According to the European Flood Directive 2007, the flood hazard must be accurately assessed in high-risk areas over the river floodplains, during extreme flood events. However, the prediction of the very high flows is not an easy task due to the lack of field data and to the strong link between flow resistance and land occupation of the floodplain. With increasing return period, the confinement decreases and the heterogeneity in lateral and longitudinal directions of the hydraulic roughness elements (e.g. trees, houses) strongly increases. The physical processes are complex, still largely unexplored, and the assumptions linked to numerical modeling cannot be validated without field data. The project aims at improving the flood hazard assessment in floodplains, by relying on experimental data base and on one field case (floods at Besançon, France). 

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Analyzing in laboratory flumes the flow structure associated with extreme events, assessing and improving the existing modeling practices for such events. 

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Flows associated with low to extreme flood events have been experimentally studied in laboratory flumes (Task n°1). Relying on measurements at a large scale (at the river reach scale) or at a small scale (scale of a hydraulic roughness element, e.g. a tree or a house), flow structure has been analyzed for various types of land occupations. The focus was on the study of the effects of (i) longitudinal and transverse transitions in hydraulic roughness (from a woodland to a meadow or urbanized area), (ii) the confinement magnitude of the roughness elements (emergent/submerged), and (iii) of their spatial distribution. During the task n°2 (“Assessment and improvement of the modelling practices”), the various experimental data sets were compared to numerical simulations. Some classical modelling methods of flow resistance have been tested and improved to capture the physical processes irrespective of the discharge magnitude. The codes and methods have been then applied to the floods of the Doubs river at Besançon. Floods with a return period of 100-, 1000- and 10000-year, have been simulated with the classical and improved methods, and the discrepancies on flow depths and velocities were estimated. Uncertainties related to the hydrological computation of discharges and to the computation of the hydraulic flood hazard were compared.  A set of recommendations for extreme flood modeling has been drawn from this work.