Figure 1 - uploaded by Denis Moiriat
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Map of the study area on the Garonne river including the location of the modelled storage areas and the main levees.
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... study area is a 50 km long reach on the Garonne river between Tonneins and La Réole (Figure 1). The area has been settled to protect floodplains by organizing flooding and flood storage, between 1760 and 1850, where many earthen levees were built to protect the harvest against spring floods [10,11]. ...
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... Therefore, there is a need to carry out UQ and GSA on levee breach parameters to fill this gap. Previous studies have been carried out to investigate levee breach influences on flooded areas with HEC-RAS 1D model along the Garonne River, between Tonneins and La Réole [11][12][13]. First, local sensitivity analysis has been conducted to assess the influence on five breach parameters related to overtopping (triggering water depth above levee crest, final length, formation time, crest lowering and breach weir coefficient). Influences of breach parameters on the water levels within each storage area have been investigated [12]. ...
... First, local sensitivity analysis has been conducted to assess the influence on five breach parameters related to overtopping (triggering water depth above levee crest, final length, formation time, crest lowering and breach weir coefficient). Influences of breach parameters on the water levels within each storage area have been investigated [12]. Then, uncertainty quantification on the assessment of the flood hazard, generated by a flooded watershed, has been conducted to evaluate the sensitivity of flooding associated to levee breaches, modelled with a 1D hydraulic model [11]. ...
... This modelling software is commonly used and integrates a levee breach module. The model construction is detailed in [12]; therefore here we only describe the main aspects. ...
... Therefore, there is a need to carry out UQ and GSA on levee breach parameters to fill this gap. Previous studies have been carried out to investigate levee breach influences on flooded areas with HEC-RAS 1D model along the Garonne River, between Tonneins and La Réole [11][12][13]. First, local sensitivity analysis has been conducted to assess the influence on five breach parameters related to overtopping (triggering water depth above levee crest, final length, formation time, crest lowering and breach weir coefficient). Influences of breach parameters on the water levels within each storage area have been investigated [12]. ...
... First, local sensitivity analysis has been conducted to assess the influence on five breach parameters related to overtopping (triggering water depth above levee crest, final length, formation time, crest lowering and breach weir coefficient). Influences of breach parameters on the water levels within each storage area have been investigated [12]. Then, uncertainty quantification on the assessment of the flood hazard, generated by a flooded watershed, has been conducted to evaluate the sensitivity of flooding associated to levee breaches, modelled with a 1D hydraulic model [11]. ...
... This modelling software is commonly used and integrates a levee breach module. The model construction is detailed in [12]; therefore here we only describe the main aspects. ...
In recent years, flooding hazard is usually assessed through numerical modelling. However, depending on the nature (e.g. 1D, 2D) and the breach characteristics (e.g. river geometry, bottom roughness, levees geometry) of the numerical model, the uncertainties on the corresponding parameters should be taken into account in a rigorous way, for improving the assessment of the simulated flooding hazard. In fact, levee behaviour during a flooding event is one of the major sources of uncertainties impacting the water level at a given location. In this context, the objective of our work is to better understand the impact of uncertain parameters related to levee breaches, on the generated overflows, through Uncertainty Quantification (UQ) and Global Sensitivity Analysis (GSA) of these parameters. With this purpose, two numerical models of the Garonne River were built and validated, between Tonneins and La Réole sections (for a river length of nearly 50 km): a 1D hydraulic model with storage areas, developed with HEC-RAS and a 2D model with TELEMAC-2D. These modelling approaches (1D and 2D) are classically used to carry inundation studies. Moreover, the simulated river reach is of interest as protected by a levee system to reduce the flood risk. These levees have been damaged during flood periods, by physical mechanisms as erosion due to overtopping for instance, such as during the 1981 historical flood event. The study evaluates the influence of levee breach parameters (breach triggering parameter, breach length and breach depth) on the maximum water level at four points located within the upper part of the study area, through UQ and GSA. These approaches are carried out with a meta-model built with 200 simulations runs using a Monte-Carlo approach for both models. In both cases, the breach parameters are uniformly distributed and randomly sampled in order to generate a large number of breach scenarios. Globally, the Monte-Carlo and FAST (Fourier Analysis Sensitivity Test) analyses performed have shown some differences between the results coming from both meta-models and between the upstream and the downstream storage areas, more sensitive to levee breaches. These analyses also indicated the slight effect of the breach length parameter contrary to the triggering and depth breach parameters.
... De façon générale, ce code a été retenu pour sa simplicité d'utilisation et pour son module permettant de modéliser le phénomène de rupture de digues de différentes façons (par surverse et/ou par érosion interne). La construction du modèle numérique utilisé pour cette étude est détaillée dans la référence [11] et nous nous limiterons ici à ne reporter que les aspects principaux. ...
... Les conditions aux limites sont la courbe de tarage (hauteur/débit) au niveau de la station hydrologique de « La Réole », à l'aval et l'hydrogramme de crue de 1981 enregistré à Tonneins, à l'amont. Le modèle a été calé lors d'une étude précédente[11]. Les paramètres ayant servi au calage et utilisés dans cette étude sont les coefficients de Strickler du lit mineur, égal à 40 m 1/3 /s, et de la plaine d'inondation, de 20 m 1/3 /s. Enfin, un pas de temps de calcul fin, de 10 secondes, a été choisi à l'issu des tests de sensibilité réalisés, afin d'avoir une bonne stabilité du modèle lors des ruptures de digues. ...
In a fluvial environment, the main role of levees is to canalize water downstream of rivers to reduce the risk of flooding to nearby areas. However, during flood periods, levees can be damaged by physical mechanisms as erosion due to overflowing, for instance. As a consequence, it is necessary to take into account levee breaches induced by floods in
hydraulic models. Nevertheless, levee breaches modeling is a complex phenomenon, which relies on a number of uncertain parameters, dependent from the breach geometry and from the mechanisms implemented in numerical models. In this context, the objective is to perform a sensitivity analysis to better understand the impact of breach parameters, used in a 1D hydraulic model, on the generated overflows. A model with storage areas of the Garonne River, preliminary built and validated with the HEC-RAS code, was used. The study case is based on the 1981 historical flood event between Tonneins
and La Réole, in a river section of 63 km, during which several levees failed (Figure 1). The methodology relies on an uncertainty propagation of the breach parameters, performed
through the coupling of HEC-RAS and the computational environment Promethee developed by IRSN. The breach parameters are uniformly distributed and randomly sampled in order to generate a large number of breach scenarios. The
Monte-Carlo and the FAST (Fourier Analysis Sensitivity Test) analyses performed in this study show the strong influence of the overflow parameter and breach geometry on
the water height in a given storage area. The study also highlights the major difference between upstream and downstream parts of the river. Indeed, the upstream area is
much more sensitive to levee breaches than the downstream area, so the uncertainties are higher.
Hydraulic models include many uncertainties related to model parameters. Uncertainty Quantification (UQ) and Global Sensitivity Analysis (GSA) allow to quantify these uncertainties and identify the most influent parameters. To use traditional methods of UQ and GSA, the inputs must be independent. Therefore, this research aims to show that the dependence between inputs should not be neglected in uncertainty studies. To illustrate this, we used a simple hydraulic model built with TELEMAC-2D, representing a hypothetical linear river protected by a dyke that can break. The methodology is completed following these steps: the uncertain inputs and the outputs of interest are identified. Then, the inputs margins are defined through a statistical analysis using real dataset. The dependence structure between inputs is figured using copulas. Kriging metamodels are used to increase the number of experiences in a short time period. Finally, UQ and GSA are achieved. Regarding the UQ results, the outputs distribution is different if the inputs are considered dependent or not, whereas regarding the GSA, some parameters, usually considered non-influent in the hypothesis of independent inputs, have a real impact on the outputs. These results suggest that it could be interesting to consider the dependencies between inputs for “real” applications.
