Impact of sand dune morphology on macro-tidal current dynamics: insight from PIV measurements

International audience ; Here we present an experimental campaign delving into the complex interplay between sand dune morphology and tidal current dynamics. Employing particle image velocimetry (PIV) measurements, we conducted diverse series of laboratory flume experiments to scrutinize the hydrodynamic influence of dunes on steady currents, both aligned and opposed to the dune asymmetry. Our experiments sought to replicate the hydrodynamics occurring over the tidal dune field of the Goulet de Brest, located in Brittany, France. The Goulet hosts well-formed and highly dynamic dunes shaped by intense tidal currents. Notably, these currents exhibit a distinctive asymmetry, with ebb flows prevailing towards the southwest in contrast to the opposite flood flows. The resulting tidal asymmetry is vividly mirrored in the dune profiles, showcasing an accentuated asymmetry in the southeast part of the field, where dunes grow taller in shallower waters. Conducted in a bidirectional current-permitting flume, each experimental series encompassed varied hydrodynamic conditions, such as ebb and flood, aligned or opposed to the dune asymmetry, and different bottom roughnesses. We utilized sediment mixtures that were progressively coarser, transitioning from a smooth to a fully rough flow regime. The PIV flow measurements provided unprecedented spatial resolution, slightly larger than 1 mm, and temporal resolution sampling data at a frequency of 200 Hz. These measurements were taken at the central dune crest and along the neighboring trough. At the inlet, lower-frequency point measurements were conducted using a Vectrino-II ADV Nortek, ensuring the stability of inlet currents during different PIV acquisitions. Analysis of the collected data unveiled a rich tapestry of turbulent processes and boundary layer dynamics. The identified detachment and reattachment points of the boundary layer, along with their spatio-temporal oscillations, revealed an intermittent advection of coherent turbulent structures downstream from the dune ...