Watersheds are complex systems with multiple interactions between physical processes and human-induced socio-economic dynamics. Since the 2000s, numerous flooding and mudslide events have affected the territory in Normandy (France), leading to significant damages. Therefore, a public policy was adopted with the aim to reduce runoff and erosion, it includes: (i) the building of 4,000 hydraulic infrastructures (dams, fascines, hedges, etc.), (ii) the creation of turbidity water-treatment plants and, (iii) the conduction of animation and protection programs on soil and water resources. These investments are co-funded by several local authorities. This original research project aims evaluating the effectiveness of the above-mentioned public policy. Therefore, two complementary approaches are applied: (i) at the regional scale, the investments and damages between 2000 and 2017 were assessed and, (ii) for a pilot small scaled watershed (la Lézarde, 212 km²) a coupled modeling was conducted, taking hydro-sedimentary processes (flood envelopes, diffuse and concentrated erosion, karstic transfers) and associated socio-economic dynamics into account. Our results suggest that over the study period, at the regional scale 500 M\euro were invested to reduce erosion/runoff impacts and, 300 M\euro of damage were caused. Nevertheless, the effectiveness of the public policy since 2000s must be evaluated at the watershed scale using a Cost-Benefit Analysis (CBA) according to two main scenarios: S1 = pre-development (2000), and S2 = post-development (2017). The processes that govern the surface transfer are modeled for different design floods (Q10-50-100) coupling two semi-dynamic models (MikeSHE and Watersed), and the karstic transfer using a deep learning algorithm (Tensorflow). Additionally, three long-term scenarios (until 2050) are modeled taking into account the effects of climate change (RCP scenarios), the change in land use (-33% of grassland areas), and the modification of agricultural practices that limit runoff. These projections provide key elements for decision-makers to guide future public policies controlling runoff and erosion in this territory.