Urban pavements, which help to maintain clean and secure streets, generate large amounts of runoff that aggravate flooding and degrade the quality of surface water. In the context of ecological and climatic crisis, they also contribute to the creation of heat islands in cities, as well as hindering biodiversity. So greening cities must be encouraged. Our article highlights the case of Montpellier - Mediterranean Metropolis (Montpellier 3M, Southern France), which is undergoing one of the highest rates of demographic and urban growth in France. It asks questions around the impact of increased construction on the quality of water in small and fragile coastal catchment areas by the Mediterranean. It also examines local collective action in an attempt to identify the brakes and levers on greening initiatives for the effective management of runoff water, encouraged by legislation: de -paving, setting up rain gardens and returning to permeable urban soils. We use empirical material collected during sociological research in addition to hydrological and chemical measurements. Our aim was to elucidate the perspectives of a wide range of stakeholders, since an understanding of these is crucial in adapting blue-green infrastructure to the local context so that it is both efficient and accepted as a legitimate solution. Our examination suggests ways to think democratically about the dismantling of bitumen road paving to slow down urban water flows and thus improve the habitability of Southern European cities in a context of climatic and ecological crisis.