Repenser la ville, sa forme, ses flux

Repenser la ville, sa forme, ses flux
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Sommaire de l'article
Plus haut n'est pas plus dense
La ville verticale, consommatrice d'espace
Kyoto et Paris, villes fluides
Londres, championne de l'éclairage naturel
49

Les contraintes environnementales croissantes nécessitent de repenser profondément l'utilisation des ressources naturelles dans les modes de vie urbains, et en particulier l'énergie. Aujourd'hui une série de réponses techniques permettent d'augmenter l'efficacité énergétique des bâtiments et de limiter leur empreinte écologique. Ces solutions doivent s'intégrer dans une planification plus large visant à faire évoluer les formes urbaines vers des modèles plus compacts, diminuant les besoins en déplacements.

Sustainable development respects the environmental, social and ecological dimensions of our world. The environmental dimension is the one most often addressed, as it appears most amenable to simple and universally applicable solutions that limit carbon footprints, greenhouse gas (GHG) emissions and resource consumption. We can continue growth and development partly by optimizing our resource consumption, producing more growth and wealth while consuming minimal quantities of natural resources. Much current systems and technologies research aims to achieve this outcome. However, while the work under way is constructive, it is meaningless if done in isolation. In an increasingly more urbanized world where 50% of the population already lives in cities, technology alone is no panacea for all of our problems: it is crucial that we rethink our lifestyles in terms of urban organization. Cities today expand and develop according to the twentieth-century model prevalent in industrialized countries, designed for cars and a limited increase in population. This model does not suit the exponential growth of cities in developing and emerging countries; it will not even remain viable for cities in the more-developed world. By rethinking urban form alongside lifestyles and new technologies, and by integrating forms and flows (movements of people, goods, water, electricity, waste, and so forth), we can build sustainable cities and greatly increase the savings facilitated by a single, coherent system. Analysis at the city scale means eliminating separations between social, environmental and ecological dimensions, and thereby linking aspects to multiply their beneficial effects without risking that they cancel each other out.

Building Liveable Cities

Research at the CSTB Centre scientifique et technique du bâtiment (Scientific and Technical Construction Center) Urban Morphology Laboratory measures the city - street lengths, building heights, green areas and so forth - and uses geometric data to describe its morphology and spatial organization. The data are used to construct urban parameters that affect energy consumption and environmental performance. We can compare the performances of cities across the world by integrating their morphological parameters into energy and environmental equations, helping decision-makers organize cities so they consume the fewest resources possible while remaining attractive places to live.

Acting simultaneously on urban form, building technology and systems, and people's behaviour would help reduce GHG emissions in successive, cumulative steps. By itself, well-thought-out bioclimatic design of urban morphology would cut GHG emissions in half. Optimizing building technology would further divide emissions by 2.5, while optimizing systems would halve them again. Finally, residents adopting "sober" or low-carbon-consuming behaviours would again divide energy consumption by 2.5. Ultimately, combining all of these factors would have a multiplicative effect, reducing energy consumption by 90% to 95%.

Studies have calculated new indicators for already-built cities and for those to be built, employing several scales - agglomeration, city, district, city block, individual building - to explore density, compactness, fractal form, In this paper, the fractal form refers to the form of the city's spatial limits, characterized by a mixture of irregular and ordered lines that repeat the same pattern at different scales. connectivity, design-related properties, organization and size of streets, irregularity, solar admittance, and so forth. The method's usefulness lies in its mathematical tools, which represent physical realities as numbers that are easier to measure, compare and optimize. Each parameter must be carefully handled: the interpretation of results is critical. Our laboratory has started to test these parameters in a cross-regional comparison of urban fabrics. The results show how urban forms may be transcribed as numbers, and how the built landscape shapes residents' behaviours.

Compactness, Density and Mixed-use at the Agglomeration Scale

The compactness of an urban area contrasts with the fractal form of urban sprawl, where empty spaces become visible, contours are complex and irregular, and the urban fabric becomes less and less dense. Sprawl consumes more land and generates roads of greater length, while spaces become isolated, far from amenities and shops. This type of urban fabric occurs in the suburbs of Western cities, and appears ever more widely in cities throughout the world. It can take the form of suburbs with detached houses or large-scale housing blocks. It includes little or no public transport and few local shops, making private cars indispensable and diluting the notion of a city-place. Social structures or relationships between residents are specific - neither urban nor rural - and have none of the benefits of either model, such as accessibility, familiarity or recognition in one's neighbourhood.

What we call urban density is the floor area of buildings within a given urban perimeter. We take into account infrastructure such as roads and transportation systems, whose size diminishes the built area and thus the density. This shows that maximum density is not achieved through gigantic buildings, but rather by a continuous urban fabric of average height (three to five storeys). In fact, high-rise blocks housing large numbers of people vertically have few access points - often only one road that must therefore be very wide. Consequently, roadways take up much space, breaking up the city: they do not favour "softer" forms of travel, such as walking or bicycling. Moreover, high-rise buildings must admit sunlight and therefore may not stand in too-close proximity, and this further increases distances travelled between them inside the city. Studies we have conducted in various cities show that Parisian districts in the Haussmann style, with buildings averaging six or seven storeys, have a higher density than a twenty- or thirty-floor tower-block district in Hong Kong (see Figures 1 and 2).

High density has an advantage because it limits how much land the city consumes. This preserves neighbouring farmland, and limits the distances residents must travel to obtain goods, go to work and do other activities. Restricting urban sprawl is a critical challenge for some countries. For example, China's agricultural production already lags behind its population growth, making it imperative to prioritize urban food needs and to preserve cities' agricultural hinterlands. The same phenomenon is true for Paris and its agricultural basin, to a lesser degree.

Mixed uses Mixed usage refers to areas where it is possible to live, shop, work in offices and/or factories, recreate and socialize within a small perimeter. at the city scale can prevent commuting in and out of the city - behaviour that currently produces a large share of GHG emissions and creates urban traffic jams, reducing mobility and making cities less enjoyable and efficient. Mixed usage also enhances neighbourhood security. It ensures an all-day human presence, since activities take place at different times, and prevents the soulless emptied-city and bedroom-community phenomena, so unappealing to residents. While important from the environmental standpoint, mixed usage matters most from a social perspective, injecting life into urban spaces and ensuring residents' wellbeing.

A Mobility-Friendly City

Mobility within the city is of paramount importance, not only for its substantial share of direct and indirect pollution, but also for its vital contribution to a city's economic and social development. For a city to be efficient and enjoyable, it must connect to regional, national and worldwide travel networks; and more importantly, its districts need inter-linkage to make intra-city movements as fluid as possible. A city's "connectivity" is measured by the average time it takes residents to travel to their various activities. The efficiency of transport systems is calculated by distance travelled divided by average time. Other important indicators of connectivity include the availability of public transport, the number and spacing of stops, and areas served. Studies of urban transport must take into account the variety of transport means and their spatial distribution and speed, as well as their contribution to global warming.

Cyclomatic numbers, which count the number of circuits in a network, prove very useful for measuring a city's degree of connectivity based simply on its block organization. A cyclomatic number gives us an idea of the number of possible routes between one point to another: the higher the cyclomatic number, the more diversified the possible routes and the less congested the city. Moreover, route diversity allows various forms of transport - such as walking, bicycling, or taking the bus or tram - adapted to different activities. The cyclomatic number, combined with the average distance between two intersections, has permitted study of several cities in different regions of the world, along with comparison of their urban block forms.

The study showed that traditional urban forms, such as those in the historical centre of Kyoto or in Paris, have many more alternative routes and much shorter distances between intersections than modern tower-block cities, such as Guangzhou. The first two cities have layouts that allow movement on foot or by bicycle, subsequently adapted for trams (Figure 3). Both cities were built before motorized vehicles, while modern cities develop solely to suit the needs of cars. This clearly creates problems: cars tend to exclude other people, occupy a great deal of space and concentrate high pollution levels. A sustainable city must allow individuals to choose their transport modes and adapt them to their activities, giving priority to soft, non-polluting means of transport - means that are more beneficial for health, accessible to all types of people, and independent of unproven and costly technological advances intended for less-polluting cars.

Urban Sustainability at the District and Building Scales

The study of urban forms requires thinking at the district scale. Again, mixed-use criteria come into play, allowing an arrangement of shops at street level, offices in specific buildings or on the first and second floors, and housing on upper storeys. Other facilities, such as schools and healthcare facilities, must be integrated as well. District-scale mixed uses must ensure residents can meet their daily needs by walking - avoiding social segregation, pollution and traffic jams, as well as saving time and creating local jobs. Public transport's design must reflect the district scale, e.g. in the location of bus and tram stops, as well as parking places for bicycles and cars. District-level density ensures a clientele for local trades people and shorter distances between shops. Residents do not perceive such arrangements as too dense; studies have shown that they associate negative feelings of density with tall buildings, and we have seen that six-storey buildings suffice for achieving high urban density.

District scale criteria include the fractal form, solar admittance and openness to the sky. Although fractal scaling has numerous disadvantages at the urban area level, it has advantages for analyzing districts. In fact, fractal scaling introduces complexity into building and street forms, allowing green spaces to be included in the city. Overly-simple linear streets and buildings without inner courtyards make the city unattractively uniform. By contrast, treed courtyards and greenways function as air-conditioners, provide shade, collect water, retain soil and absorb some carbon dioxide emissions. A variety of urban forms allows for a diversity of routes that makes moving around the city enjoyable.

Another key variable is solar admittance, linked to a building's openness to the sky, which makes it possible to benefit from sunshine's light and heat. It is tricky to adapt buildings, streets and green spaces to protect the city from the summer's heat and, conversely, to let in light and heat during winter (Figure 4). One solution is to plant deciduous trees along the streets: their leaves provide shade in summertime, and warmth and light from the sun after they fall in autumn.

Buildings account for a factor of 2.5 in terms of energy consumption. Their form and placement will predispose them to consume less or more energy for space heating and lighting. For example, individual detached houses have more surface areas leading to heat loss, making them consume more energy for space heating. On the other hand, contiguous buildings have fewer heat-losing surfaces and retain warmth better. Conversely, very high, large buildings may be compact and require little heating, but will be subject to ventilation problems and have a low passive volume, e.g. little area within six metres of a window that receives natural lighting and ventilation. In France, heating is one of the main sources of energy consumption in the housing sector, while ventilation and lighting are the largest in the retail and services sector. At the building level, one must therefore find a compromise between compactness and passive volume. Energy efficiency measures tend to show this trade-off prevails more often in medium-sized, contiguous French buildings than in detached houses or isolated tower-blocks.

CONCLUSION

Given the urgent need to reduce resource consumption and house a growing number of people in the world's cities, adopting urban development and planning strategies becomes crucial. They must take into account the drawbacks of private cars: not everyone can afford them; they consume major shares of urban space, pollute directly and indirectly, and tend to exclude other transport means. The keywords remain density, mixed usage, and sober energy use through passive building design. We have shown the tools available for comparing and measuring these criteria in cities. It now becomes vital to develop the city inside the city or as an extension of the urban fabric - providing spaces for all kinds of activities and all residents, and thinking about connecting these spaces from the outset. All participants and all aspects of urban life must be assessed as a whole, before building starts-to integrate forms and flows, ensuring cities develop along a harmonious and sustainable path.

Plus haut n'est pas plus dense

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La ville verticale, consommatrice d'espace

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Kyoto et Paris, villes fluides

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Londres, championne de l'éclairage naturel

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