Towards a metabolic approach to urban spaces

Note rapide n°823

Martial Vialleix, Léo Mariasine, November 2019

Institut Paris Région (IAU)

To download : nr_823_web.pdf (180 KiB)

Working to make cities more circular implies adopting a particular approach, using the concept of « territorial metabolism », which refers to « all the flows of energy and materials involved in the functioning of a given territory » [Barles, 2017]. This approach consists in understanding cities as the result of a specific socio-ecological regime, no longer solely by their functions or activities, but by their flows and stocks of materials and resources. While the origins of this conception seem to date back to the 19th century, territorial metabolism experienced its first developments with the work of engineers, chemists, biologists or ecologists such as Eugene Odum, Paul Duvigneaud or Abel Wolman. The latter was the precursor of an accounting approach to the inputs and outputs of the flows necessary for the functioning of urban spaces in the 1960s.

Nowadays, studies of metabolism tend to develop in a sustained manner, both in the academic and institutional world. They are mainly conducted in the form of material flow analyses, the most widespread and solid accounting method at present. In Île-de-France, six territories have embarked on such an approach. Beyond these exercises, the results of material flow analyses and, more generally, the metabolic approach, now need to be integrated into the field of urban planning.

Disseminating the circular economy in territorial planning

Clearly, circular urban planning will first of all require the appropriation of space in the heart of cities to guarantee the deployment and networking of circular economy facilities (sorting centres, resource centres, material recycling facilities, etc.). The needs of this economy and, more broadly, those of the ecological transition, mean that there will be increasing competition for use of land resources in the Ile-de-France region, particularly around vacant spaces, which are the object of fierce competition with other uses for climate resilience, preservation and restoration of ecological continuity, etc. Hemmerdinger et al. 2019].

The development of resource cadastres is an essential prerequisite for optimizing the « urban mine »: buildings, infrastructures or even landfills can constitute important deposits of secondary resources to be recovered and valorized. A growing number of studies are being carried out on this issue of qualifying the urban mine [Augiseau, 2017; Stephan, Athanassiadis, 2017]. However, the diagnoses of urban planning documents would not only allow for the evaluation of these resources at the parcel level, but above all to be in a position to set in their orientations measures aimed at their optimization. Urban planning documents are not only useful for the protection of land, but also for thinking about the architectural and urban design of circular economy facilities, the acceptability of which must also be improved. In the same way, they are important for guaranteeing access to exploitable resources (forest or mineral resources in particular) in the territories. The aim is to increase local extractive capacities, while working on the acceptability of these activities, which are likely to generate significant environmental impacts. Urban planning documents therefore play an essential role, both in boosting supply (capacity to develop local resources, etc.), but also in increasing demand for products from the circular economy, in construction (use of secondary or bio-sourced materials, etc.), as well as in green spaces¹ (production of fertile soil from recycled materials, etc.).

Inert city, flexible urban planning?

While urban planning documents have an essential role in the deployment of the circular economy, they have historically been built in a logic of accompanying and then regulating urban growth. Their considerations were then far removed from the issues of circular urban planning, based on sobriety, reducing the flow of materials consumed in cities, extending the life span of buildings, monitoring and supervising demolition operations, the use of precise materials from recycling, etc.

Faced with the relatively « inert » character of cities, these framework documents can promote the intensification of spaces with little or no material transformation, changes in building use, their evolutionary nature, the conservation of productive and logistical activities, urban services, etc. However, many of the issues at stake go beyond the new constructions on which urban planning documents act as a priority and lead us to focus on modularity, the optimization of the existing built stock. We can therefore wonder about the capacity of territorial planning tools, in their current design, to integrate these principles of a more circular metabolism?

Beyond the places and the material needs required by the latter, it is towards an increasing hybridization of uses and the multiplication of capacities for interaction, exchange and reciprocity that circular urbanism challenges us. Circularity here goes further than sustainability by taking up the principle of densification: this is no longer considered only on a physical level, but also in terms of use, around multi-functional and multi-service buildings, spaces where production, repair, sale, distribution, etc., are located side by side. Confronting urban planning with metabolic issues also means thinking about other means of producing more sober urban spaces, such as raising the height of housing, optimising vacant or single-function buildings (gymnasiums, offices, etc.), which can be sources of optimisation of the existing stock of buildings.

In a particularly constrained built environment (density, heritage protection, etc.), the circular city will be transformed less on the morphological level than on the organizational level, and in a way that is « invisible » for the classic quantitative tools of urban planning (2D cartography, plans, etc.). Interstices, ground floor, courtyards, stairwells, etc. Interstices, ground floors, courtyards, stairwells, etc.: these are all places that will be mobilized by cities lacking space in order to sort or share objects and ideas, to set up workshops or productive premises, to set up new ways of organizing work sites, etc.

At the operational level, project owners tend to make the circular economy compulsory through clauses on public contracts. This involves, for example, imposing a resource diagnosis² and selective deconstruction on demolition contractors, and recording percentages of materials reused in construction contracts. However, these markets mobilise many players, even different project owners, who can hardly exchange materials for legal reasons, raising the question of the creation of a common market for demolition and construction. In addition to public action, all the operational players (companies, architects, design offices, etc.) are called upon to work well beyond their primary missions to promote circular urban planning.

A necessary interterritorial approach to circularity

One of the conditions for the implementation of a metabolic approach to urban spaces is the search for cooperation based on the concept of « interterritoriality ». This is understood as « the search for the efficiency of territorial public action through the coordination, articulation and assembly of territories, such as they are » [Béhar et al., 2014], via existing tools (reciprocity contracts, for example), and others that remain to be imagined. Indeed, circularity imposes transversality from the beginning to the end of the cycles of flows, despite institutional limits, while dealing with the conflicts of interest of the actors. This is by no means a novelty: there are many examples of the reuse of secondary materials for street design, or of urban-rural symbiosis in urban history around organic flows in particular [Barles, 2005; D’Arienzo, 2017]. This system has been undermined by globalization and modern urban planning, to such an extent that today the globalized nature of the economy presupposes a networked organization of material flow management companies, whose facilities go far beyond the borders of institutional territories [Durand et al., 2016]. This reality raises questions about the challenge of creating local jobs that cannot be relocated, and also about the importance of the proximity principle in waste management, with regard to the economic structuring of the industries and the capacity of territories to consume the materials and products resulting from recycling. More broadly, the approaches according to which « cities are in power » [Barber, 2014] need to be qualified. Indeed, cities are either deprived of resources and outlets for their waste, or are unable to manage these deposits, which, as things stand, calls into question their ability to govern the flow of metabolism. Finally, the circularity applied to cities and territories always raises the same conceptual and practical questions as those relating to sustainable development, around the opposition between « serving » and « served » territories. Overcoming this opposition is a major challenge for the implementation of a more cooperative and solidarity-based approach to circular urban planning.

Towards a 100% circular and « clean » metabolism?

If the metabolic approach to urban spaces is promising, the conditions for the acceptability of this transition must be established. First of all, the absolute myth of 100% circularity, synonymous with urban self-sufficiency, must be qualified. Cities, especially metropolises, have an externalized materiality whose effects occur far beyond their institutional boundaries (extractions, emissions, landfills, etc.). Secondly, the transition will not be without impact on the environment, nor without impact on populations. For example, the deployment of circularity marks a return to a more productive conception of the city, in particular through crushing of construction waste or methanisation. These are activities likely to generate local nuisances (odours, noise, accidental atmospheric emissions, transport, etc.). In addition, the current economic situation is marked by the multiplication of large-scale urban projects in the Seine valley or along the banks of the Ourcq canal, aimed at replacing monofunctional industrial wasteland with large mixed-use districts. In this sense, urban renewal is a vector for increasing and diversifying the stock of materials in urban spaces [Fernandez et al., 2018]. Urban recycling, which is intensifying, spares agricultural and natural spaces and reduces the consumption of materials for the various networks, but it does mark an increase in the stock of resources contained in cities. At the same time, the trend towards urban intensification is likely to produce very diffuse waste resulting from renovation and rehabilitation operations, carried out in particular by private individuals. The metabolic approach thus requires going much further than the exercises of analysis of the flows of materials that are emerging today. A joint reflection on the tools and foundations of urban planning, as well as the methods of flow governance, is a particularly important field of research and work for the future of territories.

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