Turbulence and macro-turbulence in the bottom boundary layer downstream of large-scale bedforms; implications for sediment transport
Effects of large-scale topographic features on the benthic boundary layer (BBL) flow characteristics and on sediment transport are analysed in a natural shallow coastal site and in a laboratory flume. The main signature of flow/bedform interaction processes consists of largescale flow structures, generated downstream of the bedforms; these are characterised by alternating modules of lower and higher streamwise speeds than the mean flow. These structures modify the suspended sediment transport, generating high turbidity levels within macro-scale features of low flow speed, and the bedload transport, generating more frequent and larger bedload movements during macro-scale features of high flow speeds. These particularities result from variations in the turbulent bursting process within the macroturbulence structures. Ejection events generate increases in suspended sediment concentration; these are more frequent and more intense within the macro-scale structures of low flow speed. Sweeps are shown to generate most of the bedload transport; these events are dominant and are magnified during modules of increasing horizontal velocity. Enhanced (negative) contribution of outward and inward interaction events to the Reynolds stress, downstream of large-scale bedforms, compared to those reported for uniform BBLs, are observed in the field and laboratory investigations. This induces 'abnormally' low Reynolds stress values, which are believed to invalidate the use of the Reynolds stress parameter to predict sediment transport. Other aspects of the turbulence are little affected by the presence of bedforms upstream, such as the coupling between the turbulent velocities and the Reynolds stress, or, at the field site, the turbulent energy dissipation rate.