Laboratory study of the effects of flexible vegetation on solute diffusion in unidirectional flow

Abstract

Background Flexible vegetation is an important part of the riverine ecosystem, which can reduce flow velocity, change turbulence structure, and affect the processes of solute transport. Compared with the flow with rigid vegetation, which has been reported in many previous studies, bending of flexible vegetation increases the complexity of the flow–vegetation–solute interactions. In this study, laboratory experiments are carried out to investigate the influence of flexible vegetation on solute transport, and methods for estimating the lateral and longitudinal diffusion coefficients in the rigid vegetated flow are examined for their applications to the flow with flexible vegetation. Results The experimental observations find that vegetation can significantly reduce flow velocity, and the Manning coefficient increases with increasing vegetation density and decreases with inflow discharge. Under all the cases, the vertical peak of the solute concentration moves towards the bottom bed along the flow, and the values of vertical peak concentration longitudinally decreases from the injection point. The lateral diffusion coefficients Dy increase with vegetation density, while the longitudinal diffusion coefficients DL are opposite. Both Dy and DL increase with the inflow discharge. To estimate the Dy and DL in the flow with flexible vegetation, an effective submerged vegetation height considering vegetation bending is incorporated in the methods proposed for flow with rigid vegetation (Lou et al. Environ Sci Eur 32:15, 2020). The modified approach can well predict the diffusion coefficients in the experiments with the relative errors in the range of 5%–12%. Conclusions The methods proposed in this study can be used to estimate the lateral and longitudinal diffusion coefficients in flows through both rigid and flexible vegetations using the effective submerged vegetation height.

Flow Characteristics in Partly Vegetated Channels: An Experimental Investigation

“…Vegetation acts as an obstacle in the flow, causing increased friction and flow resistance which results in reduced flow velocities, increased flow depth, and altered flow patterns [5][6][7][8]. Moreover, vegetation plays an important role in stabilizing riverbanks and preventing erosion [9,10], improving water quality by acting as a natural filter [11], regulating water temperature [12], influencing sediment transport [13,14], providing important habitats for various aquatic organisms [15], and affecting flow structure and turbulence which in turn impact the transport processes and diffusion of dissolved species [16,17].…”

“…In vegetated channels, several properties need to be defined to understand the hydrodynamic flow structures, including flow regime, velocity distribution, vortical structures, turbulence intensities, shear stresses, mixing and dispersion, roughness, vegetation properties, and vegetative drag. Many previous studies have extensively investigated flow dynamics in open channels and their interactions with vegetation canopies [1][2][3][4]14,[16][17][18][19][20][21]. Various approaches, ranging from statistical methods to numerical models, were proposed in the literature to study and understand turbulent flows in vegetated channels.…”