Bed shear stress in open channel flows is often estimated from the
logarithmic vertical velocity profile, However, most measuring devices
used in the field do not allow for flow velocity to be measured very
close to the bed. The lack of near-bed measurements is a critical loss
of information which may affect bed shear stress estimates. Detailed
velocity profiles obtained from a field acoustic Doppler velocimeter
over three different bed roughnesses clearly show that the inclusion of
near-bed points is critical for the estimation of bed shear stress in a
shallow river environment. Moreover, the results indicate that using
the full flow depth instead of the bottom 20 per cent of the profile
generates an underestimation of the shear stress when flow is uniform.