Abstract
BackgroundFlexible 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.ResultsThe 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%.ConclusionsThe 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.
Highlights
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
Comparing the results in Cases AA and AB with those affected by rigid vegetation at x = 20 cm (Case E and Case F in [10]), it was found that the flexible vegetation resulted in smaller mean velocities than rigid vegetation with the same vegetation density
Laboratory experiments were carried out in this study to investigate the influence of flexible vegetation on solute transport, and the lateral and longitudinal diffusion coefficients were estimated based on the experimental measurements
Summary
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. In order to quantitatively study the influence of stem-scale turbulence caused by vegetations on the lateral diffusion of solute transport, Nepf et al [14] improved the traditional random walk method and introduced a wake coefficient as a key parameter to establish the lateral diffusion model of solute transport affected by emergent vegetation. They proposed a formula for calculating the lateral diffusion coefficient Dy which was fitted by the stem-Reynolds number (Red = ud/ν, u is the flow velocity, d is the diameter of vegetation, and ν is the viscosity coefficient) in the range of 200–1800. Further researches were still needed to determine whether it was appropriate for flexible vegetation
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