考虑河流流速和横向扩散系数变化的污染混合区理论分析及其分类

Theoretical analysis and the classification of pollutant mixing zone considering variation of flow velocity and lateral diffusion coefficient in rivers

宽阔河流的中部深槽和远岸地形变化,对排放岸附近污染混合区范围的影响很小。因此,河流断面平均流速和横向扩散系数不能很好地反映排放岸附近污染物的移流扩散特性。通过对长江黄沙溪排污混合区平水期同步观测资料的分析,提出了深度平均流速的横向指数分布和横向扩散系数的二维变化关系式。在宽阔河流顺直岸边稳定点源条件下,求解了变系数二维移流扩散方程浓度分布的解析解,进行了浓度分布的特性分析。在此基础上,推导了污染混合区最大长度、最大宽度和对应纵向坐标以及面积的理论公式,给出了污染混合区边界归一化(等浓度)曲线方程。讨论了深度平均流速和横向扩散系数分布的特性参数(m,n和α)对污染混合区形态的影响规律,提出了河流岸边排放污染混合区的类别、形状特征、分类条件和变横向扩散系数的估算方法。实例表明,α-Ⅰ型污染混合区、边界曲线方程和几何特征参数,能够较好地表征长江黄沙溪排污混合区平水期COD_(Cr)现场观测等值线形状。

Average velocity and lateral diffusion coefficient of the river section can not well reflect on the characteristics of the pollutant advection-diffusion near the discharge bank,because the middle deep slot and the far-shore topographic changes of the wide shallow river have little effect on the pollutant mixing zone near the discharge bank.Based on the analysis of the synchronous observation data of the sewage mixing zone at Huangshaxi in the Yangtze River during a flat water period,the lateral exponential distribution of depth-averaged flow velocity and the two-dimensional transformation formula of the lateral diffusion coefficient are proposed.Under the condition of stable point source on the straight bank of the wide river,the analytical solution of the concentration distribution of the two-dimensional advection-diffusion equation with variable coefficient was solved,and the concentration distribution characteristics were analyzed.On this basis,the theoretical formulas of the maximum length,maximum width,and corresponding longitudinal coordinates and area of the pollutant mixing zone were derived,and the boundary normalized(equal concentration)curve equation of the pollutant mixing zone was given.The influence of the distribution characteristic parameters(m,n andα)of the depth-averaged flow velocity and the lateral diffusion coefficient on the morphology of the pollutant mixing zone was discussed.The classification,shape characteristics,classification conditions,and variable lateral diffusion coefficients estimation methods of the pollution mixing zone on the river bank were proposed.The example shows that theα-I type pollutant mixing zone,boundary curve equation,and geometrical characteristic parameters can characterize the isoline shape of the CODCr field observation of the sewage mixing zone at Huangshaxi in the Yangtze River during a flat water period.