Shear velocity u * is an important parameter in geophysical flows, in particular with respect to sediment transport dynamics. In this study, we investigate the feasibility of applying five standard methods [the logarithmic mean velocity profile, the Reynolds stress profile, the turbulent kinetic energy (TKE) profile, the wall similarity and spectral methods] that were initially developed to estimate shear velocity in smooth bed flow to turbulent flow over a loose bed of coarse gravel (D 50 = 1Á5 cm) under sub-threshold conditions. The analysis is based on quasi-instantaneous three-dimensional (3D) full depth velocity profiles with high spatial and temporal resolution that were measured with an Acoustic Doppler Velocity Profiler (ADVP) in an open channel. The results of the analysis confirm the importance of detailed velocity profile measurements for the determination of shear velocity in rough-bed flows. Results from all methods fall into a range of AE 20% variability and no systematic trend between methods was observed. Local and temporal variation in the loose bed roughness may contribute to the variability of the logarithmic profile method results. Estimates obtained from the TKE and Reynolds stress methods reasonably agree. Most results from the wall similarity method are within 10% of those obtained by the TKE and Reynolds stress methods. The spectral method was difficult to use since the spectral energy of the vertical velocity component strongly increased with distance from the bed in the inner layer. This made the choice of the reference level problematic. Mean shear stress for all experiments follows a quadratic relationship with the mean velocity in the flow. The wall similarity method appears to be a promising tool for estimating shear velocity under rough-bed flow conditions and in field studies where other methods may be difficult to apply. This method allows for the determination of u * from a single point measurement at one level in the intermediate range (0Á3 < h < 0Á6).

Acoustic Doppler instrument measurements suffer from random spikes and Doppler noise. Using a fourreceiver ADV (acoustic Doppler velocimeter; Vectrino manufactured by Nortek) that allows recording beam velocities, we combine a spike-removal procedure on the beam velocities with a noise-reduction method on the flow velocities to improve turbulence measurements. We compare the results with those obtained from ADVP (acoustic Doppler velocity profiler) measurements under the same conditions, i.e., in turbulent open-channel flow over a coarse-grained bed. It is shown that spikes are best removed from ADV beam velocity data before calculating flow velocities, thereby correcting all three flow velocity components at the source. Spikes in beam velocities do not correlate with low correlation values. ADVP data generally have few spikes and do not need spike removal treatment, showing that spikes are instrument related. The noise reduction method is based on the decorrelation of the Doppler noise terms contained in two vertical velocities redundantly sampled in the same volume. The combined ADV data treatment is sufficient to significantly extend the resolved frequency range in the velocity spectra. It reduces RMS values by up to a factor of 2, and the corrected values agree with ADVP results and theoretical predictions, indicating that both treatments are needed. Owing to spatial averaging effects over the ADV sample volume, a sampling frequency limit of close to 50 Hz is determined by the deviation of the spectra from the -5/3 slope.

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The roughness coefficient is a significant factor in hydraulic design process, but carries vast uncertainty in practice. This paper reports the resistance coefficients of corrugated PVC pipes under different flow regimes (i.e. pressurised, gradually varied flow (GVF) and spatially varied flow (SVF) with increasing discharge). The investigated corrugated pipes are mainly used in subsurface drainage systems and carry a SVF. The influence of SVF on the roughness coefficient of such pipes has not yet been reported. The pipes were first examined to determine the roughness coefficient for pressurised flow. The influence of GVF and SVF was then studied. The results show that pipes behave differently in different flow regimes and the impacts of each flow condition on the flow resistance are reported here.