The influence of channel-skewed bedforms on flow structure in a high-amplitude meandering channel

Meandering;Flow structure;Bedforms

Bryk, Alexander Bodkin ; Best ; James L.

关键词: Meandering;    Flow structure;    Bedforms;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/101039/BRYK-THESIS-2018.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

The interaction of flow, sediment transport, and substrate morphology (morphodynamics) leads to complex boundary configurations of ripples, dunes, and bars along alluvial rivers. The instantaneous boundary morphology heavily influences the local flow structure, especially within meander bends, and the temporal evolution of these interactions determines the behavior of channels as they migrate across their floodplains. Within high-amplitude bends, large dunes often develop that have their crestlines skewed diagonally with respect to the channel boundaries. The influence of these channel-skewed bedforms on bend flow structure is poorly understood. These forms are of particular interest as they may prevent shoaling over point bars and promote erosion on the outer bank (similar to engineered bendway weirs). In addition, the geometry of these bedforms may provide nucleation points for large-scale transverse flow separation within high-amplitude bends, thus fundamentally changing the pattern of erosion and deposition within these bends. This thesis examines the primary and secondary flow structure in a high-amplitude experimental meandering channel under differing substrate conditions. Three separate substrate morphologies were tested in the Kinoshita experimental meandering channel at the Ven Te Chow Hydrosystems Laboratory. The first consisted of a flatbed condition with rectangular walls and served as a control for subsequent experiments as well as for comparison to previous studies. The second substrate included a simplified synthetic point bar and the third consisted of the synthetic point bar with added channel-skewed roughness elements. For each condition, detailed cross-sectional flow structure was measured using a Profiling Acoustic Doppler Velocimeter (PADV). Results show the strong influence of the point bar and channel-skewed roughness elements on flow structure within the Kinoshita channel. In contrast to the flatbed condition, large transverse flow separation cells were identified downstream of the bend apex in all experiments in which synthetic roughness elements were present. The zone over which the flow adjusts to bend curvature changes, known as the hydraulic transition zone (HTR), was redefined to include both the interaction of curvature-induced secondary flow cells as well as the normalized strength of secondary flow. The inclusion of secondary flow strength in the definition of the HTR demonstrates that the HTR extends farther downstream than previously thought, even under flatbed conditions. In addition, the HTR was greatly expanded when channel-skewed bedforms were present compared to both the point bar and flatbed conditions. For the point bar and bedform conditions, secondary flow strength did not lag spatially behind water surface superelevation as was observed in the flatbed case. These results have implications for bend migration and, in particular, the propensity for bends to translate versus elongate.