In open channels,floating vegetation canopies(e.g.,E.crassipes)reduce the flow velocity within canopies and increase the velocity in the free-flow region below canopies.The variation in the flow structure below the ca...In open channels,floating vegetation canopies(e.g.,E.crassipes)reduce the flow velocity within canopies and increase the velocity in the free-flow region below canopies.The variation in the flow structure below the canopies impacts both sediment resuspension and deposition onto the channel bed.Combining the random displacement model with the sediment deposition probability,a new model was proposed for predicting the longitudinal distribution of sediment deposition below floating vegetation canopies.The deposition distributions were measured below floating canopies of model vegetation(rigid cylinders)and real vegetation(E.crassipes).The measurements agreed well with the predictions of the proposed deposition model.With the use of the proposed model,the impacts of floating E.crassipes canopies on the deposition distribution were examined for different upstream mean flow velocities(U_(0)),relative flow depths(h_(g)/H),vegetation densities(a)and sediment sizes(d_(s)).The results suggested that net deposition below floating canopies decreased with increasing U_(0) and a and with decreasing h_(g)/H.Under the same flow and vegetation conditions,smaller sediment particles were more likely to become resuspended,resulting in less net deposition below the floating canopies.These results can improve the fundamental understanding of vegetation-based river management and ecological restoration.展开更多
Floating vegetation island(FVI)provides an effective way to remove excessive nutrition and pollutants in rivers.The Reynolds stress model(RSM)is employed to investigate the hydrodynamic characteristics induced by vari...Floating vegetation island(FVI)provides an effective way to remove excessive nutrition and pollutants in rivers.The Reynolds stress model(RSM)is employed to investigate the hydrodynamic characteristics induced by varied canopy densities of FVI in an open channel.In longitudinal direction,four regions are subdivided according to the flow development process:upstream adjustment region(LUD),diverging flow region(LDF),shear layer growth region(LSD),and flilly developed region.The increasing canopy density accelerates the flow adjustment in the diverging flow region and shear layer growth region,signaling a shorter distance to reach an equilibrium stage,while LUD keeps a constant.The vertical profiles of the normalized velocity are found to be self-similar downstream of the diverging flow region.In the vertical direction,the streamwise velocity profiles in the mixing layer collapse for all densities and obey the hyperbolic tangent law.Normalized penetration depth into the canopy was fitted as a function of dimensionless canopy density given by δc/hc=0.404(CDahc)^-0.316.This finding indicates a large space for rapid water renewal between the canopy region and the underlying water driven by the shear-scale vortices.In the lateral direction,the intensification of secondary current and the increasing number of secondary current cells with increasing canopy density reveal that dense floating canopies contribute to strong momentum exchange.The centers of vortices move as canopy density increases,while the vortices in canopy region do not merge with those in the gap region,as limited by the height and width of the canopy region.The distribution of longitudinal velocity in the transects is significantly influenced by secondary current.展开更多
基金Project supported by the Fundamental Research Project of China Yangtze Power Co.Ltd.(Grant No.2423020045)the National Key Research and Development Program of China(Grant No.2022YFE0128200)+1 种基金the National Natural Science Foundation of China(Grant No.52379072)the Fok Ying Tung Education Foundation(Grant No.171067).
文摘In open channels,floating vegetation canopies(e.g.,E.crassipes)reduce the flow velocity within canopies and increase the velocity in the free-flow region below canopies.The variation in the flow structure below the canopies impacts both sediment resuspension and deposition onto the channel bed.Combining the random displacement model with the sediment deposition probability,a new model was proposed for predicting the longitudinal distribution of sediment deposition below floating vegetation canopies.The deposition distributions were measured below floating canopies of model vegetation(rigid cylinders)and real vegetation(E.crassipes).The measurements agreed well with the predictions of the proposed deposition model.With the use of the proposed model,the impacts of floating E.crassipes canopies on the deposition distribution were examined for different upstream mean flow velocities(U_(0)),relative flow depths(h_(g)/H),vegetation densities(a)and sediment sizes(d_(s)).The results suggested that net deposition below floating canopies decreased with increasing U_(0) and a and with decreasing h_(g)/H.Under the same flow and vegetation conditions,smaller sediment particles were more likely to become resuspended,resulting in less net deposition below the floating canopies.These results can improve the fundamental understanding of vegetation-based river management and ecological restoration.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.11872285,11672213).
文摘Floating vegetation island(FVI)provides an effective way to remove excessive nutrition and pollutants in rivers.The Reynolds stress model(RSM)is employed to investigate the hydrodynamic characteristics induced by varied canopy densities of FVI in an open channel.In longitudinal direction,four regions are subdivided according to the flow development process:upstream adjustment region(LUD),diverging flow region(LDF),shear layer growth region(LSD),and flilly developed region.The increasing canopy density accelerates the flow adjustment in the diverging flow region and shear layer growth region,signaling a shorter distance to reach an equilibrium stage,while LUD keeps a constant.The vertical profiles of the normalized velocity are found to be self-similar downstream of the diverging flow region.In the vertical direction,the streamwise velocity profiles in the mixing layer collapse for all densities and obey the hyperbolic tangent law.Normalized penetration depth into the canopy was fitted as a function of dimensionless canopy density given by δc/hc=0.404(CDahc)^-0.316.This finding indicates a large space for rapid water renewal between the canopy region and the underlying water driven by the shear-scale vortices.In the lateral direction,the intensification of secondary current and the increasing number of secondary current cells with increasing canopy density reveal that dense floating canopies contribute to strong momentum exchange.The centers of vortices move as canopy density increases,while the vortices in canopy region do not merge with those in the gap region,as limited by the height and width of the canopy region.The distribution of longitudinal velocity in the transects is significantly influenced by secondary current.
