Summary
Summary
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.
Photo gallery
Overview
Year of issue
2025
Project
NWRM
Contact
Anais Hanus, ACTeon
Location of the project
At the junction of Chemin du Vieux-Clos and Chemin de Vert-Pré, Conches (Chêne-Bougeries); small valley bottom.
Country
Switzerland
NUTS2 Code
Ticino
Sponge measures implemented in the case study
Ponds and scrapes
Objectives
Attenuate storm peaks by ~450 m³ storage (BV retention ~279 m³), cap outflow ~48 L/s, and maintain ≥ 0.5 m permanent water to support aquatic biota; dimensioned for T=10 years events.
Involved partners
| Authority type | Authority name | Authority role | Comments |
|---|---|---|---|
| Other | Chêne Bougerie town | Initiation of the measure | — |
| — | — | Financing | Architects |
| Private property owners | Solfor SA | Determination of design details of the measure | Architects |
| Private property owners | Jacquet SA | Implementation | Engineers |
Site Information
Climate Zone
Cool temperate moist
Precipitation
942.00 mm
Temperature (°C)
10.10 °C
Runoff
54.00 mm
Runoff coefficient
0.25
Elevation range
422
Slope range
1-2%
Vegetation class
Within the pond: reeds (roseaux), water lilies (nénuphars) and other aquatic macrophytes; along the ~150 m open swale, reed-rich, moisture-loving vegetation.
WFD Ecological status
Moderate
WFD Chemical status
Good
Water quality status
Urban Seymaz: historic channelization & limited space, urban runoff, and recurrent pollution episodes; biological quality historically poor, renaturation ongoing.
Design and Implementation
Project scale
Micro
Project scale specifications
a single on-plot retention biotope within a villa subdivision (~3.2 ha parcel; BV ~30,800 m²)
Project area
3,2 ha
Area subject to Land use change or Management/Practice change (ha)
0.03 ha
Size
300.00
Size unit
m2
Favourable preconditions
A natural 3 ha depression/valley (NW–SE), clay subsoil (quasi-impermeable), and an undersized existing collector favored an on-site retention biotope over costly pipe upgrades.
Financing
Total cost
65126.00 €
Costs operation maintenance
49000.00 €
Policy and Governance
Policy description
Privately owned parcels with an internal regulation and shared maintenance; built after rezoning to villa zone (1990s) to mitigate runoff from urbanization. Aligns with canton policy promoting parcel-level stormwater retention and current “à ciel ouvert” strategy.
Land ownership
Private villa-plot owners at Conches. The pond/biotope spans parcels with an internal owners’ regulation; upkeep costs are shared pro-rata.
Policy targets
Increase Water Storage
Improved Biodiversity
Oher Societal Benefits
Part of a wider plan
Yes
Wider plan
| Wider plan type | Wider plan focus | Name |
|---|---|---|
| National | Water | federal law on water protection, 24 january 1991 |
| Local | Water | Cantonal law on water (L 2 05) |
Public consultation
No
Has contractual arrangements
No
Impacts and Monitoring
Impact Assessment Methodology
Performance impact assessment method
Unknown
Performance impact estimation information
null
Socio-economic Impacts
Direct benefits information
The pond reduce runoff in the area and the damages it can generate
Ancillary benefits information
The pond provides a "natural" area which can be used as recreative area for neighboring private owners. It also contributed to reduce mosquitos density.
Biophysical impacts: Hydrology
Information on retained water
Peak-flow attenuation and temporary storage (~450 m³ on-site; ~279 m³ at BV scale), controlled discharge (~48 L/s), and creation of a small hydromorphic biotope; infiltration limited by clay.
Increased water storage
90.00
Increased water storage unit
m3/ha
Information on increased water storage
279m3 can be retained at the watershed scale (3,1ha)
Ecosystem water supply
0
Biophysical impacts: Biodiversity
Improved biodiversity
Yes
Information on improved biodiversity
The pond provide habitat for fauna and flora, including aquatic flora
Biophysical impacts: Water & Soil Quality
Water quality overall improvements
Not relevant for this application
Soil quality overall improvements
Not relevant for this application
Biophysical impacts: Others
Ecosystem impact climate regulation
No information available
Lessons Learned
Key lessons
Example of how to retain water in urban areas.
Its success lead to the easy implementation of new water management projects in the city, for example a new retention pond project was presented to the public in 2025.
Its success lead to the easy implementation of new water management projects in the city, for example a new retention pond project was presented to the public in 2025.
Drivers
| Driver type | Driver role | Comments |
|---|---|---|
| Other | Main driver | The main driver which led to consider the measure was the new status of the land which became available to be constructed; water management became a major issue. |
| Other | Main driver | The measure was estimated cheaper than another project (re-build existing infrastructures) |
| Legal obligations | Main driver | The federal law on water protection requests rain water infiltration in urban areas through retention measures |
| Organisation committed to it | Secondary driver | Geneva Canton elaboraed recommandations regarding water retention in urban areas |
Success factors
| Success factor type | Success factor role | Comments |
|---|---|---|
| Attitude of relevant stakeholders | Main factor | Since private owners are responsible for maintaining vegetation on their plots, the measure efficiency partly depends on their attitude |
| Legal obligations | Secondary factor | Legal obligations can lead stakeholders to implemnt measures to retain water in urban areas |
Transferability
Highly transferable to low-gradient suburban plots. Key cautions: ensure legal servitudes/owner agreements, safe overflow paths, vector control by predators (avoid stagnant edges), and clear O&M.