OBSERVATION OF UNDERTOW AND TURBULENCE IN A LABORATORY SURF ZONE

Undertow and turbulence in the surf zone have been studied in a wave f lume for a spilling breaker and a plunging breaker. Fluid velocities a cross a 1 on 35 sloped false bottom were measured using a fiber-optic laser-Doppler anemometer, and wave decay and set-up were measured usin g a capacitance wave gage. The characteristics of mean flow and turbul ence in spilling versus plunging breakers were studied. The mean flow is the organized wave-induced flow defined as the phase average of the instantaneous velocity, while the turbulence is taken as the deviatio ns from the phase average. It was found that under the plunging breake r turbulence levels are much higher and vertical variations of underto w and turbulence intensity are much smaller in comparison with the spi lling breaker. It was also found that turbulent kinetic energy is tran sported seaward under the spilling breaker and landward under the plun ging breaker by the mean flow. The study indicates that there are fund amental differences in the dynamics of turbulence between spilling and plunging breakers, which can be related to the processes of wave brea king and turbulence production. It is suggested that the types of beac h profile produced by storm and swell waves may be the results of diff erent relationships between mean flow and turbulence in these waves.