Turbulence and turbulent flux events in a small estuary

Relatively little systematic research has been conducted on the turbulence characteristics of small estuaries. In the present study, detailed measurements were conducted in a small subtropical estuary with a focus on turbulent flux events. Acoustic Doppler velocimeters were installed in the mid-estuary at fixed locations and sampled simultaneously and continuously for 50 h. A turbulent flux event analysis was performed for the entire data sets extending the technique of Narasimha et al. (Phil Trans R Soc Ser A 365:841-858, 2007) to the unsteady open channel flow motion and to turbulent sub-events. Turbulent bursting events were defined in terms of the instantaneous turbulent flux. The data showed close results for all ADV units. The very-large majority of turbulent events lasted between 0.04 and 0.3 s with an average of 1 to 4 turbulent events observed per second. A number of turbulent bursting events 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 basic differences between high- and low-water periods. A comparison between the present estuary data and the atmospheric boundary layer results of Narasimha et al. (Phil Trans R Soc Ser A 365:841-858, 2007) showed a number of similarities and demonstrated the significance of turbulent events in environmental flows. A burstiness index of 0.85 was found for the present data

Turbulence in Small Sub-tropical Estuary with Semi-Diurnal Tides

In natural estuaries, contaminant transport is driven by the turbulent momentum mixing. The predictions of scalar dispersion can rarely be predicted accurately because of a lack of fundamental understanding of the turbulence structure in estuaries. Herein detailed turbulence field measurements were conducted at high frequency and continuously for up to 50 hours per investigation in a small subtropical estuary with semi-diurnal tides. Acoustic Doppler velocimetry was deemed the most appropriate measurement technique for such small estuarine systems with shallow water depths (less than 0.5 m at low tides), and a thorough post-processing technique was applied. The estuarine flow is always a fluctuating process. The bulk flow parameters fluctuated with periods comparable to tidal cycles and other large-scale processes. But turbulence properties depended upon the instantaneous local flow properties. They were little affected by the flow history, but their structure and temporal variability were influenced by a variety of mechanisms. This resulted in behaviour which deviated from that for equilibrium turbulent boundary layer induced by velocity shear only. A striking feature of the data sets is the large fluctuations in all turbulence characteristics during the tidal cycle. This feature was rarely documented, but an important difference between the data sets used in this study from earlier reported measurements is that the present data were collected continuously at high frequency during relatively long periods. The findings bring new lights in the fluctuating nature of momentum exchange coefficients and integral time and length scales. These turbulent properties should not be assumed constant

Turbulence, turbulent mixing and diffusion in shallow-water estuaries

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

Continuous high-frequency turbulence and suspended sediment concentration measurements in an upper estuary

The present study details new turbulence field measurements conducted continuously at high frequency for 50 hours in the upper zone of a small subtropical estuary with semi-diurnal tides. Acoustic Doppler velocimetry was used, and the signal was post-processed thoroughly. The suspended sediment concentration wad further deduced from the acoustic backscatter intensity. The field data set demonstrated some unique flow features of the upstream estuarine zone, including some low-frequency longitudinal oscillations induced by internal and external resonance. A striking feature of the data set is the large fluctuations in all turbulence properties and suspended sediment concentration during the tidal cycle. This feature has been rarely documented

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

High-Frequency Turbulence and Sediment Flux Measurements in an Upper Estuarine Zone

In natural estuaries, the predictions of scalar dispersion are rarely predicted accurately because of a lack of fundamental understanding of the turbulence structure in estuaries. Herein detailed turbulence field measurements were conducted continuously at high frequency for 50 hours in the upper zone of a small subtropical estuary with semi-diurnal tides. Acoustic Doppler velocimetry was deemed the most appropriate measurement technique for such shallow water depths (less than 0.4 m at low tides), and a thorough post-processing technique was applied. In addition, some experiments were conducted in laboratory under controlled conditions using water and soil samples collected in the estuary to test the relationship between acoustic backscatter strength and suspended sediment load. A striking feature of the field data set was the large fluctuations in all turbulence characteristics during the tidal cycle, including the suspended sediment flux. This feature was rarely documented

Using Turbidity and Acoustic Backscatter Intensity as Surrogate Measures of Suspended Sediment Concentration. Application to a Sub-Tropical Estuary (Eprapah Creek)

A key element in stream monitoring is the choice of a measuring technique of suspended sediment concentration (SSC). Several studies suggested that turbidity and acoustic Doppler backscattering may be suitable surrogate measures for SSC. A series of new experiments were conducted in laboratory under controlled conditions using water and soil samples collected in a small sub-tropical estuary of Eastern Australia. The tests were conducted with a microADV (16 MHz) system and a YSI6600 probe using two types of sediment material : some fine mud collected on the bed and some slightly coarser material collected on the bank slope. In addition, some experiments were repeated with the creek estuarine waters and with Brisbane tap waters. The best fit relationships were established in terms of the suspended sediment concentration (SSC) as a function of the acoustic backscatter intensity (BSI), the SSC as a function of the turbidity, and the turbidity as a function of the acoustic backscatter intensity. The present results confirmed earlier findings that the relationships presented some monotonic increase. The calibration curves were however affected by the sediment material characteristics and by the water quality. The results indicated that the calibration of an acoustic Doppler system must be performed with the waters of the natural system (creek waters) and with some bed material. Importantly the calibration of an ADV system is specific to the unit itself. Its calibration relationships are functions of the water quality and sediment properties, but also of the intrinsic characteristics of the emitter and receivers. A limited comparison between an ADV (10 MHz) and a microADV (16 MHz) indicated that the newer microADV system could detect significantly more counts per unit volume than the older unit. The results were applied to some earlier field measurements conducted continuously at high frequency for 50 hours each in Eprapah Creek with the same microADV system. For each field study, the instantaneous suspended sediment flux per unit area data showed some high-frequency bursts that were believed to be linked to some turbulent bursting phenomena next to the bed. For each tidal cycle, the suspended sediment flux data were integrated with respect of time. The results yielded a net sediment mass transfer per unit area of about -20 kg/m2 per tidal cycle during the first study conducted mid-estuary and of about -4 kg/m2 per tidal cycle for the second study performed in the upper estuary. That is, the net sediment flux over a full tidal cycle was upstream in average, and the finding was consistent with earlier studies in sub-tropical rivers during dry conditions for a similar tidal range. It must be stressed that the present work highlighted a number of limitations. The present calibration relationships might not be suitable for earlier field studies at Eprapah Creek with different water quality conditions. The calibration curves were also specific to the microADV unit at the time of the tests, and they were developed for a subtropical estuary with relatively low turbidity levels

Turbulent measurements in a small subtropical estuary with semi-diurnal tides

Since predictions of scalar dispersion in small estuaries can rarely be predicted accurately, new field measurements were conducted continuously at relatively high frequency for up to 50 hours (per investigation) in a small subtropical estuary with semi-diurnal tides. The bulk flow parameters varied in time with periods comparable to tidal cycles and other large-scale processes. The turbulence properties depended upon the instantaneous local flow properties. They were little affected by the flow history, but their structure and temporal variability were influenced by a variety of parameters including the tidal conditions and bathymetry. A striking feature of the data sets was the large fluctuations in all turbulence characteristics during the tidal cycle, and basic differences between neap and spring tide turbulence

Using turbidity and acoustic backscatter intensity as surrogate measures of suspended sediment concentration in a small subtropical estuary

The suspended sediment concentration is a key element in stream monitoring, although the turbidity and acoustic Doppler backscattering may be suitable surrogate measures. Herein a series of new experiments were conducted in laboratory under controlled conditions using water and mud samples collected in a small subtropical estuary of Eastern Australia. The relationship between suspended sediment concentration and turbidity exhibited a a linear relationship, while the relationships between suspended sediment concentration and acoustic backscatter intensity showed a monotonic increase. The calibration curves were affected by both sediment material characteristics and water quality properties, implying that the calibration of an acoustic Doppler system must be performed with the waters and soil materials of the natural system. The results were applied to some field studies in the estuary during which the acoustic Doppler velocimeter was sampled continuously at high-frequency. The data yielded the instantaneous suspended sediment flux per unit area in the estuarine zone. They showed some significant fluctuations in instantaneous suspended mass flux, with a net upstream suspended mass flux during flood tide and net downstream sediment flux during ebb tide. For each tidal cycle, the integration of the suspended sediment flux per unit area data with respect of time yielded some net upstream sediment flux in average