NetZeroCities

Sustainable urban drainage systems (SUDS) are a wide category of water interventions to align modern drainage systems with natural water processes. SUDS comprise “a sequence of management practices, control structures, solutions and strategies designed to efficiently and sustainably drain surface water, while minimising pollution and managing the impact on water quality of local water bodies” [1].

SUDS can be considered more sustainable than traditional drainage methods mostly due to the co-benefits they can bring to urban areas such as recreational benefits, biodiversity and habitat creation, and carbon storage and sequestration. SUDS are also environmentally beneficial because they cause minimal or no long-term damage.

The main types of SUDS are the following:

Hard-drainage flood prevention can be considered as a hybrid nature-based solution (NBS) that focuses on the delivery of additional permeability with respect to traditional engineered approach to water management. It supports water infiltration thus reducing pluvial flood risk in urban areas and improving the quality of water within sewerage systems [2].

Grassed swales are shallow, flat bottomed, vegetated open channels designed to convey, treat and often attenuate surface water runoff (Urban GreenUP).

Grassed swales in Izmir, Turkey. URBAN GreenUP. Source: https://www.urbangreenup.eu/cities/front-runners/izmir.kl#lg=1&slide=17.

Water retention ponds provide additional capacity to retain storm water continuously. In dry periods, they hold water, providing a water source that can be accessed. They can improve the water quality (e.g., with downstream infiltration). For example, in UNaLab, a retention pond has been built in Vuores, Finland to enable the treatment (retention and sedimentation) of urban runoff from a new housing estate. It complements the existing measures in the Vuores area for enhanced stormwater quality and quantity management.

Retention pond in Vuores, Finland. UNaLab. Source: https://unalab.eu/en/retention-pond.

Floodable parks can be designed to control flow rates and decrease flow peaks by storing excess floodwater and releasing it slowly once the risk of flooding has passed. This solution can mitigate potential impacts caused by surface run-off water from rain, flash-floods or from small- and medium-sized watercourses. Other potential benefits are reducing the water flow entering the public sewerage system and delivering amenity and biodiversity benefits (Urban GreenUp).

Water-retentive pavements, including hard drainage pavements and green parking pavements, are permeable pavements that are commonly used on roads, paths and parking lots. These solutions can help control storm water, reduce stagnation of runoff and surface water, and improve water quality via filtration.

Constructed wetland is a NBS mostly designed to increase water quality and/or support wastewater treatment, which recreates the removal processes developed in natural wetlands, exploiting complex biochemical, physical, and physiological removal processes (ReNature).

Rain garden is a bio-retention shallow basin designed to collect, store, filter and treat water runoff. To optimise its functions, it must include a porous soil mixture, native vegetation and some hyperaccumulator plants, capable of phytoremediation (cleaning up contaminants). For example, see the rain garden developed in the Gavoglio Urban Park in Genova (part of the UNaLAB project).

MATURITY:

Most SUDS are widely available on the market, with some variation in technology readiness level depending on the technological and innovation components.

Technical aspects/infrastructure:

Conditions and characteristics of soils have to be considered such as bearing capacity, soil type, and permeability, also in relation to the existing sewage infrastructure.
The level of the groundwater table should be properly considered in the technical design of the solution.

Policy and regulatory/legal framework:

Diverse policy and regulatory frameworks at different levels can enable the implementation of SUDS.

At the EU level, within the overall framework of the EU Green Deal:

The European Biodiversity strategies: cities with a population more than 20,000 are called upon ‘to develop Urban Greening Plans, including measures to create biodiverse and green urban forests, parks and gardens; urban farms; green roofs and walls; tree-lined streets; urban meadows; and urban hedges’. To facilitate this work, the Commission has set up an EU Urban Greening Platform with cities and mayors in 2021 under the Green City Accord [7].
The Water Framework Directive [16] and the Flood Directive [17] recognize the value of natural water retention measures (NWRMs) and supports their implementation through river basin management plans and the accompanying program of measures. NWRMs can be seen as SUDS, as an alternative to grey infrastructure (Article 4.7), to achieve and maintain healthy water ecosystems and offer multiple benefits [8].
The Strategic Environmental Assessment [9] procedures and the inclusion of criteria to compensate, mitigate or enhance Ecosystem Services through the increase of permeable surfaces.
Green Public Procurement [10] and specifically:
Criteria for Office Building Design, Construction and Management [11] acknowledges the integration of nature-based solutions (NBS), such as green roofs and walls, habitats in courtyards and patios, sustainable urban drainage systems (SUDS) and street trees, also including references to co-benefits and multi-functionality of NBS.
Criteria for Road Design, Construction and Maintenance [12] introduces water pollution control components and storm water retention capacity components, including soft engineered solutions (e.g., NBS) in the drainage system notably to reduce runoff into storm sewers and the overall amount of water entering local storm sewers or surface waters, thereby significantly reducing flooding-related damages.

At local level, cities may refer to diverse strategies/norms and laws:

Urban plans, local building codes and local green public procurement that would provide incentives for the implementation of permeable areas and SUDS.
An example of a successful procurement process can found in the City of Eindhoven where the city required the companies to submit documentation highlighting their vision for managing the cooperation between the municipality and construction companies, as well as provide a risk and opportunity assessment for the development of a city square greening and rainwater management [13]. Another example, Frederiksberg in Denmark used innovation partnership model to procure customised solution for heavy rain management, allowing to combine R&D activities with a purchase, while adhering to procurement rules [13].

Funding and financing:

Co-funding of solutions: the case of Dales Brow in Salford is a good example of a shared mechanism of funding from diverse actors, specifically City of Trees (civil society movement), Salford City Council and the Environment Agency, for developing a previously waterlogged greenspace into a multifunctional green asset [3].

Project governance and implementation modalities:

Market and community engagement: For example, in the procurement process of a solution for rainfall management, Frederiksberg in Denmark conducted dialogue with industry and R&D stakeholders to better understand the state-of-the art knowledge. The city also conducted co-design and awareness activities with local communities. Specifically, the city commissioned a survey to raise citizens’ awareness about the issue of heavy rainfall and then, after having defined the contractors and the conditions of the project, the city organized meetings with the community to provide clarifications about the works [13].

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