NetZeroCities

Urbanisation and industrialisation have led to reduced vegetative cover and decreased water storage in the subsurface, as well as the concentration and accumulation of surface runoff in sewage systems due to reduced infiltration into the soil [1] [2] [5]. Until now, no universally applicable urban flood management solution has been developed. However, hybrid measures that combine nature-based solutions (NBS) with conventional engineering solutions/grey infrastructure have been identified as the optimal mix of security provided by grey infrastructure with the multiple co-benefits of nature-based solutions [3].

The typical engineering solutions are rainwater systems collecting rainwater via drainage pipes, channelled to the urban rainwater system, and further to the water utilities to be purified. In older cities, rainwater is typically collected in the same water systems as sewage water, which creates large volumes of water for the local water utilities to purify.

There are a number of nature-based solutions for rainwater retention [4]:

GREEN ROOFS

Green roofs are associated with three main positive effects: 1) cooling and evapotranspiration that leads to lower local temperatures, 2) storing capacity of the excess water, and 3) sunlight absorption.

GREEN ROOFS [4]

Green roofs can be categorised as follows:

Intensive green roofs have both ecological and aesthetic requirements, as they are usually designed also for recreational purposes. The growth media is relatively thick and deeper than in extensive green roofs. Typical plants for intensive green roofs are trees, shrubs and perennials. Intensive green roofs need regular irrigation and fertilization. The water storage capacity is 30-160 l/m2.

Extensive green roofs are light weight systems, typically characterized by minimum maintenance and management. Typical plants for extensive green roofs are low growing, rapidly spreading and shallow-rooting. Extensive green roofs are defined as a roof for which the capacity to store water is at least 25 l/m2, typically between 20-50 l/m2 .

Smart roofs represent an extension of conventional green roofs because the system is equipped with a drainage system under the vegetation layer. The drainage layer retains storm water.

Constructed wet roofs connect green roofs and constructed wetlands for domestic wastewater (grey water) treatment. Wet roofs are most commonly planted evenly with wetland or marsh plants, but can also have height differences. In addition, constructed wet roofs retain storm water for some time, and gradually release rainwater and reduce the overall runoff. Constructed wet roofs have positive impacts on the microclimate and cool down the local climate.

VERTICAL GREENING in building integrated solutions

Vertical greening in building integrated solutions can be either facade-bound or ground-based. These solutions have lower capacity of water retention compared to green roofs. Since pure vertical soil is not found in nature, nearly all types of vertical greening need maintenance [4].

FAÇADE GREENING SOLUTIONS [4]

BIOSWALES

Bioswales are vegetated linear and low-sloped pits constructed near buildings or roads to reduce flood risk after heavy rains. Bioswales absorb, store, and convey surface water runoff.

BIOSWALE [4]

MATURITY:

Most rainwater retention solutions are already available on the market or very close to commercial deployment. However, it should be noted that many of the solutions are climate-dependent. Solutions that consider long-term durability and viability in northern climate zones tend to be in the demonstration phase.

Examples of solutions available on the market or close to commercial deployment:

Intensive and extensive green roofs in moderate climate zones can be found in demonstrations and commercial products in e.g., UNaLab. The solutions in UNaLab include also northern climate solutions. Moderate and hot climate solutions can be found e.g., in URBAN GreenUP and in CLEVER Cities .
Facade-bound vertical greening is implemented in e.g., GROW GREEN. Ground-based vertical greening is constructed e.g., in URBAN GreenUP.
Bioswales are implemented in CLEVER Cities and in UNaLab in the northern climate zone, and in the moderate climate zones in GROW GREEN and in UNaLab.

Examples of small-scale demonstration and piloting:

UNaLab handbook gives technical description and layout for demonstration of smart roofs and constructed wet roofs [4].

Physical health, Mental wellbeing/quality of life, Aesthetics: Urban green is proven to increase well-being and quality of life. Typically the perceived quality of living and working area is higher when urban green is applied (GROW GREEN, CLEVER Cities), even more so in low-income cities [8]. Urban greenness is therefore a tool to promote social inclusion and environmental justice (URBAN GreenUP).

Better water management: Urban green acts as a buffer zone for heavy rainwater and helps improve rainwater management and quality (CLEVER Cities).

Improved air quality: Urban green contributes to air purification, depending on species, particulate matter and harmful gases. (GROW GREEN).

Sustainable and resilient food systems: Urban green has some potential for small-scale food production, e.g., growing vegetables and herbs on green roofs. For instance, crops with shallow roots such as lettuce, kale, and radish can be produced in extensive green roofs, provided there are sufficient nutrient and moisture inputs [9].

Reduce hot spots/urban heat islands: Green façades and roofs reduce local temperature though water evaporation. A green facade can reduce the temperature by about 2-10 Celsius, compared to natural stone [4]. (UNaLab, URBAN GreenUP).

Reduced energy poverty, Energy reduction for buildings (heating/cooling): Green roofs and facades decrease heating needs by insulating buildings, and decrease cooling needs by evaporative cooling (URBAN GreenUP).

Reduced noise pollution: Urban green acts as sound insulation. Green noise barriers have a specific geometry that favours sound reflection. There are also vertical garden modules with a specific substrate that favours sound absorption. The noise reduction capacity depends greatly on the size and design of the urban green (CLEVER Cities, URBAN GreenUP). Green facades are most efficient in reducing noise when they are applied on acoustically hard materials. Greening of the highest storey on the street and façades in the courtyard are the most efficient ways to reduce noise [10].

Biodiversity conservation: Urban green provides habitat with potential to maintain and increase urban wildlife. Biodiversity conservation is highly dependent on the species chosen for the area (UNaLab). For instance, birds settle in green walls for e.g., nesting, food and shelter, whereas they are not present in bare walls Green walls can thus be an effective mean to provide resources for birds in cities [11].

Pre-conditions

Climate and geography: Climatic and geographical conditions affect greatly the performance of the solution. E.g., in colder climates during frost periods the urban green is not able to store water, as it is at least partially frozen.

Technical aspects/infrastructure: In the case of building integrated solutions, buildings’ physical and cultural conditions need to be considered and evaluated before implementation. Technical integration of the urban green needs to take into account space for rooting so that the roots are not harmful to existing infrastructures or building elements. See the UNaLab technical handbook for good design principles and technical guide [4].

Enabling conditions

Policy and regulatory/legal framework:

Public Procurement of nature-based solutions can be a tool to promote the use of such solutions in public buildings.
The design and deployment of NBS for water retention needs to be integrated into careful land use planning. It also needs to take into account equal distribution of the overall urban green in the city, to avoid socioeconomic inequalities and so that all citizens can benefit equally from the implementation of such solutions [17].

Funding and financing:

Access to EU/public funding can be crucial to support the initial investment for new solutions and also to encourage urban green for public procurements [16]. For investments costs, there are already some good approaches e.g., in urban planning and in public procurement. URBAN GreenUP used a methodology for the design of Renaturing Urban Plans [14]. UNaLab used innovation vouchers for groups of citizens to adopt nature-based solutions [15].

Project governance and implementation modalities:

Citizen engagement and collaborative design have proven to be extra beneficial for creating not only technically performing solutions, but also solutions that are creating inclusive cities. For example, URBAN GreenUP and CLEVER Cities used citizen consultation and workshops to work together with local municipalities and other stakeholders to create better environments and co-responsibility of the urban green space.

Climate and geography:

Northern climatic conditions as well as hot and dry conditions can be very challenging. The choice of species plays a crucial role for long-term sustainability of the solutions. Ground-based greening is exposed to drought stress and is dependent on natural water cycle. Climbing plants originate in forests, therefore often shaded conditions are optimal. Implementation of façade-bound greening is limited by frost risk. Climate-specific demonstrations can be found e.g., in UNaLab.

Funding and financing:

Urban green has in some cases slightly higher investment and maintenance costs compared to traditional rainwater retention systems [13], which can thus be perceived as barriers to the implementation of these solutions [12].
A challenge related to funding is that the benefits associated with nature-based systems cannot be capitalized by any organisation or company, which leads to the problem of ownership. Therefore, the new types of innovative partnerships and business models are needed [18] [19]. E.g., public-private-partnerships can be an effective tool for development.
Although the benefits of nature-based solutions are well renowned, lack of finance is still an issue [13]. In the public sector, Public Procurement of nature-based solutions can be hindered by the lack of NBS-dedicated budget at local level [16].

Technical aspects/infrastructure:

As green roofs are heavier than normal roofs, load bearing can be a barrier for implementation, especially in old buildings. Limited space for rooting may restrict implementation of intensive and extensive green roofs (although the growing media is relatively thick). Roof pitch can also be too steep for implementation. Green facades and bioswales need space as well as light, which might be a barrier in dense city areas. In terms of irrigation needs, façade-bound greening and intensive green roofs have high dependency on irrigation systems.

Urban form and layout:

Urban green systems need more space in order to function compared to traditional rainwater management with pipes and drainage systems.

Impact of water-retention solutions can be measured by the following indicators and metrics [6]:

Runoff in relation to rainfall
Metric: Run-off in relation to precipitation quantity (mm/%)
Infiltration capacity
Metric: Infiltration capacity (%; change in precipitation infiltration capacity measured using ring infiltrometer and extrapolated/modelled for full unsealed area)
Evapotranspiration: a loss of water from the soil both by evaporation from the soil surface and by transpiration from the leaves of the plants growing on it.
Metric: Measured or modelled evapotranspiration (typically expressed in mm per unit time)

Nature based solutions reduce the need for energy-intensive materials and technologies for water retention, e.g., plastic ducts, concrete structures, or other energy and fossil raw materials. They also reduce the need to treat the wastewater and thus contribute to the reduction of CO2 emissions [7].

Nature based solutions are also an important tool for carbon sequestration, actively contributing to lowering the CO2 levels in the atmosphere [7].

The water retention capacity varies greatly depending on the construction of the nature-based structure. Green facades have very limited capacity to absorb water, whereas the capacity of extensive green roofs can be up to 55 l/m2. The green roof structure is saturated roughly in 160 kg/m2 [4]. If smart roofs are used, the capacity can be even higher.

Facade-bound greening [4] :

Evapotranspiration: 5-20 % sunlight is used for photosynthesis, 20-40% is used for evapotranspiration, 10-50 % transformed into heat and 5-30% reflection
Water retention: 15-30%

Ground-based greening [4]

Green walls have good evaporation capacity (65-75 % of the annual rainwater)
Evapotranspiration: 5-20 % sunlight is used for photosynthesis, 20-40% is used for evapotranspiration, 10-50 % transformed into heat and 5-30% reflection

Intensive green roof [4]

Water retention capacity: 30-160 l/m²
Weight: 190-680 kg/m2

Extensive green roof [4]

Water retention capacity: 20-50 l/m²
Weight: 20-190 kg/m2

URBAN GreenUP aims at developing, applying and validating a methodology for Renaturing Urban Plans to mitigate the effects of climate change, improve air quality and water management, and increase the sustainability of cities through innovative nature-based solutions. Examples are vertical green, green coverage roofs, permeable green pavements, and water retention ponds. The project also provides examples of solutions for dry and hot climate. The project has three runner cities, Valladolid (Spain), Liverpool (UK) and Izmir (Turkey), and five follower cities, Mantova (Italy), Ludwigsburg (Germany) Medellin (Colombia), Chengdu (China) and Binh Dinh-Quy Nhon (Vietnam). (Horizon 2020, 2017-2022)

UNaLab contributes to the development of smarter, more inclusive, more resilient and increasingly sustainable cities through the implementation of nature-based solutions. The solution examples are green roofs, bioswales, and water retention ponds. The project also includes cold climate solutions. The project has three front-runner cities, Tampere (Finland), Eindhoven (The Netherlands) and Genova (Italy), and seven follower cities, Başakşehir (Turkey), Cannes (France), Castellón de la Plana (Spain), Prague (Czech Republic), Stavanger (Norway), Buenos Aires (Argentina) and Hong Kong (Hong Kong). (Horizon 2020, 2017-2022)

GROW GREEN aims to create climate and water resilient, healthy, and liveable cities by investing in nature-based solutions. The solutions include renovation of courtyards, and the focus is on raingardens and swales in mild and moderate climate zones. In addition, for hot climate zones, façade-integrated vertical green is implemented. The project has three front-runner cities, Manchester (UK), Valencia (Spain) and Wroclaw (Poland), and four follower cities, Brest (France), Zadar (Croatia), Modena (Italy) and Wuhan (China). (Horizon 2020, 2017-2022)

CLEVER Cities uses nature-based solutions to address urban challenges and promote social inclusion. It focusses on green solutions (e.g., bioswales, water retention ponds, and green walls) not only to prevent urban flooding from heavy rains, but also for sea level rise. The solutions are applicable also in northern climates. In addition, the focus is on greening existing urban structure and preventing urban flooding with green roofs in warm and hot climate. The project has three front-runner cities, Hamburg (Germany), London (UK) and Milan (Italy), and six follower cities, Belgrade (Serbia), Larissa (Greece), Madrid (Spain), Malmö (Sweden, Quito (Ecuador) and Sfântu Gheorghe (Romania). (Horizon 2020, 2018-2023)

ADDITIONAL INFORMATION FROM CASE STUDIES:

Approximate costs of some of the solutions [4] :

Intensive green roof: investment > 500 €/m2, maintenance 3,5-5,0 €/m2,a
Extensive green roof: investment 15-70 €/m2, maintenance 0,5-3,0 €/m2,a

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