Longitudinal and lateral diffusion of solute transport in flow with rigid vegetation

Abstract

Background: Aquatic vegetation has major influence on the local water environment, affecting flow velocities and solute mixing. Extensive research has been conducted on the flow characteristics of vegetated areas, but little is known about solute transport. In this study, Laboratory experiments were carried out to investigate how solute transport is affected by emergent and submerged rigid vegetation. Results: Vegetation greatly reduces the mean velocity, especially within the vegetated region. Near the bottom, the solute concentration is greater in the dense vegetation than in the sparse vegetation. The vertical distribution of the solute concentration decreases rapidly with the relative water depth. Generally, the longitudinal and lateral diffusion coefficients are less affected by denser vegetation, but both coefficients are strongly influenced by the relative water depth (submerged vegetation height). Conclusions: A modified function to estimate the longitudinal diffusion coefficients is proposed under both emergent and submerged vegetation conditions, including cases of variable vegetation height. The key parameters (a' and b') for the assessment of the lateral diffusion coefficients are improved considering vegetation height. Results in the present paper can be used as efficient and convenient methods to estimate the longitudinal and lateral diffusion coefficients in flow with rigid vegetation.

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

“…Nonadsorptive solute dye tracer carmine was discharged at 10.54 mL/s in all tests. Solute concentrations were analyzed at six sections P1-P6 (x = [0, 5,10,15,20,25] cm) based on the image processing technology (Fig. 1b).…”

“…1b). The calibrations of solute concentration in image processing technology and the fitting curve between the solute concentration and image intensity were described by Lou et al [10].…”

“…Nepf 's method had certain physical significance and accuracy considering the effects of vegetation density on D L . However, the application of Nepf 's method was limited, because it was proved to be only appropriate for flows affected by rigid submerged vegetation [10], and cannot be used directly to flexible vegetation due to the neglect of vegetation bending.…”

“…As the key coefficients of solute transport, descriptions of lateral and longitudinal diffusion coefficients considering both the hydrodynamic conditions and flexible vegetation features (i.e., density and bending) are still lacking, which limits the understanding of solute transport processes with a flexible vegetation canopy. In our previous paper [10], laboratory experiments were carried out to analyze the processes of solute transport affected by rigid vegetations, and modified functions to estimate the lateral and longitudinal diffusions in vegetated flows were proposed under both emergent and submerged vegetation conditions based on previous researches by Nepf [13] and Jamali et al [7]. This study will focus on the mechanism of solute transport under the influence of a flexible vegetation.…”

“…In this paper, the effects of a flexible vegetation on the vertical distributions of flow velocity and solute concentration will be investigated. Methods for estimating the lateral and longitudinal diffusion coefficients in the rigid vegetated flow [10] will be examined for their applications to the flows with flexible vegetation, and the improved method of the lateral and longitudinal diffusion coefficients considering both the hydrodynamic conditions and flexible vegetation features will be proposed.…”