Identification of intense, intermittent coherent motions under shoaling and breaking waves

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Abstract
Laboratory measurements of the instantaneous horizontal and vertical turbulent velocities,u′ andw′, induced by regular waves spilling on a rough, impermeable slope were analyzed to elucidate the nature of turbulence generated in the bottom boundary layer and by wave breaking. The analyses focused on the instantaneous turbulent events such as the absolute shear stress |τ′| = |−u′w′| and turbulent kinetic energyk′ = (u′2+w′2)/2. Below trough level inside the surf zone, the horizontal and vertical velocity records showed intense, intermittent turbulent events that did not occur with the passing of each wave, and the instantaneous quantities of |τ′| andk′ could not be explained in terms of the phase‐averaged quantities. The intermittent turbulent events extended into the bottom boundary layer inside the surf zone, and in this region the infrequent but intense turbulence generated by wave breaking was an order of magnitude larger than the turbulence generated locally at the boundary. Two techniques were used to analyze the turbulent motions. A quadrant analysis technique showed that although the large turbulent motions did not occur with each wave, they were phase‐dependent near trough level and less so near the bottom. A conditional sampling technique quantified the intensity and duration of the turbulent motions, indicating that coherent events (intense events) occurred for ∼10% (2%) of the record and accounted for approximately 50% (20%) of the turbulent motion. It is likely that the exactitude of these statistics will differ depending on breaker type. Nevertheless, these statistics indicated that large turbulent motions are infrequent but contribute significantly to the turbulence intensity and possibly the suspension of bottom sediments.

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