Turbulence observations of the nearshore wave bottom boundary layer

Abstract

Field observations of turbulence and sediment suspension in the nearshore wave bottom boundary layer obtained during the Duck94 field experiment on the North Carolina coast showed the generation of near‐bed turbulence to be highly intermittent. The intermittent nature of the flow was examined by applying homogeneous isotropic turbulence laws over small windows of data. The time‐varying estimates of turbulent kinetic energy, shear stress, dissipation, and concentration were averaged over the phase of the free stream wave. The observations show that the magnitude of the turbulent kinetic energy is largest under the wave crest and decreases over the decelerating flow phase until reversal to offshore flow. The turbulent kinetic energy observations under the crests compare favorably to those of three laboratory studies. The sediment suspension observations were shown to have an intermittent structure and to be biased toward the onshore decelerating phase of the flow. The shear stress was estimated with the orbital velocities by assuming an eddy viscosity model and was shown to be highly sensitive to the free stream velocity and less sensitive to the free acceleration. The intermittent signal of the turbulent kinetic energy and the suspension was not evident in the shear stress estimates, providing further support that eddy viscosity models are not appropriate for predicting the suspension of sediment. Finally, dissipation rate estimates increased with proximity to the bed but were several orders of magnitude lower than those found in the active breaking region.