Flow Structure over Bed Irregularities in a Straight Cohesive Open Channel

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 incr...