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Blue-Green Cities aim to recreate a naturally oriented water cycle while contributing to the amenity of the city by bringing water management and green infrastructure together. This is achieved by combining and protecting the hydrological and ecological values of the urban landscape while providing resilient and adaptive measures to address future changes in climate, landuse, water management, and socio-economic activity in the city. Designing and utilising the urban environment to manage water resources, water demand (including rainwater harvesting), and the interplay between flood and drought are key drivers. Integrating water management with urban green space provision plus the added value associated with the connection and interaction between blue and green assets are key concepts of a Blue-Green City. Blue-Green Cities generate a multitude of environmental, ecological, socio-cultural and economic benefits through integrated planning and management  and may be key to future resilience and sustainability of urban environments and processes.
Blue-Green Cities aim to reintroduce the natural water cycle into urban environments and provide effective measures to manage fluvial (river), coastal, and pluvial (urban runoff or surface water) flooding  while championing the concept of multi-functional greenspace and landuse to generate multiple benefits for the environment, society, and the economy.
Visible water in cities has massively declined in the last century  and many areas are facing future water scarcity in response to changes in climate, landuse and population. The concept of Blue-Green Cities involves working with green and blue infrastructure components to secure a sustainable future and generate multiple benefits for the environmental, ecological, social and cultural spheres. This requires a coordinated approach to water resource and green space management from institutional organisations, industry, academia and local communities and neighbourhoods.
The natural water cycle is characterised by high evaporation, a high rate of infiltration, and low surface runoff. This typically occurs in rural areas with abundant permeable surfaces (soils, green space), trees and vegetation, and natural meandering water courses. In contrast, in most urban environments there is more surface runoff, less infiltration and less evaporation. Green and blue spaces are often disconnected. The lack of infiltration in urban environments may reduce the amount of groundwater, which can have significant implications in some cities that experience drought. In urban environments water is quickly transported over the impermeable concrete, spending little time on the surface before being redirected underground into a network of pipes and sewers. However, these conventional systems (‘grey’ infrastructure) may not be sustainable, particularly in light of potential future climate change. They may be highly expensive and lack many of the multiple benefits associated with Blue-Green infrastructure.
Land planning and engineering design approaches in Blue-Green Cities aim to be cost effective, resilient, adaptable, and help mitigate against future climate change, while minimising environmental degradation and improving aesthetic and recreational appeal. Key functions in Blue-Green Cities include protecting natural systems and restoring natural drainage channels, mimicking pre-development hydrology, reducing imperviousness, and increasing infiltration, surface storage and the use of water retentive plants. A key factor is interlinking the blue and green assets to create Blue-Green corridors through the urban environment.
Blue-Green Cities favour the holistic approach and aim for interdisciplinary cooperation in water management, urban design, and landscape planning. Community understanding, interaction and involvement in the evolution of Blue-Green design are actively promoted. Blue-Green Cities typically incorporate sustainable urban drainage systems (SUDS), a term used in the United Kingdom, known as water-sensitive urban design (WSUD) in Australia, and low-impact development or best management practice (BMP) in the United States. Green infrastructure is also a term that is used to define many of the infrastructure components for flood risk management in Blue-Green Cities.
Water management components in Blue-Green Cities are part of a wider complex “system of systems” providing vital services for urban communities. The urban water system interacts with other essential infrastructure such as information and telecommunications, energy, transport, health and emergency services. Blue-Green Cities aim to minimise the negative impacts on these systems during times of extreme flood while maximising the positive interactions when the system is in the non-flood state.
Key barriers to effective implementation of Blue-Green infrastructure can arise if planning processes and wider urban system design and urban renewal programmes are not fully integrated.
Blue-Green Infrastructure Components
Many infrastructure components and common practices may be employed when planning and developing a Blue-Green City, in line with specific local objectives, e.g. water management, delivery of multi-functional green infrastructure, biodiversity action plans. A Blue-Green City actively works with existing grey infrastructure to provide optimal management of the urban water system during a range of flood events; from no flood, to minimal flooding, to extreme rainfall events where the drainage system may be exceeded.
The key functions of Blue-Green infrastructure components include water use/reuse, water treatment, detention and infiltration, conveyance, evapotranspiration, local amenity provision, and generation of a range of viable habitats for local ecosystems. In most cases, the components serve several functions.
Blue-Green infrastructure includes;
- Bioretention swales
- Swales and buffer strips
- Storage ponds, lakes and reservoirs
- Controlled storage areas, e.g. car parks, recreational areas, minor roads, playing fields, parkland and hard standing in school playgrounds and industrial areas
- Sand filters and infiltration trenches
- Permeable paving
- Rain gardens
- Stream and river restoration
- De-canalisation of river corridors and re-introduction of meanders
- Constructed wetlands
- Property level strategies to reduce surface water and manage runoff, such as water butts (or rainwater tanks in the US),
- Open green space
- Parks and gardens
- Street trees
- Pocket parks
- Vegetated ephemeral waterways
- Planted drainage assets (green roofs and green walls)
- Green outfalls
- Restored, rehabilitated and enhanced urban watercourses offering green erosion protection (see river restoration)
Benefits associated with Blue-Green Cities
A Blue-Green City contains an interconnected network of blue and green infrastructure that work in harmony to generate a range of benefits when the system is in both the flood state and non-flood state. A wide range of environmental, ecological, economic and socio-cultural benefits are directly and indirectly attributed to Blue-Green Cities. Many benefits are realised during times of no flood (green benefits), giving Blue-Green Cities a competitive edge over otherwise comparable, conventional cities. Mulit-functional infrastructure is a key to generating the maximum benefits when the system is in the non-flood state. An ecosystem services approach is frequently used to determine the benefits people obtain from the environment and ecosystems. Many of the good and services provided by Blue-Green Cities have economic value, e.g. the production of clean air, water and carbon sequestration.
The benefits include;
- Climate change adaptation and mitigation
- Reduction of the urban heat island effect
- Better management of stormwater and water supply, conservation of water resources through efficiency (increasing the resilience to drought)
- Carbon reduction/mitigation
- Improved air quality
- Increased biodiversity (including the reintroduction and propagation of native species)
- Habitat and biodiversity enhancement
- Water pollution control
- Public amenity (recreational water use, parks and recreation grounds, leisure)
- Cultural services (physical and mental health, well-being of citizens, aesthetics, spiritual)
- Community engagement
- Landscaping and quality of place
- Increased land and property values
- Labour productivity (stress reduction, attracting and retaining staff)
- Economic growth and investment
- Food production
- Healthy soils and a reduction in soil erosion and river bank retreat
- Reduction in the accumulation of sediment, debris and pollutants in urban watercourses
- Shading and shelter around rivers and the wider urban environment
- Economic benefits related to avoided costs from flooding
- Community cohesion and greater understanding of sustainable planning and lifestyle
- Possible diversification of the local economy and job creation
- Strengthening ecosystem resilience
- Ecological corridors and landscape permeability (biodiversity benefits)
- Avoided impacts of flood events, including avoided damage to the economy, wildlife, buildings and infrastructure, and avoided trauma and distress (mental health impacts) associated with flooding
The multiple benefits of adopting Blue-Green infrastructure will span both the local/regional and global/international scales. The Department of Environment, Farming and Rural Affairs’ (Defra) approach to flood and coastal risk management has been to seek multi-functional benefits from Flood and Coastal Erosion Risk Management (FCERM) interventions and enhance the clarity of social and environmental consequences in the decision making process. Defra note, however, that flood risk reduction benefits provided by ecosystems are not well understood and this is an area where more systematic research is needed.
Case Study examples
Concepts of water sensitive cities and tools for water-centric urban design are developing in many countries. During the first decade of the 21st century, Portland, Oregon, began its ‘grey to green' initiative  and Melbourne, Australia, reached the “water cycle city” stage. Few, if any UK cities have progressed beyond “the drained city“ stage, with water managed for a series of single functions (including flood risk management), mostly through distribution, collection and treatment systems and drainage infrastructure that are energy intensive and which continue to degrade urban environments in general and urban watercourses, in particular. Cities in Europe (e.g. Copenhagen, Rotterdam (Waterplan), Lodz (Blue Green Network), Graz, Berlin), North America (e.g. Philadelphia, New York) and Australia (e.g. Adelaide) are also addressing the challenges of moving towards a Blue-Green City.
- [Hoyer, J., Dickhaut, W., Kronawitter, L. and Weber B. 2011. Water Sensitive Urban Design. Jovis, University of Hamburg.
- Maksimović, Stanković, S., Xi Liu and Lalić, M. 2013. Blue Green Dream Project’s Solutions for Urban Areas in the Future. Reporting for Sustainability. http://www.sciconfemc.rs/PAPERS/BLUE%20GREEN%20.pdf
- http://www.academia.edu/2369268/Water_purificative_landscapes_constructed_ecologies_and_contemporary_urbanism. Retrieved 4 March 2014. Missing or empty
- Novotny V., Ahern J. and Brown P. 2010. Water Centric sustainable communities: planning, retrofitting and building the next urban environment. John Wiley and Sons, New Jersey.
- CIRIA (2006). Designing for exceedance in urban drainage - good practice (C635).
- http://ec.europa.eu/environment/nature/ecosystems/docs/green_infrastructure_broc.pdf. Missing or empty
- SWITCH, an EU-funded research programme with UNESCO-IHE as lead partner) (http://www.switchurbanwater.eu/)
- Howe, C. and Mitchell, C. 2012. Water Sensitive Cities. IWA Publishing, London.
- ‘Grey to green’ initiative, Portland, Oregon (http://www.portlandoregon.gov/bes/47203)
- Brown, R., Keath, N. and Wong, T. 2008. Transitioning to Water Sensitive Cities: Historical, Current and Future Transition States. Proceedings of the 11th International Conference on Urban Drainage, Edinburgh, Scotland (http://web.sbe.hw.ac.uk/staffprofiles/bdgsa/11th_International_Conference_on_Urban_Drainage_CD/ICUD08/pdfs/618.pdf)
- Rotterdam, Netherlands, Waterplan 2 (http://www.rotterdam.nl/waterplan_2_en_deelgemeentelijke_waterplannen)
- Rotterdam Waterplan 2013-2018 (http://www.rotterdamclimateinitiative.nl/en)
- Blue Green Network, Lodz, Poland (http://www.switchtraining.eu/fileadmin/template/projects/switch_training/files/Case_studies/Case_study_Lodz_preview.pdf)
- Blue-Green Cities Research Project website
- Blue Green Dream, Imperial College, London
- CIRIA – Sustainable Urban Drainage Systems (SUDS)
- CRC for Water Sensitive Cities
- Engineering Natures Way - Sustainable Drainage Systems (SUDS) and flood risk management
- International Water Association (IWA) (Cities of the Future Programme)
- MARE – Managing Adaptive Responses to changing Flood Risk
- Metropolitan Glasgow Strategic Drainage Partnership (MGSDP)
- Portland Guide to Sustainable Stormwater – City of Portland, Oregon
- Stormwater Industry Association of Australia
- Sydney WSUD program
- The River Restoration Centre (RRC)