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

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 (R-d = 2U(b)a(s)/nu > 300) in an open channel, where U-b is the bulk velocity, a(s) is stem radius, and nu is the kinematic viscosity. Longitudinal dispersion (D-l) of such seeds is shown to be given by D-l = U-p(3)tau(2)(0)eta(1 - eta)/2/S-1 + eta tau U-0(p)) thereby requiring the bulk transport velocity of particles U-p, spacing between canopy elements S-1, collision efficiency eta between a particle and the stem, and a wake trapping time scale tau(0). Linkages between U-p and U-b, and terms eta and tau(0) 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 eta whereas tau(0) is shown to be related to the bulk drag coefficient and radius of stems. The D-l is experimentally determined across a wide range of R-d and particle sizes and shown to be in good agreement with the proposed model calculations.