Experimental Research On Concentration Distribution Of Side Discharging In Complex-banked Rivers & Reservoirs

Water is an indispensable strategic resource,playing an important role in ecological environment,social production and life.For a period time,water environmental problems caused by substandard discharge of wastewater are frequent,and have become an important constraint factor of economic and social development.Scientific planning and increasing governance have become imperative.In practical project,sewage outfall is mostly located on the shore.After the sewage enters water body,influenced by bank terrain and turbulent conditions,a high concentration of pollutant mixing zone is formed near the sewage outfall.The research of pollutant concentration distribution in this area is an important basis for water environmental management,functional zoning and comprehensive treatment.The bank terrain of natural water bodies such as rivers and reservoirs are mostly irregular,and sloped stepped bank or compound bank(collectively referred to as complex bank)are common bank morphology.Based on the theory of side dischargingpollution,two-dimensional diffusion experimental device is improved and developed.Three kinds of complex banks,including sloped stepped bank,compound stepped bank and compound trapezoidal bank,are simulated respectively by changing the geometric features or size of the inner water tank.Different transverse and vertical diffusion coefficients are simulated by adjusting grid oscillation frequency.With Rhodamine-B as tracer,diffusion experiments of instantaneous line source were carried out.Using a digital camera to record the RGB color image of concentration field during diffusion process,then two-dimensional concentration distribution is obtained by digital image processing technology.According to the conversion formula between 2D line source diffusion and 3D point source advection-diffusion,the concentration distribution in 3D space is obtained.The distribution of section concentration,water surface and depth-averaged concentration in longitudinal uniform flow are analyzed,while the moment method of depth-cumulative concentration is proposed to estimate transverse diffusion coefficient.The results show that:(1)Section concentration distribution of sloped stepped bank is shown as follows:A large scale clockwise whirlpool is generated at the step,resulting in a lower concentration nearby;45°-sloped stepped bank generates a significant gravity flow atthe initial stage of diffusion when grid oscillation frequency is 15r/min.For compound stepped bank: in the floodplain,pollutant concentration varies little along the vertical direction,and pollutant cloud diffuse into the major trough almost side by side;in the major trough,concentration distribution is quite different according to operating conditions,and the morphology of pollutant cloud presents many characteristics.For compound trapezoidal bank: in the floodplain,the concentration decreases along the vertical direction,but the gradient is not big;in the transition part and major trough,the morphology of pollutant cloud is relatively simple.With the increase of oscillation frequency,the ratio of transverse to vertical diffusion scale becomes larger.(2)Water surface or depth-averaged concentration decreases along the transverse and longitudinal direction,and the transverse gradient is much larger than longitudinal gradient,and pollutant mixing zone presents zonal distribution in which the longitudinal scale is much larger than the transverse scale.For sloped stepped bank and compound stepped bank,the depth-averaged concentration decreases vertically at the step,the isoconcentration contours coincide with the step line,and pollutant mixing zone is separated by the step line.The depth-cumulative concentration of compound trapezoidal bank increases first and then decreases in water depth transition.(3)Combined with numerical simulation,the depth-cumulative concentration moment is deduced and put forward to estimate transverse diffusion coefficient.The calculation results show that transverse diffusion coefficient increases with the increase of grid oscillation frequency,but increase speed is not the same.When grid oscillation frequency is constant,the transverse diffusion coefficient of 45° sloped stepped bank is larger than that of 30° bank.For compound bank,when the width of floodplain is large,transverse diffusion coefficient is sensitive to the depth of the floodplain,and the transverse diffusion coefficient is larger when the floodplain depth is larger;otherwise,it is not sensitive,and the values of the two are relatively close.Under the same working conditions,the transverse diffusion coefficient of compound trapezoidal bank is larger than that of compound stepped bank.The experimental research on concentration distribution of side discharging in complex-banked rivers & reservoirs,and revealing the regularity of concentration distribution,can provide scientific basis for the construction of water environmental mathematical model,analysis of field observation data and calculation of waterenvironmental function area.