Flow Structure over Bed Irregularities in a Straight Cohesive Open Channel
Publication: Journal of Hydraulic Engineering
Volume 137, Issue 11
Abstract
This paper describes high-resolution in situ three-dimensional (3D) bed topography and 3D flow field measurements in cohesive, straight channel reaches and provides analysis of reciprocal relationships between flow structure and bed topography. Existence of secondary flows and their structures are shown to be associated with variety of bedforms. Classification of bed microtopography and identification of the major governing flow mechanisms are described for three different types of bed morphology: single trough, single ridge, and sequences of troughs and ridges. The maximum mainstream velocity component of secondary cells attains values as high as approximately 20% of the average mainstream velocity, and secondary cell width can reach bedform width. Analysis of lateral versus vertical fluctuations was performed, revealing a good correspondence () between cell form and temporal fluctuations of velocity attributes; with diminished size of the secondary flow cell, the number of flow reversals increases. For a channel with given cross-sectional geometry, discharge and mean velocity, the downflows are almost twice as large and have higher turbulent intensities than upflows. The three-dimensionality has vital implications for understanding the flow structure over channel beds, emphasizing the significance of the effect that secondary circulation in addition to lateral and vertical flow perturbations may have on the general flow field and associated bed topography.
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Acknowledgments
This study was in part funded by grants from the Israel Water Authority, the Dead Sea Drainage Authority and the German BMBF-funded SUMAR project. The study has benefited from work carried out by students of the Department of Geography and Environmental Development, the Ben Gurion University of Negev, particularly Y. Munwes, and I. Schwartz, whose assistance with data collection was invaluable. We thank the Ein-Fesh’ha Nature Reserve team for their support, and Tamir Grodek (Hebrew University of Jerusalem), Uri Shavit (Department of Civil and Environmental Engineering, Technion, Israel) and Roey Egozi (The Soil Erosion Research Station, Israel) for lending the side-looking ADV. We thank two anonymous reviewers whose comments helped to improve this manuscript.
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Published In
Journal of Hydraulic Engineering
Volume 137 • Issue 11 • November 2011
Pages: 1335 - 1346
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© 2011 American Society of Civil Engineers.
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Received: Apr 12, 2010
Accepted: Apr 28, 2011
Published online: Apr 30, 2011
Published in print: Nov 1, 2011
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