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On the development of bed surface structures in response to variable flow regimes Wang, Yinlue


The objective of this research is to use flume experiments to investigate the formation and development of bed surface structures and their impact on channel stability in response to variable flow regimes. A wide range of bed structures (clusters, stone cells, and transverse ribs) was reproduced in five sets of experiments, and data of sediment transport, bed surface grain size, and channel bed topography were systematically collected. A semi-automated method was developed to identify the bed structures, and used to record their temporal evolution regarding numbers and areal coverage. After the initial period of bed coarsening, D₅₀ and D₈₄ of surface grain size remained almost unchanged. In contrast, ongoing bedform development, longitudinal grain segregation, and evolution of bed structures were observed throughout the experiments. Bed structures developed and coexisted, and they constantly formed, disappeared, and transformed into other structures. The bed structure dynamic cannot be simply explained by flow regimes, but was significantly driven by particle interactions. The dynamic behaviour of bed structures concurred with the fluctuation of grain stability on the bed surface. Despite the active evolution of individual structures, their overall bed coverage remained around 15-20% most of the time. It can be inferred that the dynamic development of bed structures can be linked fundamentally to increasing channel stability, and that the fluctuation of bed mobility is partly associated with the development of bed structures.

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