Hydrodynamic and sedimentary processes of tidal bores: Arcins Channel, Garonne River in August-September-October 2015

A tidal bore is a compressive wave of tidal origin, propagating upstream as the tidal flow turns to rising. It might be observed when a macro-tidal flood flow enters a funnel shaped river mouth with shallow waters. The occurrence of tidal bores has a significant impact on the natural systems, the bore propagation being associated with intense sediment scouring and suspension of bed materials. The tidal bore of the Garonne River was extensively investigated in the Arcins channel in 2010, 2012 and 2013, typically over one to two days. Herein new field measurements were repeated systematically at the same site on 29 August-1 September 2015 and on 27 October 2015. The nature of the observations was comprehensive, encompassing hydrodynamics and turbulence, sedimentology and transport. The aim of the study was to comprehend the temporal evolution of hydrodynamics and sediment processes in the Garonne River estuarine zone during a spring tide period. Instantaneous velocity measurements were performed continuously at high-frequency (200 Hz) prior to, during and after each afternoon tidal bore. Instantaneous sediment concentration and suspended sediment flux data were derived from careful calibration of acoustic backscatter and checked against water sample concentration. Between 29 August and 1 September, the sediment material characteristics showed some temporal trend: (a) the bed material granulometry data showed a progressively broader grain size distribution associated with some increase in median sediment size; and (b) the apparent yield stress and effective viscosity of bed materials increased over the first four days of tidal bore occurrence. The tidal bore occurrence had a marked effect on the velocity and suspended sediment field, including a rapid flow deceleration and flow reversal during the bore passage. The turbulent Reynolds stress data indicated large shear stresses, together with large and rapid fluctuations, during the bore passage and the early flood tide. A turbulent event analysis was further conducted in the highly-unsteady rapidly varied tidal bore flow. The suspended sediment concentration (SSC) data indicated a gradual increase in initial mean SSC estimate prior to the bore from 29 August to 1 September 2015, and very large SSCs during the passage of the tidal bore front and early flood tide. A comparison between suspended sediment flux data showed very significant suspended sediment flux on 29 August 2015, i.e. on the first day of tidal bore occurrence, with a decreasing magnitude over the next three days. The data suggested a two-stage bed scour process: at each tidal bore event, surface erosion occurred initially, in the form of stripping; the first stage was followed by delayed mass erosion, occurring about 5-15 minutes after the tidal bore. The sediment flux data yielded a mass transport of about 500 tonnes of sediments per second in the 70 m wide channel, in average for the first hour of flood tide. The present work culminates a 5-year research project at the same site, showing a progressive siltation of the channel, particularly during the last three years

Field Measurements in the tidal bore of the Garonne River after a recent flood

A tidal bore is a natural phenomenon associated with the rising flood tide. Composed of surface waves, it may occur in estuaries and propagate up rivers. The present study was conducted in the Garonne River (France) in the Arcins channel. Using an ADV unit and further recording equipments, experimental data are collected during a flat undular bore with a bore Froude number close to unity. Velocity analysis and sediment characterisation revel a slight rise in water elevation starting about 70 s prior to the front and a flow reversal about 50 s after the bore front. The turbulent transport of suspended sediment is presented in term of mass flux per unit area highlighting a negative (upriver) sediment mass transfer

Field Measurements of Unsteady Turbulence in a Tidal Bore: the Garonne River in October 2013

A tidal bore is an unsteady rapidly-varied open channel flow generated by the swift advance of the early flood tide in a funnel-shaped river estuary when the tidal range exceeds 4.5 to 6 m. This contribution presents a detailed field investigation conducted on the tidal bore of the Garonne River (France). The bore was undular and the bore's leading edge was followed by well-defined secondary waves, or whelps. The instantaneous ADV 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. The investigation characterized some unusual transient turbulence caused by the bore propagation in a large river system, and the results suggested the advection of large-scale eddies in the wake of the bore front. The present study highlighted the need for detailed field measurements with fine temporal resolution, to characterize the highly unsteady rapidly-varied nature of tidal bore flows

Sediment processes and flow reversal in the undular tidal bore of the Garonne River (France)

A tidal bore is a series of waves propagating upstream as the tidal flow turns to rising, and the bore front corresponds to the leading edge of the tidal wave in a funnel shaped estuarine zone with macro-tidal conditions. Some field observations were conducted in the tidal bore of the Garonne River on 7 June 2012 in the Arcins channel, a few weeks after a major flood. The tidal bore was a flat undular bore with a Froude number close to unity: Fr1 = 1.02 and 1.19 (morning and afternoon respectively). A key feature of the study was the simultaneous recording of the water elevation, instantaneous velocity components and suspended sediment concentration (SSC) estimates, together with a detailed characterisation of the sediment bed materials. The sediment was some silty material (d50 ≈ 13 μm) which exhibited some non-Newtonion thixotropic behaviour. The velocity and SSC estimate were recorded simultaneously at high frequency, enabling a quantitative estimate of the suspended sediment flux at the end of the ebb tide and during the early flood tide. The net sediment flux per unit area was directed upstream after the bore, and its magnitude was much larger than that at end of ebb tide. The field observations highlighted a number of unusual features on the morning of 7 June 2012. These included (a) a slight rise in water elevation starting about 70 s prior to the front, (b) a delayed flow reversal about 50 s after the bore front, (c) some large fluctuations in suspended sediment concentration (SSC) about 100 s after the bore front and (d) a transient water elevation lowering about 10 min after the bore front passage. The measurements of water temperature and salinity showed nearly identical results before and after the tidal bore, with no evidence of saline and thermal front during the study

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

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 800m 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. Copyright (C) 2015 John Wiley & Sons, Ltd

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

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)

Étude expérimentale de la turbulence de grille sur le transport sédimentaire

L’objectif de cette étude est double : caractériser le transport sédimentaire dans un écoulement turbulent homogène et isotrope afin de valider les modèles utilisés dans les simulations numériques sous des conditions d’écoulement contrôlées et stabilisées ; développer une méthodologie de mesures transposables à un canal long (10 m) dans le cas d’un écoulement turbulent à surface libre généré par une onde de propagation représentant les conditions d’un mascaret. L’écoulement est généré dans une veine hydraulique rectangulaire de 1.5 m de long de section carrée (10 cm de côté). La turbulence est développée par une grille à mailles carrées (maille de 6.6 mm) choisie de manière à obtenir une faible décroissance spatiale de l’intensité turbulente soit environ 4-5% sur la majeure partie de la veine. Dans une première partie, une analyse sans particules est faite sur la base de mesures LDV (Laser Doppler Velocimetry) afin de qualifier l’écoulement de transport. Le taux de dissipation d’énergie turbulente est estimé et les différentes échelles de la turbulence sont analysées. Le mouvement des particules injectées (PMMA) est ensuite quantifié par des mesures PIV (Particle Image Velocimetry) pour différentes concentrations. Une technique de mesures par fluorescence permet de distinguer la contribution de chaque taille de particule injectée. Une analyse en termes de flux de concentration, champs de vitesses et structure de l’écoulement lors du transport sédimentaire de deux tailles caractéristiques de particules (20 et 200 µm) est présentée. Les résultats sont comparés avec des données numériques

Etude expérimentale de la sédimentation de suspensions polydisperses dans un tube

Les études expérimentales disponibles sur la sédimentation de particules sont restreintes aux suspensions mono et bidisperses à de faibles taux de charges. L'étude proposée concerne la sédimentation de particules polydisperses dans un tube (phi>30%). Les déplacements et les vitesses de particules sont obtenus par une technique de «fluorescence Laser». L'analyse des cellules de convection, des profils de concentration et des vitesses de fronts permet de discuter de l'impact de la polydispersité sur le processus de sédimentation