资源环境科技发展态势分析平台

Improving the quantification of the transverse mixing coefficient in rivers is essential for a better estimation of pollutant dispersion. This coefficient is evaluated from tracing experiments or from semi-empirical equations assuming uniform flow conditions. This assumption, verified for laboratory experiments, is questionable for river studies. For example, piedmont rivers exhibit non-uniform flows due to their longitudinal morphological variations. This work aims at studying the longitudinal variability of the transverse mixing coefficient in a river with longitudinally varied morphologyand the overlooked impact of the flow non-uniformity on the coefficient's values. Tracing experiments were conducted in the Durance River in France using slug injections and concentration measurements made at the cross-sections delineating homogeneous sub-reaches. Using the well-known 1D transverse diffusion model, values of the transverse mixing coefficient are determined for all consecutive sub-reaches. These values substantially vary along the studied river, with stronger mixing efficiency at pools and complex riffles and weaker mixing efficiency in straight sub-reaches with emerging obstacles. The dimensionless transverse mixing coefficient is strongly related to a parameter quantifying the deviation from flow-uniformity, i.e. indicating flow acceleration, deceleration or a succession of both within the sub-reaches. While usual formulas from the literature fail to predict our empirical results, a new equation including the flow non-uniformity parameter fits our experimental data with high accuracy. After further validation, such equation may be used for predicting the transverse mixing coefficient in rivers with longitudinally varied morphology.

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