Extreme precipitation events are the climatic drivers of urban runoff and urban flooding, and are expected to increase in the future due to climate change. Urban runoff does not only depend on the intensity of rainfall events but also on the degree of soil permeability. In the natural environments the meteoric waters are washed and filtered slowly from and through the soil. In the urban environment the impermeable surfaces hamper the natural water infiltration and cause a rapid water runoff towards the final receptor systems. In case of extreme precipitation events, the excessive runoff and the limited capacity of receptors can cause temporary flooding of the urban spaces. Due to climate change, extreme events are expected to intensify, generating additional pressure on the urban drainage systems and exacerbating their inefficiency.
A wide range of measures for the reduction of urban runoff are already available and applied, while others have been tested in pilot cases and are ready to become common practice.
Sustainable urban drainage systems (SuDS) are systems of structures built to manage urban runoff that tend to mimic natural drainage and exploit the drainage provided by natural elements. SuDS usually incorporate vegetation and soil in artificial structures with the aim of increasing the natural soil permeability, with positive effects also on groundwater recharge. In this sense, SuDS is an overall approach to urban runoff management, including a wide range of specific measures, such as green urban areas, green roofs, permeable flooring, unsealing of impermeable surfaces and artificial structures (e.g. leaking or infiltration wells, modular geo-cellular systems, filtering trenches, infiltration and bio-retention basins, etc.). More about unsealing may be found in Reduction of land consumption and surface unsealing in urban areas.
Green areas are much more permeable than urbanized ones and can be used in cities to reduce the water runoff. Trees, shrubs and plants in general can improve soil permeability and groundwater recharge. However, they have a limited effect due to their relatively small extension. Green areas can be created in various forms, including vegetated parking lots, tree-lined avenues, green mitigation of urban works, etc. In some cases, wider forest protection zones can be established, with greater beneficial effects in terms of increased infiltration, soil porosity and organic carbon accumulation. Besides creation of new vegetated urban areas, the conservation and maintenance of the existing ones also assume high relevance to cope with the impacts of extreme rain events and the related runoff.
Permeable flooring makes use of particular materials to improve rainwater infiltration through the urban surface into the underlying layers (soils and aquifers). This allows for storing water and releasing it slower, with controlled flow systems. There are two main types of permeable flooring: (i) porous flooring uses highly porous materials enabling water to infiltrate over the entire surface; (ii) permeable flooring uses materials that provide empty spaces (such as bricks) along the covered surface through which the water can infiltrate. The permeable flooring stores the outflow of precipitation below the surface and releases it at controlled speed, or allows a slow infiltration into the underground layer. Outflow reduction values may vary between 10% and 100%, but the effectiveness may decrease significantly over time without proper management.
Many Italian cities have started reconverting sealed surfaces into green areas and promoting other de-sealing actions, increasing their resilience to climate change. For example, the studio LandShapes located in Ravenna has worked on a project of urban regeneration along the Viale Matteotti in Milano Marittima, which includes the installation of rain gardens with positive aesthetic co-benefits. Other significant examples implemented in Italy are: the infiltrating trench installed at the private research centre of Kerakoll in Sassuolo (Modena) aiming to reduce the risk of flooding and pollution due to surface drainage of rainwater, Parco Catene in Marghera (Venezia) or the Vasca Milano-Parco Nord for the hydraulic management of the Seveso river (Milano). The last two examples prove how urban basins or wide green filters can reduce the risk of flooding and can increase water storage. Good practice examples for de-sealing of urbanized areas can be found at https://www.sos4life.it/
As for the area of sustainable urban drainage and climate change mitigation in the Republic of Croatia, most progress has been made in the City of Pula. Back in 2011, the Croatian company Starum, based in Pula, proposed the concept of stormwater and surface water drainage management according to the principles of SUDS. Since 2011, Starum designed dozens of projects, most of which were implemented in Pula, including rain gardens, infiltration trenches along the bypass and at the roundabout at the entrance to the city. A newly created park stretches along the Šijana area, as well as the rain gardens, infiltration trenches, lagoons, and contour swales. Rain gardens and retention areas have been built in King Tomislav square, Vladimir Nazor Street and at the Valdebek Children’s Playground. Rain gardens and porous parking surfaces have been created in Proštinske bune Street and by the Ribarska Koliba Resort (Pula). In the municipality of Stupnik, the installation of rain gardens along a 10-kilometer section of a roadway is currently being prepared, while rain gardens have been integrated into the landscape of the Croatian shipyard ‘Viktor Lenac’ (Rijeka).
Costs and benefits
Costs are variable and depend on the specific adaptation actions designed and implemented. Higher costs refer to complex interventions of renovation of the urban space and its infrastructure. The transformation of a wide urban area requires a long-term vision and regular funding. Small-scale measures (e.g. vegetated strips along roads) are cheaper and easier to implement; however, they provide more local benefits if compared to transformative approach taken at a larger scale.
Benefits include the overall reduction of vulnerability of urban areas to flooding, thus also reducing malfunctions, inefficiencies and interruptions of services in case of extreme climate events. These adaptation measures provide important climate change mitigation co-benefits as all measures including vegetation contribute to CO2 absorption. Improving green spaces also provides habitats for urban species and opportunity for recreational activities. Green areas can also contribute to reducing air pollution, decreasing urban heat-island effect and other climate change effects on health.
As some of the measures dealing with reduced urban runoff have positive effects on the ecological and social quality of urban areas, they also increase the values of the built capital and may attract investment. For example, the Green Infrastructure Plan of New York aims at reducing water purification costs by 2.4 billion dollars over 20 years and saving 7.5 billion litres of fuel by 2030. The Plan estimates the savings of 23,000 dollars/year in energy, emissions and air quality and an increase of 11,600 dollars in real estate values for each additional hectare of urban green infrastructure.
Implementation time and lifetime
The implementation of measures for the reduction of urban runoff can take several years (approximately 2-5 years) or less. The implementation time does not only depend on the scale of application, but is also determined by the availability of economic resources. The full transition to a more climate proof city/territory is a long and slow process, made of spread and systemic actions. Such a long time can act as a barrier, together with economic costs, for this kind of interventions. However, very specific and local measures can be quickly implemented. The maintenance is very important: it can be burdensome but ensures long-term lifetime of the implemented measures.
Source for more detailed information
Aad, M.P.A., Suidan, M.T., Shuster, W.D. (2010) Modeling techniques of best management practices: rain barrels and rain gardens using EPA SWMM-5. J. Hydrol. Eng. 15, 434-443.
Bedan, E.S., Clausen, J.C. (2009) Stormwater runoff quality and quantity from traditional and low impact development watersheds, J. Am. Water Resour. Assoc., 4, 998-1008.
Comune di Mantova, (2018) Mantova Resiliente: Verso il Piano di Adattamento Climatico. Linee Guida, Mantova
Field, R. (1986) Urban stormwater runoff quality management: low- structurally intensive measures and treatmen, In: Torno H.C.,
Foster J., Lowe A., Winkelman S., (2011), The value of green infrastructure for urban climate adaptation, The Center for Clean Air Policy