Cross-issues in the restoration of a heavily modified watercourse

The Vignois district is a housing estate built in Gonesse in the 1980s, bordering the communes of Arnouville and Bonneuil-en-France, in the Val-d'Oise region. Situated on the banks of the River Croult, the homes in this neighborhood have been regularly flooded by direct overflow from the river and/or by the backflow of stormwater networks into basements.

The Sarcelles-village sector, meanwhile, is the historic heart of the town, whose river finally disappeared, buried after the Second World War for essentially hygienic reasons.

Launched in 2007, the restoration projects for these two stretches of river are the responsibility of the Syndicat mixte pour l'aménagement hydraulique des vallées du Croult et du Petit Rosne (SIAH) as part of its remit, which in 2018 became GEMAPI (gestion des milieux aquatiques et prévention des inondations - management of aquatic environments and flood prevention).

Initiated by the SIAH, and with the support of its project manager, the two projects quickly became fully-fledged hydro-ecological projects. The opportunity to place these operations within a broader framework of watercourse restoration and wetland creation gave these projects a character that, while still somewhat experimental at the time for the SIAH, was nonetheless strategic in terms of the issues assigned to the "Croult" water body, whose good ecological potential, as a highly modified water body, is set for 2027.

The scope of the Vignois project in Gonesse is not only local, but also communal, with links to communal and regional parks. As such, it is a forerunner of future SIAH projects, some of which now have a purely ecological vocation.

Throughout the project, the river gradually reappears as a potential asset for enhancing the area, restored and remeandered. Paradoxically, however, the added landscape value is such that some local residents have expressed fears, echoing questions of public safety that the SIAH has been observing on the site of the Petit Rosne reopening in Sarcelles-village since 2014. The attraction of visitors (walkers, squatters...) has led to debate about the peace and quiet that local residents legitimately wish to enjoy.

On a newly restored "wet" site, the question of site management thus becomes a major issue, not only in terms of the site's longevity, but also in terms of the project's acceptance by the local population. A good understanding of the ecological issues at stake, both in terms of green space management and ecosystem balance, quickly becomes a strategic issue for the project owner, so that management rules are understood and accepted by all, and the site is respected and even defended.

In this way, the role...