Field observations of turbulence in the intertidal and shallow subtidal zones

Turbulence is critical for sediment mobilisation in shallow water and hence beach evolution. Present large-scale morphological models often exclude the effects of bed- and surface-generated turbulence on the short-wave sediment transport, which might be part of the reason why they tend to result in incorrect estimates of the onshore-directed sediment transport under low-energetic conditions. A first step for model improvements is to be able to couple turbulence intensity and orbital motion at intra-wave time scale. Field measurements of flow velocities at three vertical elevations at two cross-shore locations in the intertidal and shallow subtidal zones were used to examine turbulence beneath shoaling waves, spilling breakers and surf bores. Beneath shoaling waves, near-bed turbulent kinetic energy (TKE(t)) was generated by bed friction and phase-coupled to the cross-shore orbital velocity (u(t)). Beneath spilling breakers, turbulence intensity increased, especially in the upper part of the water column due to injection of breaker-generated turbulence. The surface-induced TKE(t) peaked just after the wave crest. Near the bed, TKE(t) had a broad maximum under the wave trough possibly due to increased shear from stronger offshore-directed mean currents. Beneath surf bores, TKE(t) peaked at the wave front throughout the water column which could be due to both acceleration skewness and a turbulent bore front.