Z. Ahmad∗
River engineering, longitudinal dispersion, pollutant, mixing length, mixing coefficient, flow measurement
The need for effective control of pollution in streams has increased because of rapid industrialization and growth in urban population. This requires a proper understanding of pollutants mixing in these streams. Mixing length, defined as the length measured from the source of pollutants upto the distance where these pollutants get completely mixed over the cross section of the stream, must be known for the application of longitudinal dispersion models as well as for the measurement of flow in streams using chemical method in which dilution of given solution is considered. Charts have been developed in this paper for the computation of mixing length for the plane source of finite length with different injection locations. Available methods for the estimation of mixing length consider depth and velocity of flow, and transverse mixing coefficient as constant across the width of stream. However, in practice these parameters are not constant and rather vary in the transverse direction. Ahmad’s finite volume model is used for the computation of mixing length for variable mixing coefficient. Analysis of data reveals an overestimation in mixing lengths computed on the basis of constant mixing coefficient. Relationship for the mixing length for varying transverse mixing coefficient for the plane source of finite length located at the bank and in the middle of stream has been proposed. The proposed relationship for the mixing length produces satisfactory results.
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