Longitudinal dispersal properties of floating seeds within open-channel flows covered by emergent vegetation

Abstract

The effect of emergent stems on the transport of downstream floating particles (e.g., buoyant seeds) is explored theoretically and experimentally at moderate to high Reynolds number (Rd=2Ubas/υ>300) in an open channel, where Ub is the bulk velocity, as is stem radius, and ν is the kinematic viscosity. Longitudinal dispersion (Dl) of such seeds is shown to be given by Dl=Up3τ02η(1−η)/2/(S1+ητ0Up) thereby requiring the bulk transport velocity of particles Up, spacing between canopy elements S1, collision efficiency η between a particle and the stem, and a wake trapping time scale τ0. Linkages between Up and Ub, and terms η and τ0 are then derived using inhomogeneous flow characteristics adjacent to the stem with physical properties of particles. A semiempirical model with potential flow theory around the stem is introduced to estimate η whereas τ0 is shown to be related to the bulk drag coefficient and radius of stems. The Dl is experimentally determined across a wide range of Rd and particle sizes and shown to be in good agreement with the proposed model calculations.