Turbulence characteristics of a small subtropical estuary during and after some moderate rainfall

Abstract

Keywords: turbulence small subtropical estuary covariances triple correlations field measurements acoustic Doppler velocimetry rainfall runoff a b s t r a c tIn natural estuaries, scalar diffusion and dispersion are driven by turbulence. In the present study, detailed turbulence measurements were conducted in a small subtropical estuary with semi-diurnal tides under neap tide conditions. Three acoustic Doppler velocimeters were installed mid-estuary at fixed locations close together. The units were sampled simultaneously and continuously at relatively high frequency for 50 h. The results illustrated the influence of tidal forcing in the small estuary, although lowfrequency longitudinal velocity oscillations were observed and believed to be induced by external resonance. The boundary shear stress data implied that the turbulent shear in the lower flow region was one order of magnitude larger than the boundary shear itself. The observation differed from turbulence data in a laboratory channel, but a key feature of natural estuary flow was the significant threedimensional effects associated with strong secondary currents including transverse shear events. The velocity covariances and triple correlations, as well as the backscatter intensity and covariances, were calculated for the entire field study. The covariances of the longitudinal velocity component showed some tidal trend, while the covariances 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 tides. The acoustic backscatter intensity data were characterised by large fluctuations during the entire study, with dimensionless fluctuation intensity I 0 b =I b between 0.46 and 0.54. 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.

Turbulence measurements in a small subtropical estuary under king tide conditions

“…The water surface texture highlighted some network of braided "smooth" channels as opposed to the rest of the river as previously observed by TREVETHAN et al (2008b) during some rainy periods.…”

Characteristics of flow fluctuations in a tide-dominated estuary: Application of triple decomposition technique

“…The streamwise velocity showed strong tidal incursion about low tide with the maximum velocities around 0.19 m/s and 0.16 m/s during the late ebb and early flood tides, respectively. Similar flood and ebb tide maxima were observed at various locations in Eprapah Creek [Trevethan et al, 2008a[Trevethan et al, , 2008bChanson et al, 2012]. The maximum velocity was on an average greater during the ebb than the flood tide.…”

Transient secondary currents behind a compression wave in an irregular channel

“…5B). This may be associated with the existence of streamwise vortices, as a result of momentum transfer from high velocity regions to low velocity regions, previously documented in laboratory and in the field (TAMBURRINO and GULLIVER 2007, TREVETHAN et al 2008, REUNGOAT et al 2016.…”