The Po River Delta in Italy is an important example of how such complex issues have been intertwined over time with urban, social and economic developments, which have characterised the evolution of the so-called “Megalopoli Padana” (or “Po Valley Megalopolis”). This geographical notion (Gottmann, 1978) has been used by Eugenio Turri in 2000 to describe the peculiar urban growth through the entire alluvial valley of the Po River, from Turin to the Adriatic Sea. In fact, according to Turri, the whole territory included between the river and the Via Emilia road can be described as a continuum, where agricultural fields have today become interstitial areas incorporated within dense urbanizations across the plain. A system that has developed following partial and heterogeneous interests at the local scale, rather than an overall design, resulting in a “vast machine” exploiting a territory of extraordinary resources, able to offer an amazing variety of industrial and agricultural products, and profoundly shaped by man to the needs of anthropization.

In this framework, the Po Delta and its present fragility represent the outcome and the clear evidence of the major changes that have affected the whole fluvial system during the last century to say the least. 

Anyway, such condition of instability is not new. The current layout of the Po Delta of Northern Italy is the result of continuous anthropic actions started in 1604 with the so-called “Porto Viro cut,” which enabled the main distributary channel to flow southward, in order to protect the Venetian Lagoon from silting (Fig. 1). 

Against this logic, the Retreat Strategy aims to provide a different approach to environmental management and a long-term vision driven by pragmatic evaluations. Through the last few years, a certain ambiguity has grown in the landscape planning of coastal and deltaic contexts. Scientific studies and planning procedures on deltas have mainly focused on environmental remediation, ecological restoration and the so-called “re-naturalisation” processes. According to this approach, the landscape should somehow be brought back to a previous “natural” state, whose characteristics however actually belong to a very specific evolutionary phase. Fast evolving contexts such as deltas have often been considered and managed as if their dynamic attitudes were something to be fixed or eliminated rather than being included into planning policies. The European and national regulatory framework itself, following the same logic, identifies protected areas and high environmental interest zones (known as “Natura 2000” sites, under the 92/43/CEE “Habitat” Directive) as permanent in time and space. This is in sharp contrast with the high natural mobility of the deltaic systems. Littoral zones, dune systems and wetlands need to be identified and protected through more flexible instruments, assuming their mobility and the chance to be relocated elsewhere. In a near future, the delta planning authorities will not be able to manage environmental changes anymore without being allowed to take more radical and faster decisions. Therefore, environmental protection instruments, such as the ones mentioned before, need to be updated and adapted to highly mobile context framework.

The comparison of different generations of aerial photos, taken since 1950, shows the ongoing process of the sea retaking its space on the delta’s top plain, re-flooding broader and broader areas and affecting human activities, settlements and their safety (Fig. 3).

The combination of fast subsidence, eustatic rise, climate change, and sediment input starvation, water pollution, and deterioration of the infrastructures will soon make maintaining the current delta configuration very difficult. Even more rigid protection works will be required, causing a further stiffening of the entire hydro-morphological lagoon system. Persisting on this course of action implies building in the future even higher embankments to protect sunken and unprofitable areas.

The futile attempt to crystallize and fully control such a changeable system is a losing approach, as events have already shown. In the long-term, such efforts will not be sustainable, neither environmentally nor economically. Innovative strategies grounded on the intrinsic dynamism of the Po Delta system are therefore strongly needed. It is clear that the present rigid environmental framework will soon become untenable and a retreat of human activity from the artificially reclaimed areas, well below sea level, will become unavoidable. A project of Selective Retreat Strategy implies policies aiming at abandoning to the marine transgression selected delta areas and at re-organizing the whole delta dynamic landscape.

Various desertion plans have already been carried out in several regions of the world, yet trying to give an answer to different issues, needs, and objectives. These examples are mostly the result of powerful economic interests, but they are also useful to envisage the social and economic impacts of transferring people, activities and urban services from one place to another.

It is not so uncommon to relocate entire villages and communities, in order to allow the construction of highways and dams (Égré and Senécal, 2003). This is the case of a southern Australian town that has been moved for about 8 km [4.97 mi.] in the ’50s to make way for the expansion of the massive Lake Hume Dam. More recently, in 2008, the Hubei Province of China witnessed the relocation of 1,24 million Sandouping residents, due to the construction of the Three Gorges Dam, a major infrastructure crosscutting the Yangtze River.

In others cases, a crisis resulting from aggressive mining close to urban areas forced the relocations of entire towns (Speller, Lyons, and Twigger-Ross, 2002). For instance, in the late 1910s, the Minnesota town of Hibbing was moved about 3 km [2 mi.] southward because of unstable ground caused by the town burgeoning iron mine. A similar destiny is now affecting the northernmost town in Sweden, Kiruna, which is currently caving in of the over-mining in the large Precambrian iron deposit. Officials are now tasked with moving the town about 4 km [2.48 mi.] eastward, in order to enlarge the mining site, keep the inhabitants safe, and develop an improved modern version of the city. Institutions, stakeholders and citizens have been engaging during this process in several common initiatives, with the overall objective of building a roadmap for the village transition; such roadmap is the outcome of several workshops aimed at informing the urban and infrastructural planning (Leonhardt, 2015) according to different scenarios. Even though the adopted methods are not exempt from criticisms, and the whole debate - as pinpointed by the Bo Nilsson’s article (Nilsson, 2010) - has been affected by the ideological bias that characterizes various opinions in relation to relocation plans, the Kiruna case-study shows how democratic processes in the context of urban and territorial transformations could be managed and adopted to inspire a shared vision of the future.

The proposed Selective Desertion Strategy for the Po Delta aims at integrating such external forces into a long-term sustainable scenario of growth for the deltaic territories. Such hypothesis explores a retreat scenario, choosing certain areas to selectively flood or further protect, in order to develop new landscapes and new socio-economic opportunities. As evidenced by Ivan D. Correa and Juan Luis Gonzalez in 2000, such goals, in particular conditions, can be achieved only by taking radical choices rather than continuous technical compromises. In this perspective, a deeper awareness of Po Delta dynamics and more flexible planning rules are however needed. Nowadays, the whole delta territory is still highly fragmented in terms of planning and management authorities and this is a notable weakness not only for the proposed strategy implementation, but also for any kind of policy, even less radical, that will be applied in the future. For these reasons, a closer cooperation between public and private stakeholders, investors, enterprises and citizens is needed, as well as the rendering of more extreme scenarios could be useful to test the political will in tackling the issue, instead of reiterating the current attitude at postponing it.

These main steps will be leading to increasingly safer configurations of the delta system. 

The planning has been traced to reach a progressive economic balance in the Po River Delta hydro-morphologic management:

1. Emerging lands are planned to decrease by 13% in the first phase, and 63%, in the last one.
2. The embankments length will decrease by 48%, in the first phase, and 71%, in the last one. The associated maintenance cost-saving will be effective from the beginning of the process.
3. The delta distributary channels will shorten by 11% and 56% in the first and last phase, respectively. The channel shortening will increase the extremely low gradients, supporting an improved water flux.
4. Lagoon area is expected to decrease by 74% in the first phase, then to increase through the flooding of new areas in the following project phases.
5. The spontaneous wave erosion of the newly flooded coastal area will mobilize millions of cubic meters of sediment, now stored beneath reclaimed area, already below sea level. The sediment will be involved in the reshaping of the coastline, through the sea wave action, locally matched with artificial nourishment of sandy beaches. 
6. Inhabitant displacement would involve 2% of the delta region population at the first stage, up to 11% during the possible next retreat phases, involving a maximum of about 8.600 people. The ongoing decrease in population and the incomplete demographic turnover of the elder component will likely further reduce the number.

In many cases, persisting in the protection of anthropic habitats would be economically unfavourable if compared to balancing their loss with the re-design of similar new sites elsewhere; in other cases, flooding an area would not be economically convenient. For instance, the huge dismissed thermoelectric plant in Porto Tolle, if flooded, would have to be completely remediated: an expense far greater than the investment required for keeping it dry and re-functionalizing it in a touristic perspective. At every stage of the retreat process, some sites are planned to be preserved from flooding, to form an artificial archipelago in front of the delta where several services and infrastructural facilities will be developed. Existing embankments or harbour works could be saved by becoming marinas or offshore cruise docks, with the aim of boosting the maritime accessibility to the area. Small isolated villages, such as Santa Giulia, San Rocco and Gorino, and other sites below sea level, could be networked and converted into diving parks. The whole archipelago system, besides its value as a focal element within the landscape, would probably act as a catalyst for the development of the delta tourism. 

The project is now focusing on two main operative layers: the mobility infrastructure network and the wetlands system.

The infrastructural system needs to be reset before the starting of the regulated flooding process. Both existing and new infrastructures have to be connected to manage the population and economic activity resettlement. Several interventions are planned for resetting the territory according to the new scenarios, which would imply, for example, an increase in population in some areas and function losses in others. Roads will be designed following short and long-term previsions on rising relative sea level and catastrophic events. The roads already well under sea level and exposed to a high flooding risk will be abandoned. The new mobility network is planned through two types of roads corresponding to an increasing resiliency level. The first level consists of seasonal connections, conceived to be flooded under extreme overflow conditions, being just over the average sea level. They will shape the wetland landscape when the most advanced protections are dismantled. The second one is designed to be a long-lasting infrastructural level. This road system is patterned after existing main roads and embankments, set up to a safety elevation, and will ensure durable connections between dry lands. Road infrastructure will become the supporting framework of the retreat process, catalysing new functions and re-shaping the Po Delta’s identity. Unlike today, the future Po Delta landscape will lay on a “branched” fruition system, rather than on the monotonous zoning of single-purpose areas (Fig. 6).

The idea of Hyper-Natural Environments informing the research on the Selective Retreat Strategy for the Po Delta is an attempt to define a new conceptual framework for landscape planning policies. In fact, it also aims at generating a change of attitude in the diverse territorial institutions involved in the delta management.

The neologism “hyper-natural” describes environments generated by the strong interaction between human actions and “natural” processes. Within this framework, the opposition between natural and artificial no longer applies. Persisting in this opposition could lead to misunderstandings or paralysis. In the case-study of the Po Delta, the overcoming of such an antithesis allows to plan a progressive site re-organization of a landscape shaped over the centuries by the continuous interaction of natural phenomena and human intervention. Since the effort to “freeze” the environmental evolution is no longer sustainable, dynamic scenarios should be planned to manage a soft and resilient transition to future arrangements (Nicolin, 2014).

The recent declaration by UNESCO of the Po Delta as one of the 651 Biosphere Reserves of the world ratifies the importance of this region and presents a new challenge. According to the MAB Program (the “Man and the Biosphere Programme” launched in 1971 by UNESCO), the proclamation purpose is to recognise and promote a balanced relationship between human communities and ecosystems. It also promotes the creation of on-site infrastructures for scientific research, environmental training and education, as well as experimental hubs for spatial planning policies. The Selective Retreat Strategy proposal is aimed at supporting a sustainable delta development, by exploiting its intrinsic changeability. The strategy supports an effective adaption strategy to climate change, eustatic sea level rise, fast subsidence, reduced fluvial sediment input and socio-economic expectations.

Far from being a rigid and definite design, the proposal wants to give alternative standpoints from which to start rethinking the future of the Po Delta. In fact, since the political feasibility of such a proposal could seem to be still difficult to implement, the added value of the Selective Retreat Strategy consists in being potentially used as a term of comparison: with the actual more conservative policies, as well as with other less radical options. Within this framework, the Selective Retreat System can be considered as an “exploratory scenario” aimed at grounding the debate around the delta on a more prospective outlook.