Extreme Mixing Events in Rivers

摘要

Although much effort has been expended on quantifying dispersion in simple channels, much less work has focused on compound channels, particularly for over-bank flows. Following an earlier study of a hypothetical compound channel, this paper provides further evidence for the significantly different flow dependence of dispersion coefficients for in-bank and over-bank conditions. The study is based on a natural cross-section of the River Severn, UK, and uses the same method as the earlier work, by combining the Shiono-Knight hydraulic model with the Fischer flow structure integral. The results show that dispersion coefficients are typically two orders of magnitude larger during over-bank flows compared to in-bank flows. Importantly, the maximum dispersion does not occur under the maximum flow. Instead, the maximum dispersion occurs when the flood plain inundation reaches the far edge of the flood plain.