A field study of turbulent flows in shallow gravel-bed rivers

The study of turbulent flows has always been a challenge for scientists. Turbulent flows are common in nature and have an important role in several geophysical processes related to a variety of phenomena such as river morphology, landscape modeling, atmospheric dynamics and ocean currents. At present, new measurement and observation techniques suitable for fieldwork can be combined with laboratory and theoretical work in order to advance in the understanding of river processes. In this Ph.D. dissertation, an Acoustic Doppler Velocity Profiler (ADVP) suspended from a deployable structure allowed the investigation of turbulent gravel-bed river flows. The ADVP, which was developed by the Laboratoire d'Hydraulique Environnementale (LHE), permits to obtain over the entire water depth, three-dimensional quasi-instantaneous information on the fluctuating velocity flow field in the production and inertial subranges of the spectral space. Improvements on the ADVP data quality were made with the implementation of a correction methodology for errors due to data aliasing. This reduced the range-velocity ambiguity in Doppler-based instruments. The results presented in this dissertation contribute to the understanding of transport and mixing processes in river flows. They are based on three sets of field measurements made in gravel-bed rivers with blockage ratios of h/D84≈3.0 and aspect ratios B/h between 23 and 32. The measurements were made during low-water periods. The fieldwork provided results on the mean and instantaneous velocity field. The flow was divided into three inviscid vertical layers with different mean field and Reynolds stress characteristics: the roughness layer, the blending or intermediate layer and the surface layer. In the lower layers of the flow three types of mean velocity profile were found: mono-logarithmic, s-shaped due to bed perturbations and double-logarithmic downstream bed perturbations. The determination of the shear stress distribution for each of these profile types is studied. In double-log profiles, the friction velocity and roughness length determined for the outer logarithmic layer are required for the velocity profile parameterization. The s-shaped profiles are described by a tangent-hyperbolic function in the lower layers compatible with an external log layer. Limitations of 2D open-channel theories to parameterize the velocity distribution and to characterize the bottom drag are discussed. Bottom drag occurs in the predominant momentum direction. The direction changes as a function of the local bed forms. To estimate bottom drag one has to consider the actual momentum transport direction which varies with the flow depth. The wall effect of the riverbank is visible until y/h≈5. Bottom topography produces important secondary mean motion in the flow. A permanent structure of the flow was described in the upper layers, near the surface (z/h