Abstract
Plants in natural water flow can improve water quality by adhering and absorbing the fine suspended sediments. Dense plants usually form an additional permeable bottom boundary for the water flow over it. In the flow layer above dense plants, the flow velocity generally presents a zero-plane-displacement and roughness-height double modified semi-logarithmic profile. In addition, the second order shear turbulent moment (or the Reynolds stress) are different from that found in non-vegetated flow. As a result, the turbulent momentum diffusivity of flow and thus the diffusivity of sediment will shift, which will cause the vertical profile of suspended sediment and the corresponding Rouse formula deform. A set of physical experiments with three different diameters of fine suspended sediments was conducted in an indoor water flume. These experiments investigated a new distribution pattern of suspended sediment and the correspondingly deformed Rouse formula in the flow layer over the dense plants. Experimental results showed that above the dense plants, the shear turbulent momentum of flow presented a plant-height modified negative linear profile, which has been proposed by a previous study, and the vertical distribution of fine suspended sediments presented an equilibrium pattern. Based on the plant-modified profiles of flow velocity and the shear turbulent momentum a new zero-plane and plant-height double modified Rouse formula were analytically derived. This double-parameter modified Rouse formula agrees well with the measured profile of suspended sediment concentration experimentally observed in the present study. By adjusting the Prandtl–Schmidt number, i.e., the ratio of sediment diffusivity to flow diffusivity, the double-parameter modified Rouse formula can be applied to submerged dense plant occupied flow.
Highlights
Aquatic plants in natural water flow as are present in a river, lake or reservoir is usually used as a key phytoremediation element in eco-restoration projects because of its many benefits to an aquatic environment [1,2,3]
The objectives of the present study are: (1) to investigate the profile of suspended sediment concentration (Cs ) in dense plant occupied flow under a high submergence ratio, with suspended sediment coming from upstream instead of coming upwards from the flow bed; (2) to seek suitable formulae of the profiles of flow shear and the shear turbulent momentum (−u0 w0 ); and (3) to derive an analytical formula of the profile of suspended sediment concentration
This d–hp double modified Rouse formula is suitable for describing dense plants with a high submergence ratio where the d–hp double modified logarithmic law of flow shear (Equation (5)) and the negative linear profile of shear turbulent momentum (Equation (7))
Summary
Aquatic plants in natural water flow as are present in a river, lake or reservoir is usually used as a key phytoremediation element in eco-restoration projects because of its many benefits to an aquatic environment [1,2,3]. In the above-plant region, aquatic plants influence the transport and movement of fine suspended sediment. These fine suspended sediments increase water turbidity, prevent photosynthesis in plants, affect the accessibility of food for larvae, and impact the quality of the aquatic habitat [8,9]. Water 2020, 12, x; doi: FOR PEER REVIEW distribution pattern of in suspended sediments in the above-plant is important for developing suspended sediments the above-plant region is important for region developing our understanding the our understanding aquatic eco-system.the aquatic eco-system. In a natural water flow environment, aquatic aquatic plants plants usually usually grow grow in in dense dense groups groups on on the the flow bed, thereby forming an additional permeable bottom boundary for the flow layer above it. The flow flow shear, i.e., the vertical gradient of flow velocity velocity (du/dz)
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