2DH two-layer shallow-water model for flows over mobile beds with erodible banks

The collapse of a dam induces a fast transient flow that may interact with the erodible bed and banks of the downstream river, entraining large amounts of sediments and modifying the valley morphology. It may constitute a danger for the surrounding inhabitants and infrastructures. To reduce the catastrophic consequences of such an event, the decision-makers need tools to assess the risk posed by a dam and to prepare emergency plans. This thesis presents a numerical model aimed at simulating such transient flows over mobile beds. The two-dimensional horizontal (2DH) two-layer shallow-water model considers an upper clear-water layer and a lower bed-load transport layer flowing on a motionless sediment bed. The depth-averaged velocities in both layers are assumed distinct in norm and direction. Distinct granular concentrations are assumed in the bed-load layer and in the motionless bed. The mass and horizontal momentum balances are written in each layer. Through the source terms, vertical mass and momentum transfers may occur between the layers following erosion / deposition. The conservative part of the governing equations is shown to be hyperbolic. The set of equations is solved on unstructured triangular meshes by a finite-volume scheme with a LHLL solver. Several boundary conditions can be handled. A bank-failure operator is inserted in the model. It compares local bank slopes with threshold values and tilts the unstable elements until they reach a repose angle. The model is validated against theoretical and experimental results of dam-break flows over sand beds, e.g. in an enlargement, on a floodplain and in a prismatic flume.(FSA 3) -- UCL, 201

Experimental assessment of alluviation downstream of Hun-lock, Belgium

To maintain inland navigation, a deposition crest in the Hunlock area, in the Upper Meuse River, Belgium, has to be dredged almost every two years. A long-term solution is searched for, based on the understanding of water velocity field and solid transport processes, to avoid the silting and the expensive dredging. The physical process is analysed thanks to a 1:50 small-scale model of the 500 m-long Meuse reach with Hun movable dam as upstream boundary. The velocity fields, recorded with an electromagnetic probe displaced automatically, are compared for several configurations. Three discharges and distinct dam openings are tested in the existing configuration. Modifications of the downstream guard wall, by perforating ports, are also considered. Building groynes in the right side of the river is analysed. Finally, flushing water through the lock is tested. As conclusions, alluviations are due to the localisation of the lock in an enlarged area of the Upper Meuse and in a meander intrados, inducing decreasing flow velocities. None of the tested solutions is completely convincing according to 4 criterions: (1) ability to decrease the deposition crest, (2) navigation requirements, (3) safety in case of high-stage discharges, and (4) acceptable cost

Experimental assessment of alluviation downstream of Hun-lock, Belgium

To maintain inland navigation, a deposition crest in the Hunlock area, in the Upper Meuse River, Belgium, has to be dredged almost every two years. A long-term solution is searched for, based on the understanding of water velocity field and solid transport processes, to avoid the silting and the expensive dredging. The physical process is analysed thanks to a 1:50 small-scale model of the 500 m-long Meuse reach with Hun movable dam as upstream boundary. The velocity fields, recorded with an electromagnetic probe displaced automatically, are compared for several configurations. Three discharges and distinct dam openings are tested in the existing configuration. Modifications of the downstream guard wall, by perforating ports, are also considered. Building groynes in the right side of the river is analysed. Finally, flushing water through the lock is tested. As conclusions, alluviations are due to the localisation of the lock in an enlarged area of the Upper Meuse and in a meander intrados, inducing decreasing flow velocities. None of the tested solutions is completely convincing according to 4 criterions: (1) ability to decrease the deposition crest, (2) navigation requirements, (3) safety in case of high-stage discharges, and (4) acceptable cost

Two-dimensional two-layer shallow water model for dam break flows with significant bed load transport

River flow dynamics and sediment transport are intimately interdependent. Their interaction governs a great diversity of flows with significant morphologic consequences. To predict the bed load transport induced by dam break waves, the proposed two-dimensional (2D) two-layer shallow water description considers an upper layer made of clear water and a lower layer made of a dense mixture of water and moving grains. Continuity and 2D momentum conservation are written for each layer, which allows the depth-averaged velocities to be distinct in magnitude and direction in both layers. The model accounts for the grain entrainment across the bed interface and for the mass and momentum exchanges between the flowing layers. The system of governing equations, written so that no loss of hyperbolicity occurs in the conservative part, is solved by a Harten-Lax-Van Leer finite volume scheme on an unstructured triangular mesh. The numerical model is tested against four dam break flows over mobile beds: a theoretical radial problem and three laboratory experiments in 1D and 2D configurations. © 2013 John Wiley & Sons, Ltd

Le défi pétrolier : questions actuelles du pétrole et du gaz

Analyse des grands enjeux pétroliers et gaziers internationaux : les prix, les marchés à terme, les principaux acteurs

Two-dimensional operator for bank failures induced by water-level rise in dam-break flows

Bank failure in a stream generally originates from bed erosion at the bank toe. This research proposes a novel two-dimensional (2D) bank-failure operator, based on a simple local failure criterion and on the conservation of sediment mass. This operator is inserted into a 2D Saint-Venant-Exner model to simulate both bed and bank geomorphic processes and their interaction with the flow, especially during dam-break events. This shallow-water model is solved by a first-order finite-volume scheme on an unstructured triangular mesh. After an erosion or deposition update, the bed inclination is compared locally with the sediment stability angles, considering the impact of water-level rise. Assuming material homogeneity, the unstable bed elements are tilted around an appropriate axis of rotation, ensuring mass conservation. The numerical model is tested for a collapsing circular hole and against laboratory tests of a dam-break flow: first in a prismatic channel made of coarse sand and then over an initial bed step

Le défi pétrolier : questions actuelles du pétrole et du gaz

Analyse des grands enjeux pétroliers et gaziers internationaux : les prix, les marchés à terme, les principaux acteurs

2D two-layer shallow-water model with a bank-failure operator for dam-break waves on mobile beds

To consider both bed and bank erosion due to a dam-break induced wave, a bank-failure operator is inserted into a two-dimensional (2D) two-layer shallow-water model. This model accounts explicitly for the inertia of the bed-load transport, considering an upper layer made of clear water and a lower layer, called the bed-load transport layer, made of a mixture of water and moving grains. These layers flow on a motionless bed and are assumed to present distinct depth-averaged velocities. The model accounts for the grain entrainment across the bed interface and for the mass and momentum exchanges between the flowing layers thanks to the definition of an erosion rate. This shallow-water model is solved by a first-order finite-volume scheme on an unstructured triangular mesh. The bank-failure operator consists in comparing locally for each computational cell the bed inclination to the sediment stability angles, considering the impact of a water-level rise as a destabilizing phenomenon. After an erosion or deposition update due to the bed-load transport, the unstable bed elements are tilted around an appropriate axis of rotation, ensuring mass conservation of the assumed homogeneous material. The numerical model is tested for a collapsing circular hole and against laboratory tests of a dam-break flow; firstly in a prismatic channel made of coarse sand, and then over an initial bed step

Transporte de la carga del fundo y erosion de las riberas en un modelo bidimensional para ondas consecutivas a la rotura de una presa

Para tomar en consideración la erosión del cauce y de las orillas debida a una onda inducida por la rotura de una presa, un operador de rotura de las riberas se inserta en un modelo bidimensional de aguas poco profundas llamado “modelo con dos capas”. Este modelo considera una capa superior hecha de agua y una capa inferior constituido por una mezcla de agua y de sedimentos en movimiento. Estas capas están colocadas sobre el cauce y se supone que presentan velocidades medias distintas. Un intercambio de masa y de momento es posible entre las capas y el cauce durante la erosión o deposición de los sedimentos. El modelo se resuelve mediante un esquema de volúmenes finitos de primer orden sobre una malla triangular. El operador de rotura de las riberas consiste en una serie de comparaciones locales de la inclinación del cauce a los ángulos considerados estables, teniendo en cuenta el impacto de un aumento del nivel del agua como un fenómeno desestabilizador. El modelo se prueba en el caso del colapso de un pozo circular lleno de agua y en dos casos de rotura de presas en laboratorio.To consider both bed and bank erosion due to a dam-break induced wave, a bank-failure operator is inserted into a two-dimensional two-layer shallow-water model. This model considers an upper layer made of clear water and a lower layer made of a mixture of water and moving grains. These layers flow on a motionless bed and are assumed to present distinct depth-averaged velocities. The model accounts for the grain entrainment across the bed interface and for the mass and momentum exchanges between the flowing layers. This model is solved by a first-order finite-volume scheme on an unstructured triangular mesh. The bank-failure operator consists in comparing locally for each computational cell the bed inclination to the sediment stability angles, considering the impact of a water-level rise as a destabilizing phenomenon. The numerical model is tested for a collapsing circular hole and against laboratory tests of a dam-break flow