Internal architecture and evolution of bioclastic beach ridges in a megatidal chenier plain: Field data and wave flume experiment

International audienceBeach ridges in macrotidal environments experience strong multi-annual to multi-decennial fluctuations of tidal inundation. The duration of tide flooding directly controls the duration of sediment reworking by waves, and thus the ridge dynamics. Flume modelling was used to investigate the impact of low-frequency tidal cycles on beach ridge evolution and internal architecture. The experiment was performed using natural bioclastic sediment, constant wave parameters and low-frequency variations of the mean water level. The morphological response of the beach ridge to water level fluctuations and the preservation of sedimentary structures were monitored by using side-view and plan-view photographs. Results were compared with the internal architecture of modern bioclastic beach ridges in a macrotidal chenier plain (Mont St. Michel Bay, France) surveyed with ground-penetrating radar. The experimentally obtained morphologies and internal structures matched those observed in the field, and the three ridge development stages identified in ground-penetrating radar profiles (early transgressive, late transgressive and progradational) were modelled successfully. Flume experiments indicate that flat bioclastic shapes play a key role in sediment sorting in the breaker zone, and in sediment layering in the beach and washover fans. Water level controls washover geometry, beach ridge evolution and internal structure. Low water levels allow beach ridge stabilization and sediment accumulation lower on tidal flats. During subsequent water level rise, accumulated sediment becomes available for deposition of new washover units and for bayward extension of the beach ridges. In the field, low-frequency water level fluctuations are related to the 4 4 year and 18 6 year tidal cycles. Experimental results suggest that these cycles may represent the underlying factor in the evolution of the macrotidal chenier coast at the multi-decadal to centennial time scale

Etude de couches limites oscillantes par vélocimétrie laser Doppler

International audienceLe transport sédimentaire induit par les vagues à l'approche de la côte est piloté par des processus non linéaires et turbulents. Les non-linéarités des vagues se caractérisent par une dissymétrie de vitesse (les crêtes hautes des vagues sont de courte durée et les creux peu profonds de longue durée) et une asymétrie de vitesse (ou dissymétrie d'accélération, caractérisant la raideur des fronts). Des études récentes indiquent que des fronts raides (vagues asymétriques) produisent des vitesses dissymétriques dans la couche limite. Ainsi, pour développer des formules de prédiction de transport des sédiments, la compréhension détaillée de la dynamique de la couche limite de fond et des contraintes de cisaillement sous les ondes de surface apparaît essentielle. Ceci justifie de chercher à réaliser des mesures de vitesse dans les tous premiers millimètres au-dessus du lit.Par ailleurs, la caractérisation de la turbulence sous les vagues déferlantes reste une question ouverte, en particulier pour chercher à évaluer la part provenant de la vague déferlée de celle produite par frottement au fond.Actuellement, l’essentiel de notre connaissance des couches limites oscillantes est issu de mesures réalisées sur des fonds fixes horizontaux. En laboratoire, l’évolution des non-linéarités des vagues, lors de leur propagation et de leur déferlement, a été principalement étudiée pour des plages de pentes relativement fortes (> 1:40). Cependant, des études de terrain récentes sur des plages réelles de pentes moins raides (1:80) ont montré que certains processus non-linéaires sont différents par rapport aux cas des plages de pentes relativement raides (> 1:40). Cette constatation a motivé le lancement d’une série d'expériences de laboratoire dans le cadre du projet européen GLOBEX sur une plage à fond fixe de pente 1:80

Flow visualisation around a horizontal cylinder near a plane wall and subject to waves

International audienceThis study is motivated by the description and the understanding of the vortices formation and development near a cylinder subject to waves and under the influence of the bed. The results presents a classification of flow types for different gap-to-diameter ratio and for two cylinder diameters ðD 1 ¼10 cm and D 2 ¼4 cm, 0.8 m long) according to a systematic wave conditions range. The flow visualisations reveal different mechanisms of separation, development and growth of vortices depending on the Keulegan Carpenter number (0.5 , KC , 26) and the influence of the bed proximity. A flow asymmetry was observed between the crest and the trough of the wave and a stronger vortex activity downstream the cylinder especially for the highest gap-diameter ratios. Furthermore the vortices are rotating preferentially in the same sense than the orbital motion. The flow tends to become more similar to a planar oscillating flow when the cylinder is closer to the bed. The emergence of instabilities in the wake of the cylinder for the highest wave amplitude leads us to measure the velocities in the cylinder axis in order to quantify 3D effects

Wave-induced boundary layer flows over a flat and rippled bed

International audienceThe wave-induced boundary layer flow over a rippled bed is studied. A two-dimensional model aiming to solve the flow in the vicinity of the bed has been developed. First the model is tested for a flat bed case and comparison is made with experimental data obtained by laser Doppler velocimetry (LDV) in two wave flumes presenting different characteristics. As the numerical and experimental results are compared, particular attention is given to the mean Eulerian velocity profiles calculated from the two sources and the theoretical solution from Longuet-Higgins [Philos. Trans. Roy. Soc. Lond. A 245 (1953) 535]. Discrepancies are explained and the use of the available theory is discussed. The numerical model is then adapted to a rippled bed case and compared to experimental data obtained by LDV in a wave flume fitted with a rippled bed. The dynamical consistency of the model is tested and the appearance of a vortex created at each half-wave cycle is shown. Mean velocity profiles are plotted at different locations along the ripple profile. Different types of profile are obtained depending on the location. Mean velocity profiles from the model are studied for other flow and ripple conditions and it is shown that the profiles located at mid-distance between the crest and the trough always featured negative overshoot amplitude while the profiles located above the crest and the trough could be of either sign. An example of flow visualizations is also shown and qualitative comparisons are made with the numerical model run for a similar case

Theoretical calculation of wind (Or water) turbine considering kinetic and potential energy to exceed the Betz limit

The Betz limit sets a theoretical upper limit for the energy efficiency of turbines. The energy efficiency of turbines, expressed as a maximum power coefficient of 16/27. Betz's theory is precise and is based on the calculation of kinetic energy. However, if the potential energy is taken into account the theoretical energy efficiency of a turbine can be higher. Fast wind turbines recover the kinetic energy of the wind in an optimal way. A large amount of potential energy is created without being recovered. The notion of potential energy is fundamental, it is not possible to recover energy, if we do not create a constraint. This article examines this potential energy and the possibility for a wind turbine to transform it into kinetic energy. The Betz theory has been defined from the model of fast moving turbines. This theory has been generalized to slow and fast moving turbines and it has been defined as a law. The conservation of energy implies that if a variation of kinetic energy increases, the variation of potential energy decreases. In the case of slow moving turbines, the conservation of energy applies, but not for the case of fast moving turbines, however this is the reality. This paper proposes a new formulation of the turbine power with a notion of temporal, in order to be able to verify the conservation of energy

Sedimentary signatures of tidal bores: a brief synthesis

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Simplified theory of an active lift turbine with controlled displacement

It is presented in this article, a simplified theory of the active lift turbine which has been the subject of several patent[4, 5, 11]. A simplified theory is proposed to extend the Betz limit of the yield on vertical axis wind turbine. This work can be extended either on wind driven or marine current turbine. Based on kinetic energy calculation , that theory demonstrates that the radial force acting on the blade can be used to extend the maximum recoverable power, mainly by transforming a linear motion into a rotating motion. The geometry of this new type of vertical axis turbine is described. Finally the driving power is calculated through the tangential power coefficients. Consequences on the velocity variation generation and induced vibrations are exposed. Two parts is treated in this article : The calculation of power coefficient and the calculation natural modes of vibratio

Flow regime features on a variable rough rippled bed: Application to a coastal environment

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