drag forces (and hence rates of wave dissipation) are conventionally parameterised using a drag coefficient (Cd). Existing empirical models for predicting Cd are usually dependent solely on either Reynolds number (Re) or Keulegan–Carpenter number (KC), and hence neglect the potential effect of vegetation canopy density (λ) and the interactions between adjacent stems. This study uses high-resolution numerical simulations to investigate the dynamics of wave-driven oscillatory flow through emergent canopies (modelled as arrays of rigid cylinders). The simulations cover a wide range of KC. The influence of two mechanisms in modifying canopy drag, namely the effects of blockage and sheltering, are evaluated. The blockage effect is found to be the dominant mechanism responsible for increasing the canopy drag coefficients at high KC for medium to high density canopies; conversely, the sheltering effect plays only a minimal role in reducing the drag coefficient of the very sparse canopies. We show that Cd for canopies at high KC can be robustly estimated using a new modified drag formulation for unidirectional flow. Conversely, in the limit of the low KC, Cd is close to that of a single isolated cylinder. The results of this study can be used as a basis for developing new predictive formulations for specifying bulk canopy drag coefficients, and in turn quantifying wave attenuation by vegetation.