Vertical Distribution of Suspended Sediments above Dense Plants in Water Flow

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.

Flow dynamics and sediment transport in vegetated rivers: A review

“…As for the determination of sediment diffusion coefficient, it is usually linked to the turbulent momentum diffusion coefficient  m via a turbulent Schmidt number St . Sediment diffusion coefficients were mainly derived by fitting the vertical distribution of SSC to experiments [121][122][123][124][125] and deviations of  from unity linked to sediment or flow properties. The main results are listed in Table 3.…”

“…They first optimized the expression for  m using prior studies for turbulent flow within vegetated systems and then fitted  to different experiments. Li et al [124] conducted flume experiments on the vertical distribution of SSC in a channel with flexible vegetation. The flexible vegetation is more likely to represent the natural vegetation in rivers compared to rigid cylinders.…”

Study of Suspended Sediment Diffusion Coefficients in Submerged Vegetation Flow

“…Recently, the research community has focused extensively on the transport and diffusion processes of the sediment in vegetated rivers (Huai et al., 2021) and evaluation of the vertical distribution of the SSC. Several prediction models have been proposed based on the diffusion theory, gravitational theory and random displacement model (Huai et al., 2020, Huai, Yang, et al., 2019; D. Li et al., 2018, 2020; Y. Li et al., 2020a, 2020b; Tseng & Tinoco, 2021; Yang & Choi, 2010). By summarizing the vertical profile of $${\varepsilon }_{s}$$ adopted in these SSC profile models for submerged vegetation sediment‐laden flow, the adopted profile of $${\varepsilon }_{s}$$ can be divided into two layers: submerged vegetation layer (0 < z < h ), and free water layer ( h < z < H ).…”

“…In addition, it is usually considered that $${\varepsilon }_{s}$$ is maximized at the submerged canopy height ( z = h ). Specifically, for the free water layer, $${\varepsilon }_{s}$$ exhibits two vertical distribution forms: linear profile (Huai et al., 2020; Huai, Yang, et al., 2019; Y. Li et al., 2020a), and parabolic profile (D. Li et al., 2018; Y. Li et al., 2020b; Yang & Choi, 2010). Moreover, in terms of the submerged vegetation layer, two types of vertical distribution forms for $${\varepsilon }_{s}$$ can be observed: linear form (Huai et al., 2020; Huai, Yang, et al., 2019; Yang & Choi, 2010), and exponential form (D. Li et al., 2018).…”

Assessment of flow characteristics through a grassed canal

“…The presence of vegetation intensified the flow in un-vegetated areas. Li et al [32] investigated a new distribution pattern of suspended sediments and the correspondingly deformed Rouse formula in the flow layer over dense plants. The findings indicated that above dense plants, the shear turbulent momentum of flow achieved a plant-height-modified negative linear profile and the vertical distribution of fine suspended sediments showed an equilibrium pattern.…”