Study On Flow Structure And Pollutant Transport At The Asymmetric River Confluences

River confluences are typical geomorphic units in the river system,where two streams converge and mix with each other,resulting in complex and special flow structure,bed morphology,material mixing process and water environment characteristics,which are of great significance to river connectivity,navigation,sediment and pollutant transport and bed evolution of the river network.The confluences often have complex and changeable configurations and hydrological conditions.However,the previous studies were mainly focus on the qualitative description of the open-channel confluences and lacked of quantitative research results.Moreover,the flow structure at the confluence shows strong three-dimensional(3-D)character and complexity,especially the real 3-D motion characteristics of the natural river confluence needs to be further studied.The main factors affecting the flow structure and pollutant transport at the confluence and the quantitative response relationship between them are not clear.The water surface morphology,bed topography,and spatiotemporal distribution,mixing and transport of pollutants still need to be deeply studied.The relevant research can further enrich the knowledge of fluid dynamics and pollutant transport at the confluence,can contribute a lot to the in-depth understanding of the flow structure and bed morphology at the confluence with wide shallow channel,and can provide references for river channel control and environmental management in the river network.In this paper,the influence of junction angle(a),momentum flux ratio(Mr),width-depth ratio(b/h)and concentration gradient(Cg)on flow structure,pollutant transport and mixing dynamic at the asymmetric river confluence is systematically studied by flume experiments,numerical simulation and field measurement.The research mainly revolves around the following points:(1)Flow structure at the asymmetric open-channel confluenceThe water depth and 3-D velocity at the asymmetric open-channel confluence were measured in detail by the particle image velocimetry(PIV)system,to analyze the water surface morphology,flow field characteristics,vertical average velocity distribution and 3-D flow structure,and to study the influence of junction angle,momentum ratio and width-depth ratio on the above flow structure.The results showed that,the convergence of the two streams produces a jacking effect that causes the water surface elevation within the stagnation zone and adjacent water area,and the water surface rapid decline near the separation zone.The flow structure represents strong 3-D characteristics and conforms to the previous conceptual model of the confluence hydrodynamic zone.The junction angle,momentum ratios and width-depth ratios have great influence on the flow structure.With the increase of the three factors,the area of the stagnation zone increased,the flow deflection enhanced,the size of the separation zone changed,the maximum velocity zone shifted toward the right bank,and the distance of the recovery zone was shortened(2)Hydrodynamic characteristics and channel morphodynamics at the confluence of the Yellow River and Fen RiverSeveral field measurements were conducted by the Acoustic Doppler Current Profiler(ADCP)at the confluence of the Yellow River and Fen River,which with the characteristics of large width-depth ratio,small momentum ratio,asymmetric confluent planform,discordant bed and high sediment concentration.The water surface morphology,bed topography,flow field structure and suspended sediment particle size distribution characteristics of were studied at the confluence in different momentum ratios.And a conceptual model of flow structure and bed morphology was established for the asymmetric confluence with high sediment load.The results showed that the momentum ratio has a great influence on the flow structure and bed morphology at the confluence.There was no obvious separation zone and helical motion in the water flow.There was a scour hole located in the immediate downstream of the channel,maximum scour depth is approximately 3 m.The riverbed was complex and changeable,which was easy to be affected by water flow and produce large area of deposition or erosion(3)Spatiotemporal distribution and mixing dynamics of pollutants at the asymmetric open-channel confluenceThe pollutant concentration at the asymmetric open-channel confluence was systematically measured by plane laser induced fluorescence(PLIF)measurement system and numerical simulation method.The distribution of the mean concentration and the mixing interface trajectory of pollutants at the confluence were studied,to reveal the influences of junction angles,momentum ratios,width-depth ratios and concentration difference on concentration field,mixing interface trajectory line,mixing rate and process.The function expressions of mixing rate and mixing degree of pollutants are given by multiple regression analysis.The results showed that the junction angles,momentum ratios and width-depth ratios had great influence on the transport of pollutants.The increase of the three factors could enhance the mixing.However,the concentration gradient had little influence on mixing.When the momentum ratio is greater than 1,complete mixing is basically achieved within the length of 4-5 branch widths.The four intersection factors had a linear correlation with the degree of mixing.The influence degrees were ranked from the largest to the smallest as b/h,? and Cg.Mixing rate(Rm)and mixing degree(??xn)are functions of a,Mr and blh.The mixing rate characterization function is Rm=-0.036·exp(?')-0.022·ln Mr-0.027·In(blh).The mixing degree characterization function is ??xn=0.432·?'·Mr-0.084·(b/h)2+0.523 b/h.(4)Spatiotemporal distribution and mixing dynamics of pollutants at the confluence of the Yellow River and Fen RiverThe typical water quality indexes at the confluence of the Yellow River and Fen River were monitored in field for a long time,to illustrate the water pollution status and the spatiotemporal distribution of pollutants,to study the effects of discharge ratios and concentration ratios on the mixing rate and deviation from complete mixing of pollutants,and to reveal the potential key factors affecting the mixing of pollutants at the confluence.The results showed that the flow of Fen River joined into the Yellow River produced a long narrow pollution zone within the downstream channel,seriously impacting the water quality of the Yellow River,mainly in terms of the pollutions of organic matter,ammonia nitrogen,total phosphorus and dissolved oxygen.The pollutants at the confluence had obvious seasonal variability.The spatial distribution and mixing pattern of the several pollutants were consistent.The discharge ratio was the key factor to control the mixing rate and mixing degree.