Turbulence measurements in a small subtropical estuary under king tide conditions

Chanson, Hubert, Brown, Richard and Trevethan, Mark (2010) Turbulence measurements in a small subtropical estuary under king tide conditions. Hydraulic Model Report CH CH77/10, School of Civil Engineering, The University of Queensland.

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Author Chanson, Hubert
Brown, Richard
Trevethan, Mark
Title Turbulence measurements in a small subtropical estuary under king tide conditions
School, Department or Centre School of Civil Engineering
Institution The University of Queensland
Open Access Status Other
Report Number CH77/10
Series Hydraulic Model Report CH
Publication date 2010
Start page 1
End page 82
Total pages 82
Publisher School of Civil Engineering, The University of Queensland
Editor Hubert Chanson
Language eng
Subject 09 Engineering
0905 Civil Engineering
090509 Water Resources Engineering
Abstract/Summary In natural waterways and estuaries, an understanding of turbulent mixing is critical to the knowledge of sediment transport, storm-water runoff during flood events, and release of nutrient-rich wastewater into ecosystems. In the present study, some detailed turbulence field measurements were conducted in a small subtropical estuary with semi-diurnal tides during king tidal conditions: i.e., the tidal range was 2.78 m, the largest for both 2009 and 2010. The turbulent velocity measurements were performed continuously at high-frequency (50 Hz) for 60 hours. Two acoustic Doppler velocimeters (ADVs) were sampled simultaneously in the mid-estuarine zone at z = 0.12 and 0.32 m above the bed. A third ADV was deployed in the upper estuary and sampled continuously at 50 Hz for 12 hours only. The results provided an unique characterisation of the turbulence in both mid- and upper estuarine zones under the king tidal conditions. The extreme low and high water levels had some impact on the estuary hydrodynamics. At high tides, the mangroves and surrounding mudflats were inundated by the high water levels. At low tides, the upper estuarine zone became practically disconnected from the lower estuary. The physio-chemistry data indicated that the water column was well mixed throughout the study period. However a transient front was observed in the upper estuary during the early flood tide on 2 February 2010. The formation and passage of the front were associated with a sudden increase in turbulent velocity fluctuations with sustained fluctuations of all three velocity components for more than 25 minutes. During the 60 hours period, the largest longitudinal velocities occurred around the low tide, while a very rapid flow reversal took place at low tide slack. The early flood tide current was very intense with a strong upstream current in the mid-estuary but a much slower flow motion was observed in the upstream estuary. The standard deviations of all velocity components were the largest during the first half of the flood tide. A second period of large standard deviations of all velocity components was the early ebb tide, linked the emptying of the inundated mangroves and surrounding mudflats into the main channel. The horizontal and vertical turbulence intensities vy'/vx' and vz'/vx' showed no discernable tidal trend. Similarly the integral turbulent time scale and dissipation time scales did not present any tidal trend. The acoustic backscatter intensity data showed some strong correlation with the turbidity data. In the mid-estuary, the turbulent shear stresses were comparable to the boundary shear stress. The observations differed from the earlier neap tide data in the same estuary. Overall the present data showed some marked differences between king tide and neap tidal conditions. During the king tidal conditions, the tidal forcing was the dominant water exchange and circulation mechanism in the estuary. In contrast, the long-term oscillations linked with internal and external resonance played a major role in the turbulent mixing during neap tides. The data set showed further the upper estuarine zone to be drastically less affected by the spring tidal range: the flow motion remained slow, but the present turbulent velocity data were clearly affected by the propagation of a transient front during the very early flood tide motion at that sampling site.
Keyword Turbulence
Small subtropical estuary
Turbulent mixing
King tides
Spring tides
Field measurements
Mid-estuary
Upper estuary
Acoustic Doppler velocimetry (ADV)
Transient front
Eprapah Creek
Additional Notes The full bibliographic details are: CHANSON, H., BROWN, R., and TREVETHAN, M. (2010). "Turbulence Measurements in a Small Subtropical Estuary under King Tide Conditions." Hydraulic Model Report No. CH77/10, School of Civil Engineering, The University of Queensland, Brisbane, Australia, 82 pages (ISBN 9781864999969).

Document type: Department Technical Report
Collection: School of Civil Engineering Publications
 
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Created: Fri, 30 Jul 2010, 15:39:58 EST by Hubert Chanson on behalf of School of Civil Engineering