River bio-engineering for bank protection in urban context: measured and perceived ecosystem services

International audienceIncreasing biodiversity and associated ecosystem services through ecosystem restoration has become a new challenge for urban planning. Riparian ecosystems are known to host an important biodiversity, to fulfill key ecosystem services, especially flood control and riverbank protection. Their restoration with soil bioengineering techniques offers a nature based solution for erosion and flood protection, while (unlike civil engineering) maintaining a significant level of biodiversity and associated services, including the improvement of the living environment of residents. In the GeniEaux project we followed an interdisciplinary approach to assess the ecological and social services of soil bioengineering in comparison with civil engineering in the context of riverbank protection. Those services were assessed: 1. through biophysical field measurements, 2. through the perceptions of urban residents or urban water stakeholders. The study was carried out in two stages. 12 structures – 5 civil engineering, 3 mixed engineering and 4 soil bioengineering, were selected in urban areas in Rhône-Alpes, near Lyon, Chambéry and Grenoble. The structures have been built between 2006 and 2017 on small streams. The assessment of biodiversity and associated ecosystem services was carried out in spring 2020, using botanical survey (floristic composition, presence of Invasive alien species, biodiversity indicators), topographical and landscape integration descriptions (proxies of lateral, longitudinal continuity, and shading).Then, in January 2021, an online photo-questionnaire based on these same structures was distributed to residents and urban water stakeholders. This questionnaire dealt with identifying the perceived services associated to those structures, assessing their assigned values (aesthetics, recreational and safety); and determining the criteria used by surveyed people to define a successful riverbank protection. About 500 answers were received, from urban water stakeholders or residents - neophyte to citizens aware of the biodiversity issues - half of which were living in urban areas. The first results showed that the most integrated riverbank protection structures clearly maximise plant biodiversity in urban environments, as well as some ecosystem services such as shading and connectivity. They are also more appreciated than civil engineering by residents and urban water stakeholders, mainly due to the increase in the presence of vegetation which induces a higher perceived naturalness in cities