Abstract
A selectable frequency acoustic profiler and electromagnetic current meter were deployed in the lower Fraser River, Canada, to examine large turbulent structures. Features were generated on the lower stoss sides of dunes and appeared at the surface tens of metres downstream as strong upwellings (‘boils’). They were identified with acoustic ‘clouds’ in the acoustic profile.Event recurrence periods measured from acoustic time series and from ‘ejection’ events identified in the current meter record had similar two-component frequency distributions, but the acoustic values lacked the highest frequencies. A comparison of measured periods with predictions from the Strouhal law indicate that both dune amplitude and the internal boundary layer developed over the dunes are significant length scales associated with the generation of the macroturbulence. Our data give no support to the notion that boils develop from microturbulent bursts.
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