Turbulence dissipation under breaking waves and bores in a natural surf zone

Abstract

Wave breaking is the primary driver of beach erosion, injecting breaking-induced turbulence at the sea surface and diffusing bed boundary layer turbulence at the sea bed. The limited understanding of the vertical turbulence structure under natural breaking waves, and hence sand entrainment, is one of the reasons that coastal-evolution models produce inadequate estimates of storm response. Here we use a recently collected field dataset to analyze turbulence dissipation under breaking waves and bores on the intertidal beach at Truc Vert, France. The vertical structure of the turbulent dissipation rate indicates that wave breaking is the dominant source of turbulence dissipation. The current-induced turbulence represents no more than 50% of the turbulent dissipation rate close to the bed (at 10% of the water column), even when alongshore currents reach 1 m/s. The data further illustrate that the turbulent dissipation rate is almost depth-uniform under breaking waves, whereas it decreases profoundly toward the bed under bores. Moreover, we found that the fraction of wave energy flux decay dissipated below wave-trough level is about 1% under breaking waves and about 10% under bores. These results imply that the turbulent dissipation rate in the surf zone is severely underestimated by coastal-evolution models that do not consider breaking-induced turbulence as a surface boundary condition. Consequently, they will underestimate sand stirring and transport by mean currents during severe storms.