Predicting the flow in the floodplains with evolving land occupation during extreme flood events

Improving the flood hazard assessment in the floodplains

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 (Figure 1). 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).

Figure 1. Land occupation of the floodplains along the Moselle river, Uckange (France). Flow resistance caused by bed friction (cultivated land and meadow) and by the drag forces on the roughness elements like trees and houses.

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

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.

Main results of the project:

Experimental data associated with low to very high flows have been collected in four laboratory flumes, and will be made available to the international scientific community, in particular via the European database The predominant flow resistance processes were analyzed and quantified, theoretically and numerically modelled. A recommendation guide for the simulation of extreme flood flows, intended for operational staff, is being written and will be presented at the Simhydro international conference in Nice in June 2019. Finally, four new partnerships have emerged from this project.

Scientific production since the beginning of the project:

All the work has been (or will be) highlighted in 21 articles in scientific journals, 26 scientific communications with proceedings (mainly international conferences), and 1 scientific book chapter. Of these publications, 36 focus on the physical processes associated with extreme floods, particularly in the presence of bottom roughness combined with weakly submerged or emergent roughness elements; 10 focus on the comparison of simulations / experiments; and 4 focus on the simulation of the field case.  

Factual informations:

The Flowres project is a fundamental and applied research project coordinated by the Institut national de Recherche en Sciences et Technologies pour l’Environnement et l’Agriculture (Irstea), Lyon-Villeurbanne (coordinator: S. Proust, UR Riverly). It also includes the Institut National des Sciences Appliquées de Lyon (INSA), the Institut de Mécanique des Fluides de Toulouse (IMFT), the Laboratoire d’Hydraulique Saint-Venant (LHSV – EDF R&D), Chatou, as well as the Instituto Superior Técnico (IST), Lisbon, Portugual, the National Laboratory for Civil Engineering (LNEC), Lisbon, Portugual, the Université catholique de Louvain (UCL), Louvain-la-Neuve, Belgium, the Direction des Recherches Hydrauliques du Service Public de Wallonie (SPW), Châtelet, Belgium, the University i Agder (UiA), Norway, the Karlsruhe Institut of Technology (KIT), Germany, the Ecole Nationale Polytechnique d’Alger (ENPA), the National Institute of Technology de Rourkela (NIT), India and Aberdeen University, United Kingdom. The project started on 1 January 2015 and lasted 48 months. It received an ANR grant of € 584 810 for an initial total cost estimated at € 1 532 650.