Analyse coût-bénéfice du programme d’actions visant à réduire les impacts du ruissellement et de l’érosion en Haute-Normandie : évaluation des actions passées et projections futures sur le bassin versant de la Lézarde

The Seine-Maritime and Eure areas are highly vulnerable to surface runoff impacts, such as floods and mudflows. Recent years saw massive investments in public policies to reduce the risks associated with those hazards, however, the policies effectiveness has never been assessed. This original study combines the parallel modelling, at the watershed scale (Lézarde, 212 km2), of runoff (MIKE11) and sediment transfer (WaterSed), with a cost-benefit analysis of the mitigation program for different scenarios (past, present, future, and modified agricultural practices scenarios), with three designed storm project simulations for each scenario. The results from the simulations suggest that structural hydraulic structures (i.e. dams and retention ponds) are effective in terms of damage mitigation and economically efficient (benefit/cost ~ 2). As a result of global warming it is very likely that flood damages increase. At the same time, it is very likely that the amount of avoided damages also increases due to more frequent use of existing hydraulic structures. While soft erosion control measures can trap a significant amount of sediment, the lack of sufficient economic data did not allow us to integrate all the benefits from those measure. The economic efficiency of soft erosion control measures, in addition to structural hydraulic structures, was not demonstrated by our simulations. The modification of agricultural practices has a large influence on flood’s potential damage: (i) a significant reduction of damage is expected by modifying agricultural practices (-250 k€2000 yr−1), (ii) a significant decrease of damage is observed by reducing grassland areas by a one third (+400 k€2000 yr−1). This study shows the relevance of parallel modelling to assess the effectiveness and economic efficiency of anti-erosion/surface runoff actions. It also shows that it is necessary to develop new damage functions associated with these processes. Finally, the methodology developed in this study can be applied to other territories to evaluate public intervention strategies for erosion and runoff control. Les départements de Seine-Maritime et de l’Eure sont très sensibles aux ruissellements. Des investissements ont été réalisés ces dernières années pour réduire les risques associés à ces processus. Pourtant l’efficacité de cette politique publique n’a pas été évaluée. À l’échelle d’un bassin pilote (la Lézarde, 212 km2), cette étude originale combine la modélisation en parallèle du