Brèche dans une digue causée par une surverse : une comparaison entre deux méthodes de modélisation

International audienceIn order to model the process of flooding induced by fluvial dike breaching, 2-D shallow water equations are often used. They can be coupled with a sediment transport model simulating the dike erosion and the erosion of the neighbouring bed areas. However, such a model is not dedicated to the specific process of embankment erosion. One may prefer using a specific breach model, simulating the breach expansion and outflow hydrograph, associated to a 2D shallow water model simulating the flood propagation in the areas protected. In the present work, the two approaches are compared on a field case and on laboratory experiments. For the field case, the short overtopping duration involves that the results are very sensitive to the erosion parameters associated to the dike breaching, particularly the critical shear stress necessary for the initiation of surface erosion. The breach development depends on the modelling approach that can favour either the breach deepening or widening. With a fast breach deepening (breach model), the breach peak outflow is high whereas a high breach widening (sediment transport model) yields a peak flow reduced by more than 90%. The laboratory experiments represent a long flood event leading to a total erosion of the dike over a long distance with a large part of the upstream flow being deviated into the floodplain through the breach. The two approaches (breach model and sediment transport model) provide rather similar results and close to the laboratory measurements although the breach model provides faster initial erosion and is less sensitive to the type of erosion rate equations

Sensibilité de la modélisation hydrodynamique 2D des inondations urbaines aux variables de forçage: leçons de deux cas de terrain

International audienceThis paper reports two urban floods in France: the September 2000 flood at the city of Marseille caused by heavy rainfall, and the November 2008 flood in Oullins induced by the Yzeron River overflow. A two-dimensional (2D) depth-averaged model was used to simulate flood hazard and to investigate the uncertainty associated to the forcing inputs. Using a rainfall simulator, several distributions of rainfall inputs over the whole domain were produced for the event of September 2000 in Marseille. Results showed these latter distributions changed the average peak water depth more than other input parameters. The Yzeron River water elevation was the main source of uncertainty in the case of Oullins, but the flow pattern in the floodplain was also influenced by the method representing the urban areas. For both cases, average uncertainty remains high (up to 20% for peak water depth or velocity) and local uncertainty is almost much higher.Cet article traite de crues urbaines en France: la crue de septembre 2000 à Marseille et la crue de novembre 2008 à Oullins. Un modèle 2D a permis de simuler l'aléa d'inondation et étudier l'incertitude associée aux variables d'entrée. En utilisant un simulateur de pluie pour Marseille, on a pu montrer que la répartition de la pluie influait plus que les autres paramètres. A Oullins, le niveau d'eau dans la rivière était le principal paramètre mais le parcours de l'eau dans la zone urbaine était aussi important. Dans les deux cas, l'incertitude restait en moyenne de l'ordre de 20% sur les hauteurs d'eau et les vitesses maximales mais pouvait localement être bien plus forte

Division d'un écoulement critique dans une intersection de 3 canaux

International audienceAn experimental study of the characteristics of dividing critical flows in a 90° open-channel junction formed by three horizontal equal-width branches is presented, conducted for various inflow discharges and downstream boundary conditions. Outflow discharges and flow depths were also measured. Four main flow patterns are identified considering the location and length of the hydraulic jumps that develop across the main and lateral channels. A relationship between the discharge division ratio and the tailwater Froude number is found reproducing well the experimental data. Practical applications of this relationship include the design of open channel and pipe networks and one-dimensional numerical modeling of flood propagation in urban areas. The study shows also that a two-dimensional depth-averaged numerical model hardly gives better predictions of the discharge ratios because of the presence of three-dimensional features at the junction.Une recherche expérimentale sur les caractéristiques d'écoulements critiques se divisant dans une intersection de trois canaux est conduite pour différentes conditions limites. Les caractéristiques expérimentales des écoulements sont indiquées dans les différents types d'écoulements identifiés. La répartition des débits est comparée à une formule analytique et aux résultats d'une modélisation 2D

Comparaison entre les modèles numériques mono-couche et à deux couches pour la simulation du transport sédimentaire induit par une rupture de barrage

International audienceIn this paper, we investigate the extent to which in numerical modelling of dam-break waves over movable beds. The models considered here are one-layer or two layers based on one-dimensional or two-dimensional depth-averaged shallow water equations. Three idealized laboratory cases on dam-break waves over sandy beds. Comparisons show that for each case better predictions are obtained using a particular formula, but overall, formulas proposed by Meyer-Peter and Müller (1948) (with the factor 8 being replaced by 12). Moreover, results show that in the cases where a bed step exists, implementing a mass failure mechanism in the numerical modelling plays an important role in reproducing the bed and water profiles.Nous examinons les modèles numériques utilisés pour la propagation des ondes de rupture de barrage entrainant des sédiments : modèles à une ou deux couches de sédiments en une ou deux dimensions. Trois expériences idéalisées sur fond sableux permettent une comparaison qui ne conduit pas à des préférences marquées mais l'utilisation de la formule de Meyer Peter and Muller avec un facteur 12 et l'utilisation d'un écroulement brutal dans le cas d'une marche initiale sont à retenir

A one dimensional (1-D) numerical modelling of pesticide transfer through the wetland drainage channels of Breton-Vendéen marsh (west coast of France)

International audiencePredicting pollutant transport in coastal ecosystems has become an important topic in many industrial and environmental projects, because of the degradation of water quality and conflicts related to multiple use of coastal resources. Most numerical studies on coastal catchment do not consider flow and pollutant fluxes through wetlands, or simulates theses transfers using “black box” based on linear regressions that greatly simplifies the complexity of the underlying processes.This paper presents a numerical study on the simulation of pesticide transport and degradation in the artificial wetland drainage channels of the Breton-Vendéen marsh (west coast of France). The one-dimensional (1-D), open source MASCARET modelling tool is used. The 1-D unsteady water flow through a network of open channels is represented by the shallow water equations. The water quality module of MASCARET simulates solute transport processes, consisting of advection, dispersion and mass reduction/generation by physical, chemical and biological mechanisms. Results show that the 1-D model is a reliable tool for simulating pollutant transfer through the coastal wetlands drainage channels. Moreover, it is possible to link landward and seaward models to MASCARET, which allows simulating water and pollutant propagation from watershed to coastal sea through wetland drainage channels with a high level of confidenc