Impact of sediment size, channel velocity and plant density on sediment deposition inside Phragmites australis canopies

Abstract

In this study, laboratory experiments were performed to investigate the impacts of sediment size, channel velocity and plant density on sediment deposition inside an emergent canopy constructed with Phragmites australis (P. australis) that included real plant morphology. Three sediment sizes (ds = 10, 22 and 48 μm), three plant densities (n = 0.006, 0.018 and 0.027 cm−2) and various mean channel velocities (U0 = 6 to 17 cm/s) were considered. Different mean channel velocities corresponded to the presence or absence of sediment resuspension in the bare channel, which led to distinct deposition patterns. Inside canopies, sediment deposition was dominated by the near-bed turbulent kinematic energy (near-bed TKE). At very low mean channel velocities (i.e., no resuspension in the bare channel), the pure deposition inside the canopy was the same as that in the bare channel. As the mean channel velocity increased but resuspension did not occur in the bare channel, the local vegetation-induced turbulence led to an increase in near-bed TKE within the canopy leading edge region, further causing the formation of a diminished deposition region. When the mean channel velocity was sufficiently large to produce resuspension in the bare channel, diminished and enhanced deposition regions were observed inside the canopy compared to the bare channel. A prediction method considering the impacts of sediment size, channel velocity, plant density and plant morphology was proposed to predict the length of the in-canopy diminished deposition region. The predictions and measurements were consistent. Different sediment sizes, channel velocities and plant densities led to two distinct sediment supply conditions inside the canopies. When sediment supply was not limited, finer sediment and/or sparser canopies led to longer diminished deposition regions, but the pure deposition inside the canopies remained constant. When sediment supply was limited, a different deposition pattern was observed. The pure deposition decreased along the canopy length, but the resuspension in the bare channel and canopy leading edge region might compensate for the limited sediment supply inside the canopy. A threshold representing the ratio of the advection time throughout the canopy to the particle settling time, κ, was defined to evaluate whether the sediment supply inside the canopy was limited. The sediment supply was not limited at κ < 0.4 but was limited at κ > 1.