Bed shear stress is a fundamental variable in river studies to link flow conditions to sediment transport. It is, however, difficult to estimate this variable accurately, particularly in complex flow fields. This study compares shear stress estimated from the log profile, drag, Reynolds and turbulent kinetic energy (TKE) approaches in a laboratory flume in a simple boundary layer, over plexiglas and over sand, and in a complex flow field around deflectors. Results show that in a simple boundary layer, the log profile estimate is always the highest. Over plexiglas, the TKE estimate was the second largest with a value 30 per cent less than the log estimate. However, over sand, the TKE estimate did not show the expected increase in shear stress. In a simple boundary layer, the Reynolds shear stress seems the most appropriate method, particularly the extrapolated value at the bed obtained from a turbulent profile. In a complex flow field around deflectors, the TKE method provided the best estimate of shear stress as it is not affected by local streamline variations and it takes into account the increased streamwise turbulent fluctuations close to the deflectors. It is suggested that when single-point measurements are used to estimate shear stress, the instrument should be positioned close to 0·1 of the flow depth, which corresponds to the peak value height in profiles of Reynolds and TKE shear stress.

The spatial distribution of climatological variables such as rainfall and temperature is needed whenever hydrological modelling is undertaken at the watershed scale. These models can be used to simulate hydrological processes at a daily or hourly time step and the interpolation of climatological variables (in particular precipitation) at this time scale poses a particular problem due to its large spatial variation. This work analyzes the temporal variation of both minimum and maximum temperature and rainfall, its correlation with elevation and whether or not this relationship should be used when daily data are interpolated. In order to achieve this, the monthly distribution of these variables is derived from daily interpolations, which is compared to their monthly accumulated value for each climatological station. The interpolation methods used to undertake the analysis were Ordinary Kriging (OK), Kriging with External Drift (KED), Block Kriging with External Drift (BKED), Ordinary Kriging in a local neighborhood (OK l ) and Kriging with External Drift in a local neighborhood (KED l ). This analysis used daily climatological data from approximately 200 stations located in the Basin of Mexico for June 1978 and June 1985, from which accumulated monthly data were derived. The results of this analysis show that the interpolation of daily events is improved by the use of elevation as a secondary variable even when these variables show a low correlation. ª

The effect of background turbulence on a turbulent jet was investigated experimentally. The primary objective of this work was to study the effect of different levels of the background turbulence on the dynamics and mixing of an axisymmetric turbulent jet at different Reynolds numbers. The secondary objective, which arose during the experiments, was to improve the acoustic Doppler velocimetry measurements which were found to be inaccurate when measuring turbulence statistics.In addition to acoustic Doppler velocimetry (ADV), flying hot-film anemometry was employed in this study. To move the hot-film probe at constant speeds, a high precision traversing mechanism was designed and built. A data acquisition system and LabVIEW programs were also developed to acquire data and control the traversing mechanism. The experiments started by benchmarking the two measurement techniques in an axisymmetric turbulent jet. Comparing the results with those of the other studies validated the use of flying hot-film anemometry to estimate the mean and the root-mean square (RMS) velocities.The experiments also validated the use of ADV for measurement of the mean velocities (measured in three Cartesian directions) and the RMS velocity (measured in the z-direction only). RMS velocities measured by the ADV along the x-and y-direction of the probe were overestimated.Attempts to improve the turbulence statistics measured by the ADV using the post-processing and noise-reduction methods presented in the literature were undertaken. However, the RMS velocities remained higher than the accepted values. In addition, a noise-reduction method was presented in this study which reduced the RMS velocities down to the accepted values. It was also attempted to relate Doppler noise to current velocity, and thus improve the results by iv subtracting the Doppler noise from the measured RMS velocities in the jet. However, no relationship was found between the Doppler noise and the mean velocity.The effect of different levels of background turbulence on the dynamics and mixing of an axisymmetric turbulent jet at different Reynolds numbers was then investigated. The background turbulence was generated by a random jet array. To confirm that the turbulence is approximately homogeneous and isotropic and has a low mean flow, the background flow was first characterized. Velocity measurements in an axisymmetric jet issuing into two different levels of background turbulence were then conducted. Three different jet Reynolds

In some regions the clay banks of the St. Lawrence River along the Montréal to Lac St. Pierre reach have recession rates of up to 1–3 m/year. The banks are formed of structured marine clays of the Champlain Sea (Leda clay). In this laboratory study, undisturbed samples of this high-plasticity inorganic clay taken at Îles de Verchères were subjected to a unidirectional current and a constant wave climate to investigate the mechanisms of erosion and the factors influencing erosion rates. Initially, surface erosion resulted in the formation and enlargement of cracks and the smoothing of competent surfaces. The dominant erosion process was a mass erosion of the blocks of clay delineated by the cracks. Desiccation or weathering significantly increased erosion rates, as tension cracks formed due to drying, and upon rewetting, the formation of microfissures resulted in disintegration into small, easily erodable flakes. The estimated critical shear stress of the samples was 6–20 Pa. For the St. Lawrence River, these results suggest that waves are the dominant erosion mechanism, with shipping contributing significantly to the erosion of banks close to the navigation channel. Weathering caused by wetting and drying from changing water levels or wave runup greatly increases erosion rates.Key words: erosion, Leda clay, undisturbed clay, natural clay structure, St. Lawrence River, waves, weathering, desiccation, vegetation.

To validate the use of acoustic Doppler velocimeters (ADVs) for the measurement of turbulent flows, experiments were conducted in i) an axisymmetric turbulent jet, and ii) approximately homogenous isotropic turbulence with zero mean flow. The jet experiments show that the horizontal RMS velocities measured by the ADV were overestimated when compared to both flying hot-film anemometry measurements and 1