Turbulence, turbulent mixing and diffusion in shallow-water estuaries

Chanson, Hubert and Trevethan, Mark (2010). Turbulence, turbulent mixing and diffusion in shallow-water estuaries. In P. R. Lang and F.S. Lombargo (Ed.), Atmospheric turbulence, meteorological modeling and aerodynamics (pp. 167-204) Hauppauge, NY, U.S.A.: Nova Science Publishers.

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Author Chanson, Hubert
Trevethan, Mark
Title of chapter Turbulence, turbulent mixing and diffusion in shallow-water estuaries
Title of book Atmospheric turbulence, meteorological modeling and aerodynamics
Place of Publication Hauppauge, NY, U.S.A.
Publisher Nova Science Publishers
Publication Year 2010
Sub-type Research book chapter (original research)
ISBN 9781607410911
Editor P. R. Lang
F.S. Lombargo
Chapter number 4
Start page 167
End page 204
Total pages 38
Total chapters 16
Collection year 2011
Language eng
Subjects 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. The predictions of contaminant dispersion in estuaries can rarely be predicted analytically without exhaustive field data for calibration and validation. Why ? In natural estuaries, the flow Reynolds number is typically within the range of 105 to 108 and more. The flow is turbulent, and there is an absence of fundamental understanding of the turbulence structure. Any turbulent flow is characterised by an unpredictable behaviour, a broad spectrum of length and time scales, and its strong mixing properties. In small estuaries, the predictions of scalar dispersion can rarely be estimated accurately because of a lack of fundamental understanding of the turbulence structure. Detailed turbulent velocity and suspended sediment concentration measurements were performed simultaneously and continuously at high frequency for between 25 and 50 hours per investigation in shallow-water estuaries with semi-diurnal tides in Australia and Japan (Fig. 18). The detailed analyses provided an unique characterisation of the turbulent mixing processes and suspended sediment fluxes. Continuous turbulent velocity sampling at high frequency allowed a detailed characterisation of the turbulence field in estuarine systems and its variations during the tidal cycle. The turbulence was neither homogeneous nor isotropic. It was not a purely Gaussian process, and the small departures from Gaussian probability distribution were an important feature of the turbulent processes. A striking feature of the present data sets was the large and rapid fluctuations in all turbulence characteristics and of the suspended sediment fluxes during the tidal cycles. This was rarely documented, but an important characteristic of the newer data sets is the continuous high frequency sampling over relatively long periods. The findings showed that the turbulent properties, and integral time and length scales should not be assumed constant in a shallow estuary. The integral time scales for turbulence and suspended sediment concentration were similar during flood tides, but differed significantly during ebb tides. It is believed that the present results provided a picture general enough to be used, as a first approximation, to characterise the flow field in similar shallow-water estuaries with semi-diurnal tides. It showed in particular a different response from that observed in larger, deep-water estuaries. A turbulent flux event analysis was performed for a 50 hour long field study. The results showed that the large majority of turbulent events had a duration between 0.04 s and 0.3 s, and there were on average 1 to 4 turbulent events per second. A number of turbulent bursting events consisted of consecutive sub-events, with between 1 and 3 sub-events per event on average for all turbulent fluxes. A comparison with atmospheric boundary layer results illustrated a number of similarities between the two types of turbulent flows. Both studies implied that the amplitude of an event and its duration were nearly independent. Overall the present research highlighted some turbulent processes that were rarely documented in previous studies. However an important feature of the present analysis was the continuous high frequency sampling data sets collected during relatively long periods, as well as the simultaneous sampling of both turbulent velocities and suspended sediment concentrations.
Keyword Turbulence
Small estuaries
Small subtropical estuary
Shallow water lake
Turbulent mixing
Turbulent busting events
Turbulent subevents
Field measurements
Eprapah Creek
Hamanako Lake
Q-Index Code B1
Q-Index Status Confirmed Code
Institutional Status UQ

Document type: Book Chapter
Collections: School of Civil Engineering Publications
Official 2011 Collection
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Created: Wed, 09 Jun 2010, 14:38:38 EST by Hubert Chanson on behalf of School of Civil Engineering