François Marin

Bedload transport for heterogeneous sediments

A large panel of instruments was deployed in the Eastern English Channel to measure the evolution of bedload fluxes during a tidal cycle for two different sites. The first one was characterized by a sandy bed with a low dispersion in size while the other study site implied graded sediments with grain sizes ranging from fine sands to granules. The in situ results obtained were compared with predictions of total bedload fluxes by classical models. A good agreement was found for homogeneous sediments with these formulas. In the case of size heterogeneous sediments, a fractionwise approach, involving a hiding-exposure coefficient and a hindrance factor, provided better predictions of bedload fluxes, but still some discrepancies were noticed. Present results revealed that the consideration of particle shape in formulas through the circularity index enhanced the estimations of bedload transport rates. A new adjustment of Wu et al.’s (J Hydraul Res 38:427–434, 2000) formula was proposed and a very good agreement was obtained between the measured and predicted values.

Formation of Sand Bedforms Under Surface Waves

Surface waves often generate bedforms at the seabed. Small structures such as ripples with a typical wavelength between ten centimeters and one meter are very common structures in the coastal zone. The formation of these structures under nonlinear surface waves is considered in this chapter. Under regular waves, two modes of pattern formation from a flatbed in a wave flume are reported for well-sorted grains and mixtures of grains. Sand ripples can form uniformly or from isolated ripples spreading on the bed while growing. In this latter case, front propagation speed is measured and a simple model based on the quintic complex Ginzburg-Landau equation can explain features of front propagation on the granular bed. The profile of surface waves propagating in shoaling water approaches the solitary waveform before wave breaking. The main characteristics of solitary waves are presented. The effect of the high nonlinearity of these waves may be very significant on bedforms induced in the nearshore zone. The interaction between solitary waves and a sandy bed is reported. Sandy ripples induce a strong energy dissipation of solitary waves. When solitary waves propagate on the background of a standing harmonic wave, bars are formed with crests located beneath the nodes of the harmonic surface wave. In the case of harmonic standing waves alone, the bar crests are positioned beneath the antinodes of the harmonic surface wave. Grains with different densities may be found on the seabed. The concentration of light sedimenting particles on ripple crests is explained by a simple theoretical model.

Pattern formation in a granular medium excited by waves in a flume

This work deals with the dynamics of sand ripples which occur on a flat sandy sea bed excited with an oscillatory flow in a wave flume. The bathymetry of the bed is reported since the first stages of ripple formation with an optical technique. A statistical approach is developed to study the dynamical evolution of ripples. With an identical mobility number and different Reynolds number, two tests leading to 2D and 3D pattern formation are compared. Two stages are detected during ripple formation independently of pattern morphology. However, pattern tridimensionality can be detected with statistical tools. 2D and 3D equilibrium ripple geometric data are compared with the literature.