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TeMA Journal of
Land Use, Mobility and Environment
1 - TeMA Journal of Land Use Mobility and Environment 2 (2012)
THE RESILIENT CITY
2 (2012)
Published by
Laboratorio Territorio Mobilità e Ambiente - TeMALab
Dipartimento di Pianificazione e Scienza del Territorio
Università degli Studi di Napoli Federico II
Publised on line with OJS Open Journal System by Centro di Ateneo per le
Biblioteche of University of Naples Federico II on the servers of Centro di Ateneo
per i Sistemi Informativi of University of Naples Federico II
Direttore responsabile: Rocco Papa
print ISSN 1970-9889
on line ISSN 1970-9870
Registrazione: Cancelleria del Tribunale di Napoli, n° 6, 29/01/2008
Editorials correspondence, including books for review, should be sent to
Laboratorio Territorio Mobilità e Ambiente - TeMALab
Università degli Studi di Napoli “Federico II”
Dipartimento di Pianificazione e Scienza del Territorio
Piazzale Tecchio, 80 - 80125 Napoli - Italy
Sito web: www.tema.unina.it
info: redazione.tema@unina.it
TeMA Journal of
Land Use, Mobility and Environment
2 - TeMA Journal of Land Use Mobility and Environment 2 (2012)
TeMA - Journal of Land Use, Mobility and Environment offers researches, applications and contributions with a unified approach to
planning and mobility and publishes original inter-disciplinary papers on the interaction of transport, land use and Environment.
Domains include: engineering, planning, modeling, behavior, economics, geography, regional science, sociology, architecture and
design, network science, and complex systems.
The Italian National Agency for the Evaluation of Universities and Research Institutes (ANVUR) classified TeMA s one of the most
highly regarded scholarly journals (Category A) in the Areas ICAR 05, ICAR 20 and ICAR21. TeMA Journal has also received theSparc
Europe Seal for Open Access Journals released by Scholarly Publishing and Academic Resources Coalition (SPARC Europe) and the
Directory of Open Access Journals DOAJ). TeMa publishes online in open access under a Creative Commons Attribution 3.0 License
and is double-blind peer reviewed at least by two referees selected among high-profile scientists, in great majority belonging to foreign
institutions. Publishing frequency is quadrimestral. TeMA has been published since 2007 and is indexed in the main bibliographical
databases and present in the catalogues of hundreds of academic and research libraries worldwide.
EDITORIAL MANAGER
Rocco Papa, Università degli Studi di Napoli Federico II, Italy
EDITORIAL ADVISORY BOARD
Luca Bertolini, Universiteit van Amsterdam, Netherlands
Virgilio Bettini, Università Iuav di Venezia, Italy
Dino Borri, Politecnico di Bari, Italy
Enrique Calderon, Universidad Politécnica de Madrid, Spain
Roberto Camagni, Politecnico di Milano, Italy
Robert Leonardi, London School of Economics and Political Science, United Kingdom
Raffaella Nanetti, College of Urban Planning and Public Affairs, United States
Agostino Nuzzolo, Università degli Studi di Roma Tor Vergata, Italy
Rocco Papa, Università degli Studi di Napoli Federico II, Italy
EDITORS
Agostino Nuzzolo, Università degli Studi di Roma Tor Vergata, Italy
Enrique Calderon, Universidad Politécnica de Madrid, Spain
Luca Bertolini, Universiteit van Amsterdam, Netherlands
Romano Fistola, Dept. of Engineering - University of Sannio - Italy, Italy
Adriana Galderisi, Università degli Studi di Napoli Federico II, Italy
Carmela Gargiulo, Università degli Studi di Napoli Federico II, Italy
Giuseppe Mazzeo, CNR - Istituito per gli Studi sulle Società del Mediterraneo, Italy
EDITORIAL SECRETARY
Rosaria Battarra, CNR - Istituito per gli Studi sulle Società del Mediterraneo, Italy
Daniela Cerrone, TeMALab, Università degli Studi di Napoli Federico II, Italy
Andrea Ceudech, TeMALab, Università degli Studi di Napoli Federico II, Italy
Rosa Anna La Rocca, TeMALab, Università degli Studi di Napoli Federico II, Italy
Enrica Papa, Università degli Studi di Napoli Federico II, Italy
ADMISTRATIVE SECRETARY
Stefania Gatta, Università degli Studi di Napoli Federico II, Italy
TeMA Journal of
Land Use, Mobility and Environment
3 - TeMA Journal of Land Use Mobility and Environment 2 (2012)
THE RESILIENT CITY 2 (2012)
Contents
EDITORIALE
The Resilient City
Rocco Papa
3
EDITORIAL PREFACE
The Resilient City
Rocco Papa
FOCUS
FOCUS
Searching for Ariadne’s Thread
Giovanni Rabino, Valerio Cutini 7 Searching for Ariadne’s Thread
Giovanni Rabino, Valerio Cutini
City and Mobility. Towards an
Integrated Approach to Resolve
Energy Problems
C
armela Gar
g
iulo, Valentina Pinto, Floriana Zucaro
23
City and Mobility. Towards an
Integrated Approach to Resolve
Energy Problems
Carmela Gargiulo, Valentina Pinto, Floriana Zucaro
Systemic Resilience of Complex
Urban Systems. On Trees and Leaves
Serge Salat, Loeiz Bourdic
55
Systemic Resilience of Complex
Urban Systems. On Trees and Leaves
Serge Salat, Loeiz Bourdic
Enhancing Urban Resilience in Face
of Climat Change
Adriana Galderisi, Floriana Federica Ferrara
69
Enhancing Urban Resilience in Face
of Climat Change
Adriana Galderisi, Floriana Federica Ferrara
Il sistema ospedaliero e la resilienza
urbana
Francesca Pirlone
89
The Hospital System and the Urban
Resilience
Francesca Pirlone
Towards Resilient City:
Comparing Approaches/Strategies
Angela Colucci
101
Towards Resilient City:
Comparing Approaches/Strategies
Angela Colucci
TeMA Journal of
Land Use, Mobility and Environment
4 - TeMA Journal of Land Use Mobility and Environment 2 (2012)
Strumenti di valutazione della
resilienza urbana
Giovanna Saporiti, Gianni Scudo,
Cynthia Echave
117
Assessment Tools of Urban
Resilience
Giovanna Saporiti, Gianni Scudo,
Cynthia Echave
Spatial Resilience of Outdoor
Domestic Spaces in Mozambique
Céline F. Veríssimo
131
Spatial Resilience of Outdoor
Domestic Spaces in Mozambique
Céline F. Veríssimo
Enhancing Resilience of London by
Learning from Experiences
Funda Atun
147
Enhancing Resilience of London by
Learning from Experiences
Funda Atun
Urban Resilience and Ecosystem
Services: How can e Integrated in the
Case of Istanbul – SultanBeyli District?
Azime Tezer, Zeynep Deniz Yaman, Ayse
Ozyetgin Altun, Ilke Albayrak
159
Urban Resilience and Ecosystem
Services: How can e Integrated in the
Case of Istanbul – SultanBeyli District?
Azime Tezer, Zeynep Deniz Yaman, Ayse Ozyetgin
Altun, Ilke Albayrak
La resilienza:
futuro della protezione civile
Fulvio Toseroni
177
Resilience:
the Future of Civil Protection
Fulvio Toseroni
TERRITORIO, MOBILITA’ E
AMBIENTE
LAND USE, MOBILITY AND
ENVIRONMENT
The Effect of Central Metro Stations
on Real Estate Values
Agapi Xifilidou, Nikolaos Karanikolas,
Spyridon Spatalas
185
The Effect of Central Metro Stations
on Real Estate Values
Agapi Xifilidou, Nikolaos Karanikolas,
Spyridon Spatalas
I finanziamenti europei per l’ambiente
e la mobilità
Michele Macaluso, Nicola Clemente,
Nadijara Alves Acunzo, Giulio Guarracino
195
European Funds for Environment and
Sustainable Mobility
Michele Macaluso, Nicola Clemente,
Nadijara Alves Acunzo, Giulio Guarracino
OSSERVATORI
Daniela Cerrone, Fiorella De Ciutiis,
Rosa Alba Giannoccaro, Giuseppe Mazzeo,
Valentina Pinto, Floriana Zucaro
213
REVIEW PAGES
Daniela Cerrone, Fiorella De Ciutiis,
Rosa Alba Giannoccaro, Giuseppe Mazzeo,
Valentina Pinto, Floriana Zucaro
TeMA
Journal of
Land Use, Mobility and Environment
TeMA 2 (2012) 55-68
print ISSN 1970-9889, e- ISSN 1970-9870
DOI: 10.6092/1970-9870/918
review paper. received 07 June 2012, accepted 23 July 2012
Licensed under the Creative Commons Attribution – Non Commercial License 3.0
www.tema.unina.it
SYSTEMIC RESILIENCE OF
COMPLEX URBAN SYSTEMS
ON TREES AND LEAVES
ABSTRACT
Two key paradigms emerge out of the variety of
urban forms: cert ain cit ies resemble trees, others
leaves. The structural difference betw een a tree and
a leaf is huge: one is open, the other closed. Tr ees
are entirely disconnect ed on a given scale: even if
tw o tw igs are spatially close, if they do not belong to
the same branch, to go from one t o the ot her
implies m oving down and t hen up all t he hierarchy
of branches. Leaves on the contrary are entirely
connect ed on inter mediary scales. The veins of a
leaf are disconnected on t he two larger scales but
entirely connected on the t wo or three follow ing
intermediary scales before presenting t iny t ree-like
structures on the f inest capillary scales.
Urban system’s structural resilience is highest when
it is configur ed according to a scale f ree struct ure
for its part s and for its connections. The spatial
distribution and the intensity of connections in such
a struct ure obeys a scale-free distr ibution. I t stat es
the frequency of an elem ent ’s appearance and t he
span of a connect ion based on its hier archic level:
the smaller an element is, the more oft en it w ill be
encountered in the system; the bigger an element is
the rarer it will be. This fundamental law defines in
itself the m anner in which living organisms and
things should be organized to optimize their access
to energy, t he use that t hey make of it , and their
resilience. The history of urban planning has evolved
from leaf-like to t ree-like patterns, with a
consequent loss of efficiency and resilience.
KEYWORDS:
Urban resilience, Complex systems, Scale hierarchy,
Urban systemic
SERGE SALAT1, LOEIZ BOURDIC2
Urban Morphology Lab
CSTB, Paris
URL: www.urbanmorphologylab.com
e-mail: (1)serge.salat@free.fr (2) loeiz.bourdic@m4x.org
S. Salat, L. Bourdic – Systemic resilience of complex urban systems
56 - TeMA Journal of Land Use Mobility and Environment 2 (2012)
1 URBAN RESILIENCE THROUGHOUT HISTORY
Historical cities had the capacity to absorb successive transformations without losing their essential
structure. In Paris, deemed capital of the 19th century by Walter Benjamin, no more than half of the
buildings predating 1900 subsist within its historical boundaries and yet t he cit y has managed to maintain its
character thanks t o the tenacious hold of t he structure creat ed by Baron Haussmann. In the historical
European cit y, t he extremely complex substrat e, t he subdivisions and the street grid can be traced back to
the Middle Ages and sometimes even t o the Roman Empire ( Salat, 2011) . The capacity of the city to retain
its identity despit e changes has vanished from t he modernist city, since it has lost it s distinctive character
and its transformative power. The capacity to survive disasters and even t o rise out of its ashes, like Lisbon
after the 1755 earthquake, London after the Great Fire in 1666, Kyoto after the fires in the Middle Ages,
Tokyo after the 1923 earthquake, is what we call urban resilience – a complex concept related to the
permanence of a memory at once social, symbolic and mat erial. The vast maj ority of historical cit ies is
resilient and has managed to survive t he cent uries, often out lasting the civilizations t hat gave rise t o them.
Can modernist cities survive? Will they withstand the t est of time like Rome and the great many cities that
the Romans founded around the Mediterranean? Will they even manage to survive t he century and hold out
against the grow ing risks linked to climat e change?
Fig. 1 Haussmannian Paris
The question is all the more important insofar as the fragility of modern cities is struct ural: they have
exposed themselves more to risks by becoming more and more artificial and incorporating energies t hat are
S. Salat, L. Bourdic – Systemic resilience of complex urban systems
57 - TeMA Journal of Land Use Mobility and Environment 2 (2012)
hard t o predict and control. This paper aim s introducing urban resilience through t he prism of history and
progressively shift to a more dynamic understanding of t his concept, using analyt ical insight s from complex
systems theory.
2 HYBRIDIZING THE NATURAL AND THE ARTIFICIAL
In cutting them selves off from nature, cities have become fragile. I ndeed, they have internalized the most
destructive dynamics of nature wit hout learning how to regulate them. I f nat ural elements are not
incorporated int o the planning and construction of cit ies, t hey risk collapse.
The mat erial met abolism of cities is founded on the redeployment of the energy of nature t hrough the
construct ion of hybrids. The infrastructures of m odern cit ies combine human dynamics and natural forces in
ways that transform nature and change societ y. This phenomenon, verifiable in all cities since t he birt h of
the urban world five thousand years ago, has become a predominant fact or in modern cities. The
redeployment of t he forces of nature provides the energy for processes in which complex physical hybrids
(motors, telecommunications, heating, lighting, water distribution systems, air-conditioning, et c.) and
complex social structures (governments, national and t ransnat ional companies, universities, etc.) are built
out of simpler components. The I ndustrial Revolution developed such hybrids on an unprecedented scale
and they relied on massive injections of energy, mainly from fossil fuels.
Massive flows of energy from nature can travel across these hybrids in catastrophic ways, breaking them
down int o simpler element. Indeed, in t hese hybrid construct ions, natural forces do not lose t heir potential
autonomy. Despite human efforts, hybrids corrode, rot, explode, etc. But t here is worse. These hybrids of
nature and artifice exist in a much wider context of forces over which human beings have no control, like fire
or ice storms, earthquakes, and floods. Modern technological hybrids, like dams, that oppose the resistance
of a human art ifact t o the colossal pressure of masses of wat er, are much more fragile in the face of nat ural
forces than older technologies, like the floating houses in the Mekong Delta that went with the movement of
the water instead of resisting it . I n both cases, t here is a hybridization of the natural and the artificial, but
tradit ional technologies construct with nat ure, whereas modern technologies construct against nature for the
purpose of harnessing its forces.
2.1 THE LAWS OF EVOLUTION
Ecology was long dominated by a paradigm of stability but now we know that all natural systems are
unstable. Nat ure’s unpredictable character is not a temporary state in t he construction of human knowledge;
it is a fundamental feat ure of nat ure, as t heories of chaos and dynamical systems have demonstrated. Cities
exist in a vortex of continually changing dynamic energy flows that we call nature. One fundamental reason
for the fragility of hybrids built by human beings is t hat they are informed by a simple mechanical logic
whereas nature is organized in a much more complex way. The fragility today comes from the coexistence of
two very different levels of complexity within a single hybrid construction. Consequently the complexity of
urban systems must be enhanced t o approximat e the complexit y of natural systems.
Living systems, because they developed and became more complex over four billion years of evolution,
serve as the best model for t he conception of a complex system that can enduringly survive the biological
conditions of our planet. Local ecosystems in part icular tell us much about the optimal organization for
maintaining life in a particular region of the planet. We can look at living systems to understand how to
design sustainable buildings, district s, cities and regions.
Evolution permitted t he survival of species t hrough constant transformations. We can thus find a functional
order in nat ure without an architect or planner. Adaptation via incremental changes can lead to great
S. Salat, L. Bourdic – Systemic resilience of complex urban systems
58 - TeMA Journal of Land Use Mobility and Environment 2 (2012)
transformations and great formal diversity. Evolut ion involves a combination of continuity and change that
occurs in response to the environment. I t allow s us to understand why organisms differ and yet are
connect ed over time and space (Dawkins, 1986).
Can evolutionary theories be applied to cities? Despit e evident differences between t he evolut ion of living
organisms and the development of cit ies, t here are some commonalit ies. Cities can be classified by type.
They change over time and the t ypes also change even as they maintain great stability. Emerging schemes,
however, are never simple. The global scheme of the city emerges from its agreement with local orders. A
complex order is created from t he evolut ion of the small scale and it s influence on higher scales.
The evolutionist perspective can help us understand why the crisis of cities is so profound. Never w ere cit ies
confronted with such massive changes on such vast scales in so little t im e. Thus we may be witnessing a
radical break in an evolutionary process thousands of years old and even an end to the history of cities.
2.2 THE PERSISTENCE OF THE IDEA OF A CITY ACROSS THE METAMORPHOSES OF ITS
FORMS
Cities are t he physical human creations that have persisted over the longest period of time, more than two
thousand years insofar as the Greek and Roman cities are concerned. The historical city was a “ standard
ideal” but never the sterile repetition of a model. Cities of Roman origin share certain qualities and elements
that derive from common principles rat her than from a rigid preconceived plan. Historical cities were
changing organisms, all dif ferent. Over time, the city grew and became more complex in its own right. I t
came to incorporate conscious and unconscious memories, traces of forgotten rituals and forms along with
original patt erns that remained embedded in its construction. The destruct ion of its memory is the worst
crime t hat can be committ ed against a city. To deprive a city of its memory is to destroy its identity and its
singularity, to shatt er t he distinctive lines of its development, and eradicate its identity and its values. “The
city of Florence is a concrete reality,” writes Aldo Rossi. “But the memory of Florence and its image are
loaded with values that ref lect other experiences. In addition, t he universal value of its experience can never
completely explain that special something that makes Florence Florence.” (Rossi, 1981)
By replacing the organic morphogenesis of cit ies by normative plans abstractly projected ont o areas relieved
of the weight of culture and history, Le Corbusier’s modernism replaced t he infinit e variety of the human
world with the serial character of mechanical production. Normative processes did not form historical cities.
They are not the stat ic product of such rational plans as Lucio Costa’s for Brasilia. They are t he outgrowt h of
creative evolution.
Time, temporality, and durat ion – all have a decisive impact. We have forgott en the virtues of a slow pace.
The long t erm and the gradual spread of information in a fragment ed world creat ed the diversity of Western
cities. A rich mant le of cities with complex pat t erns covered both sides of the Mediterranean at a time w hen
centralized China had already developed a more homogenous urban system. I n contrast to the European
complexit y and variety, the city patt erns of urban America, which were planned at a time when information
spread more rapidly, are more homogenous, ordered as they are by an omnipresent obsession with grids.
One cannot simply opt for uniformity or variety as a mat ter of choices in urban design; they are the product
of political centralization versus fragmentat ion, and cultural homogeneity versus diversity.
However, no mat ter how many dif ferent forms a city went through, its initial founding phase will be the most
tenacious attribute of its morphology. Take a cit y like Bath in England. The street plans laid down by the
Romans at the time of its foundation have survived thousands of years, despite periods of destruction and
S. Salat, L. Bourdic – Systemic resilience of complex urban systems
59 - TeMA Journal of Land Use Mobility and Environment 2 (2012)
adversity. And even though the buildings of Turin t oday reflect the Baroque designs of t he Piedmont kings,
its inhabit ants are still walking in the footsteps of the Romans on the same streets.
The cities of Magna Græcia are still standing around the Mediterranean. The mosques of I stanbul took over
the great concave spaces of Byzantine churches. Nation-states pass while cit ies remain. “Soon you will have
forgot t en the world and soon the world will have forgotten you,” Marcus Aurelius famously remarked but his
Rome is still present in Fellini’s Rome. And although Rome may be et ernal, w ho believes in the eternity of
Italy? (Vance, 1990)
But is t his still true t oday? Satellite images show us the inexorable dilut ion of the form of cities. At what
point will the age-old balance of Dutch cit ies, as Vermeer and Peter de Hooch knew them, be destroyed by
the massive emergence of t he conurbation of Randstad? The slab blocks of the modern housing projects can
be seen from the tower of Delft’s Neue Kerke and Vermeer’s famous view of Delft, still visible only fift een
years ago, is now disfigured.
2.3 MORPHOGENESIS AND EVOLUTION
Cities have changed more in the past twenty years than they had in t wo thousand years before. Their
evolution can be described in ten processes:
1. Phases of evolution are connect ed more to the f unctional life of t he cit y than to chronological time.
2. Even though the city changes over t ime, certain physical features like t he netw ork of street s or the
urban fabric are remarkably unchanging. The cit y can be deformed, somet imes impoverished like
the center of Boston, but without ever totally shedding its past. This is why t he blank slate
approach to urban planning that destroys the city or razes whole districts brutally int errupts and
damages the city’s evolution.
3. Whereas forms tend to persist, functions change.
4. The capacity t o have different successive funct ions fulfilled by the same forms or by gradually
modified forms is the adaptation that characterizes historical cities.
5. The adaptation is not only a matt er of the city’s physical structures. It is a process of continually
adjusting form and funct ion – a matter of mutual transformat ion rather t han the primacy of
functions over f orms. The fundamental persistence of adaptation is the basis of the evolution and
continuity of cit ies.
6. Throughout history cities have spread out in continuous processes in which changes of scale and
size gave rise to int egrated wholes. After the Second World War, modernism caused an explosion
of the urban form and huge breaks in scale before the phenomenon of urban sprawl came to dilut e
the form and dissolve the scale in endless repetition.
7. Morphological dynamism was one of the characteristic t raits of historical cities until serial spraw ling
cities put an end to the creative moment um of most cit ies and destroyed the cont inuit y of their
morphology.
8. The dynamics of historical cities made forms and activities converge, while modernism and urban
sprawl separated the two.
9. The historical city increased its complexity and connectivit y as it grew, whereas the morphology of
the modernist city was simplified and it s connectivity was reduced by a factor of twenty w hen
measured with t he help of graph t heory.
S. Salat, L. Bourdic – Systemic resilience of complex urban systems
60 - TeMA Journal of Land Use Mobility and Environment 2 (2012)
The evolving nature of cities is linked not only to forms and f unctions but also to a key third element:
connect ivity.
An analysis of resilience has t o be based on the forms, f unctions and connections. The connections are no
doubt the most fundamental element for creating a living and sustainable city. Some periods are dominat ed
by the creation of new forms, functions or connections while others are characterized by the persistence of
existing pat t erns ( Tannier, 2009). We have witnessed over the last thirty years the destruction of historical
urban forms or t heir dilution in vast formless agglomerat ions, along with t he destruction of connections
(divided by two in t he historical cent er of Boston), the erasure of forms and t he segregation of functions.
The accelerated urbanization of the planet is paradoxically a huge anti-urban production.
2.4 THE PERMANENCE OF THE PLAN
The study of city plans affords valuable indications as to their t ype and level of connectivity. Not withstanding
differences between periods and civilizations, historical cities display relative unity insofar as connectivity is
concerned – a unity with which modernism made a radical break. We can analyze the resilience of an urban
form by looking at t he role of the street. The cit y is born in a precise place but it is the street t hat gives it
life. “The association of t he destiny of the city with communication art eries becomes a fundamental principle
of development.” ( Rossi, 1981)
The urban land is at once a fact of nature and a product of civilization. It is linked to the urban composition
where each element must be the most faithful expression of the life of this collect ive organism, which is t he
city. According to Aldo Rossi, “at t he basis of t his organism that is the city is the persistence of the plan.”
(Rossi, 1981) The concepts of persistence and of memory are essential t o the resilience of cities. Rapid,
brutal t ransformations of urban fabrics destroy the continuity and resilience of cities. Persistence is in fact
the generator of t he plan. The urban structure is a material structure formed of streets, monuments, and so
on, but it is also a structure that internalizes continually changing social forces along with t he forces of
nature, subject to the unpredictability of det erministic chaos. Amid transformations, and sometimes amid
catastrophic breaks, what persists is the urban fact. What constit utes an urban fact par excellence is t he
capacity to subsist within a totality in transformat ion. The functions, single or plural, t hat the city fulfils over
time are only tem porary mom ents in the reality of its structure. Resilient living cities maintain their axes of
development; they preserve the placement of their arteries; they grow while continuing to conform to an
orientation and a sense determined by older facts whose memory has often been erased.
To survive a city has to be able to evolve in a continuous met amorphosis and adapt to new needs, which
necessarily implies deformations to it s init ial plan. The evolution of cit ies shows that successful urban
developments are based on an interaction between urban planning and processes of self-organization that
make the overly regular aspects of t he initial organization more complex. In addition, the original form of the
founded city must be able to deform successfully. The capacity of urban structures to last over time depends
on the complexit y of their organization, the intricacy of their network, t he richness of their connectivity, and
the creation of a fract al order of the same level of complexit y on several very distinct scales. A city can be
said t o be resilient if the idea of its form is maint ained through successive met amorphoses but not fixed for
all eternity in an unchanging order. Cities like Turin, Florence or Rome survived the centuries and different
civilizations. Wit h each metam orphosis enough of their different successive forms was maintained to keep
their memory alive w hile leaving their future open.
S. Salat, L. Bourdic – Systemic resilience of complex urban systems
61 - TeMA Journal of Land Use Mobility and Environment 2 (2012)
3 THE RESILIENCE OF FRACTAL URBAN FORMS
3.1 THE STABILITY OF FRACTAL URBAN SYSTEMS – EMERGING PROPERTIES
The fundament al notion that defines t he stability of physical systems is that states are only stable if minor
pert urbations reinforce rather t han destroy them. Dynamically stable urban stat es are those t hat display an
enormous number of geometric and functional connections on different scales. When some connections are
cut, others are created. These connect ive forces act on urban morphology to generate unique cit ies every
time and transform them following singular trajectories. The process is exactly the opposite of the utopian or
imaginary orders that architects try t o impose on cities and that offer few connections. Huge quantit ies of
energy are needed in such imaginary orders to maintain the urban system in a stable state. Modernist cities,
wit h forms imposed from the outside, obstruct the emergence of connections whereas the continuous
creation of connections in historical cit ies favored their evolution and hence their survival. Traditional
buildings, because of their connective forces, have a stabilizing impact on the urban fabric and system.
Modernist buildings do not connect into the urban fabric. They have a destabilizing impact and fail t o creat e
a human environment. Indeed modernist architects tried t o reverse the laws of urban growth by working
wit h large-scale elements. The brutal j uxtaposition of vast homogeneous zones made of a repetition of very
big object s hinders the appearance of emerging properties.
Emerging properties are propert ies that were not integrated into the initial conception of the system . For a
property to emerge on a big scale, small scales need to exist t o support it . Each scale support s t he higher
scales in the hierarchy. The fact that a system has emergent properties is what allows it to repair and
stabilize itself and to evolve. We cannot understand emergent propert ies by breaking down the system and
analyzing its parts. Emergent properties are analogous to t he human brain (Edelman & Tononi, 2000). The
three conditions needed for emergence to appear in a system are: a high connectivit y, the presence of a
mechanism that creates new connections and a sufficiently low degree of cont rol, since less control im plies
more emergence and vice versa.
3.3 COMPLEXITY, COHERENCE AND URBAN RESILIENCE
A fundamental attribute shared by resilient living cit ies is a high degree of organized complexity. The
geometric assemblage of element s constitutes a series of organized wholes on each successive scale and
across the progression of scales. This fractal harmony is what distinguishes a coherent urban m orphology
from the repetitive serial din of modernist non-compositions. Urban morphology is fract al by nature.
Modernist cit ies, on t he other hand, are incapable of generating urban coherence. Geometric coherence is an
indispensable quality insofar as it connects the city through forms across all scales. It is crucial to t he vitality
of the urban fabric.
S. Salat, L. Bourdic – Systemic resilience of complex urban systems
62 - TeMA Journal of Land Use Mobility and Environment 2 (2012)
Fig. 2 The map of Roma drawn by Giambattista Nolli in 1748 (details)
In a fractal morphological field like the one we have j ust described the position and even the form of each
element are influenced by its interaction on different scales with all ot her elements. When the result of all
these interactions creat es a form, it is neither symmetrical nor fixed. I t displays a degree of plasticity that
allows it to evolve. Evolution is only possible if the large scale is correctly defined on the basis of a great
many connections obeying a hierarchy of scale.
The structure of the connections is what matters and not the nature of the component s. I n a multiply
connect ed, living organic structure, t he smaller components can be changed without affecting the overall
structure. Building the whole from t he parts in an organic way leaves room for evolution. Arriving at the
part s, on the other hand, in a rigid way start ing from t he whole creates struct ures t hat cannot evolve. I n
concret e terms, modifying the w hole once it has been established involves destroying a great many
components on very different scales. It is, to the cont rary, very easy to modify smaller components, like the
arrangement of rooms in a house or the nature of buildings along a street. The streets themselves
part icipate in the structure of t he whole and display remarkable permanence over time.
According to Nikos Salingaros (Salingaros, 1998), the idea underlying the resilience of fractal street patt erns
is very simple: a complex city is a network of paths t hat are topologically deformable. This is particularly
evident in Tokyo and Kyoto where, despite differences in form – in one case very regular, in t he ot her,
curved, labyrinthine and deformed by t he topography – the topological structure of the graph of the two
cities is identical and translates a fundamental anthropological dim ension of Japanese society. I n the same
way, to be resilient, the urban form must be deformable and display a high degree of plasticity. I t must be
capable of accompanying the t orsion, extensions and compressions of paths without tearing. To be
deformable, the urban fabric must be strongly connected into the smaller scales and weakly connected int o
the large scale. This is also a characteristic of the Japanese city with its multiplicity of short-range
connect ions and average distances betw een intersections of around 50 meters in Tokyo as in Kyoto.
Connectivity on all scales following the inverse power law produces urban coherence. Tokyo and Kyoto are
thus part icularly coherent cit ies because they display a great number of small connections.
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3.4 RESILIENCE AND MULTIPLE CONNECTIVITY
The multiplicity of connections enhances the resilience of a city and its possibilities of evolution, change and
adaptation. In fact, the more connect ions there are, t he more likely they are to be redundant. Thus, if the
connect ions are cut, as they were for example during the successive fires in Kyot o, t he cit y can continue to
exist even as it changes. Districts can be reborn from their ashes, sometimes after centuries. They persist
simultaneously t he same and different, like Rome’s Forma Urbis that cont inued to exist in Renaissance Rome
and has survived in contemporary Rome.
Salingaros demonstrates that complex cities are those whose network displays a large degree of
redundancy. I f there are a great number of ways of getting from one point of the city to another passing
through different nodes, then cut ting a connection bet ween two nodes will not keep the network from
working.
Multiple connectivity also presents many functional advantages. Too many connections of the same type in a
single channel can overload the channel’s capacity. We can see this in overly hierarchical systems with the
problem of collectors that gather the t raffic from lower-level paths. On the other hand, connections of a wide
variety of types create a less hierarchical network but one t hat is connect ed in a much more diversified way
and this prevents t he saturation of a single channel or gridlock caused by congestion at an unavoidable
node. The different networks, on different scales, need not coincide. If t hey do, net work saturation will t ake
place faster. A good example of a resilient network is the Tokyo metro, which consists in multiple
superimposed and intertwined net works.
4 SCALE HIERARCHY AND STRUCTURAL OPTIMIZATION
How are we t o approach new urban proj ects in ways that embeds cities in the long term, and factors in the
constraints we are facing in a finite world and the risks of climate change? Cities will have t o reinforce their
efficiency and resilience to meet t hese changes. They will have to be more efficient in their use of material
and energy resources to reduce their ecological footprint and their climate impact. They will also have to
rediscover the resilience of historical cities, in order t o withstand climatic and natural shocks, and to absorb
fluctuations in their environment, which will increase in number and int ensity as the Earth’s atmosphere
warms.
We will show that for an urban fabric to be efficient and resilient, it must be structured in a complex way,
strongly connected in t he manner of a leaf, and hierarchized in a f ractal way according to the Pareto scale-
free distribution.
4.1 SIMON’S WATCHMAKERS’ PARABLE
In a seminal paper, Herbert A. Simon (1962) introduced the topic of complexit y architecture with a parable
that has since largely influenced complexity sciences. He told the story of t wo highly regarded watchmakers,
who constantly had to pick up t he phone to answer client s. One of the two fine watch businesses, run by
Hora, prospered, w hile the other, run by Tempus went bankrupt.
The tw o watchmakers had to construct wat ches out of 1000 parts each. Tempus’ watch was designed so
that if he had partly assembled it and had to put it dow n to answer the phone, it immediately fell to pieces.
The more clients he had, the more they phoned him, the more difficult it became to have enough time to
finish a watch. On the contrary, Hora’s watch were designed so that he could put t ogether subassemblies of
about ten elements each, t hen put together ten of those subassemblies into a larger subassembly, and so
on. Whereas a phone call caused Tempus’ work to fall entirely into pieces, it only causes Hora a
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subassembly to fall into pieces. No need t o argue further that the probability for Hora to finish a w atch is
much higher than for Tempus.
This parable was meant by Herbert A. Simon to highlight the role of hierarchy within complex systems: a
complex system made up of coherent subassemblies has a great er abilit y to evolve and adapt quickly. And
as we will see later in this paper, adaptability has crucial implications on resilience ability.
4.2 SCALE HIERARCHY, FLUCTUATIONS, AND RESILIENCE
Historical cities, over the course of t heir long history, were slowly transformed by incremental phenomena of
destruction and reconstruction of the urban fabric. Structures that were not resilient enough were
eliminat ed. And so historical cities have come down to us with extraordinary capacities of efficiency and
resilience. I n a process of ongoing, spont aneous self-organization t o adapt their forms to fluctuations in their
environment, historical cities acquired t he capacit y to absorb fluctuations by reinforcing their structure and
order, and becoming more complex and richer as a result of t he changes t hat take place in them.
Fig. 3 A free scale network (left) and the Parisian street network (right)
Scale hierarchic structures optimize urban flows and are also vit al in giving cities the resilience that they are
lacking today. The more structured and complex the city, the more readily it can be nurtured by the
pert urbations t o which it is subjected, absorbing them without lett ing them upset t he stability of it s
structure. And it is in assimilating the fluctuations and t ensions t hat it complexifies and absorbs t hem all the
more easily. Hence, t here is an ongoing dialogue between the city’s capacities of resilience and the
constraints to w hich it is subj ected, between t he fluct uations f rom the outside environm ent and it s
resistance t o these fluctuations.
The resilience of a city is int rinsically linked to it s self-organizing capacities. But self-organization is inevitably
lodged in time, and the long span of natural fluctuations is not t hat of contemporary cities; t he lat t er are
designed and built very rapidly by authoritarian, rigid forms of urban planning to accommodat e an ever
growing number of rural migrants irresistibly at tract ed to cities. These cities are designed nearly
instantaneously in emerging countries, without t he time and distance needed to evaluate the quality of their
interactions wit h the environment, the adaptat ion of their forms to the f lows that run through them, and the
systemic ef ficiency t hat det ermines their resilience. These are cit ies that are expected to survive for
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centuries but the long span of their existence is almost never t aken into consideration when they are
designed.
Alongside these long fluctuations, whose effect s over cent uries are somet imes impercept ible, there are
short -term, even catastrophic fluctuations, which are becoming more frequent today, with their share of
deaths and destruct ions. Cities were always subjected to them. Cases in point are the Great Fire of London
in 1666 and the earthquake in Lisbon that out raged Voltaire so. But London and Lisbon both managed to live
through these disasters and maintain their form, whereas contemporary cities are more and more vulnerable
to earthquakes, droughts, f loods, and natural and energy crises. They are vulnerable, to begin w it h, due to
their low efficiency, and t heir voracity in energy and resources. They are also vulnerable because they are
not adapted to their sites, to the environm ent they inhabit all in t he same way and which, from one day to
the next, may violently remind t hem of its existence and its identity, like the Chao Phraya delt a int o which
Bangkok is inexorably sinking. Finally, t hey are vulnerable because of the disordered uniformity of t heir
urban fabric, its absence of hierarchized structure, of identity based on the complexif icat ions of a long
history t hat forges a city’s capacities of resilience.
Following Simon’s parable (1962), the resilience of scale hierarchic structures is linked to their power to
complexify so as to absorb fluctuat ions, to t ransform t he currents of t he waves of history and time into a
construct ive rather than a destructive force. Urban resilience can be understood as the robustness of urban
structures and netw orks against random failures. Such failures might be small-scale failures (local transport
network disruption, local energy supply disrupt ion, etc.) or large-scale ones. According to Buhl et al. (2004,
2006), t he resilience of a netw ork – its robustness – can be evaluated by studying how fragmented t he
structure becomes as an increasing fraction of nodes is removed. The network fragmentation is usually
measured by the fraction of nodes contained by the largest connected component (Buhl, et al., 2004) . The
move removal can be chosen either randomly or selectively. According to Albert et al. (2000) and Holme et
al. (2002) real networks clearly deviat e from t he prediction made for random graphs. Moreover, several real
networks have proved t o be highly resilient to random node removal and highly vulnerable t o selective node
removal. Although they might not be the unique ones (Newman, 2002; Dunne, et al., 2002), scale-free
networks do exhibit this specific feature (Albert, et al., 2000).
Counteracting the vulnerability of contemporary cities requires a real paradigm reversal, and a shift from a
mono-scale conception to a scale hierarchic conception of cities. Only scale hierarchic structures in t he case
of flow networks can secure optimal efficiency and resilience, while limiting the propagation of local
pert urbations. But another parameter is j ust as fundamental for t he capacities of resilience of cities, and that
is the fine-grained connectivity of their subjacent structures. This parameter entails pushing our thinking
beyond the tree-like structures prescribed by simple thermodynamic considerations.
4.3 ARBORESCENCES AND LEAF STRUCTURES
An arborescence is a highly hierarchic structure, and this hierarchization is precisely w hat causes it s
efficiency ( Salat & Bourdic, 2011) . This then is the first element we are seeking for the sustainable structure
of the urban system: a strong scale hierarchy ensuring system efficiency. However, the connectivity of a t ree
is low: between two point s t here is only one possible path. And connectivity is an essential param eter of
cities. For a city to be connected, it must be struct ured not like a tree but like a leaf.
A series of connections whose int ensity obeys a Pareto distribution – scale hierarchic - increases resilience by
preventing rapid and catastrophic fluctuat ions from spreading quickly through the system and disorganizing
it. There should be few long-range connect ions and these connections should be weak to prevent the spread
of disrupt ing fluctuations. Indeed, weak connections are what allow the fluctuat ions to be absorbed. On the
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other hand, a great many strong short -range connect ions ensure the system’s deformability. I f efficiency is
linked to the arborescence of elements, resilience seems to be linked to a more abstract arborescence, that
of the system of connections between elements the int ensities of which should also obey a Pareto
distribution.
As Alexander has noted (Alexander, 1965), one can readily see that street net works are not structured like
trees: small streets are more often linked to one another or to several higher level streets, which is not the
case in a tree structure. In fact, t he underlying struct ure of these networks is w hat is called a “ semilattice”.
A striking image of this type of structure is the system of veins on the leaves of most deciduous trees. Their
leaves manifest a remarkable exception to the many tree-like systems observed elsewhere in nature. They
display t he same scale hierarchy, which proves again the universality of the Pareto distribution, but the
midsize veins and the venules connect to one anot her, like the street s of a city, and so the connectivity is
much stronger than in a t ree-like structure.
4.4 THE MULTIPLE PATHS OF LIFE
The multiple connectivit y and scale hierarchy that leaves and cit ies have in common enhance both their
efficiency and their resilience.
Firstly, the loops that these struct ures contain, as Francis Corson has demonstrat ed, (Corson, 2010) manage
variable flows more efficiently. The t ree structure is most efficient when it comes to distributing stationary
flows. But one of t he characteristic feat ures of urban flow s is their extreme variability, both in time and in
space. The semilat tice structure absorbs these variations by distribut ing flows along different possible paths.
This is impossible in a tree-like structure, where there is only one path between two points.
Secondly, the semilattice structure imparts greater resilience to a network. When a branch of a tree is cut,
all those t hat grew from it will die t oo. In a leaf, if a vein is interrupted, the redundancy of t he netw ork will
allow the flow to get around the interruption via secondary paths, so that it will only be partly slowed down
by the degradation of the network. This is why cit ies structured like leaves are more resilient. Just imagine
that a path is blocked by an accident: the flow is simply deviated ont o ot her paths t o irrigate t he far side of
the perturbat ion. A part of the leaf’s network can be amputated and the leaf will go on living and converting
light energy into nut rient s. Thanks t o the dilatation symmetry or the scale invariance linked to the Pareto
distribution, nat ure has provided for redundancy on all scales to ensure the permanence of it s structures.
The simultaneous existence of small and big nervures having the same function contains a natural
redundancy for living organisms that answers the objective of efficiency and resilience with an economy of
volume.
5 CONCLUSIONS
We have discussed the t heoret ical underpinnings of what a sustainable and resilient city should be. This is a
concept ual framework, governed by fractal geometry for spatial planning, t he power law for distributions,
and leaf structures for connect ions. The scale relationships between the different hierarchic levels of an
arborescence, a leaf, and the blood and oxygen circulation systems in our bodies obey such a mathematical
law. It states the frequency of an element’s appearance and the span of a connection based on it s hierarchic
level: the smaller an element is, t he more often it w ill be encountered in the system; the bigger an element
is the rarer it will be. This fundamental law defines in itself the manner in which living organisms and things
should be organized to optimize their access to energy, the use that they make of it, and t heir resilience.
City planning t oday has lost all its complexity and hierarchy of scale. I t has become so simplistic,
mechanical, and functional t hat its structural ineff iciency causes an enormous waste of energy. It should
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possess the qualities that history has conferred upon cit ies: complex, connect ed, and structured according t o
scale hierarchies based on the Pareto distribut ion.
To reach these high levels of connectivity, complexity and scale hierarchy that make t he efficiency and the
resilience of historical urban fabrics, a set of innovative tools based on the science of complexity has to be
settled. It is meant t o be applied to the design of new cit ies, but also to the restructuring of hastily built
cities, denatured by the ideas of modernism, mechanical bodies completely disconnected from t he time of
historical, organic cities.
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AUTHORS’ PROFILE
Serge Salat
Serge Salat is an architect , a graduat e of the École Polytechnique and t he ENA. He also earned one PhD in economics and
one in art history from EHESS. He is t he founding direct or of the Urban Morphology Laboratory. Serge Salat is the author
of more than 20 books on art and architecture. He has been a pr acticing architect and the project director of large
infrastructure projects such as inter nat ional airport s and TGV train stat ions. Presently Direct or of the Urban Morphology
Laboratory in Paris, he is gr ouping the research efforts on sustainable forms and met abolisms of cities of main French
National Research Centers such as CSTB, Universities, engineering schools, and urban planning agencies in the field of
energy, carbon and economic efficiency of urban forms. He is the author of tw o maj or books on urban morphology, as
well as numerous publications and communications. He is a member of the editorial board of several major int ernational
scientif ic journals.
Loeiz Bourdic
Loeiz Bourdic holds a Master in Engineering from the École Polyt echnique and a Master of Science in Environmental
Economics & Policy from Imperial College, London. He is currently a Ph.D. candidate in economics at the Urban
Morphology Lab. He is studying t he links betw een urban morphology, urban complexit y, energy eff iciency and economic
value creat ion on the city scale. This theoretical research aims at applying result s f rom the complexit y theory (fract als,
complex systems) to urban analysis. He is also working on t he transposition of scientific findings int o assessment t ools for
urban policies.