Characterisation of unsteady turbulence in breaking tidal bores including the effects of bed roughness

Docherty, Nicholas J. and Chanson, Hubert (2010) Characterisation of unsteady turbulence in breaking tidal bores including the effects of bed roughness Brisbane, Australia: School of Civil Engineering, The University of Queensland

Attached Files (Some files may be inaccessible until you login with your UQ eSpace credentials)
Name Description MIMEType Size Downloads
ch7610.pdf Full-text application/pdf 6.93MB 0
Author Docherty, Nicholas J.
Chanson, Hubert
Title of report Characterisation of unsteady turbulence in breaking tidal bores including the effects of bed roughness
Parent publication Hydraulic Model Report CH
Editor Hubert Chanson
Publication date 2010
ISBN 9781864999884
Year available 2010
Open Access Status File (Publisher version)
Publisher School of Civil Engineering, The University of Queensland
Place of publication Brisbane, Australia
Start page 1
End page 112
Total pages 112
Collection year 2010
Language eng
Subjects 09 Engineering
0905 Civil Engineering
090509 Water Resources Engineering
Abstract/Summary A tidal bore is an unsteady flow motion generated by the rapid water level rise at the river mouth during the early flood tide. With time, the leading edge of the tidal wave becomes steeper and steeper until it forms a wall of water that is the tidal bore. Herein the turbulence in breaking tidal bores was investigated experimentally under controlled conditions with two types of bed roughness: smooth PVC and fixed gravel bed (ks = 3.4 mm). Some qualitative observations were conducted with both undular and breaking bores. The tidal bore flow patterns were independent of the bed roughness, as well as of the initial steady flow conditions, while the free-surface properties were close to earlier findings. Using an ensemble-averaging technique, the free-surface fluctuations of breaking bores were characterised. Immediately prior to the roller, the free-surface curved gradually upwards and the gentle surface elevation rise was about 0.1×do where do is the initial water depth. The passage of the bore roller was associated with some large water depth fluctuations. Some detailed turbulent velocity measurements were performed at several vertical elevations during and shortly after the breaking bore passage. Both the instantaneous and ensemble-averaged velocity data highlighted some seminal features of breaking bores. Namely a strong flow deceleration was observed at all elevations during the tidal bore passage. Close to the bed, the longitudinal velocity component became negative immediately after the roller passage implying the existence of a transient recirculation "bubble". The vertical velocity data presented some positive, upward motion during the bore front passage with increasing maximum vertical velocity with increasing distance from the bed. The vertical motion was believed to be linked with some streamline curvature. The transverse velocity data presented some large fluctuations with a non-zero ensemble-average after the roller passage that highlighted some intense secondary motion advected behind the bore front. A comparison between ensemble-average (EA) and variable interval time average (VITA) velocity data was performed. The EA and VITA results showed some comparable velocity pattern with some relatively-long-term data trend superposed to some high-frequency turbulent fluctuations. The data showed however that the VITA calculations for a single experiment presented some non-negligible difference with the EA median value for all velocity components. Overall the study demonstrated the intensive turbulence and turbulent mixing under a breaking tidal bore.
Keyword Tidal bores
Breaking bore
Unsteady turbulence
Turbulent mixing
Acoustic doppler velocimetry (ADV)
Signal processing
Ensemble average (EA)
Variable interval time average (VITA)
Fixed gravel bed roughness
Turbulent shear stresses
Q-Index Status Provisional Code
Additional Notes The full bibliographic details of the peer-reviewed research report is: DOCHERTY, N.J., and CHANSON, H. (2010). "Characterisation of Unsteady Turbulence in Breaking Tidal Bores including the Effects of Bed Roughness." Hydraulic Model Report No. CH76/10, School of Civil Engineering, The University of Queensland, Brisbane, Australia, 112 pages (ISBN 9781864999884).

Document type: Research Report
Collection: School of Civil Engineering Publications
 
Versions
Version Filter Type
Citation counts: Google Scholar Search Google Scholar
Created: Fri, 28 May 2010, 04:37:01 EST by Hubert Chanson on behalf of School of Civil Engineering