Velocity, Turbulence, and Sediment Deposition in a Channel Partially Filled With a Phragmites australis Canopy

Laboratory experiments examined the longitudinal evolution of near-bed velocity, turbulent kinetic energy (TKE), and net deposition in a model Phragmites australis canopy occupying 1/3 of the channel width. The canopies were constructed from model P. australis with real morphology and a solid volume fraction between 0.003 and 0.018. An exponential model was modified to predict the longitudinal evolution of near-bed velocity inside the canopy, from which the near-bed TKE can be predicted. By combining the predicted TKE and a deposition probability, we proposed a model to predict the distribution of net deposition inside the canopy. The predicted velocity, TKE, and deposition were in good agreement with the measurements. Relative to an upstream reference, the net deposition within the canopy was enhanced when two conditions were met: the in-canopy, near-bed TKE was smaller than the critical value for resuspension, and resuspension took place in the bare channel. Above a critical vegetation density (defined by a critical solid volume fraction phi(c)), the spatially-averaged deposition inside P. australis surpassed that in the adjacent bare channel. The proposed model provides a way to estimate phi(c). Relative to the upstream reference, deposition inside the canopy was always diminished over some fraction of the flow adjustment distance, L-d (distance from canopy leading edge to fully developed flow). When the canopy length was greater than 0.4 L-d, canopy-averaged deposition was enhanced relative to the bare channel. Finally, for the same canopy length, differences in plant morphologies did not have a strong impact on the in-canopy deposition distribution.