Blue City

Traditional cities are often designed to drain rainwater away as quickly as possible. Impermeable roofs, roads, car parks and squares collect rainfall and channel it into drains, pipes and waterways.

Today, this model is showing increasingly evident limitations. Heavy rainfall, heatwaves, land consumption and drainage systems under growing pressure call for a different approach. A Blue City does not treat rainwater simply as a problem to be disposed of, but as a resource to be slowed down, retained, filtered, infiltrated and, whenever possible, reused.

Why is an impermeable city fragile?

When the ground is covered by asphalt, concrete and other impermeable surfaces, water cannot infiltrate into the soil. Instead, it flows more rapidly, increases peak runoff, overloads drainage systems and sewers, and can cause localised flooding.

The problem is not only related to water management. Dark, impermeable surfaces absorb and retain heat, worsen thermal comfort during the summer and reduce the space available for vegetation and biodiversity. As a result, cities become both hotter and more vulnerable to heavy rainfall.

Technical guidelines on sustainable urban drainage describe a shift from an approach based primarily on pipes and rapid water disposal towards a more distributed model, in which water is managed at surface level through green and blue infrastructure.

What are the SUDS

SUDS, or Sustainable Urban Drainage Systems, are solutions designed to mimic some of the natural processes of the water cycle. They do not always replace traditional drainage networks, but complement them and help reduce the pressure on existing infrastructure.

Their purpose is to reduce surface runoff, slow down water flows, promote infiltration and retention, filter out some pollutants, and improve urban comfort and biodiversity.

The most effective solutions for creating more permeable and resilient cities include permeable car parks, green street designs, tree-lined avenues with structural soils, depaving interventions, parks with water retention areas, sponge squares, and buildings equipped with drainage strips.

Parkings, roads and green streets

A parking area can be designed to infiltrate and retain part of the rainfall by using permeable paving, grass grids, stabilised gravel, drainage sub-bases, trees and bioretention areas.

Streets can become blue-green infrastructure. Green verges, sunken planting areas, bioswales, vegetated swales and filtered drains intercept rainwater before it reaches the drainage network. These solutions are particularly suitable for residential streets, school areas, public car parks, roundabouts and roadsides.

Trees work effectively only when their roots have enough space to grow. Along urban streets, structural soils and adequate soil volumes can support paving and light traffic while maintaining the porosity and space needed for healthy root development.

Depaving, sponge parks and sponge squares

Depaving involves removing unnecessary asphalt and impermeable surfaces and replacing them with green areas, stabilised gravel, permeable paving or porous soils. It is particularly well suited to existing urban areas, where surfaces that have been unnecessarily sealed can often be reclaimed.

A park can be designed to temporarily accommodate water during heavy rainfall without losing its recreational function. Shallow depressions, temporary wetlands, sunken lawns and floodable areas can become light and effective water management infrastructure.

A square can also serve as a public space under normal conditions and as a temporary water storage area during intense rainfall events. So-called water squares demonstrate how high-quality urban spaces and effective stormwater management can coexist.

Maintenance and water quality 

SUDS are not decorative green features: they are infrastructure. They need to be properly designed, sized and maintained.

To function effectively, they require permeability testing, first-flush management, safe overflow systems, regular cleaning of filters and drainage chambers, vegetation management, and maintenance of permeable surfaces.

Water quality is essential. In car parks, busy roads or industrial areas, rainwater can carry oils, dust, metals and other pollutants. In these cases, appropriate treatment systems, filters, separators or other solutions are needed to prevent the direct infiltration of contaminated water.

Best practices:

· Italy/ Bologna - Guidelines SUDS: The Municipality of Bologna has developed guidelines with technical specifications and practical applications for car parks, streets and urban spaces, following an integrated approach that combines green infrastructure, urban quality and stormwater management.

· Italy/Padua - Sustainable urban drainage in urban planning: The guidelines developed by the Municipality of Padua within the framework of the LIFE Veneto ADAPT project link sustainable urban drainage with the planning and design of streets, squares, car parks and major maintenance works.

· Italy/Upper Vicenza Area - LIFE BEWARE - Natural Water Retention Measures: The LIFE BEWARE project implemented natural water retention measures in urban and rural areas, combining demonstration interventions with the active involvement of local communities.

· Europe/Copenaghen- Copenaghen - Cloudburst Management Plan: Following extreme rainfall events, Copenhagen adopted an urban strategy to manage cloudbursts through surface-based blue-green solutions, including channels, parks, water storage areas and controlled water flow routes.

· Europe/Rotterdam- Rotterdam - Benthemplein Water Square: Benthemplein combines public space with temporary rainwater storage. It is considered a best practice because it makes urban water management both visible and multifunctional.

· World/Singapore- Active, Beautiful, Clean Waters Programme -The Active, Beautiful, Clean Waters (ABC Waters) Programme transforms urban canals, rivers and reservoirs into accessible and liveable spaces, integrating water quality, landscape design and public use.