Mathieu Mory

On the determination of the settling flux of cohesive sediments in a turbulent fluid

In this paper we compare the settling flux of a cohesive sediment mixture measured in a quiescent fluid with that achieved in a turbulent flow. Experiments were performed in a mixing tank. The turbulence produced mechanically by an oscillating grid maintains a stationary, highly concentrated fluid mud layer in which the concentration is almost uniform. In this layer the turbulence decay with increasing distance from the grid is similar to that obtained in clear water. For steady conditions the settling flux of the fluid mud mixture is balanced by the upward turbulent flux, and its value can be determined from the measured depth of the fluid mud layer. At high concentrations (10–200 g L−1), we show that the settling flux in a turbulent fluid is much larger than estimated in a quiescent fluid (up to two orders of magnitude). Hindering effects in the settling process of cohesive sediments may thus be considerably reduced by turbulence.

CBS layers in a diffusive turbulence grid oscillation experiment

The generation of a Concentrated Benthic Suspension (CBS) of a fluid mud mixture is investigated in the laboratory in a grid stirred experiment. A sediment concentration in the CBS layer in the range 3 g/l to 200 g/l was considered. The sediment concentration, the turbulence properties, and the settling velocity have been measured. The decay of turbulence with increasing distance from the grid is not found to vary with the concentration inside the CBS layer except in the vicinity of the lutocline. The settling velocity decreases rapidely with increasing sediment concentration and appears to be the most sensitive parameter in the experiment. A clear difference in the value of the flux Richardson number at the lutocline is observed depending on the sediment concentration. Rif is about 0.1 for the highest concentrations and increases above 0.5 for the low concentration cases.

Wave Scour Around Piles

This paper reports the results of an experimental study on the scour around a vertical pile and in groups of piles submitted to wave forcing. In particular, the influence of the wave non-linearity on local scour around single piles has been studied. In fact, as the characteristics of non-linear waves evolves in the domain the scour processes shall depend on the position of the piles, i.e., on the local characteristics of the waves. The scour processes around vertical piles located in the surf zone are also discussed in this paper.

Nouvelle technique de mesure locale de l'évolution du fond en zone de surf

ABSTRACT The high frequency bathymetric surveys in the surf zone are difficult and hazardous because of the breaking waves. It is however in these intense energy zones that the morphological evolutions are typically useful to know. A local measurement technique of the bed level evolution that can be used in the surf zone has been developed. This is a resistivity measurement based method and we have here the preliminary results of a 30 days in situ measurement. The extended fields of application of this method are multiple. We present how they can be extended to water level measurement and under specific conditions, to give information on the variations of ground resistivity under wave action.

An Optical System for Monitoring Seabed Evolutions at a Coastal Structure

An optical system has been designed for measuring the bed level, sand grain motions inside the soil and the presence of suspension at the toe of a coastal structure exposed to breaking waves. These measurements were conducted in the framework of the LIMAS project. We show that the soil is not strictly stable even at great depths (about 1 m). The generation of pressure differences in the soil is the main mechanism for this instability at the wall facing the waves. At the corner of the structure, the shear induced by large longitudinal velocities enhances the sediment transport and scour.

OBSERVATIONS OF MOMENTARY LIQUEFACTION CAUSED BY BREAKING WAVES AROUND A COASTAL STRUCTURE

This paper aims at demonstrating the occurrence of momentary liquefaction in a sandy soil around a coastal structure subject to wave forcing. This was made in the field, using as coastal structure a bunker from the Second World War that is currently situated on the beach at Capbreton on the Atlantic coast of south-west France. By positioning pressure sensors at different levels inside the soil, time series of simultaneous pore pressure measurements were acquired while the position of the soil level was monitored. Analysis of the pressure measurements demonstrated that the sediment layer on top of the bed does not remain continuously in static equilibrium. An upward pressure gradient was sometimes observed, which could overcome the effective weight of the sediment. Pore pressure variations inside the soil were compared, using a Fourier analysis of the time series, with the theory put forward by Sakai et al. (1992) whereby the damping of pore pressure variations inside the soil can be quantified. A surprisingly good agreement is found when the gas content is adjusted in the model to 1%. This indicates the existence of a significant amount of gas inside the soil. This property is confirmed in the field by geo-endoscopic videos.

Coherent vortices in a turbulent and rotating fluid

The emergence of coherent, intense and quasi-steady vortices in a rotating fluid under the action of turbulence is a phenomenon which is not yet well accounted for. A first deterministic approach by Maxworthy, Hopfinger and Redekopp (1985) related it to the occurence of mixed density fluid intrusions in a linearly stratified fluid. In the first part we present visualizations of the vortex genesis verifying qualitatively Maxworthy et al.s ideas. In the second part, a linear instability model is proposed, very similar to the thermal convection instability in a rotating fluid. For marginal stability conditions, the model shows the occurrence of a regular spatial distribution of steady vortices. This new approach does not contradict the visualizations presented in the first part.