The refurbishment of Gomeznarro park in Madrid focused on storm water retention

The park is characterised by steep slopes which, combined with compacted soils and extensive sealed surfaces, resulted in little water infiltration and intense runoff and erosion during heavy rainfall. Nearby housing was regularly affected by flooding, with damp basements and ground floors causing inconvenience and health hazards, while runoff carried heavy sediment loads that burdened the city's wastewater system. Low soil moisture also limited vegetation growth and the park's cooling capacity. Under the high-emission scenario (RCP8.5), extreme heatwaves are projected to occur every two years by the end of the century (strongest impacts in southern Europe), with decreasing mean summer precipitation and more intense heavy-rain events expected across most of Europe.

Restore the natural water cycle in the urbanised park: regenerate the landscape, eliminate or significantly reduce runoff, cut the volume and suspended-matter load reaching the urban drainage system during heavy rain, and demonstrate water-sensitive materials and rainwater-management techniques.

Across the park's 10,000 m2 (Hortaleza District, southern Madrid), works included restructuring/replacing eroded and compacted soil, removing impervious pavement and replacing it with permeable surfaces to aid drainage and collection, installing underground storage tanks with associated collection/distribution systems beneath the pavements, and re-vegetating eroded areas. The project won a 2004 UN-Habitat 'good practice' award, and its water infiltration/collection technology has since been replicated across Spain, including park creation in Barcelona (Barrio les Roquetes, 2017), streetscape greening (Barrio Bon Pastor, Barcelona, 2016), permeable transport infrastructure (Logrono train station green cover 2012-13; A-6 highway permeable shoulder in Valladolid, 2008; Torre Baro street improvement in Barcelona, 2008), and rainwater-recycling-for-irrigation projects (a Las Palmas sports field, 2016-17; a Madrid elderly-care facility, 2017-18).

Madrid's municipality financed the project at a construction cost of 343,600 EUR (about 34 EUR/m2). Benefits were not monetised, but the park now has no additional maintenance cost versus traditional landscaping, and erosion, flood risk and drainage-system pressure have been reduced. The park now absorbs about 5 million litres of rainwater annually into the groundwater rather than the drainage system, lowering the need for supplemental irrigation, alongside improved local air quality and reduced temperatures in and around the park.

Resident complaints about flooded buildings drove inclusion of drainage improvement in the urban rehabilitation programme. The technology and materials were novel to Spain at the time, causing implementation delays (careful design, defining approval criteria for the private technology provider/consultant, contractor briefing), and required unusually intensive coordination across multiple municipal departments (water management, construction, planning, environment). The approach has since been replicated elsewhere in Madrid and Spain.