The turbulent structure of the bottom boundary layer in a tidal current

Summary Measurements of turbulence in the neutrally stratified bottom boundary layer of a tidal current are described. It is shown that whereas the bottom boundary layer is similar to its atmospheric counterpart, in terms of appropriately scaled spectra of the turbulent velocity fluctuations and the Reynolds stress, there is less evidence to suggest that it is structurally similar when measured in terms of the Reynolds stress tensor components. These differences appear to be more pronounced in deep flows where there is some evidence to suggest that the indices of structural similarity may vary over the tidal cycle; this behaviour may be due in part to the presence of inactive motions in the boundary layer. The statistical properties of the turbulence are described and in particular the high sampling variability of the Reynolds stress is shown to be associated with intermittently large momentum fluxes occurring in the boundary layer. The use of an intermittency factor in determining the burst period is also discussed. Suspended sediment is shown to have no effect on the turbulent structure of this example of a boundary layer. Estimates of the drag coefficient for a range of sediment types and bed forms have been obtained and finally it is shown that levelling errors are a major source of inaccuracy in boundary layer stress measurements.