Reducing the imperviousness of urban soils: a way of improving the quality of runoff that is struggling to impose itself in the Mediterranean area

Coastal areas are under the influence of runoff that wash out urban soils that are mostly impervious. Our study reports on interdisciplinary research combining hydrology and sociology, to understand the impact of rain runoff on the quality of coastal waters, and to identify brakes and levers for improving urban infrastructure that would slow down the transfer of runoff and associated contaminants to the environment.

After measuring and modelling the transfer of contaminants from impervious urban soils to the environment, semi-structured interviews were conducted in the Montpellier metropolitan area with scientists, local water management stakeholders (elected officials, technical services, state services) and representatives of civil society (associations). They were completed by a focus group with a panel of people previously interviewed and selected. The collection of testimonies allowed us to understand that the pollution generated by urban stormwater was not, with some exceptions, identified as a public problem. The renaturation of urban spaces, to limit the transfer of contaminated water, is rarely considered as a solution, whatever the incentives of the Water Agencies in this field. Preference is generally given to heavy techniques (retention basins and special coatings) rather than to soft alternative developments (rain gardens, etc.). Beyond the organizational reluctance of city's technical services, contextual factors explain the lack of political support for change: the issues of the degradation of the quality of rainwater runoff, which is difficult to perceive, appear negligible compared to the risks of flooding in a Mediterranean region subject to extreme rainfall events. In this particular context, scientists must redouble their efforts to challenge and interest decision-makers in order to convince them to take steps to improve the quality of coastal waters.

The results of our research enabled us to test several runoff management scenarios: the installation of permeable pavements on road surfaces or the connection of roof runoff to permeable areas. They also opened up a broader socio-hydrological reflection on the role of scientists in the city, and their necessary involvement with stakeholders to jointly imagine a concerted approach to water issues.