Vertical Structure of Dissipation in the Nearshore

TLDR

Surface and bottom boundary layer dissipation scaling concepts overlap in this nearshore region. The study measured turbulence kinetic energy dissipation vertically in a 3.2‑m deep nearshore area using a tripod of three acoustic Doppler current meters. The measurements revealed that dissipation peaks near the surface, dips at middepth, and rises again near the bed, with wind‑induced whitecapping breaking present but no depth‑limited breaking, and the pattern does not conform to surfzone or log‑layer scaling but follows a modified deep‑water breaking‑wave scaling at the upper meters, indicating vertical diffusion dominates over shear production.

Abstract

Abstract The vertical structure of the dissipation of turbulence kinetic energy was observed in the nearshore region (3.2-m mean water depth) with a tripod of three acoustic Doppler current meters off a sandy ocean beach. Surface and bottom boundary layer dissipation scaling concepts overlap in this region. No depth-limited wave breaking occurred at the tripod, but wind-induced whitecapping wave breaking did occur. Dissipation is maximum near the surface and minimum at middepth, with a secondary maximum near the bed. The observed dissipation does not follow a surfzone scaling, nor does it follow a “log layer” surface or bottom boundary layer scaling. At the upper two current meters, dissipation follows a modified deep-water breaking-wave scaling. Vertical shear in the mean currents is negligible and shear production magnitude is much less than dissipation, implying that the vertical diffusion of turbulence is important. The increased near-bed secondary dissipation maximum results from a decrease in the turbulent length scale.