José M. Alsina

SEDIMENT TRANSPORT NUMERICAL MODELLING IN THE SWASH ZONE

A numerical model of sediment transport in the swash zone has been recently developed. The followed approach relies on the solution of the vertical distribution of suspended sediment concentration and incorporates the horizontal advection of suspended sediment from the bore collapse and local pickup using a Lagrangian computing scheme. Computed sediment transport rates are compared to measurements on a reflective beach on the central east coast of Australia. The model has proved to contribute to the explanation of the sedimentary process in the swash zone, predicting accretion on a reflective beach where classical swash sediment transport concepts would give erosion. Copyright ASCE 2006.

Transfer and dissipation of energy during wave group propagation on a gentle beach slope

The propagation of bichromatic wave groups over a constant 1:100 beach slope and the influence of the group modulation is presented. The modulation is controlled by varying the group frequency, fg, which is shown to remarkably affect the energy transfer to high and low frequency components. The growth of the high frequency (hf) wave skewness increases when fg decreases. This is explained by nonlinear coupling between the primary frequencies, which results in a larger growth of hf components as fg decreases, causing the hf waves to break earlier. Due to high spatial resolution, wave tracking has provided an accurate measurement of the varying breakpoint. These breaking locations are very well described ( R2>0.91) by the wave-height to effective-depth ratio (γ). However, for any given Iribarren number, this γ is shown to increase with fg. Therefore, a modified Iribarren number is proposed to include the grouping structure, leading to a considerable improvement in reproducing the measured γ-values. Within the surf zone, the behavior of the Incident Long Wave also depends on the group modulation. For low fg conditions, the lf wave decays only slightly by transferring energy back to the hf wave components. However, for high fg wave conditions, strong dissipation of low frequency (lf) components occurs close to the shoreline associated with lf wave breaking. This mechanism is explained by the growth of the lf wave height, induced partly by the self-self interaction of fg, and partly by the nonlinear coupling between the primary frequencies and fg.

Swash zone response under various wave regimes

The modelling of swash zone (SZ) sediment transport and the resulting morphodynamics have been areas of active research over the last decade. However, many details are still to be understood, whose knowledge will be greatly advanced by the collection of high-quality data under the controlled large-scale laboratory conditions. The research describes tests carried out in the large wave flume of the Maritime Engineering Laboratory at Catalonia University of Technology, to investigate the SZ under the storm conditions. Its main aim was to compare beach-profile responses for monochromatic waves, monochromatic waves plus free long waves, bi-chromatic waves and random waves. Both erosive and accretive conditions were considered. Results discussed here were derived from the analysis of only a part of the whole data set.

Sediment transport and shoreline erosion induced by bichromatic waves with varying group period

In the present paper, large scale experimental data are presented showing sediment transport and beach profile evolution at the inner surf zone, close to the shoreline. Four different bichromatic wave conditions have been generated characterized by a similar energy content but varying the wave group period (frequency bandwidth). The differences in the shoreline evolution and associated sediment transport are explained from the differences in the wave group period. It has been shown that increasing the wave group period promotes a seaward horizontal displacement in the shoreline erosion pattern. The inner surf zone shows larger erosion for increasing wave group periods. However at the swash zone larger wave group periods show a berm located further seaward with a relatively less shoreline retreat. Differences in the beach profile evolution are explained from the bandwidth influence in the propagation of wave groups and associated long wave. Larger wave group periods showed a progressive long wave pattern with the associated long wave out of phase with the wave group envelope. This resulted in negative long-wave induced sediment transport within the inner surf zone. Reduced wave group periods have shown a standing pattern that resulted in long wave motions in and out of phase with the wave group envelope depending on the cross-shore location. Within the inner surf zone this resulted in mostly positive long wave induced sediment transport for reduced wave group periods. It was also shown that the long wave induced sediment transport represented on average 18% of the total mobilized sediment transport.

Lagrangian modelling and direct bed shear stress measurement in the swash zone

Direct measurements of swash zone bed shear stress obtained with a shear plate are presented and a new swash zone boundary layer model is introduced. The model considers boundary layer growth in a Lagrangian framework, in terms of fluid particle displacement and flow history. The model is based on the momentum integral analysis of the smooth, flat-plate boundary layer and predicts the observed onshore asymmetry in bed shear stress. Friction coefficients back-calculated from the measured bed shear stress display strong temporal variation. In the absence of bore-induced turbulence, the uprush friction coefficient is predicted well by the smooth, turbulent boundary layer model. Early in the backwash phase, the friction coefficient may be better predicted by a laminar boundary layer model. Measurements suggest that the backwash boundary layer transitions from laminar to turbulent. It is proposed that the backwash boundary layer may remain laminar for a longer period than expected due a favorable pressure gradient.