Claude, Nicolas

Magnier, Benjamin

Rodrigues, Stéphane

Tassi, Pablo

Villaret, Catherine

English

Hydraulic Engineering Repository

Conference Paper, Published VersionMagnier, Benjamin; Claude, Nicolas; Villaret, Catherine; Rodrigues,Stéphane; Tassi, PabloNumerical Simulation of Flow Structures in the Presence ofAlternate and Transverse Bars: Application to the LoireRiver (France)Zur Verfügung gestellt in Kooperation mit/Provided in Cooperation with:TELEMAC-MASCARET Core GroupVerfügbar unter/Available at: https://hdl.handle.net/20.500.11970/100424Vorgeschlagene Zitierweise/Suggested citation:Magnier, Benjamin; Claude, Nicolas; Villaret, Catherine; Rodrigues, Stéphane; Tassi, Pablo(2013): Numerical Simulation of Flow Structures in the Presence of Alternate and TransverseBars: Application to the Loire River (France). In: Kopmann, Rebekka; Goll, Annalena (Hg.):XXth TELEMAC-MASCARET. User Conference 2013. Karlsruhe: Bundesanstalt fürWasserbau. S. 31-36.Standardnutzungsbedingungen/Terms of Use:Die Dokumente in HENRY stehen unter der Creative Commons Lizenz CC BY 4.0, sofern keine abweichendenNutzungsbedingungen getroffen wurden. Damit ist sowohl die kommerzielle Nutzung als auch das Teilen, dieWeiterbearbeitung und Speicherung erlaubt. Das Verwenden und das Bearbeiten stehen unter der Bedingung derNamensnennung. Im Einzelfall kann eine restriktivere Lizenz gelten; dann gelten abweichend von den obigenNutzungsbedingungen die in der dort genannten Lizenz gewährten Nutzungsrechte.Documents in HENRY are made available under the Creative Commons License CC BY 4.0, if no other license isapplicable. Under CC BY 4.0 commercial use and sharing, remixing, transforming, and building upon the materialof the work is permitted. In some cases a different, more restrictive license may apply; if applicable the terms ofthe restrictive license will be binding.Numerical simulation of flow structures in the presence of alternate and transverse bars:  application to the Loire river (France)  B. Magnier1,2, N. Claude2, 3 , C. Villaret2 , S. Rodrigues4 and P. Tassi2 1 Institut de Physique du Globe de Paris Paris France 2 EDF R&D - Laboratoire Saint Venant Chatou, France   3 Ecole des Nationale des Ponts et Chaussées Champs sur Marne, France 4 E.A 6293 GéHCO, Université François Rabelais Tours, France   Abstract—In this study, the module TELEMAC-3D is used to analyse the flow behaviour in the presence of alternate and transverse bars. This particular bar organization has been poorly studied in the literature and presents great interest for the planing of dredging operations in rivers like the Loire in France. Three-dimensional numerical results showed that the main flow structure is defined by the channel planform and bars configurations. The roughness-length distance strongly influences the velocity near the bed and the increasing of the mesh resolution impacts the bed resistance mechanisms and, in consequence, the distribution of the velocity profiles.  I. INTRODUCTION Bars are ubiquitous morphological macroforms that interact with the flow and sediment transport processes in rivers with important bedload transport rates [2]. Alternate bars, whose structure is characterized by a sequence of steep consecutive diagonal fronts and deep pools at the downstream face, have been extensively investigated both theoretically and through flume studies, e.g. [1, 2, 3, 4]. More complicated macroform patterns, such as transverse, central or multiple configurations can be found in wider reaches, e.g. [5, 6]. Bars can be classified as forced or free. Forced bars arise from a forcing effect that can have an anthropic or natural origin, such as channel curvature, width variations or the presence of confluence/diffluence zones in the channel. Free bars, on the other hand, can arise from an inherent instability of the erodible bottom subject to a turbulent flow [5]. Further details can be found in the companion paper presented in this conference [7]. Depending on the forcing mechanism, bar patterns show different configurations. Forced bars that are associated with a flow in a variable-width channel appear as mid-channel bars or transverse bar, or as two symmetrical lateral bars [8]. The formation of central or transverse bars could be associated to an antisymmetric forcing such as a channel curvature, while alternate bars can appear as a response to instability in the channel bed or by the presence of a repetitive sequence of width variations.  Furthermore, Wu et al. [8] pointed out that the presence of free bars in a “forced bar” configuration could give rise to a “mixed” organization (free + forced), arranged in alternate or transversal configurations. Coexisting free and forced bars have been extensively studied in meandering channels. In contrast, the presence of mixed bars in widening/narrowing channels has received little attention [9]. As highlighted by Claude et al. [10], the site of Bréhémont in the middle reach of the Loire river in France presents forcing effects caused by channel width variations that induce the presence of alternate and transverse bars over short periods of time, even in a slow hydrodynamic context. To the best of our knowledge, this particular bar organization has been poorly studied in the literature.  In this work, the three-dimensional (3D) hydrodynamic models TELEMAC-3D is calibrated for the site of Bréhémont on the Loire River from a large dataset of high-quality field surveys based on Acoustic Doppler Current Profiler (ADCP) measurements. The validated model is then used to analyze the flow behaviour in the presence of alternate and transverse bars. The plan form and variable bed topography (obtained from multibeam echosoundings) present a complex hydrodynamic behavior in which a number of physical processes are investigated, such as the impact of roughness-length treatment due to “sub-grid” or non-resolved effects of bed roughness [11].  II. SITE AND FIELD MEASUREMENTS The Loire River, the largest river in France, is 1,020 km long and drains a catchment area of 117,000 km². The study site is located at the village of Bréhémont (47°17’43.31’’N, 0°20’33.80’’E) in the middle reaches of the Loire River. At the study site, the Loire system presents a multiple-channel pattern. Bed materials are constituted by sands and gravels. The main channel is composed of an expansion zone followed by a contraction area occupied by migrating bars (Fig. 1). These macroforms can adopt successively an alternate or a transverse configuration (Fig. 1) [9]. The channel width varies between 175 and 300 m.  TUC 2013, 16-18/10/2013, Karlsruhe - Kopmann, Goll (eds.) - © 2013 Bundesanstalt für Wasserbau ISBN 978-3-939230-07-631XXth TELEMAC-MASCARET User Conference Karlsruhe, October 16–18, 2013    and, like for the hydraulic reversal theory (velocity shear stress), could affect the formation of pools and riffles [13].  Furthermore, for a water discharge and a bar configuration, the vertical velocity profiles obtained from the models are in relative agreement with the field measurements according to their location (thalweg or bar) (Fig. 3a, 3c and 5). These results may indicate a spatial variability of the bed roughness. Thus, the bars and the thalweg could be associated to different roughness-lengths in order to improve the hydrodynamic calibration. This observation is supported by the results of a study on the dunes dynamic on the site [9] which show that the mesoforms adopt different sizes on the thalweg and on the bars. Using several bed roughness on the domain is probably more pertinent. No relationship seems to link the bed roughness coefficients obtained after the calibration and the water discharge (Table I). This illustrates the complexity of the interactions between hydrodynamic and bed roughness (characterized during floods by complex hysteresis [14]), which depends strongly to the dunes dynamic (in sand-bed rivers as the Loire). The present study highlights this point by showing the influence of the small dunes on the bed roughness and the flows. Indeed, only the dunes longer than 10 m have been integrated in the spatial discretization. It is admitted that the influence of the bedforms smaller than the elements of the meshes is parameterized through the roughness length coefficient [11]. Sensitivity analysis have shown that for a constant ks, the use of refined meshes and re-interpolated on a more detailed bed topography, gives values of velocities similar to those predicted by calculations with the basic meshes (Fig. 5b). Normally, the bed roughness of the refined meshes should be increased (roughness of the small dunes +  ks of the models) and the velocities decreased. As it is rarely the case, the results of the sensitive tests indicate that the small dunes seems to not contribute significantly to the bed roughness. Nevertheless, in general, the integration of the small bedforms in the meshes improves the representation of the flow structure. Thus, with the refined and re-interpolated meshes, the distribution of the velocities follow vertical and crosswise variations close to those observed in the field data (Fig. 2e). The integration of the small dunes also improves significantly the calculation of the vertical velocities (Fig. 2f). This indicates that the small bedforms strongly influence the vertical velocities; the latter being linked to the turbulences generated by these small dunes. Thus, if the turbulence affects significantly the sediment dynamics (especially for the suspended sediment and therefore, the von Karman constant), and by extension the morphology of the bed, it seems necessary to integrate the small bedforms into the morphodynamic models. However, it should be noticed that in some cases, the refinement of the mesh modified the bed topography so much that the results of the simulation can be locally degraded. Calibration and sensitivity tests have showed a systematic overestimation of the velocities predicted for a discharge of 2000 m³/s applied on alternate bars configuration (Fig. 3b and 3d). This can be explained by the not-representation of a secondary channel in the models on the left bank close to the profile P90. Indeed, this secondary channel is totally connected above 1700 m3/s [9]. Thus at 2000 m³/s, the connection of the secondary channel should decrease the water discharge in the main channel (from P90). As the reducing of the water discharge in the models is no considered, the predicted velocities are overestimated compared to the field data. A deeper investigation of the influence of the connection/disconnection of the secondary channel on the main channel’s hydrodynamic should improve our understanding of bifurcations. The results of the models show that the general flow is defined by the channel planform. However, the bars modify the hydrodynamic. Indeed, the macroforms deflect the streamlines and concentrate the flows in the thalweg. Thus, the large flow velocities (Fig. 6) are constrained on the edge of the bars which follow the thalweg. It also seems that the macroforms constitute a physical limit which gives the currents a different structure on either side of the bars. The bars control the location of the divergence and convergence zones, respectively caused by the channel widening and narrowing (Fig. 7). Thus, the separation and the mixing of the currents always take place, respectively, on the crest and downstream of the macroforms. The bars’ configurations (i.e. alternate or transverse) in the channel partly determine the location of the large flow velocities and those of the divergence and convergence zones. This study allowed the validation of TELEMAC-3D to reproduce the hydrodynamics in a complex morphological environment in the presence of transverse and alternate bars. Future work include the coupling to the morphodynamic model (SISYPHE) in order to simulate the sedimentary processes around the bars, for a better understanding of their formation and evolution in natural systems [7]. ACKNOWLEDGEMENT The measurements were funded by the Master Plan Loire Grandeur Nature-FEDER (Presage n° 30809). The authors thank Philippe Jugé, Yann Guerez, Vincent Bustillo, Benoît Delplancque, Benjamin Gandubert and Olivier Guillemet for their help during the measurements. The four month research scholarship of Benjamin Magnier is gratefully acknowledged. REFERENCES [1] M. Colombini, G. Seminara and M. Tubino (1987). "Finite-amplitude alternate bars." Journal of Fluid Mechanics, 181, 1, pp. 213-232. [2] G. Seminara and M. Tubino (1989). "Alternate bars and meandering: free, forced and mixed interactions." River meandering, S. Ikeda and G. Parker, Washington, American Geophysical Union - Water Resources Monographs 12, p. 267-320. [3] M. Tubino (1991). "Growth of alternate bars in unsteady flow." Water Resour. Res., 27, 1, pp. 37-52. [4] S. Lanzoni (2000). "Experiments on bar formation in a straight flume: 1. Uniform sediment." Water Resour. Res., 36, 11, pp. 3337-3349, doi:10.1029/2000WR900160. [5] M. Tubino, R. Repetto and G. Zolezzi (1999). "Free bars in rivers." Journal of Hydraulic Research, 37, 6, pp. 759-775. [6] A. Crosato and E. Mosselman (2009). "Simple physics-based predictor for the number of river bars and the transition between 35XXth TELEMAC-MASCARET User Conference Karlsruhe, October 16–18, 2013    meandering and braiding." Water Resour. Res., 45, 3, pp. W03424, doi:10.1029/2008WR007242. [7] F. Mattia, D. Wang, J.-M. Hervouet, A. Leopardi, K. El Kadi Abderrezzak and P. Tassi (2013). "Numerical simulations of bar formation and propagation in straight and curved channels." XXth TELEMAC-MASCARET Users conference, Karlsruhe, Germany. [8] F. C. Wu, Y. C. Shao and Y. C. Chen (2011). "Quantifying the forcing effect of channel width variations on free bars: Morphodynamic modeling based on characteristic dissipative Galerkin scheme." J. Geophys. Res., 116, pp. F03023, doi:10.1029/2010JF001941. [9] N. Claude (2012). "Processus et flux hydro-sédimentaires en rivière sablo-graveleuse : influence de la largeur de section et des bifurcations en Loire moyenne (France)." PhD, University of Tours, Tours, p. 366. [10] N. Claude, S. Rodrigues, V. Bustillo, J.-G. Bréhéret, P. Tassi and P. Jugé (2013). "Interactions between flow structure and morphodynamic of migrating bars in a channel expansion/contraction, Loire River, France.", unpublished. [11] S. D. Sandbach, S. N. Lane, R. J. Hardy, M. L. Amsler, P. J. Ashworth, J. L. Best, A. P. Nicholas, O. Orfeo, D. R. Parsons, A. J. H. Reesink and Szupiany R.N. (2012). "Application of a roughness-length representation to parameterize energy loss in 3-D numerical simulations of large rivers." Water Resour. Res., 48, 12, doi:10.1029/2011WR011284. [12] J.-M. Hervouet ( 2007). "Hydrodynamics of free surface flows, modelling with the finite element method", John Wiley & Sons. [13] E. A. Keller (1971). "Areal sorting of bed-load material: the hypothesis of velocity reversal." Bulletin of the Geological Society of America, 82, 3, pp. 753-756.  [14] R. M. Frings and M. G. Kleinhans (2008). "Complex variations in sediment transport at three large river bifurcations during discharge waves in the river Rhine." Sedimentology, 55, 5, pp. 1145-1171, doi:10.1111/j.1365-3091.2007.00940.x. 36

Numerical Simulation of Flow Structures in the Presence of Alternate and Transverse Bars: Application to the Loire River (France)

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Numerical Simulation of Flow Structures in the Presence of Alternate and Transverse Bars: Application to the Loire River (France)

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