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Title: 2D and 3D numerical simulations of morphodynamics structures in a large-amplitude meanders

Author
item WANG, DONGCHEN - Electricite De France (EDF)
item TASSI, PABLO - Electricite De France (EDF)
item EL KADI ABDERREZZAK, KAMAL - Electricite De France (EDF)
item MENDOZA, ALEJANDRO - University Of Pittsburgh
item ABAD, JORGE - University Of Pittsburgh
item Langendoen, Eddy

Submitted to: Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 7/15/2014
Publication Date: 8/27/2014
Citation: Wang, D., Tassi, P., El Kadi Abderrezzak, K., Mendoza, A., Abad, J., Langendoen, E.J. 2014. 2D and 3D numerical simulations of morphodynamics structures in a large-amplitude meanders. In: Proceedings of River Flow 2014, September 3-5, 2014, Lausanne, Switzerland. A. J. Schleiss, G. De Cesare, M. J. Franca, & M. Pfister (eds). pp. 1105-111.

Interpretive Summary: To appropriately assess planform adjustment of large rivers under varying sediment transport loads using computer models, it is necessary to accurately simulate the bottom topography of rivers. Meandering streams with large bends are characterized by multiple bars unlike the typical single pointbar located in a bend itself. The differing bed topographic provides a steering of the flow that forces different bank erosion patterns. Current vertically-integrated, two-dimensional (2D) computer modeling technology has difficulties in simulating the different bed topographies accurately. Scientists of the USDA-ARS National Sedimentation Laboratory in collaboration with researchers of the University of Pittsburgh and the French National Hydraulics Laboratory-Chatou, have developed a detailed computer simulation of both the two-dimensional and the three-dimensional (3D) flow, sediment transport, and bed morphology in large-amplitude meandering rivers. The computer simulation was validated against laboratory experiments conducted at the University of Washington in the 1990s. The depth-averaged 2D model, even with appropriate parameterizations of relevant 3D effects, failed to capture some 3D patterns. On the other hand, the 3D numerical simulation was able to capture most of the observed flow and morphodynamic patterns. It was further found that the bed topography in large amplitude, symmetric planforms can be divided into zones according to the different morphodynamics patterns: a zone with an almost zero bed load discharge and bed evolution, an “alternate bars” zone, a “shingle-bars zone” and a “multiple-pools” zone. The findings will help with the improving of computer models to evaluate dam decommissioning scenarios during which large amounts of sediments are released that can adversely change the morphodynamics of downstream river reaches.

Technical Abstract: In the pioneering study of the Ishikari River, Japan, Kinoshita (Kinoshita 1957, 1961) described two types of meandering channels: (1) channel with two bars per meander wavelength (one bar per bend), and (2) channel with three or more bars per meander wavelength (multiple bars per bend). Based on the study of Whiting & Dietrich (1993a, b), we assess the capabilities of the Telemac-Mascaret modelling system (2014) to reproduce Two-Dimensional (2D) and Three-Dimensional (3D) flow and morphodynamics structures in large-amplitude meandering channels. The large-amplitude meander setup of Whiting & Dietrich (1993a, b) was shown to be a difficult test for a depth-averaged 2-D model: even if appropriate parameterizations of relevant 3D effects are incorporated into the model, numerical simulations failed to capture some 3D patterns, such as the first well-defined pool observed in the experiments. However, 3D results obtained from the numerical solution of the RANS equations showed that most of the observed flow and morphodynamics patterns (e.g., series of shingled bars with pools along the concave bank, depositional bank fronts along the inner bank) are well captured by the model. In agreement with experimental observations, the bed topography in large amplitude symmetric planforms can be divided into different zones according to the different morphodynamics patterns: a zone with an almost zero bed load discharge and bed evolution, an “alternate bars” zone, a “shingle-bars zone” and a “multiple-pools” zone.