A Nortek Vector Acoustic Doppler Velocimeter was deployed in February 2010 near the DDL platform and set to sample the near bottom currents at 4 Hz. An RDI 150 kHz ADCP was also deployed on the DDL platform, recording 30s ensembles (Figure 1). The spectra of the vertical velocity component (w’) from the Nortek Vector was then fit to the Nasmyth universal spectra to obtain an estimate of the local turbulent dissipation rate (epsilon). Assuming a local balance of turbulence production and dissipation, we can then estimate the bottom stress and the friction velocity (Figure 2, this figure). The friction velocity determines when and to what extent the bottom sediments become re-suspended by the tidal currents. Depending on how compact the sediments are, re-suspension can occur for frictions velocities above 0.5 cm/s.
Top panel is the time series of the turbulent dissipation rate (epsilon, W/kg) as measured by the Nortek Vector. The middle panel are the raw time series of the UVW currents measured by the Vector, and the bottom panel is the estimated friction velocity derived from the dissipation rate and assuming a simple balance of production and dissipation at 1.3m height.