Near‐bed turbulence dissipation measurements in the inner surf and swash zone

Abstract

Near‐bed turbulence dissipation rates were quantified in the swash zone and inner surf zone of a dissipative beach. The vertical structure and temporal evolution of the dissipation rate were calculated using the structure function applied to profiles of the vertical velocity collected by three profiling acoustic Doppler velocimeters. Dissipation rates between and were observed. Turbulence dissipation rates increased with diminishing water depth but showed no significant correlation to offshore wave height. Temporal phasing of strong turbulence dissipation events agreed well with image‐based observations of pixel intensity associated with wave breaking. Bore‐generated turbulence decayed rapidly following bore arrival and followed a decay rate similar to grid turbulence. The turbulence dissipation was dominated by (advected) bore‐generated turbulence during the uprush and initial stages of the backwash, and by bed‐generated turbulence during the later stages of the backwash. A scaling analysis shows that near the bed, sediment‐induced density stratification effects may have been an order of magnitude larger than turbulence dissipation.