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Lagrangian measurements of turbulent dissipation over a shallow tidal flat from pulse coherent Acoustic Doppler Profilers

Abstract
We present high resolution (25 mm spatial, 8 Hz temporal) profiles of velocity measured over a shallow tidal flat using pulse-coherent Acoustic Doppler Profilers mounted on surface drifters. The use of Lagrangian measurements mitigated the problem of resolving velocity ambiguities, a problem which often limits the application of high-resolution pulse-coherent profilers. Turbulent dissipation rates were estimated from second-order structure functions of measured velocity. Drifters were advected towards, and subsequently trapped on, a convergent surface front which marked the edge of a freshwater plume. Measured dissipation rates increased as a drifter deployed within the plume approached the front. A drifter then propagated with and along the front as the fresh plume spread across the tidal flats. Near-surface turbulent dissipation measured at the front roughly matched a theoretical mean-shear-cubed relationship, whereas dissipation measured in the stratified plume behind the front was suppressed. After removal of estimates affected by surface waves, near-bed dissipation matched the velocity cubed relationship, although scatter was substantial. Dissipation rates appeared to be enhanced when the drifter propagated across small subtidal channels.
Type
Conference Contribution
Type of thesis
Series
Citation
Mullarney, J. C., & Henderson, S. M. (2012). Lagrangian measurements of turbulent dissipation over a shallow tidal flat from pulse coherent Acoustic Doppler Profilers. Proceedings of the 33rd International Conference on Coastal Engineering 2012, 33.
Date
2012
Publisher
Costal Engineering Research Council
Degree
Supervisors
Rights
This work is licensed under a Creative Commons Attribution 3.0 License. http://creativecommons.org/licenses/by/3.0/