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Urban Planning and Sustainable Development in The 21st Century, Conceptual and Management Issues

Urban areas in historic cities resemble a living organism that evolves in parallel to social transformation processes, shaping the material substrate that expresses identity and collective memory. In the twenty-first century, exponential population growth, globalization and the information society have resulted in many of these socio-economic processes accelerating, with consequences that we are not yet able to discern in their entirety. In this context, cities need to adapt to the general dynamics of urban development by incorporating the environmental, economic and social aspects of the "sustainability paradigm". With good planning, urban heritage is a key sustainable resource that needs promoting as part of the existing territorial competitiveness in a scenario marked by an increase in rivalry between cities. This requires the development of a conceptual framework that, based on a global, holistic and integrative approach, covers equity and social justice, respect for human rights, the gender perspective, public health and environmental quality, among other aspects. In this sense, the purpose of this paper is to study the concept of landscape applied to urban planning in greater depth, paying special attention to the analysis of the notion of Historic Urban Landscapes from a critical point of view, since the economic pressures arising from the reality of today's globalized world pose a serious threat that hinders their custody and protection, complicating this new comprehensive approach: how to bring this new systemic and transversal concept to the current regulatory framework in order to achieve real legal protection and effective governance models in urban areas? What should be the acceptable limits to ensure that "managing this change" does not result in public spaces being at the service of the interests of financial capitalism? These, along with many other questions, make the work of the professionals in charge of urban conservation more challenging in their aim to establish a sustainable dialogue to clear the complex equation between historical city and development. With a view to try to answer these and other questions, this paper sets out need to design comprehensive urban policy and legislative frameworks coordinated, and sets out proposals regarding the development of acceptable change limitation methods and indicators, and the application of the concept of buffer zones to the conservation of urban heritage that can become the basis for urban management policies and models.


Introduction
Interpreting urban transformation processes has always posed substantial difficulties. Over the last few decades we have also been witnessing the transformation of the classical paradigm of "urban conservation" by that of change management. The whole process has taken place through an intense international debate with continual reformulations and multiple approaches from various spheres, and this has built up a veritable cluster of meetings that has failed to follow a clear guiding principle. This panorama becomes even more complicated if we consider the multiplicity of institutions, bodies and organisations involved as well as the heterogeneity in the typology and format of the events held. Among all the approaches used to tackle the study of the sphere that concerns us, our analysis will be specifically focussing on the regulatory evolution taking place around the concept of Historic Urban Landscapes as the basis on which to undertake its conceptual study per se. All this is with a critical approach that highlights the risks and difficulties that its scope and content in the sphere of urban management and planning may entail. Proposals are being put forward to develop strategies for urban management focussing on the analysis of "acceptable limits of change". All this is with a view to coming up with measures geared towards implementing the plan to execute the new urban agenda for the 21 st century that is being outlined within the framework of the Habitat III Global Summit 1 , and for the purpose of trying to reconcile conservation, innovation and development through planning systems that from an integrating perspective will enable the values and significances present in historic cities to be conserved and transmitted to future generations. In our view, the assumption of change as an inherent part of the urban condition is the source of all the problems and difficulties posed by the management and stewardship of contemporary cities since there is no consensus on how far these transformations can go as far as Historic Urban Landscapes are concerned.

Regulatory developments
Historic cities, their spaces and their landscapes, are not static. They constitute a complex system in which the historic dimension lives alongside the contemporary. Appropriate management of urban heritage requires clear awareness of this dichotomy in order to tackle, with some degree of certainty, the issues affecting its development: the new role of cities in the global economy, the need to foster urban sustainability, the pressure of tourism, climate change, a concept of cultural heritage in a continual state of transformation, the significant weight of intangible value, the insertion of contemporary architecture etc. All these factors, have given rise to a profound reflection from which a concept of historic districts has emerged and gradually developed, until the Vienna Memorandum (UNESCO, 2005) expressly catapulted the notion of historic urban landscapes to the international arena 2 . Following the "Vienna Memorandum", the notion of Historic Urban Landscapes, which is still in the process of being formulated and consolidated, has been gradually shaped through a large number of international meetings encouraged mainly by the World Heritage Centre and by ICOMOS. In this respect, the "Recommendation on Historic Urban Landscape" submitted at the 36th General Congress of UNESCO held in Paris in November 2011 stands out as the most recent reference 3 .

Results and discussions
The all-encompassing, transversal character encouraged by UNESCO with the incorporation of the concept of landscape has produced a very complex situation which has promoted a vision of the urban sphere as a global, socio-systemic phenomenon that has been tackled from all the heritage angles. As a result of the "European Landscape Convention of Florence" (Council of Europe, 2000), the landscape is conceived as a point of intersection in which all the dimensions that form it, be they of a physical, economic, social or symbolic nature, meet dynamically with different temporalities. It will be this hybrid dimension of the landscape that positions it on the crossroads of many academic disciplines wherever disciplinary frontiers become blurred and where, inevitably, we will all be obliged to develop transversal, cross-disciplinary approaches, [2].
In section 8 of the Paris Recommendation on Historic Urban Landscapes (UNESCO, 2011), the latter are described as "urban areas resulting from a historic layering of cultural and natural values and attributes, extending beyond the notion of "historic centre" or "ensemble" to include the broader urban context and its geographical setting". The notion of Historic Urban Landscapes goes beyond the idea of the building to take into consideration the space with all its attributes: the city profile, the visual perspectives, the topography, the environment and the whole human perception present in the urban 1 On the basis of resolution 66/207 and in line with the series of conferences every two decades (1976, 1996 and 2016), the United Nations General Assembly has called the Habitat III Global Summit on "Housing and Sustainable Development", which will be held in Quito (Ecuador) in October, 2016.
2 In this respect, the reader is advised to consult the analysis made in Azkarate and Azpeitia, [1]. 3 It is a regulatory tool designed to promote an integrated vision of the safeguarding and management of urban heritage in the institutional and legal mechanisms designed to protect and conserve cultural heritage and a natural insert in the urban territorial sphere. That is why it urges the use of the concept of historic urban landscapes as the basis on which to plan urban development policies and strategies from a holistic approach. space. However, as Lalana Soto [3] points out, the notion of historic urban landscapes entails, at the same time, a degree of ambiguity and intrinsic vagueness that raises difficulties and problems of interpretation and practical application that have led to the justification of indiscriminate actions contrary to all the principles of urban conservation. The regeneration, functional recovery and revitalisation of the core fabric of cities have become fundamental aspects to achieve the models for sustainable development that are currently being pursued. The controversy surrounding the integration of contemporary architecture into the heritage areas is one of the main problems that experts in the matter need to tackle. It is an important but by no means new issue, and right now the theoretical development relating to the notion of Historic Urban Landscapes has not managed to address it and provide a clear solution. What is more, we must not forget that although in the field of architectural conservation there is the background of significant practical experience that has been built up, the situation is confrontational in the sphere of the multitudinous landscape inherent in the historic city (social structures, activities, customs, rituals, etc.). Recent actions such as, for example, the ones carried out in the city of Seville with the building of the Metropol Parasol on the Encarnación square, or the Pelli Tower on the Isla de la Cartuja, demonstrate that the controversy surrounding integrating contemporary architecture into Heritage of Humanity cities remains a problem that is still a long way from being solved.
It is necessary to stop regarding the old quarters of historic cities as spatial units separate from the rest of the urban sphere and start to accept that they are immersed in the flows that drive forward current urban dynamics. That way, it will be possible to prevent fossilisation and combat, among other things, the gentrification processes and changes in customs as a result of tourist pressure. At the same time, however, it is fundamental that the development of indicators for managing historic urban landscapes should focus on guaranteeing, in any case, harmony with the irreplaceable heritage values and their transmission to future generations. In this respect, the applying of Buffer Zones appears to us to be a basic management tool to achieve this objective. This concept, traditionally linked to the sphere of nature and environmental sciences, was specified in 1977 in the "Operating Guidelines" of the "Convention on World Heritage" (UNESCO 1972) as "areas surrounding the property which has restrictions of a legal and/or customary nature placed on its use to give an added layer of protection (…) which are established when required by the heritage property".  The Buffer Zones enable us to make use of some of the considerations that currently stand out in the sphere of urban conservation: the setting up of flexible, multifunctional spaces where the territorial scale and modernization of the historic fabric fuse 4 . The diagram of circles presented below ( Figure. 2) formulates a proposal of management model in relation to this idea whose application needs to be adapted at all levels 5 . Cultural assets that individually display different values and which require specific strategies to guarantee their conservation and protection coexist inside old quarters. In turn, this compendium of assets forms an indivisible unit in which these values intermingle and act as a single unit and give rise to an amalgam of cultural interests that give each old quarter added value and a specific personality. They function as complex organisms particularly sensitive to the alteration of any of the elements that comprise them; as a result, not only the indicator systems but also the "limits of acceptable change" have to pay particular attention to the needs arising out of this systemic nature. In this respect, buffer zones acquire great importance as they constitute a tool allowing the spatial organisation to be regulated while delimiting an area of harmonisation that acts at the unifying factor and test bench for integrating cultural heritage and urban development into each other. The delimitation of these buffer zones also involves a limit on change by contributing thus towards guaranteeing the preservation and transmission of the main asset of historic cities: their heritage in all cultural aspects.

Conclusions
This synthesis on the concept of historic urban landscapes has set out to reflect on the contradictions and complexities inherent in the contemporary approach to the urban dimension, and, in particular, to highlight the gap existing between the theoretical approach of the notion of Historical Urban Landscapes and a reality heavily influenced by the weight and influence of globalised financial interests in the sociocultural transformation processes characterising cities in the 21 st century.
Some questions remain, including the task of defining certain "limits of acceptable change" that can reconcile the management of ancient districts with modern aspects of historic cities, the definition of new heritage values that incorporate contemporary expression, the immaterial dimension and natural elements, or the current lack of agreement between this conceptual field's holistic and cross-cutting vision and the current administrative regulation, structured by different sectoral standards. In our view, such questions hinder the plausible and effective application of the objectives set out in the notion of historic urban landscapes. The controversy surrounding the integration of contemporary architecture into heritage areas and the protection of the visual perspectives that gave rise to the holding of the "Memorandum of Vienna" (UNESCO 2005) and the subsequent theoretical development of the concept of historic urban landscapes continue to call for an approach capable of specifying the level of homogeneity and continuity that needs to be respected as it is routinely underestimated.
These issues must therefore be key aspects to be developed in the implementation plan for the new "Agenda 2030 for Sustainable Development" in the sphere of "Urban Planning and Administration" 6 .