Series of Detailed Turbulence Measurements in a Small Subtropical Estuarine System

Abstract

In natural waterways and estuaries, the momentum mixing is a turbulent process directly relevant to the understanding of sediment transport, release of wastewater into ecosystems and storm-water runoff during flood events. The predictions of contaminant dispersion in estuaries are highly sensitive to specific features of the natural system. They cannot be predicted analytically and must rely upon exhaustive field data for accurate calibration and validation. Herein detailed turbulence and physio-chemistry field measurements were conducted in a small subtropical estuary of Eastern Australia with a semi-diurnal tidal regime. The turbulent velocity measurements were performed continuously at high frequency for 25 hours during spring tide conditions and for 50 hours during neap tide conditions. The velocities were measured with acoustic Doppler velocimetry and a thorough post-processing technique was applied to the velocity signals. The measurement technique used in this study was well suited to the investigation of momentum transport in small estuarine systems with shallow water depths (e.g. less than 0.5 m at low tides).
During the field studies, some tidal asymmetry was observed systematically. The time-averaged streamwise velocity during the ebb tides was smaller than that during the flood tide. The turbulent fluctuations next to the bed were highly intermittent. The third (skewness) and fourth (kurtosis) moments of velocity fluctuations were within the ranges of -0.6 to +0.6 and -1 to +2 respectively. The horizontal and vertical turbulence intensities were typically within the range of vy'/vx' ~ 1 and vz'/vx' ~ 0.3 to 0.8 implying some turbulence anisotropy. The magnitudes were similar to earlier field work results in estuaries and experimental measurements in turbulent open channel flows. The time-averaged Reynolds stresses vx*vz were proportional to minus the time-averaged streamwise velocity, and the dimensionless Reynolds stresses Rvxvz = vx*vz/(vx'*vz') were similar for both spring and neap tide conditions. The probability distribution functions of tangential Reynolds stresses differed substantially from Gaussian distributions. Based upon auto-correlation analyses, the integral and dissipation time scales were systematically analysed. The dimensionless integral time scales were found to be about : TEy/TEx ~ 1 and TEz/TEx ~ 2 to 3 through all the field studies. The findings were consistent with the anisotropy reported above and with an earlier study in a tidal channel. The dissipation time scales were typically between 1 and 10 ms. They were basically independent of the tidal phase, tidal range and sampling elevation. Some observations of momentum mixing coefficients showed a large data scatter spreading over more than three orders of magnitudes, associated with relatively rapid variations with time. The results demonstrated that the assumption of "constant" mixing coefficient was simply untrue.
In addition, some unusual events are reported including some multiple flow reversal events at slack tides, probably induced by some form of resonance. A front was also observed once. The data suggested that the front passage was associated with some temperature anomaly next to the free-surface, and some secondary transverse circulation next to the bed.
A striking feature of the analysed data sets is the large fluctuations in all turbulence characteristics, including the momentum exchange coefficients, during the tidal cycles. This feature was rarely documented, but an important difference between the ADV data sets used in this study from earlier reported measurements is that the present data were collected continuously at high frequency for relatively long periods. 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 small subtropical estuaries. A new set of field deployments is planned to clarify the upper estuary dynamics in more details.