Turbulence and sediment processes in The tidal bore of the garonne river: First observations

Chanson, Hubert, Lubin, Pierre, Simon, Bruno and Reungoat, David (2010) Turbulence and sediment processes in The tidal bore of the garonne river: First observations. Hydraulic Model Reports CH 79/10, School of Civil Engineering, The University of Queensland.

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Author Chanson, Hubert
Lubin, Pierre
Simon, Bruno
Reungoat, David
Title Turbulence and sediment processes in The tidal bore of the garonne river: First observations
School, Department or Centre School of Civil Engineering
Institution The University of Queensland
Report Number CH 79/10
Series Hydraulic Model Reports
Publication date 2010-10
Start page 1
End page 99
Total pages 99
Publisher School of Civil Engineering, The University of Queensland
Editor The University of Queensland, St Lucia QLD
Language eng
Subject 0905 Civil Engineering
020303 Fluid Physics
0907 Environmental Engineering
Abstract/Summary A tidal bore is a series of waves propagating upstream as the tidal flow turns to rising, forming during spring tide conditions when the tidal range exceeds 4 to 6 m and the flood tide is confined to a narrow funnelled estuary. After the formation of the bore, there is an abrupt rise in water depth at the bore front that is discontinuity in the water depth, and pressure and velocity fields. To date, the field observations of tidal bores are very limited, and most studies were conducted with a verycoarse resolution in terms of temporal and spatial scales: it is challenging to analyse conclusively these data. In the present study, some detailed turbulence field measurements were conducted continuously at high-frequency (64 Hz) in the tidal bore of the Garonne River in September 2010. The turbulent velocity components were sampled with an acoustic Doppler velocimeter (ADV) with its sampling volume located 0.8 m beneath the free-surface. The tidal bore propagation in the Garonne River was observed on both 10 and 11 Sept. 2010. The tidal bore was undular as it passed in front of the sampling site. The passage of the tidal bore was characterised by a pseudo-chaotic wave motion lasting for several minutes after the bore. At the sampling location, the free-surface elevation rose very rapidly. The tidal bore Froude number was estimated from the channel bathymetry and tidal bore observations: it was equal to 1.30 and 1.20 on 10 and 11 Sept. 2010 respectively. The turbulent velocity data showed the marked impact of the tidal bore propagation. The longitudinal velocity component highlighted some rapid flow deceleration during the passage of the tidal bore, associated with a sudden rise in the free surface elevation, and a flow reversal after the tidal bore front passage. The observations were consistent with some earlier field and laboratory results. The tidal bore passage was further characterised by some large fluctuations of all three turbulent velocity components. The Reynolds stress data indicated some large and rapid turbulent stress fluctuations during the tidal bore and flood flow. The Reynolds stress magnitudes were significantly larger than during the ebb tide, and some substantial normal and tangential stress fluctuations were observed. The ADV backscatter intensity was calibrated in terms of the suspended sediment concentration in laboratory using the soft mud bed material. The results provided an unique characterisation of the turbulence and sediment flux beneath to the free-surface during the tidal bore. The arrival of the tidal bore was characterised by a rapid reversal in suspended sediment flux. Prior the tidal bore, the net sediment mass transfer per area was positive downstream. After the passage of the bore, the net sediment mass transfer per unit area was negative and its magnitude was 30 times larger than the ebb tide net flux. A striking feature of the present field data set was the large and rapid fluctuations in turbulent velocities and suspended sediment flux during the tidal bore and flood flow. This was not documented to date, but an important difference between the present ADV data set from earlier reported field measurements was that the present data were collected continuously at relatively high frequency (64 Hz) during a relatively long period (at least 2 hours).
Keyword Tidal bore
Garonne River
Field measurements
Turbulence
Turbulent mixing
Suspended sediment concentration
Sediment processes
Acoustic Doppler velocimetry
Additional Notes ISBN: 9781742720104

Document type: Department Technical Report
Collection: School of Civil Engineering Publications
 
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Created: Wed, 03 Nov 2010, 14:23:42 EST by Ms Lynette Adams on behalf of School of Engineering