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The retention of particles within meadows of submerged aquatic vegetation impacts the fate of organic matter, pollen, and larvae. Because flow conditions near the leading edge differ from those over the bulk of the canopy, particle retention is likely to differ as well. In particular, near the leading edge of a wide meadow, flow deceleration generates a vertical updraft, which impacts particle fate. In the fully developed region of the meadow, shear layer vortices at the top of the meadow may also influence particle fate. In this study, the retention of particles was measured along the length of a 10-m model meadow (height h = 0.1 m) and was connected to the evolving flow field. Two particle sizes, with settling velocity w[subscript s50] = 0.00075 , 0.018 m s[superscript −1], were released at two heights within the model meadow (Z[subscript rel]/h = 0.31, 0.81). The retention of particles was measured using microscope slides distributed along the flume bed. Retention increased with distance from the leading edge, associated with the decrease in vertical updraft. Retention was also greater for the particles with higher settling velocity. In the fully developed region of the meadow, particle retention was lower for particles influenced by the shear layer vortices at the top of the meadow (Z[subscript rel]/h = 0.81)., National Science Foundation (U.S.) (Grant AGS-1005480)

There is a need for more practical tools for estimating spore escape from crop canopies, which is essential in forecasting the propagation of disease to other fields. In this paper, we evaluated whether a random displacement model (RDM) parameterized with an eddy diffusivity could be used to predict spore escape probability. The proposed RDM does not require detailed turbulence measurements for parameterization. Instead, it constructs profiles of velocity and eddy diffusivity from a simple set of parameters [canopy height, canopy density, vegetation length scale, and wind speed]. The RDM was validated using field measurements of spore concentration. On average, the model predictions matched the field measurements within 28% inside the canopy and 42% above it, comparable to LES results over the same canopy. Once validated, the RDM was used to explore particle escape across a range of canopy densities and particle settling velocities, in order to inform estimates of particle escape from crops of varying maturity or area density. Escape fraction as calculated by the RDM increased as canopy density decreased, as the ratio of particle settling velocity to turbulent shear velocity ratio decreased, and as the source height within the canopy increased. Keywords: Particle transport; Escape of particles from canopy; Eddy diffusivity; Random displacement model; Maize, National Science Foundation (U.S.) (Grant AGS-1005480)

Flexible terrestrial and aquatic plants bend in response to fluid motion and this reconfiguration mechanism reduces drag forces, which protects against uprooting or breaking under high winds and currents. The impact of reconfiguration on the flow can be described quantitatively by introducing a drag coefficient that decreases as a power-law function of velocity with a negative exponent known as the Vogel number. In this paper, two case studies are conducted to examine the connection between reconfiguration and turbulence dynamics within a canopy. First, a flume experiment was conducted with a model seagrass meadow. As the flow rate increased, both the mean and unsteady one-dimensional linear elastic reconfiguration increased. In the transition between the asymptotic regimes of negligible and strong reconfiguration, there is a regime of weak reconfiguration, in which the Vogel number achieved its peak negative value. Second, large-eddy simulation was conducted for a maize canopy, with different modes of reconfiguration characterized by increasingly negative values of the Vogel number. Even though the mean vertical momentum flux was constrained by field measurements, changing the mode of reconfiguration altered the distribution, strength, and fraction of momentum carried by strong and weak events. Despite the differences between these two studies, similar effects of the Vogel number on turbulence dynamics were demonstrated. In particular, a more negative Vogel number leads to a more positive peak of the skewness of streamwise velocity within the canopy, which indicates a preferential penetration of strong events into a vegetation canopy. We consider different reconfiguration geometry (one- and two-dimensional) and regime (negligible, weak, and strong) that can apply to a wide range of terrestrial and aquatic canopies., National Science Foundation (U.S.) (Grant AGS1005363)

This laboratory study describes the sediment patterns formed in a sand bed around circular patches of rigid vertical cylinders, representing a patch of reedy emergent vegetation. The patch diameter was much smaller than the channel width. Two patch densities (solid volume fraction 3% and 10%) and two patch diameters (22 and 10 cm) were considered. For flows above the threshold of sediment motion, patterns of sediment erosion and deposition were observed around and within the patch. Scouring within the patch was positively correlated with turbulent kinetic energy in the patch. For sparse patches, sediment scoured from within the patch was mostly deposited within one patch diameter downstream of the patch. For dense patches, which experience greater flow diversion, sediment scoured from the patch was carried farther downstream before deposition along the patch centerline. Differences between the sparse and dense patch patterns of deposition are explained in the context of flow diversion and wake structure, which are related to a nondimensional flow blockage parameter. While sediment was redistributed near the patch, observations suggest that net deposition was not recorded at the reach scale., National Science Foundation (U.S.) (STC Center for Earth-surface Dynamics Agreement EAR-0120914), National Science Foundation (U.S.) (Grant EAR 0738352)