The structure and dynamics of the thin near-bed layer in a complex marine environment: a case study in Beatrix Bay, New Zealand.

The 1-3 cm thick near-bed layer at the interface between sea-water and the sea-bed was studied using two acoustic Doppler velocimeter (ADV) deployments in Beatrix Bay (New Zealand), which relate to late autumn and early winter conditions. The data strongly support the multi-mechanism nature of the near-bed layer and show that the turbulent stresses and energy may be non-zero even at nearly zero or zero mean velocities. The turbulent fluctuations in the near-bed layer are highly intermittent and consist of two components: (1) low energy 'background' fluctuations; and (2) relatively rare high-energy events (HEE). The structure, dynamics, and influence of these components on bed erosion, particle aggregation and re-suspension are significantly different. Our results suggest that the revealed high-energy events are the main contributors to bed erosion and sediment re-suspension. The estimates of the local Reynolds number and other turbulence parameters show that HEE may change the near-bed flow regime from viscous or transitional at low-energy background conditions to fully turbulent during the high-energy events. Although the physical origin of HEE is not yet clear, most probably they are generated by intermittent highly-energetic internal waves or near-bed convective mixing, or both. These results provide a base for future research in this complex near-bed region. (C) 2002 Elsevier Science Ltd. All rights reserved.