A Fundamental Study of Turbulence and Turbulent Mixing in a Small Subtropical Estuary

The flow of water in natural systems such as estuaries is a turbulent process, with Reynolds numbers greater than 1E+5. Understanding the turbulence properties of an estuary is important for the investigation of mixing, dispersion and sediment transport within an estuary. This study investigates the turbulence properties in a typical small subtropical estuary (Eprapah Creek, Australia). Small coastal plain type estuaries constitute approximately 60 % of all estuaries in Australia, yet no thorough study of the turbulence properties of this type of estuary has been conducted. To date only a limited number of turbulence studies in estuaries have been undertaken. One reason for this was the lack of appropriate instrumentation to collect turbulent velocity measurements with fine spatial and temporal resolutions. This is especially true of turbulence studies in small estuaries, with the majority of published turbulence studies being performed in relatively large systems. Previous studies of turbulence in estuaries were mostly conducted for relatively short periods (up to 6 hours) and by collecting data over long periods in bursts of several minutes. The present study showed that acoustic Doppler velocimetry (ADV) was well suited for the measurement of turbulence properties in small estuarine systems. A unique aspect of this study was the continuous collection of turbulence data at high frequency (fscane ≥ 25 Hz) for relatively long periods (Tstudy up to 50 hours). This approach characterised the estuarine turbulence properties for up to two complete tidal cycles. The data analysis showed that continuous collection of high frequency turbulence data was essential to accurately characterise the rapid fluctuations of turbulence in an estuary. The field data showed that the turbulence flow properties were highly fluctuating in a small estuary. All turbulence properties exhibited large and rapid fluctuations over the investigation period of each field study. The variations in time scales were related to both the instantaneous local flow properties and the tidal fluctuations. Some long period oscillations observed in the water level and velocity data also had a significant impact on the turbulence properties. These long period oscillations had periods similar to resonance periods generated both internally in and externally to the creek. Many turbulence properties showed an asymmetrical response to the tidal forcing, especially under spring tidal conditions. Large turbulent velocity fluctuations were, however, observed throughout all investigation periods, including during the slack tides. Substantial fluctuations in the normal and tangential Reynolds stresses were observed in the middle and upper estuarine zones. The turbulent velocity data showed some non-Gaussian behaviour and the Reynolds stresses were non-Gaussian throughout all investigation periods. Field data collected mid estuary under spring and neap tidal conditions showed two distinctly different turbulence responses for spring and neap tides. During spring tides, the magnitudes of all turbulence properties were up to an order of magnitude larger than for neap tides. The turbulence properties showed some increased tidal asymmetries under spring tidal conditions. Further two field studies were conducted under similar neap tidal conditions in the middle and upper estuarine zones. A comparison of these two data sets showed that the turbulence properties in the middle and upper estuaries differed substantially. Mid estuary, the magnitude of the turbulence properties were up to an order of magnitude larger than those observed in the upper estuary, thereby indicating that at least three distinct sets of turbulence properties existed in this small subtropical estuary. This study included some turbulence data collected in a large tidal lake with a restricted entrance (Hamana Lake, Japan). This turbulence data was compared with the data measured in Eprapah Creek. The comparison showed similar tidal patterns in some turbulence properties were observed close to the bed despite the distinct topography and hydrodynamics of the two estuaries. Some of these patterns in turbulence properties were also observed in previous studies of turbulence in estuaries. However, the spring tide data collected in the small estuary showed an increased magnitude of turbulence and a more asymmetrical response than other estuarine turbulence studies. The ratio of local tidal amplitude and local mean depth was used to characterise the local turbulence properties for a certain tidal range. A critical value of the ratio a1/h1 was 0.5, corresponding to the local tidal range being equal to the local mean depth. If the tidal range was greater than the local mean depth (i.e. a1/h1 > 0.5), a more asymmetrical tidal response and some increased turbulence property magnitudes were observed. A comparison of the turbulence properties in two distinct tidal systems (Eprapah Creek and Hamana Lake) showed similar tidal patterns for a1/h1 < 0.5