The surf zone is a region of high turbulence generated primarily through wave breaking and, to a lesser extent, by shear at the bottom boundary layer. Proper parameterization of this turbulent kinetic energy is important to three main processes. First, turbulence provides the horizontal and vertical mixing for both currents (undertow, and longshore currents) as well as passive tracers (for example, pollutants). Second, water column turbulence supports the suspension of sediments and the associated sediment transport. Third, delays in turbulent decay associated with advection in a surface roller can significantly modify forcing patterns for mean flows (for example, longshore currents, as discussed above).
It is widely acknowledged that wave breaking at the surface provides the dominant source of water column turbulence [ Svendsen, 1987]. Near the zone of initial breaking, lab experiments have shown this turbulent generation to be initially an intermittent and two-dimensional process that becomes three-dimensional as the turbulence decays after passage of the wave front [ Nadaoka, et al., 1989]. Large scale eddies are produced that can cause significant mixing. After further propagation, the breaking wave may become a steady bore whose bulk turbulent properties can be parameterized based on large-scale conservation laws. The advection of this turbulence in a roller introduces an important temporal and spatial lag on the driving of mean flows.
The generation of turbulence through the shear in an oscillatory bottom boundary layer has been studied extensively [see Nielsen, 1992 for a good review], although the work has almost exclusively been theoretical (generally based on monochromatic waves) or laboratory based. The focus has generally been on the nature of the turbulent closure, particularly the vertical and temporal (over a wave period) dependencies.
Because of the harshness of the environment, very little work has been done on the measurement of surf zone turbulence in natural field settings. Agrawal and Aubrey [1992] deployed a Laser Doppler Velocimeter (LDV) in the boundary layer region of a very low-energy environment, although their methodology for the separation of turbulent and wave-associated energies was later questioned by Herbers and Guza [1993]. George et al. [1994] deployed hot film sensors to measure surf zone turbulence through the water column under typical ocean wave forcing. Magnitudes and scales of turbulence were found to be consistent with prior laboratory results, encouraging the continuation of those controlled experiments.
Because of the observed intermittency of sediment suspension (and presumably the associated bottom boundary layer turbulence), it seems that further work is needed on the nature of turbulence in the vicinity of the bed. One potential area of future interest is the modification of the boundary layer behavior by sediment-induced stratification, suggested by Glenn and Grant [1987].