Dynamics of surf-zone turbulence in a spilling breaker

Abstract

The structure of turbulence in a spilling breaker has been studied experimentally based on the transport equation for turbulent kinetic energy (the k-equation). We study turbulence transport in the evolving flow from the breaking point to the inner surf zone in the region below trough level and above the bottom boundary layer. The study shows that turbulence transport processes are similar in the outer and inner surf zones. It is found that diffusive transport plays the most important role in the distribution of turbulence, while advection is important mainly near the surface. It is also found that although turbulence production below trough level amounts to only a small portion of the wave energy loss, the production term is not small compared to the dissipation term and the major terms in the k-equation and thus it cannot be neglected. The mixing length is estimated based on the measured rates of vertical advance of the turbulent front, and comparisons of turbulence production and energy dissipation. The results are similar to those found in previous studies. It is shown that the length scale and velocity scale of the large eddies are subject to turbulence transport processes, therefore their distributions cannot be prescribed in an easy way. The relative values of the components of the Reynolds stress tensor are examined. The results of analysis supports the notion that surf-zone turbulence created by spilling and plunging breakers differ primarily in the method of energy transfer from organized wave-induced motion to turbulent motion, and the constraints imposed by the mean flow and the solid bottom on the large-scale turbulence.