Secondary Flow and Shear Stress at River Bends

Field measurements are presented to illustrate the pattern of secondary circulation at river bends, the effect of the circulation on boundary shear stress disbribution, and the way in which that effect changes with discharge. The field results show that, in addition to the classical main cell, there can be a small cell of opposite circulation close to the outer bank provided the bank is steep. The circulation distorts the primary isovel pattern and thereby affects the positions and relative magnitudes of the boundary shear stress peaks associated with regions of downwelling and high velocity. Secondary circulation appears to be weakest at low and high discharges and to be strongest, with its greatest effect on the shear stress distribution, at medium discharges. Generally the distribution of shear stress at a section becomes more uniform as discharge increases and at high discharges is determined mainly by primary flow effects.