Flow and mass transport in vegetated surface waters
This chapter describes flow and mass transport under conditions relevant to surface water systems with emergent vegetation. It is convenient to conceptualize vegetated surface waters as homogeneous arrays of discrete, rigid, two-dimensional plant elements, e.g., stems. The integral length scale of turbulence within a homogeneous emergent canopy, which has traditionally been taken to be the canopy element diameter, is constrained by the interstitial pore size in dense canopies. The canopy-averaged integral length scale is well-described by the average surface-to-surface separation between a canopy element and its nearest neighbour. One proposed consequence of this reduction in the integral length scale is that turbulent diffusion decays with increasing canopy density at solid volume fractions above O(3)%. This explains the intermediate regime exhibited by net lateral dispersion, in which the dispersion coefficient, normalized by the interstitial flow velocity and canopy element diameter, decreases with increasing canopy density. Net longitudinal dispersion increases with increasing canopy density in sparse (φ< 6.4%) canopies; its behaviour in more dense canopies have not been established. In sparse canopies at transitional Reynolds numbers, the dominant contributions to longitudinal dispersion are associated with transient trapping in the unsteady wake of the elements and the velocity deficit downstream of the elements.