The effect of dead zones on longitudinal dispersion in streams

Dead zones tend to hold back the downstream travel, to increase the longitudinal spreading and to provide a long tail of low concentration for passive contaminant releases in natural streams. Here it is shown how the presence of a random distribution of dead zones can be accommodated into the method of moments by choosing an appropriate composite averaging. The individual roles of the cross-stream velocity shear, the dead-zones mean volume fraction and the dead-zones probability distribution are clearly revealed in the longitudinal shear-dispersion coefficient. The inevitable deviations from Gaussianity are examined by means of skewness and kurtosis. Simple examples are used toquantify the effects of the dead zones upon contaminant dispersion in Couette flow, pipe and plane Poiseuille flows.