Blue–green architecture: A case study analysis considering the synergetic effects of water

One circumstance that will in any case be aggravated by climate change is the rise in extreme weather events, such as heavy rainfall and long drought . In July 2011, during a cloudburst in Copenhagen, over 150 mm of rain fell within 2 h, resulting in extensive flooding. Copenhagen City decided to initiate the development of a city transformation plan that would optimize water retention in the city by integrating blue–green elements (American Society of Landscape Architects, 2016). Other cities must also revise their water management schemes due to heavy rain and floods (Stokman et al., 2015). Other meteorological changes place cities under stress. Persistent periods of heat and drought have a negative impact on energy consumption and health. The heatwave that hit Europe in 2003 is estimated to have caused over 30,000 deaths (Mann, 2018). Dryness leads to an imbalance between water availability and water demand. The need for plant irrigation increases in dry phases, whereas water supplies simultaneously decrease .

The integration of natural elements into cities is an effective method to counteract the effects of UHI and the consequences of extreme weather events (Gunawardena et al., 2017). Urban green provides water retention and slow evaporation, which in turn has a positive impact on the microclimate. The term “blue–green infrastructure” (BGI) refers to the combination of blue (water) and green (vegetation) components in distinction to gray infrastructure, such as roads, settlements, and canals. The concept was developed at the level of infrastructure and landscape. This concept has already been successfully applied on that scale. Architecture and buildings can be an integral component of BGI if they embody the inherent principles and objectives and thus become part of the network.

1.1. Definitions and functions of BGI
The concept of BGI has yet to be generally understood. The term “green infrastructure” (GI), also known as “urban green infrastructure” (UGI), is common. In the case of GI, the definitions in the literature also vary, especially with regard to the integration of “blue” components. A commonly cited definition of GI comes from the European Commission. “Green infrastructure can be defined as a strategically planned network of valuable natural and semi-natural areas with further environmental elements, which is designed and managed in such a way that a broad spectrum of ecosystem services is guaranteed in both urban and rural areas and biological diversity is protected” .

A good overview of the spectrum and possibilities of BGI results from projects that have already been successfully implemented. For this reason, the definitions of the planning and execution of engineering offices are also relevant. The engineering company, Arup, which has implemented numerous notable projects in this field, defines GI as follows: “the system of open spaces, natural areas, urban woodland and parks; green streets, squares and public realm; rivers and waterways; and smaller scale interventions such as green roofs, walls and façades – all of which lie within the physical networks of cities themselves and their immediate hinterlands, and perform essential ecosystem services” . The following subcategories are listed in the “Green Infrastructure Cards” developed by Arup: demographics, urbanization, water, climate change, waste, energy, and food . The integration of water is an optional component but not mandatory. This list also illustrates the area of tension in which the projects operate in a great variety of requirements.

Ramboll, a company that implements climate adaptation and flood risk management projects worldwide, provides another definition for BGI in which the interaction of green and blue becomes the key feature: “BGI combines hydrological functions with urban nature, landscaping and urban planning. Blue (water) and green (nature, squares and parks) serve to protect against floods and other impacts of climate change.” (Ramboll, 2018). This description shows the interdisciplinary approach of BGI, but it remains at the conceptual level on a large scale. Small interventions are not considered.

In the following, BGI is used as an umbrella term for projects of all scales in the urban environment, where blue and green elements are combined and have a multifunctional impact in urban space. Under this definition, a wide range of issues and solutions is addressed. In addition, BGI cannot be assigned to one profession. The perspective on the topic also changes depending on the discipline.

The combination of blue and green elements depends on location, scale, season, and weather; moreover, it can provide various functions regarding social, ecological, and design aspects and make cities resilient to climate change . The resulting multifunctionality is a mandatory component of BGI . Some core functions are the improvement of microclimate, air quality, and public health . On an urban scale, the positive effects of BGI include sound insulation, air cleaning, fine dust binding, rain water retention, and aesthetic aspect. These blue–green functions regarding supply, regulation, and culture can also be summarized as ecosystem services. Such functions enable urban nature and biodiversity that have a considerable value for society .

1.2. Application of BGI in architecture
The use of blue–green systems at the building level is less common than in landscape planning. Rather, many individual components can be part of a blue–green network. Sustainable water management in buildings includes, for example, the collection and reuse of rainwater as service water for irrigation or cooling. When rainwater runoff seeps into the ground, resulting in the enrichment of groundwater, the aim is often to compensate for the negative effects of the ground being sealed by the building. The separate collection of gray water remains an absolute exception. The internal treatment of gray water in buildings remains to be a cost-intensive specialized solution even though it is amortized over the life cycle by saving drinking water .

Building greening is an important component of BGI. Green roofs and façades appear in many different variations. Green roofs are usually classified in accordance with their established categories (extensive/intensive). Nevertheless, extended concepts, such as marsh plant and retention roofs, have been developed over time.

The systematic installation of roof and façade greening and the plantation of trees and green spaces near a building can positively affect the direct and indirect energy consumption of buildings. Heating and cooling energy can be saved through shading (trees and facade greening), cooling by evapotranspiration (all green elements), and insulation (green roofs) .

2. BGI-related planning concepts
   BGI bundles planning concepts that deal with the blue (water) and green (vegetation) aspects. The differences of the subcategories lie in motivation and approach. Blue-motivated projects mostly result from urban water management, flood protection, and decentralized rainwater management. By contrast, green-motivated projects aim for the densification and optimization of urban vegetation. For this reason, blue-driven projects can be grouped under water sensitive urban design (WSUD) and green-driven projects under GI. Fig. 1 shows the classification through which BGI is based by breaking down the contained planning concepts into blue and green approaches. All concepts can include blue and green components. The differences do not lie in the applied elements but in the questions from which the projects are motivated. The interconnection of these objectives comes together in BGI.