Longitudinal dispersion in natural channels

The longitudinal dispersion of fluid particles in natural channels is investigated in an extensive series of experiments in small mountain streams (New Zealand). These experiments extend over a maximum distance of 2250 m, a mean velocity range of 0.32–1.57 m/s, and a mean flow width range of 2.7–21.8 m. It is conclusively shown for these channels and others as well that the spread or standard deviation of an initially concentrated mass increases linearly with distance and not as its square root, as is necessary for the application of Taylor's mixing model. One consequence of the linearity is an ever-increasing dispersion coefficient along the channel. It is also shown that the time-concentration curve of a dispersing tracer mass maintains a persistent asymmetry. This persistent asymmetry and the continued linear spreading appear to be characteristic of dispersion in natural channels and as such show the inadequacies of applying Taylor's analysis.

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