Effects of tidal bores on turbulent mixing: a numerical and physical study in positive surges

Simon, Bruno (2014). Effects of tidal bores on turbulent mixing: a numerical and physical study in positive surges PhD Thesis, School of Civil Engineering, The University of Queensland. doi:10.14264/uql.2014.19

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Author Simon, Bruno
Thesis Title Effects of tidal bores on turbulent mixing: a numerical and physical study in positive surges
School, Centre or Institute School of Civil Engineering
Institution The University of Queensland
DOI 10.14264/uql.2014.19
Publication date 2014-04-06
Thesis type PhD Thesis
Open Access Status Other
Supervisor Pierre Lubin
Hubert Chanson
Total pages 259
Total colour pages 134
Total black and white pages 125
Language eng
Subjects 0406 Physical Geography and Environmental Geoscience
0905 Civil Engineering
Formatted abstract
Tidal bores are positive surges created by the flooding of tides inside estuaries and rivers. They propagate upstream in an estuary sometimes as several metre high waves. Tidal bores are fascinating phenomenon with some impact on the wildlife, the mixing and re-suspension of sediments, and river constructions. Field studies remain challenging due to the violent nature of some bores. Herein, the hydrodynamics of tidal bores was investigated in a complementary study with experimental measurements and numerical simulations of idealised models of tidal bores. While laboratory experiments offer an accurate way to measure the flow properties, numerical simulations provide rather detailed information on the hydrodynamics of bores. These simulations require detailed experimental data to accurately replicate the experiments and validate the numerical methods.

Investigations were conducted at the University of Queensland in a large rectangular channel partially covered by a fixed gravel bed. The steady flow condition properties were investigated using a Pitot tube and acoustic Doppler velocimeters (ADVs). The unsteady flow conditions were obtained by the fast closure of a downstream gate, which led to an upstream propagating bore. Free surface characteristics were studied for several types of bores, from undular to breaking. Those measurements showed large free surface fluctuations during the bore passage in agreement with earlier findings. Velocity measurements were conducted at high frequency (200 Hz). In breaking bores, the ensemble averaged data exhibited large velocity reversals near the bed beneath the bore front and in its wake, whereas, in undular bores, a lesser deceleration was observed. The vertical velocity data trend closely followed the water depth time derivative for both breaking and undular bores. Overall, the velocity measurements displayed large velocity fluctuations during the passage of the bore which were believed to induce strong turbulent mixing. The turbulence was further investigated by a two point correlation technique to estimate both turbulent integral time and length scales. The turbulent integral length scales values were similar in the steady and unsteady flows, whereas the turbulent time scales were larger in the unsteady flow. The experiments were limited by the instrumentation since the ADV systems interacted with each other and lacked accuracy for measurements performed in the near vicinity of physical boundaries.

A numerical investigation was then performed using the computational fluid dynamics (CFD) code Thétis developed in the I2M laboratory of the Université de Bordeaux. Numerical simulations reproduced experiments performed in laboratory prior to this study (Chanson, 2010a,b). The fluid was modelled by the Navier-Stokes equations in their incompressible two-phase form. The turbulence model used a large eddy simulation and the interface air-water was tracked by using a volume of fluid model. Two dimensional and three dimensional simulations were performed to study the complete flow evolution beneath undular bores for different bore configurations. Two and three dimensional simulations were used to validate the capacity of the CFD code. The analysis of the two dimensional simulations confirmed some previous experimental findings, detailing data unreachable by ADV units and showed the strong effects of the bore on the flow velocity, including intense flow reversals very close to the bed. Three dimensional simulations were conducted with and without initial steady turbulent conditions. A synthetic eddy method (Jarrin, 2008) was implemented to recreate realistic turbulent steady flow conditions. The different effects of turbulent and non-turbulent flows were analysed with a particular focus on the lateral walls and channel bed effects. The simulations showed a strong flow reversal very close to the bed and the lateral walls, inducing a strong shear and turbulence generation.

The findings of both experimental and numerical studies offered accurate and complementary methods to quantify information on the flow evolution and to analyse the flow structure during positive surge propagation. The strong shear next to the wall would induce some scouring in rivers with a movable bed, associated with sediment re-suspension. These results are a step toward a better understanding of tidal bores and could be used to explain pollutant dispersion in rivers affected by tidal bores. A better understanding of the bore hydrodynamics would improve the management of estuarine systems. For example, dredging operations or river constructions (pontoon, dyke or bridge) could be performed at selected sites to ensure the safety of navigation and infrastructure as well as to preserve the phenomenon, unlike the Seine River bore which nearly disappeared after estuarine settlements. These findings might also help with the preservation of wildlife for which feeding and breeding depend on tidal bore turbulent mixing.
Keyword Tidal bore
positive surge
physical modelling
numerical simulation
unsteady open channel flow
coherent structures
large eddy simulation
two-point correlation

Document type: Thesis
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Created: Thu, 03 Apr 2014, 20:46:39 EST by Bruno Simon on behalf of University of Queensland Graduate School