Nature-based solutions as alternative to tackle urban water and soil management challenges: A distributed experiment in three climate regions
Résumé
Urbanization leads to sealed surfaces that are poorly covered with vegetation; consequently, the amount of water that can infiltrate soils is limited compared to rural area. In this context, nature-based solutions (NBS) are a new way to deliver multiple environmental benefits especially for urban space management through suggesting best practices. Nowadays, the use of innovative procedures will help to promote the social acceptability of water management and soil challenge by the city users. In the Horizon 2020 Nature4Cities project aims at a multi-thematic and multi-scalar evaluation of all the NBSs and their capability in addressing different urban challenges (UC) on a platform used by urban planners. To question this crucial issue, we chose to focus our study on the capability of NBSs to address UC from physical, chemical and ecotoxicological aspects. To do so, different NBSs [e.g. swales, family gardens (FG), green roofs (GR)] have been studied in different cities (City of Paris, Nantes and Nancy). Several indicators relating to environmental issues were selected and, were linked to threshold values and baselines. The results exhibit contrasted behaviours and have confirmed that NBS have a relevant impact on studied UC. NBSs are of purifying interest by using the soil as a trap for micropollutants (GR and swales) and, have good fertility through the consideration of amendments or the initial presence of organic matter (FG). In term of best practices, we must pay attention to the deleterious impact of contaminants on soil microorganisms, which in turn may reduce the positive benefits of NBS and to occasional pollutant discharge from swales. We also have suggested different ways to improve the soil fertility and at the same time to mitigate contaminations in soils by: (i) adding mulching over the NBS, (ii) adding a certain concentration of biochar and (iii) using pre-existing vegetation. The outcome is a set of suggestions for reference values for the selected indicators. The objective is to define new threshold values adapted to urban context.