Inception of bed load motion beneath a bore

Khezri, Nazanin; Chanson, Hubert

doi:10.1016/j.geomorph.2012.02.006

Cited by 24 publications

(28 citation statements)

References 10 publications

(10 reference statements)

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“…Once the breaking roller formed, some air entrainment took place in the bore front and downstream the surge inducing a chaotic flow motion (Figure IV.14b). Similar flow patterns were observed in earlier studies using similar configurations with variation of the Froude number corresponding in the transition of undular bores to breaking bores Khezri and Chanson, 2012a;Koch and Chanson, 2009) or for propagation of dam break waves (Hornung et al, 1995). In rivers and estuaries, undular and breaking bores are observed for similar Froude numbers than the bore studied in laboratory (Chanson, 2011a, and Table II.1).…”

Section: Iv31 Description Of the Flow Free Surfacesupporting

confidence: 85%

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

Simon

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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) ii 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 comp...

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Section: Iv31 Description Of the Flow Free Surfacesupporting

confidence: 85%

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

Simon

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“…; Chanson ; Khezri and Chanson, ). One study on sediment scour used clear‐water with a live bed to study the scour inception process (Khezri and Chanson, ). Data from field studies have demonstrated the massive impact of tidal bores on sedimentary processes, e.g.…”

Section: Introductionmentioning

confidence: 99%

“…Laboratory studies of tidal bores carried out to date have been conducted without suspended sediments, however recent results highlight their importance to erosional processes (Macdonald et al 2009;Chanson 2011b;Khezri and Chanson, 2012a). One study on sediment scour used clear-water with a live bed to study the scour inception process (Khezri and Chanson, 2012b). Data from field studies have demonstrated the massive impact of tidal bores on sedimentary processes, e.g.…”

Section: Introductionmentioning

confidence: 99%

Fluid flow and sediment entrainment in the Garonne River bore and tidal bore collision

Keevil

Chanson

Reungoat

2015

Earth Surf Processes Landf

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Abstract:A detailed field study was carried out on a tidal bore to document the turbulent processes and sediment entrainment which occurred. The measured bore, within the Arcins Channel of the Garonne River (France), was undular in nature and was followed by well-defined secondary wave motion. Due to the local river geometry a collision between the Arcins channel tidal bore and the bore which formed within the main Garonne River channel was observed about 800 m upstream of the sampling site. This bore collision generated a transient standing wave with a black water mixing zone. Following this collision the bore from the main Garonne River channel propagated 'backward' to the downstream end of the Arcins channel. Velocity measurements with a fine temporal resolution were complemented by measurements of the sediment concentration and river level. The instantaneous velocity data indicated large and rapid fluctuations of all velocity components during the tidal bore. Large Reynolds shear stresses were observed during and after the tidal bore passage, including during the 'backward' bore propagation. Large suspended sediment concentration estimates were recorded and the suspended sediment flux data showed some substantial sediment motion, consistent with the murky appearance of the flood tide waters.

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“…For sediment transport scenarios where the fluid accelerations (∼ the sreamwise pressure gradients) are more important than drag, like coarse sediment under bore fronts, as observed by Khezri and Chanson (2012), Sleath (1999) suggested the acceleration parameter…”

Section: Sleath's Acceleration Parametermentioning

confidence: 97%

Basic Coastal Sediment Transport Mechanisms

Nielsen¹

2015

International Compendium of Coastal Engineering

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Inception of bed load motion beneath a bore

Cited by 24 publications

References 10 publications

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

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

Fluid flow and sediment entrainment in the Garonne River bore and tidal bore collision

Basic Coastal Sediment Transport Mechanisms

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