Mixing Length for Establishment of Longitudinal Dispersion in Streams

Z. Ahmad (India)


River Engineering, Longitudinal dispersion, Pollutant, Mixing length, Mixing Coefficient, Flow measurement.


Understanding of mixing of pollutants is a matter of concern in recent years for effective control of pollution in streams. Mixing length for complete mixing of pollutants over the cross-sectional area of a stream must be known for application of longitudinal dispersion model. The same is also required for the measurement of flow in streams. The present paper deals with computation of mixing length for constant and variable transverse mixing coefficients for plane source of finite length across the stream. Charts have been developed for computation of mixing length for plane source of finite length and for different injection locations using transverse mixing equation with constant mixing coefficient. Ahmad's finite volume model is used for computation of mixing length for variable mixing coefficient. Mixing length for a typical cross-section of a stream reveals that mixing lengths computed for constant and variable mixing coefficients are not comparable. It is thus concluded that mixing length in a stream for known cross-section should be calculated using variable mixing coefficient. However, a preliminary estimate of mixing length for plane source of finite length may be computed using charts developed for constant mixing coefficient.

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