Flow Structure over Bed Irregularities in a Straight Cohesive Open Channel

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 1/2 bedform width. Analysis of lateral versus vertical fluctuations was performed, revealing a good correspondence (r2=0.67) 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.