Study On Flow Movement And Pollutant Dispersion In Vegetated Channel

As the core carrier of water,river is an important channel of material circulation energy and information transmission in nature,as well as an important lifeblood of human activities,so the healthy development of river is particularly important.As an important component of river ecosystems,aquatic plants enhance riverbank stability by consolidating soil,reducing erosion and reducing water turbidity,while providing habitat for aquatic and terrestrial wildlife,mitigating flooding,providing aesthetic properties and filtering pollutants carried by runoff.In recent years,aquatic vegetation has been widely used in ecological restoration and has achieved remarkable results.Therefore,it is of great significance for ecological protection to study the interaction mechanism between water flow and pollutants in vegetated river channels.In this thesis,two types of simulated vegetation and one type of real vegetation were used for flume experiments,while the flume was arranged as a compound channel and a rectangular channel according to the research needs.Based on the experimental research,a combination of numerical simulation and theoretical analysis was used to study the water movement and pollutant transport characteristics of the vegetated river channel.The main research results achieved in this thesis are as follows.(1)for the rectangular channel under full coverage of submerged flexible vegetation,in the vertical direction,the water flow is divided into vegetation layer and free water layer,and the vertical distribution of longitudinal flow velocity is obtained by using the first-order closure model of Reynolds stress and the assumption that the vegetation resistance is proportional to the primary side of the flow velocity,and the wake effect is introduced,and the error analysis shows that the measured values agree well with the predicted values.Correlation analysis of k_v with parameters such as maximum bending angle,flooding and vegetation morphology showed that the Carmen coefficient in the vegetation layer could behave approximately as a constant.(2)The vertical distribution of longitudinal flow velocity in each lateral under simulated and real vegetation was analyzed for the compound channel and rectangular channel under partial vegetation cover.In the main channel zone of the compound channel,the vertical velocity distribution shows a"J"shape;in the side slope zone,the velocity decreases as it gets closer to the vegetation;in the floodplain zone,the velocity decreases as it gets closer to the wall,and the velocity distribution shows different patterns due to the different inundation levels of the vegetation.In the rectangular channel,the velocity in the vegetated area shows an"S"distribution,while the velocity in the non-vegetated area shows a"J"distribution.(3)Based on the depth-averaged Navier-Stokes momentum equation,the analytical solution model for the transverse distribution of longitudinal flow velocity in complex and rectangular channels was solved by applying the eddy viscosity theory.The error analysis shows that the measured values agree well with the predicted values,indicating that the model can predict the lateral distribution of longitudinal flow velocity in a partially vegetated channel.Based on this,a new method is proposed to obtain the analytical solution for the mean velocity of the cross-section.The measured values and the calculated values of the analytical solution are well fitted,and the errors are within reasonable limits.(4)The time-concentration curves of rhodamine were obtained in the upstream and downstream reaches of the river by conducting rhodamine concentration tests in a partially vegetated compound channel.The test results show that the upstream of the pollutant concentration process shows a small range with high concentrations and the downstream shows a large range with low concentrations under the same working conditions.(5)Based on the measured pollutant concentration processes in each zone,an"algorithm"is used to calculate the measured values of the longitudinal dispersion coefficients of the compound river channel.A two-zone model was proposed to predict the longitudinal dispersion coefficients in vegetated channels,and it was found that the two-zone model could reasonably and effectively predict the longitudinal dispersion coefficients in vegetated channels with partial cover.By comparing with previous studies,it is shown that the presence of vegetation can have some influence on the longitudinal dispersion coefficient.