Salat, S. & Vialan, D.,
Sustainable Mediterranean Urban Development Affordable to All, a Morphological Approach
SB10 Conference, Algarve, 2010
By 2030 the Mediterranean cities will count 100 million new inhabitants. This new urbanization will be mostly informal and in the Southern part of the Mediterranean region. Moreover, it will have to meet with the challenge of sustainable development and with a more and more arid climatic situation due to Global Warming. On the other hand, the Mediterranean region presents a huge potential for solar energy and has a long tradition of bioclimatic urbanism. To cope with those challenges and to seize these opportunities we propose to analyze urban morphology, which is an influential factor on the energy performance and livability of a city. We will question what lessons are to be learned from traditional vernacular dwellings.
1.1 Different scales and different layers in the city
Urban morphology describes the city with the help of different interdependent layers:
1) Human interactions and activities are the first level studied, it corresponds to the upper part of the scheme below.
2) Streets network and urban pattern are the second center of focus; they imply means of transportations in the long run and are the first choice to be made to design a city or a district.
3) The third layer is the plot. The administrative and historical divisions of the city are a constraint that often induces a certain type of houses. For example, small plots discourage the construction of high rise buildings.
4) Then topography and relief are studied, they also influence flows in the city along with the urban form.
5) The fifth layer comprises the activities and land use. This is very important for the flow of people, the equilibrium, and dynamic of the city. It is crucial on a financial point of view as well as on a social point of view. 1.2 Measuring the city to limit GES emissions through the urban form
Research at the CSTB Urban Morphologies 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, in order to help decision-makers organize cities so they consume the fewest resources possible while remaining attractive places to live.
2 MORPHOLOGY, CONTINUITY, CONNECTIVITY
2.1 Morphological discontinuities
Whereas in Europe urban morphology of cities changed slightly along time for centuries, the South of the Mediterranean Sea experienced a huge morphological discontinuity between the colonization and the creation of new cities as in Morocco. Barcelona is a typical example of a smooth change in its urban morphology between the old, medieval Barri Gothic and the new, modern extension designed by Cerda during the 18th century. In Fes, the old medieval city (the medina) is closed by high walls and the French created a new city a few kilometers away from the medina with typical French urbanism. Today, both North and South shores of the Mediterranean Sea are experiencing a new morphological change with the international modern urbanism inherited by Le Corbusier’s theories. The morphology is characterized by high rise skyscrapers with no connection to their environment, and urban sprawl. With the same population (2.1 million) the Great Tunis is 25 times bigger than Paris.
2.2 Our approach
The Laboratory of Urban Morphology uses mathematical theories, like the graph theory, to analyze different urban textures and their connectivity. In order to analyze street patterns, three levels of analysis must be distinguished with different degrees of abstraction.
– Composition of the street network. This is the first impression anyone has, when he comes into a new city. The connection between the human being and its environment are the core part of the composition: how does the space physically or visually impact the man? For example, to describe Torino the narrowness of streets, the relative heights of buildings, the impression of surprise created by the sudden discovery of huge places around the corner of a small street, must be discussed on a composition point of view.
– Configuration. The form is taken out of the analyzis and the focus is put only on topologies, that is to say the connections between the different elements of the city. Ratios are calculated, indicators of continuity and connectivity.
– Constitution. The structure of links and nodes is then the center of focus. Hierarchy and constraints are the two central topics discussed in this part. Typologies of street patterns are built and used. This part helps to understand the fundamental choices of urban designers.
Our approach implies the definition of indicators, such as the cyclomatic number. Cyclomatic numbers, which count the number of circuits in a network, prove very useful to measure 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 is, the more diversified the possible routes and the less congested the city will be. 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 been used to study several cities in different regions of the world, allowing comparisons of their urban block forms.
The Urban Morphology Lab studies 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 Le Corbusier modernist archetype City of 3 Million inhabitants. The first two cities have layouts that allow movements on foot or by bicycle, subsequently adapted to trams. Both cities were built before motorized vehicles, while modernist 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 to adapt them to their activities, giving priority to soft, non-polluting means of transport – means that are more beneficial to health, accessible to all types of people, and independent of unproven and costly technological advances intended for less-polluting cars. A sustainable city must allow individuals to choose their means of transport.
2.3 Measuring the connectivity of Mediterranean urban forms: the example of Fes, Morocco
Islamic patterns can easily be set apart from other patterns. Below are represented Islamic Patterns in comparison with a modernist pattern (Brazilia) and a new urbanism of sprawl in the US. Moreover these specificities can be mathematically proved and analysed. That is the object of the very interesting work of Kubat & Asami, who created a mathematical function to discriminate Islamic patterns. Their work outlined the following characteristics of Islamic patterns:
- Numerous cul-de-sacs
– Huges urban blocs
– Few X-junctions
– Narrow and curved streets
– An opaque network that cannot be apprehended totally easily.
We applied our metrical method to four samples of urban morphologies in Fes, Morocco. Traditional Islamic patterns are characterized by a very thin urban grid. You can measure this by looking at the mean distance between intersections. It is 10 meters in Fez, 40 meters in Toledo and almost all European medieval cities, which are very closed to the medina type, whereas the mean distance between intersections is 150 meters in Paris, Melbourne and Hong Kong. It is a first morphological change in the urban form, from the medieval type, with a very thin thin grid, and the first extensions of this grid. Then another huge morphological change appeared in the second half of the 20th century with the new modernist urban forms and the urban sprawl. The distance between intersections in Brasilia is 400 to 500 meters. We can find this kind of typology in Fes, with the old medina, the new town built by the French settlers, and new developments.
This multiplication by more than ten of the basic size of the city, creates a city made for the car and not for the pedestrian. It produces a city which doesn’t reach the density required to protect itself from the sun through the clumping of housing units with each other. The cyclomatic number is also very important. It is an indicator of the number of different possible paths through the city. The cyclomatic number is crashing down in the modern town, creating a monotonous repetitive city from a pedestrian point of view.
The old medina
– cyclomatic number : 200
– density of intersections : 6 per ha
– mean distance between intersections : 10m
The French colonial city
- cyclomatic number : 40
– density of intersections : 1,4 per ha
– mean distance between intersections : 50m
- cyclomatic number : 40
– density of intersections : 1 per ha
– mean distance between intersections : 75m
- cyclomatic number : 6
– density of intersections : 0,3 per ha
– mean distance between intersections : 100m
3 MORPHOLOGY AND VERNACULAR BIOCLIMATIC CITIES IN THE MEDITERRANEAN AREA
Current design guidelines used in Northern Europe do not apply to the Southern shore of the Mediterranean Sea, where the main question is to deal with hot and dry climate. Lessons for the design of urban forms can be found in traditional organization of human settlements, which were very efficient to protect from light and to use wind to refresh the city at different scales thanks to a very porous urban texture (with the traditional courthouse for example) which creates a dense (not compact) city. This peculiar texture manipulates the climate to create a more livable and sustainable city, and it could be used to design new urban developments. The ancient Greeks, as well as Mesopotamians and Egyptians, designed their towns as tools able to organise symbiotic exchanges with their environment. Ancient Thera, now Santorin, which we have chosen, formed part of a national identity whose cosmogonic centre was Delphi, which in Antiquity represented the pivotal point in Greek urban planning. Delphi supervised the planning for siting new towns. This centralised planning entity, using oracles and placed under the aegis of Apollo, made decisions concerning the dispatching of expeditions and controlled the growth of the urban population which otherwise would have led to overcrowded cities and uncontrolled migration.
3.2 Greek and Roman development of the “bioclimatic housing” concept
In Greece, it was above all the Economics by Xenophon (born around 340 BC) that led to the birth of “bioclimatic housing”. He saw the need to orient buildings and make nature and land bend to production requirements while continuing to respect their natural characteristics and potential. Around 350 BC, Aristotle made a number of observations concerning the relation between the healthiness of the air and the prevailing winds, reconciling the need of defence and harmony with nature. One of the most important treatises by Hippocrates, Treatise on air, water and places, formulated basic public hygiene concepts linked to the choice of where to build and urban planning: “there is a need to orient streets and buildings in such a way as to avoid the summer sun and take advantage of cooling winds, to build away from mosquito infested areas and unhealthy places and have sources of clean water”. This became a town planning standard in the creation of Greek cities and the same principles were subsequently used by Vitruvius.
These characteristics continue to remain strongly anchored in innumerable examples of ancient architecture and in today’s towns around the Mediterranean Sea. Given that they were designed to provide a maximum level of comfort in a world without fossil fuels, they provide examples of complete urban complexes based on zero energy bioclimatic urban morphologies. Priene was developed in Western Asia Minor in 400 B.C. as a new, entirely Hellenistic city. At the centre of the city grid are the expected municipal buildings and the market place. The southern section of the city was designed for recreational activities, with a stadion, or racetrack, and a gymnasion, an open court for sports. Recent excavations have shown that almost all buidings at Priene were the same, with almost identical plans, sections, and elevations and similar orientation.
The plans and axonometrics show this simple structure. Every unit was organised around a courtyard. The buildings to the north were used for living. The main room had a shaded porch facing south.
The citizens of Santorin have transformed the forces of nature into groups of shapes designed to defend and provide protection. They have made use of the climatic specificities of their island to insulate their houses. On Santorin, each house is oriented to take full advantage of the wind. All towns, from Pirgos to Thera, from Emporeion to Ia, make use of the predominant onshore winds, which run parallel with the street networks in the towns and provide natural ventilation. Each house uses the breeze provided to ventilate its rooms through a basic system of openings and/or connections to the streets. In the Cyclades, the street represents the main bioclimatic element used by the houses. All new buildings built by the islanders are designed to assure the passage of fresh air and avoid creating any physical barriers that might block the wind.
3.3 The sustainable town is a complete and organic bioclimatic unit: the example of Santorini,Greece
The organic functioning of the towns on Santorin results in the creation of informally shaped built masses that provide an unbroken continuity between the houses. Each housing unit is organically connected to its neighbours, resulting in an air filtered by the preceding houses being introduced to circulate through the rooms and beyond. The formal consequence of this bioclimatic orientation is that the urban grid is given a unitary appearance: houses interpenetrate like organic cells, creating a dense, tight fabric amalgamated by its own heterogeneity.
The urban layout of the towns on Santorin gives them the appearance of being a single large structure. The widths of the streets are designed to prevent the walls of the houses from overheating during the summer. The continuous house elevations act as wind tunnels and their configuration, using the Venturi effect, increases the speed of the onshore wind along the steep streets. As in many other old Mediterranean cities, bioclimatic design underlines the construction of the urban groupings on Santorin. The relationship between the urban morphology and the sun, wind and the use of local materials, the interaction of the houses with the ground and the urban morphology results from an attentive design approach that has always been closely linked to local environmental resources.
3.4 Conclusion: the sustainable bioclimatic town is naturally cooled without any energy cost.
The houses are equipped with underground air conditioning, as each house has “roots” in the basement formed by small rooms dug out of the rock and equipped with a well in which rainwater is stored. These cavities, which create an environment where the temperature remains cool and constant throughout the year, have connecting passageways leading to the house’s upper levels. The higher temperature of the rooms resulting from the daily sunlight sets an internal ventilation level thanks to exchanges with the air currents provided from the underground chambers
The cooling is based on three physical laws:
– the natural thermal inertia and insulating quality of the rock,
– the heat exchange between water and air,
– the slow endothermic evaporation of the water.
This process is able to maintain the temperature of the environment and the air at a satisfactory level throughout most of the year without any energy cost: during the winter, the high thermal inertia of the thick walls retains the heat while, in the summer, it cools the rooms. This phenomenon is facilitated by the natural movement of the air inside the house. Thermal mass in vernacular Mediterranean architecture, Mykonos, Paraportiani Church, Greece. In harmony with Mediterranean traditions, the external skin of the buildings also has an influence on the climate of the rooms in the house. The interaction of a mixture of mud and straw has a very good thermal coefficient and the natural very pale colours form a sort of skin that protects the architecture from extremely high and low temperatures.
This analysis aim at encouraging new researches and quantitative evaluations to invent new urban forms as sustainable as the old medina, but suited to the modern way of life and which can cope with the 100 billions new inhabitants in the Mediterranean area. Given the urgent need to reduce resource consumption and to 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 starting to build— to integrate forms and flows, so ensuring that cities develop along a harmonious and sustainable path.
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