SENSITIVITY OF BED SHEAR-STRESS ESTIMATED FROM VERTICAL VELOCITY PROFILES - THE PROBLEM OF SAMPLING RESOLUTION

Bed shear stress in open channel flows is often estimated from the log arithmic vertical velocity profile, However, most measuring devices us ed in the field do not allow for flow velocity to be measured very clo se to the bed. The lack of near-bed measurements is a critical loss of information which may affect bed shear stress estimates. Detailed vel ocity profiles obtained from a field acoustic Doppler velocimeter over three different bed roughnesses clearly show that the inclusion of ne ar-bed points is critical for the estimation of bed shear stress in a shallow river environment. Moreover, the results indicate that using t he full flow depth instead of the bottom 20 per cent of the profile ge nerates an underestimation of the shear stress when flow is uniform. ( C) 1998 John Wiley & Sons, Ltd.