Study On Release And Transport Of Phosphorus Under Hydrodynamic Disturbance

As one of the main causes of lake eutrophication and agricultural non-point source pollution,phosphorus content directly affects the quality of lake and river system.Healthy lakes and river systems have certain self-purification ability,but if the content of pollutants exceeds the upper limit of self-purification ability,lakes and river systems will not be able to self-restoration,which will lead to the deterioration of water quality in lakes and river system.Studying on the release of phosphorus in lakes and the transport of phosphorus in channels is the basis for solving the problems of lake eutrophication and agricultural non-point source pollution.In this paper,phosphorus in lake sediment and in channel current are taken as the research subjects,and a series of experiments and numerical simulations are carried out to study the release and movement of phosphorus under varying hydrodynamic conditions.For lake phosphorus,the exchange law of phosphorus between bottom sediment and overlying water under constant and variable hydrodynamic disturbance was studied by using self-assembled experimental device,and a new treatment method of endogenous phosphorus pollution,ecological corridor method,was suggested.For phosphorus in channel current,transverse distribution of phosphorus concentration in a compound channel with artificial vegetation on the floodplain was further studied,and the ecological drainage ditches of Qujialing Experimental Base,State Key Laboratory of Water Resources and Hydropower Engineering Science,Wuhan University,were adopted to conduct field experiments.Under varied hydrodynamic disturbance,retention efficiencies of phosphorus of the four ecological ditches are different,and in wich the two-stage channel with woodpile revetment has the highest efficiency.Under constant shear velocity conditions,the variation of phosphorus concentration in overlying water generally presents a logarithmic profile with time.The release rate of phosphorus was highest in the early stage of the experiment,and decreased slowly in the middle stage.At the later stage,the phosphorus concentration in overlying water tended to be stable,and the release rate of phosphorus increased with the increase of shear velocity.Under the changing shear velocity disturbance,the release curve of phosphorus is quite different from that under the constant hydrodynamic disturbance.The gradual increase of shear velocity has a positive effect on the release of phosphorus.The sudden decrease of shear velocity will lead to the decrease of phosphorus release rate.To effectively eliminate the excessive phosphorus content in lake sediment,we suggested a method by inserting porous pipes into the sediment to increase the release speed of phosphorous.This method imitates the principle of biological corridor,enlarges the interface area between sediment and water,and increase the exchange of phosphorus between sediment and overlying water.The experimental results show that the porous pipe can promote the release of phosphorus from sediment to overlying water,and the release rate of phosphorus increases with the increase of the number of porous tubes.The simulated ecological corridor can accelerate the release of phosphorus in sediment and reduce the phosphorus content in sediment.When combined with the "water diversion and recharge" project,the presented method can efftively dilute phosphorus concentration in the lake and reduce the risk of secondary pollution of the sediment.Two-stage ditch has been proved to have a higher efficiency in phosphorus retention,yet the inner mechanism is still confused.Current studies were mainly focused on the laterral distribution of velocity,while the theoretical study of the transverse distribution of sediment and phosphorus is rare.Based on the laboratory flume experiments,the lateral distributions of velocity,suspended sediment and phosphorus in the compound section were analyzed.Based on the N-S equation,the suspended sediment diffusion equation and the phosphorus diffusion equation,the analytical solution of the transverse distribution of phosphorus concentration in the compound section was given with appropriate boundary conditions.The analytically predicted results agree well with the experimental results.As a new way of agricultural drainage,the important role of ecological ditch in retention of phosphorus has no doubt.Based on the field experiments in Qujialing field base,the retention efficiency of phosphorus in four different ecological revetment channels(including trapezoidal ditch with geotextile bag revetment,trapezoidal ditch with hollow hexagonal brick revetment,trapezoidal ditch with reinforced vegetative bank revetment and two-stage ditch with woodpile revetment)were analyzed.The experimental results show that the two-stage ditch with woodpile revetment has the most obvious effect on reducing phosphorus.Drainage also has a significant impact on the intention capacity of phosphorus of ecological ditches.With the increase of drainage discharge,the retention efficiency of suspended sediment,dissolved phosphorus and particulate phosphorus decreases.Lattice Boltzmann method is a novel and promising numerical method.In recent decades,many scholars have tried to introduce Lattice Boltzmann method into computational hydraulics.By introducing additional terms of hydrodynamic conditions into the governing equation of diffusion and convective diffusion,we numerically simulated the exchange of phosphorus between lake sediment and overlying water interface and the transport of phosphorus in ecological channels with 1D-3D lattice type models,respectively.The trails and comparison results show that the numerical model can successfully simulate phosphorus transport in lakes and channels under hydrodynamic disturbance with a high accuracy and reliability.