Secondary Motion in Meandering Streams: An Experimental Study

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Authors
Sagnak, Alper
Keyword
Meandering Streams , Sine-generated Channel , Experimental Study , Secondary Motion , Flow Structure
Abstract
Meandering is a common plan shape acquired by rivers. Its prevalence in the environment and highly dynamic nature make the mechanics of flow in meandering streams a great interest for engineers and scientists. Meandering streams are characterized by continuous variation of curvature in the downstream direction, with subsequent loops of alternating curvature. This curvature invariably results in the emergence of secondary motion, which is motion in the direction normal to that of the stream. This work concerns the mechanics of flow in meandering rivers, focusing specifically on secondary motion. A series of laboratory experiments was carried out in a meandering channel to resolve the three-dimensional flow structure and secondary flow field of meandering streams. The meandering channel had smooth boundaries, a flat bed, and an initial deflection angle of 70°. The width-to-depth ratio in the experiments was varied from 5.82 to 14.55. The results suggest that in meandering streams with low width-to-depth ratio, a long transition region can exist where there is simultaneous growth of the secondary motion induced by the local curvature, and decay of the residual secondary motion induced by the curvature of the upstream loop. This results in the emergence of a complex secondary motion pattern, characterized by two large counter-rotating cells in cross-sectional view, contrary to classical descriptions of cross circulation. The length of the transition region was found to decrease with increasing width-to-depth ratio in the experiments. Vertical profiles of longitudinal velocity were deformed in the region of the residual secondary motion of the upstream loop, with the maximum longitudinal velocity often occurring at the lower half of the water column in these regions. The region of deformation of longitudinal velocity started in the upstream loop, near the inner bank immediately downstream of the apex. The increased strength of the residual secondary motion from the upstream loop in tests with lower width-to-depth ratio was found to dampen the intensity of secondary motion in the upper flow regions.
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