Local processes of bed destabilization in the surf zone

• Published in: Journal of coastal research 2011-01, Vol.SI 64, p.2017-2021
• Main Authors: Berni, C., Michallet, H., Barthélemy, E.
• Format: Article
• Language: English
• Subjects: Engineering Sciences; Fluid mechanics; Fluids mechanics; Mechanics; Physics; Sedimentary processes and morphodynamics of sandy beaches on short time response
• ISSN: 0749-0208; 1551-5036

In the surf zone, most of the sediment transport occurs close to the bed as sheet flow. Laboratory experiments were designed to reproduce natural beach dynamics with a beach of lightweight sediment to fulfill Shields number and Rouse number scaling requirements. Previous studies have shown that an equilibrium beach profile results from a constant wave climate. The wave climate for this experiment involved a succession of two groups of waves of period 2.5 and 3 s. The resulting profile is a barred beach with a long surf zone with spilling waves. Near bed sediment concentration and velocity measurements were performed within the surf zone. The positions of both immobile bed and top sheet flow were also measured with an acoustic sensor. The sediment transport was computed as the product of the instantaneous velocity and concentration. It appears that the sheet-flow is thicker under the wave crests and the sediment transport larger. The free stream cross shore velocity is comparable in magnitude in both crests and troughs, but the shape of the vertical profile differs close to the bed. This results from the strong acceleration at the wave front related to a large wave asymmetry typical of surf zone waves. The vertically integrated net sediment flux over the whole sequence is very weak, which is consistent for a nearly equilibrium beach profile.