Located in Chêne-Bougeries (Geneva canton), this 1995 retention pond demonstrates NWRM in a dense urban plot. The small biotope (~300 m²) stores about 450 m³ on-site and contributes ~279 m³ at the 3.1-ha catchment scale, reducing storm runoff and flood risk. Design details include a lined basin (plastic membrane over sand; clay subsoil), a 1.2-m earth embankment with two overflows (primary outlet limiting discharge to ~48 L/s; emergency crest), and a ~150-m open swale collecting parcel runoff. The pond maintains permanent water (~0.5 m; max depth ~1.6 m); the safety overflow reportedly activated only once, and the basin has not dried out. Works were completed in ~2–3 months (1995) for ≈ CHF 60,000. Maintenance is shared by adjacent owners (annual vegetation removal). Reported benefits include habitat for amphibians, fish and odonates and a reduction in mosquito nuisance. Subsequent ecological survey work in Geneva (2015 sampling; 2018 publication) confirms continued use by amphibians at Étang Vieux-Clos. The approach aligns with the canton’s ongoing strategy to manage stormwater “à ciel ouvert”, with new retention/biotope projects under development nearby.