Under the circumstances that equal intensity point source is discharged off-bank in rivers of medium width with the consideration of the reflection of both banks,a calculation method of pollutant mixing zone was derived from the analytical solutions of the simplified two-dimensional advection-diffusion equation.The form of the boundary shape curves and the non-dimensional equation of the pollutant mixing zone were obtained.Using the methods of iterative calculation and trial calculation,pollutants mixing zones of river bank and off-bank were identified.The relationship between the geometric characteristic parameters (coordinates),the area and area coefficient of the zone,and the relative off-bank distance were found.As a result,the relative off-bank distance a'was proposed as a classification criterion of discharge types.The classification criteria for river bank,off-bank and river center discharge types are as follows:①for 0 ≤ a'≤ a'1,the outfall can be simplified as the type of river bank (near-bank) discharge;②for a'1 ＜a'＜a'2,it is named as the type of off-bank discharge;③for a'2≤a'≤0.5,the outfall can be simplified as the type of river center discharge,and the expressions of a'1 and a'2 have been put forward to represent the classification lines of the river bank and off-bank discharge,off-bank and river center discharge respectively.By regression analysis,an empirical formula of the main characteristic parameters for off-bank discharge in pollutant mixing zone in rivers of medium width is proposed,which provides the basis for the optimization of outfall locations and for the calculation of pollutant mixing zone.%在等强度点源离岸排放条件下,从简化二维移流扩散方程的解析解出发,探讨了考虑两岸反射作用的中宽河流污染混合区的计算方法.给出了污染混合区边界形状曲线与量纲一方程的形式;采用迭代计算和试算比较法,给出了靠岸与离岸两种类型混合区各几何特征参数(坐标)、面积及面积系数与相对离岸距离的关系曲线,提出了以相对离岸距离a'作为判据的排放类型分区条件.给出了河流岸边、离岸与中心排放的分类判别准则:①当0≤a'≤a'1时,简化为岸边(近岸)排放类型;②当a'1＜a'＜a'2时,称为离岸排放类型;③当a'2≤a'≤0.5时,简化为中心排放类型,并给出了岸边排放类型与离岸排放类型的临界相对离岸距离a'1和中心排放类型与离岸排放类型的临界相对离岸距离a'2的表达式.通过回归分析,给出了中宽河流离岸排放类型混合区主要特征参数的实用计算公式,可作为中宽河流污染混合区范围计算和排污口位置优化设计的依据.
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