Turbulence and Turbulent Flux Events in a Small Subtropical Estuary
Trevethan, Mark, Chanson, Hubert, & Brown, Richard J. (2007) Turbulence and Turbulent Flux Events in a Small Subtropical Estuary. University of Queensland.
In natural estuaries, scalar diffusion and dispersion are driven by turbulent momentum mixing. To date, relatively little systematic research was conducted on the turbulence characteristics of small estuaries. In the present study, detailed turbulence measurements were conducted in a small sub-tropical estuary with semi-diurnal tides under neap tide conditions. Three acoustic Doppler velocimeters were installed in the mid-estuary at fixed locations close together. The ADV units were sampled simultaneously and continuously at high-frequency (50 Hz) for 50 hours. The velocity co-variances and triple correlations, as well as the backscatter intensity and pseudo-sediment flux co-variances, were calculated for the entire field study. The co-variances of the longitudinal velocity component showed some tidal trend, while the co-variances of the transverse horizontal velocity component exhibited trends that reflected changes in secondary current patterns between ebb and flood tides. The triple correlation data tended to show some differences between ebb and flood tide. The acoustic backscatter intensity data were characterised by large fluctuations during the entire study, with dimensionless fluctuation intensity between 0.45 and 0.55. The co-variances of backscatter flux intensity showed little tidal trend although larger co-variance values were observed at high tide. A turbulent flux event analysis was performed for the entire study following the technique of NARASIMHA et al. (2007). Turbulent bursting events were defined in terms of the instantaneous turbulent flux. The method was extended to the unsteady estuarine flow motion. The data showed close results for all three ADV units. The very-large majority of turbulent events lasted between 0.04 s and 0.3 s, with an average of 1 to 4 turbulent events observed per second. A number of turbulent bursting event consisted of consecutive turbulent sub-events, with between 1 and 3 sub-events per main event on average. For all ADV systems, the number of events, event duration and event amplitude showed some tidal trends, with key differences between high- and low-water periods. An unusual feature of the field study was some moderate rainfall prior to and during the first part of the sampling period. Visual observations showed some surface scars and marked channels, while some mini-transient fronts were observed. It is believed that the freshwater runoff induced some difference in turbulence properties during the early part of the field work.
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