Effects of Vertical Variation in Vegetation Density on Wave Attenuation

Abstract

A physical model experiment was used to investigate irregular wave attenuation through emergent vegetation with variations in stem heights. The experiment was conducted with six peak periods, six incident wave heights, and two schemes of vegetation. One scheme used uniform vegetation (constant stem height) and the other scheme approximated vegetation with larger biomass near the bottom and decreasing linearly to the surface, consistent with observed biomass patterns in the field. Although distributions were different, the total vertical projected area was kept constant between the two schemes. The cross shore variation of wave heights across the vegetation were measured, and losses due to sidewall and bottom friction effects were measured and removed from the wave attenuation in the vegetated cases to isolate the influence of vegetation. The normalized wave height attenuation for each case was fit to the decay equation to determine the difference of vegetation transmission coefficients Kv and damping factor α. The drag coefficients CD were estimated under the assumption of uniform, rigid vegetation and linear wave theory. This paper shows that in deeper water, the damping coefficient for the uniform vegetation is higher than that of the nonuniform vegetation because the wave-induced velocity decays exponentially with depth and is less affected by the biomass near the bottom. In shallower water, the ratio of the damping coefficients tends toward unity for the two schemes because the wave-induced velocity is nearly uniform and the vertical projected area was the same between the two schemes. In addition, this study highlights that the effect of vertical biomass distributions can result in changes in CD ranging from 140% to 170% between the two schemes and that the assumption of a uniform distribution of vegetation biomass is likely more critical than the assumption of rigid vegetation to model wave attenuation due to vegetation.