ABSTRACT
Field measurements over cross sections in cobble-bed rivers have allowed patterns of boundary shear stress distribution to be delineated at various discharges. In straight pool reaches the cross-sectional distribution of shear stress is characterized by peaks and troughs. These may result from the action of multicell, strcss-induced, secondary circulation systems which create alternate regions of up welling and down welling flow. Upwelling and down-welling flow produces alternate regions of low and high shear stress respectively. At sections just upstream of riffles the shear stresses have a major peak in the center of the channel. This peak may be caused by the accentuating effect of flow acceleration on the shear stress peak associated naturally with the core of maximum velocity. At sections across bends, shear stress peaks are associated with the core of maximum velocity and with down-welling of flow caused by skew-induced secondary circulation.
Little change in shear stress with discharge was observed over cross sections on straight reaches, but at bends the positions and relative magnitudes of the peaks change with discharge.
Uniformity of shear stress distribution over the channel cross-section generally increases as Reynolds number increases. This is because at high Reynolds number the capacity of secondary circulation to decrease the uniformity of distribution is relatively reduced.
Scouring can occur where there is a longstream increase in shear stress. At bends the positions of such regions change with discharge, so the position of maximum scour along a channel bend is likely to depend on the dominant range of discharges.
