资源环境科技发展态势分析平台

The vertical distribution pattern of the fine suspended sediment concentration (SSC) plays an important role in the aquatic ecosystem of shallow water. To investigate the SSC profile in submerged vegetated river flow, a series of indoor flume experiments was conducted. Glass spherical balls of three different diameters (40, 77, and 90 mu m) with a uniform specific gravity of 2.5 were used for the model sediments. A set of sampling devices was designed for the simultaneous measurements of SSCs at multipoints along a vertical line. Experimental results show that the SSC profile was re-distributed in the vegetated region, gradually forming a new equilibrium pattern in the overcanopy flow layer with its maximum occurring near the canopy top. From this maximum point near the canopy top, the suspended sediment formed a dynamic balance between upward turbulent diffusion and downward gravitational settling. Meanwhile, the momentum diffusivity was observed to linearly decrease upward from the maximum-SSC elevation toward the water surface, thereby allowing us to deduce an empirical negative linear formula. Based on the convection-diffusion equilibrium mechanism and with assistance of the empirical formula of flow momentum diffusivity, a new form of the Rouse formula was derived for the overcanopy flow layer. In general, this equation agrees well with the measured data. Some undercalculated deviations appear in the well-above canopy zone due to the upskewed turbulence and the weak upward secondary flow. This deviation decreases with increasing sediment diameter. The bed deposition mainly comes from the deep part of canopy-occupied flow layer.

Key Points

The SSC profile is re-distributed in vegetated reach, forming an equilibrium pattern under the dynamic balance of diffusion and settling The new equilibrium pattern of SSC has a peak point near the top of vegetation canopy The equilibrium equation of SSC over vegetation canopy was derived based on newly proposed empirical formula of flow momentum diffusivity