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.