Céline Berni

Measurements of surf zone sand bed dynamics under irregular waves

Measurements of velocity profiles, sediment concentration, pore-pressure and sheet flow layer dynamics are analysed in order to better assess the relative importance of the processes that contribute to destabilise the bed of a sandy beach. These measurements were conducted in the surf zone of irregular waves, in a 30 m × 30 m wave tank. The effects of flow acceleration, excess pore-pressure vertical gradients across the bed and infiltration/exfiltration seepage flow are studied. The appearance of a sheet flow layer is apparently initiated by the strong accelerations in the wave fronts. These results are compared to field measurements at Truc-Vert beach (Atlantic coast of France). Probably due to the partial saturation of the bed made of finer sand in the wave tank experiments, the upwards-directed excess pore-pressure gradient could enhance the bed destabilisation compared to that observed in the field. Des mesures de profil de vitesses, de concentration en sédiment, de pressions intersticielles et de la dynamique de la couche de fond sont analysées afin de mieux juger de l’importance relative des processus susceptibles de déstabiliser le lit d’une plage. Ces mesures ont été réalisées en laboratoire, dans un bassin de 30 m × 30 m, dans la zone de déferlement de vagues irrégulières et sur la plage du Truc Vert (côte Aquitaine, France). Les effets d’accélération, du gradient vertical de pression à travers le lit et de l’écoulement d’infiltration/exfiltration sont étudiés. L’apparition d’une couche mobile et fortement concentrée semble initiée par les fortes accélérations précédant le passage de la crête des vagues. Probablement dû à un lit partiellement saturé et constitué de grains plus fins dans les expériences en bassin, la déstabilisation du lit par un gradient de pression interstitielle orienté vers le haut n’apparaît pas négligeable comme dans les mesures de terrain.

122. A LARGE-SCALE LABORATORY EXPERIMENT OF RIP CURRENT CIRCULATIONS OVER A MOVEABLE BED: DRIFTER MEASUREMENTS

The present paper presents a laboratory experiment of rip current circulations over a moveable bed. The rip current characteristics over four distinct beach morphologies, exhibiting more or less developed nature-like bar-rip morphology, were investigated. For each video run, the same offshore shore-normal waves were generated by the wavemaker with the same mean water level in order to study the sensitivity of the rip current characteristics as a function of the beach morphology only. In each case, a 1-hour video run was used to track a large number (~30) of drifters released within the surf zone. Image coordinates were then rectified to still water level Cartesian coordinates to compute drifter velocities, mean characteristics and surf zone retention rates. Results show the presence of classic rip current patterns with counter-rotating cells and a relatively narrow offshore-directed jet with, for three of the situations, a reasonably symmetric shape. Non-surprisingly, it was found that rip current intensity increases with increasing relative depth of the rip channel. The wave-driven circulations were strongly unstable. Computed standard deviation in flow intensity and direction provides high resolution information on the spatial variability of the rip current instabilities with, for instance, highly-pulsating and weakly directionally variable offshore-directed flow in the rip channel. Conversely to what was previously hypothesized in the literature, there was hardly trace of vortices being shed offshore and drifters exiting the surf zone compartment were not systematically caught by a pulsating jet. The cause for drifter exiting the semi-enclosed surf zone compartment remains, however, elusive and deserves further investigations. The computed surf zone retention rates (~90%) were of the order of those previously observed in the field, with no clear relationship with the mean rip current velocity or relative depth of the rip channel. Further video-runs will have to be analyzed to explore potential explanations.