A laboratory study of gradually varied flow through semi-rigid emergent blade-type vegetation.

Abstract

Vegetation is an important component of an aquatic ecosystem and contributes to the natural defense of coastal areas from currents, waves and storm surges. This study investigates the hydrodynamics of blade-type vegetation under gradually varied flow. In the laboratory cable tile blades are used to simulate vegetation elements. The blades are fixed to PVC boards of length 2.4m. The spacing of blade elements is different for different boards and the finite artificial vegetation patches are of densities 𝜆=72,48,24,18,𝑎𝑛𝑑 15 and blade Reynolds number Re = 510 – 1220. Six flow rates are used in each set of experiments. The longitudinal water surface profiles have been measured and the effect of increasing roughness density under the flow conditions examined. A theoretical equation that relates vegetation resistant force and water surface profile (Li and Tam, 2002) has been modified and used for determining the averaged drag coefficient Cd. The results show that Cd varies with the vegetation density with an apparent trend due to the change in the number of shedding eddies per unit area. A better understanding of the resistance mechanism of the geometrical shape of vegetation elements is obtained.