Turbulent and Macro-turbulent Structures Developed in the Benthic Boundary Layer Downstream of Topographic Features

The characteristics and effects of large-scale flow structures developed in the benthic boundary layer downstream from large topographic features were analysed throughout a tidal cycle. The observed signature of the macro-turbulent features consisted of streamwise modules of low horizontal velocity and high suspended sediment concentration (SSC), alternating with modules of high horizontal velocity and low SSC. These modules extended 10 to 20 m streamwise and exceeded 1 m vertically, and are believed to be related to flow separation effects over large bedforms upstream of the deployment site. The macroscale flow modules intensified the ‘ burst-like ’ turbulent events and favoured sediment transport. ‘ Ejection-like ’ events were magnified during modules of decreasing horizontal velocity and increasing turbidity, whereas ‘ sweep-like ’ events were magnified during modules of increasing horizontal velocity and decreasing SSC. The enhanced turbidity of the macroscale modules may be the result of enhanced upward diffusion of sediment by ejection events, whereas the low-turbidity modules may be induced by increased downward transport of suspended sediment by sweep events. These hypotheses were supported by cross-spectral analysis performed on velocity and suspended sediment concentration time-series recorded at the site. An enhanced (negative) contribution of outward and inward interaction events to the Reynolds stress, compared to those reported in uniform BBLs, resulted in ‘ abnormally ’ low stress values.