Abstract
Longitudinal dispersion is the spreading of suspended or dissolved substances caused by the combined action of differential advection and lateral mixing. Tracer tests in many rivers under open-water conditions have indicated that the temporal spread first increases linearly with downstream distance (linear range). It may ultimately grow in proportion to the square root of distance (Fickian range), very far below the point of release. Test data have been obtained in ice-covered rivers, starting in 1975. They show linear-range behavior even though the test reaches were as long as hundreds of kilometers. As in the case of open-water conditions, the rates of spread under an ice cover are related to the friction factor of the flow, being generally higher than the open-water values for equal friction factors. Based on early results in pipe flow, many water quality models assume Fickian dispersion, contrary to the field evidence. A more correct quantification of the dispersion process is outlined and implications to the method of computation are discussed.
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