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International audience; A prime requirement for hydrological applications, such as sediment budgeting or numerical modelling, is that produced Digital Terrain Models (DTMs) accurately represent the shape of landforms, especially for river reaches where data are not homogeneous. DTM error is a function of data point measurement accuracy and density and also of the field survey strategy when limited amounts of data will be acquired. This paper aims to advance the importance of the field survey strategy for the specific, but common cases, where only limited topographic data will be available. This methodology is based on the idea that any feature can be properly described by a set of cross sections and breaklines describing both main and secondary directions of the flow. Then, a longitudinal linear interpolation can be applied to the defined homogeneous zones. This morphologically oriented (MO) method that includes data acquisition strategy and interpolation, was validated using a reference DTM derived from LiDAR measurements. An stimation of the uncertainties also is suggested based on the distance of the nearest point and the local slope using a geographically weighted regression. The proposed MO method is typically applicable to Alpine river reaches characterized by multiple channels that may always be underwater and not navigable such as an alternate bar system with secondary and transverse channels.

Physical modelling of urban flooding usually considers that the flow is concentrated in the streets, assuming that urban blocks and individual buildings are non-porous structures. This assumption is open to criticism, as potential lateral flow exchanges through openings in the walls of an urban block (e.g. gates, windows and doors) are likely to modify the flow pattern in the block and adjacent streets. This paper reports an experimental study conducted on a 5.4 m long and 3.2 m wide physical model, representing a rectangular urban block and four surrounding streets. The focus of the study was on investigating the lateral flow exchange processes between the porous urban block and its adjacent streets during flooding, for a variable urban block porosity. Eight porosity values were studied, keeping the same boundary conditions. A porosity equals zero, i.e. a flow around a non-porous block, served as a reference flow situation. For each porosity value, the impact of the lateral flow exchanges on the spatial distribution of flow depth and velocity in the streets and within the block was assessed, as well as the risk to pedestrians. First, it was found that, in the streets surrounding the block, flow depth and depth-averaged velocity can vary by 12% and 70%, respectively, when modifying block porosity. Within the block, the most impacted flow parameter is the number and size of horizontal secondary flow cells. Second, noticeable differences in flow depth can be observed from either side of an opening between street and block. Third, when moving away from the block, flow parameters are little affected. Last, the risk to pedestrians, which is both related to local flow depth and velocity, is dependent on the block porosity value.

Urban floods cause high damages and thus mapping of flood risk is essential for protection and evacuation planning. To obtain such mapping, hydrodynamic modelling is central because it provides water depths and velocities everywhere and along the whole flood period. The case of the 1988 flood in Nîmes is presented as an example of the method to get the basis for evaluating flood risk. Once the hydro-meteorological event is selected, a lot of causes of uncertainty still exist, which are linked to either the flow processes in a complex environment or the simplifications of the 2-D models that are used for saving computation time. In order to gain information regarding the origin of such uncertainties, two series of laboratory experiments are described: one focuses on the error in the flow distribution at crossroad, the other one stresses the flow complexity in the exchange structures between the streets and the sewer network. Beyond the analysis of the causes of uncertainty, obtaining a detailed local assessment of flood risk requires a sensitivity analysis to cope with the associated uncertainty ranges; this analysis may be reduced if a relevant model is used, which means, for instance, that the processes described in the experiments are conveniently modeled.

This study aims to show the ability of 1D model to simulate the wadi flow during the flood. The wadi is a semiarid stream characterized by the non uniform geometry and an intermittent flow. These complexities can be modelled by a robust 1D model such as Rubarbe. In this article we study the 1994 flood of Ksob wadi that caused a large inundation of M’sila town in the center of Algeria. The simulated reach located downstream the Ksob dam, crosses M’sila over a length of 6800 m with a bed slope of 0.6 %. The recorded flow peak of the 1994 flood is about 1430 m3/s and flood duration is 24 days. The flood propagation is simulated through the studied reach using the Rubarbe code (1D model) developed in Irstea (former Cemagref). The results show that the wadi overflows in some sections at the upstream and the center of the reach. The major part of the overflow is observed just around the peak of the flood, but the duration is variable from a few minutes to several hours. The overflow hazard limits are plotted in each section from the intersections of the highest water elevation recorded during the flood with the ground level of the two banks of the same section, taking into account the floodplain slope. According to our investigations, the calculated flooded area is very close to the real one, especially in the central part of the reach., L'étude vise à montrer la capacité d'un modèle 1D à simuler l'écoulement dans un oued. L'article traite le cas de l'oued Ksob qui a entrainé l'inondation de la ville de M'sila en 1994 et détaille les étapes qui ont mené à une carte des zones inondées proche des observations pour cette crue de 1994.

Un code résolvant les équations de Saint venant 2D par un schéma du second ordre explicite est utilisé pour simuler la crue extrême d'octobre 1988 dans le quartier Richelieu de la ville française de Nîmes. Un calcul de référence utilisant une description détaillée du réseau de rues et des sections en travers des rues mais négligeant le réseau d'assainissement fournit une altitude moyenne du pic 0,13 m plus basse que les laisses de crue avec un écart type de 0,53 m. l'analyse de sensibilité des paramètres topographiques et numériques montre que globalement les résultats gardent le même niveau de précision, ce qui reflète la stabilité de la méthode de calcul et le lissage des résultats. Toutefois, localement des modifications d'écoulement dues à la modification des valeurs des paramètres changent fortement les niveaux d'eau, particulièrement quand le régime est modifié. En outre, la répartition des écoulements dans la partie val du quartier peut être altéré. Finalement, une seconde crue (2002) a été simulée avec le modèle calé donnant des résultats similaires à 1988. Donc, l'article montre qu'après calage, un modèle 2D peut être utilisé pour aider à la planification de mesures de prévention dans une zone urbaine dense. / A code solving the 2-D shallow water equations by an explicit second-order scheme is used to simulate the severe October 1988 flood in the Richelieu urban locality of the French city of Nîmes. A reference calculation using a detailed description of the street network and of the cross sections of the streets, considering impervious residence blocks and neglecting the flow interaction with the sewer network provides a mean peak water elevation 0.13 m lower than the measured flood marks with a standard deviation between the measured and computed water depths of 0.53 m. Sensitivity analysis of various topographical and numerical parameters shows that globally, the results keep the same level of accuracy, which reflects both the stability of the calculation method and the smoothening of results. However, the local flow modifications due to change of parameter values can drastically modify the local water depths, especially when the local flow regime is modified. Furthermore, the flow distribution to the downstream parts of the city can be altered depending on the set of parameters used. Finally, a second event, the 2002 flood, was simulated with the calibrated model providing results similar to 1988 flood calculation. Thus, the article shows that, after calibration, a 2-D model can be used to help planning mitigation measures in a dense urban area.