Taking into account changing alluvial channels in flood risk assessment – GESTRANS

River management is becoming more and more complex because it needs to integrate short-term flood risk assessment and long-term environmental objectives. These objectives may sometimes conflict, for instance when a sediment deposit, which is fully part of the river equilibrium (or disequilibrium), locally modifies the river section's flow capacity. In order to make these two demands compatible, regulations were modified in 2008, restricting human interventions inside river beds. Consequently, all new sediment management decision will have to mention its motivation (usually flood risk), the river context, its sediment budget, its equilibrium slope and the possible effects induced by removal of sediments. Despite this effort stakeholders face a number of problems when making decisions concerning such projects. Two of these can be considered as critical key challenges. First, we lack tools to predict macro-scale bedform dynamics that control flow routing and bank stability. Second, there is a gap between stakeholders decisions (for example not removing sediment deposits in order to preserve or to improve river quality) and their understanding (and acceptance) by floodplain inhabitants. Sustainable river management demands that these two challenges be addressed and solved. This project brings together stakeholders, geographers, geophysicists and engineers towards this end: improving our predictive skills on river dynamics, and encouraging acceptance by inhabitants through knowledge transfer. Because short-term risk management at the reach scale cannot be dissociated from long-term environmental aspects at the river scale and because scientific arguments cannot be dissociated from the inhabitants' perceptions, this complex and multidisciplinary problem must necessarily be treated as a whole. This is why local authorities (DDAF38 Isère), engineers (Cemagref), sociologist and social geographers (CNRS UMR5600) and geophysicists (LTHE, IPGP) seek to work together to optimize river management. Since the problem of sediment dynamics has been extensively studied in other countries, the project also extends the collaboration to a foreign partner (University of British Columbia). The project will include five interconnected work Tasks. task 1 will investigate the mechanics of transport through the development of new field survey technologies, bedload transport formulas validation and investigation of mechanisms involved at the grain scale. task 2 will investigate the morpho dynamics at the river and at the reach scale, by holistic, statistical and numerical approaches. task 3 will study the social stakes and conditions for sediment projects acceptance through a regional daily press analysis and photographe based enquiries. task 4 will integrate the results of all the other work packages in order to propose tools for river management that will prove useful to find conditions for acceptance of stakeholders' decisions by local populations. A fifth Task will be dedicated to project management.