Observations of turbulence within a natural surf zone

Abstract

Here, the Reynolds stresses <u′w′> and <v′w′>, where u′, v′, and w′ are the cross-shore, alongshore, and vertical turbulence velocities, respectively, and the angle brackets represent time averaging, are used to diagnose turbulence dynamics beneath natural breaking surf-zone waves. The data were collected at Truc Vert Beach, France, during a 12-day period in 1–3-m water depth with strong cross-shore and alongshore currents under high-energy wave conditions (offshore significant wave heights ranged between 2 and 8 m). The <u′w′> term is predominantly negative, increases with the ratio of wave height Hs to water depth h (degree of wave breaking), and decreases in magnitude toward the bed. This supports the view that the cross-shore shear stress is due to breaking-induced vortices that transport high-speed cross-shore flow downward and disintegrate close to the bed. The occasional positive sign of <u′w′> within the lower 15%–20% of the water column indicates that sometimes surface-generated turbulence is overwhelmed by bed-generated turbulence, but the conditions when this happens are not clear from the data. The term <v′w′> is persistently of opposite sign to the alongshore mean current and decreases with height above the seabed, implying that <v′w′> is due to bottom boundary layer processes rather than surface-generated turbulence. The bottom drag coefficient amounted to 1.6 × 10−3, similar to earlier observations. As in other high-Reynolds-number geophysical flows, time series of u′w′ and v′w′ comprise intermittently large, short-duration (here, 1 s) stress events that in the data contribute considerably to the net stress in only 3%–15% of the time. The data further show that the turbulent kinetic energy is depth uniform and increases with Hs/h. The depth-averaged Froude-scaled turbulent kinetic energy beneath surf-zone bores is 0.025, a factor of 2 to 3 less than observed beneath regular laboratory waves.