2.2.1. Thessaloniki’s PRA and RS
The participation of Thessaloniki in the 100RC network was itself a great challenge for the city. The lack of familiarity with resilience principles and policies has arguably been the most important challenge for Thessaloniki’s RS, as new concepts and principles introduced for the first time to local officials, needed to be incorporated to the municipality’s ‘modus operandi’. Thus, the first goal of the RS was to clarify the city’s strengths and weaknesses, identify potential opportunities and threats and create an effective and viable vision for the city.
It is safe to say that this endeavour has been largely successful, as for the first time more than 40 organisations and 2000 citizens participated in the resilience dialogue, partially shaping the base for its implementation [14
]. This wide multi-stakeholder engagement generates a new reality for the city achieving, to a certain extent, a first layer of resilience building, by creating capacities absent in the past [17
]. Moreover, it provides a roadmap for guiding developmental activities in the city, thus becoming an invaluable document governing Thessaloniki’s strategic urban planning.
Weighting its acute impact and broad influence, the PRA identifies earthquakes and surface flooding at the top of the five shocks or stresses for the city. This also comes from a review of the major shocks, either natural or human-induced, occurred in the modern history of Thessaloniki. In this regard, Figure 4
is a modified version of the Thessaloniki Resilience Timeline that is published in the PRA [13
], through which we want to highlight which of the shocks are recognised as having generated either positive or negative impacts on the urban environment, as well as having shaped city policies of disaster management and risk reduction.
This, in a sense, seems to confirm that the city has already a history of positive relationships between hazards and urban planning and design, despite the fact that this process is not always constant or linear. For example, since the 1978 earthquake the seismic code in Greece has been changed and upgraded several times [16
]. The most important reform was the release of the New Seismic Code for Building Structures in 1984, which was introduced after the earthquakes in Thessaloniki (1978) and Alkionides (Athens) 1981, and divided Greece into three different seismic zones, with more tightening the regulatory framework for new constructions around the country. Thessaloniki was included in Zone II.
However, this not necessarily brought to retrofit all the existing buildings and infrastructure (e.g., also due to the lack of investments). In this regard, both the PRA and RS mention the ageing building stock and housing infrastructure among the stresses directly related to the current economic condition and past urban development, alongside dense urban structure with limited open and green spaces.
2.2.2. Seismic Susceptibility
Thessaloniki was founded in 315 BC and has since its founding has faced continuous urbanisation. Due to its geographical location the city has historically played a dominant role as a focal trading, exchange, transportation, multinational and cultural point at both a national and an international level. Its port is the second biggest in Greece and among the most important in the Mediterranean, while the HELEXPO International Exhibition Centre plays a predominant role in the business community in southeastern Europe. The wide seafront is one of the most significant assets of the city, especially after its renovation was completed in 2011, where continuous pedestrian and bicycle paths intermingle with a series of consecutive thematic gardens [20
Thessaloniki’s current urban area ranges in a NW‒SE direction between the hills of Oreokastro and the mount of Chortiatis (Figure 1
a). In parallel, the Metropolitan Area of the city extends further in the west up to Loudias River and south, surrounding the whole areas around the Thermaikos gulf. From an administrative perspective and following the last administrative reform of the country in 2011 (Kallikratis Plan), the metropolitan area consists of 11 municipalities, with the Municipality of Thessaloniki holding the most central position among them (Figure 1
b). The Municipality’s population is 324,766, while the Metropolitan Area accounts for around 1.12 million residents (2011 census).
From a geomorphological perspective, the city’s elevation starts at sea level and reaches almost 950 m around Chortiatis, while within the urban area it does not exceed 150 m with smooth ascents [16
]. This morphology favours the outburst of surface flooding phenomena to specific downstream sites in the Metropolitan Area. For instance, the existence of streams and gullies in the relatively low elevation areas and in close proximity to residential settlements in the municipality of Thermaikos has played a central role in the accumulation and channelization of the excess of water during heavy rainfall events (see Section 3.2
). Similar issues have been identified in the Municipality of Delta in the western part of the Metropolitan Area, where three different rivers flow in (see [21
The geological map of the Metropolitan Area demonstrates the heterogeneity of lithologies upon which the city has developed throughout the years (Figure 5
a). Holocene deposits, red clay and sandstone marl dominate the southern and western parts, while the higher altitudes of the eastern part, the Chortiatis mount rises, are primarily covered by various metamorphic formations. Thessaloniki’s historic centre in particular, is located in the junction of three different lithologies, each characterised by different depth (Figure 5
a,b). Moreover, after many years of continuous inhabitancy, the historic centre consists of large areas of filled ground with different characteristics and attributes [16
]. In general, deeper urban fills are much more prone to earthquake damage [22
], and in Thessaloniki’s case they exceed the average fills of similar size and historical background cities reaching even 15 m within the limits of the ancient city walls in Zone II (Figure 6
). Makedon et al. [16
] identify this layer as the most prone to building damage from a 100-year earthquake event.
This geological heterogeneity produces divergent geographies susceptible to distinct types of geohazards. Thus, the Holocene deposits in Delta Municipality and Perea Region (Figure 5
c) lead to land subsidence whenever farming activities and urban development require high levels of groundwater abstraction [21
]. Furthermore, during the last decade, continuous pumping has led to the exposure of low-lying areas to surface flooding, due to the recovering of groundwater levels and constrained drainage of the water flow as a result of the deeper topography [23
From a tectonic perspective, Thessaloniki belongs to the Circum-Rhodope zone, which traverses the central part of Macedonia in northern Greece at a general NW–SE direction and lies between Axios zone and the Serbomacedonian massive (Figure 7
). Even though Thessaloniki is partly located in the least active seismic zone (Axios), the Serbomacedonian massive lying nearby is a geotectonic zone presenting very high levels of seismicity [16
]; as a consequence, the city has sustained numerous events of medium or higher repercussions. Apart from the 1978 earthquake, which was the highest of a series of approximately 100 earthquakes, seismic activity in Thessaloniki is still more than apparent, with several light, moderate and strong events occurring regularly.
The current seismic susceptibility of Thessaloniki’s built environment to damage has its roots in past construction practices. The city suffered wide structural damage during the 1978 earthquake from differential settlement on soft sediments, particularly within its historic district [24
]. In parallel, the absence of an organised and robust planning regulatory framework contributed in dense, unregulated and earthquake vulnerable urban planning. This absence has led to urban expansion in hazard-prone areas, particularly within the city centre (no detailed seismic micro-zonation), lack of open public spaces (potential assembly areas during evacuations) and high probability of cascading effect due to the proximity of buildings to one another.
The building stock of the city consists primarily of Reinforced Concrete (R/C) and Unreinforced Masonry (URM) buildings designed according to old codes, including pre-1959, or ‘pre-code’ buildings whose performance was found to be very similar to that of low-code ones, as reported in [25
]. Additionally, the city accommodates 340 impressive historic monuments from the Roman, Ottoman and more accurately Byzantine period. Most of them are religious settings (i.e., churches, mosques) and they are predominantly located within the Municipality of Thessaloniki.
The 1978 earthquake, which resulted in the death of 49 people, the collapse of a nine-storey R/C building and the damage beyond repair of 4.5% of the city’s building stock [25
], induced a massive reform of the building codes, especially within the historic centre of the city [24
]. Yet, despite this reform as mentioned, the majority of buildings in the historic centre are 6–9-storey residential R/C or URM constructions either designed or erected following the old code of 1959, presenting similar engineering performance [16
]. The historic centre has been predominantly built with the use of R/C as a result of the Great Fire of 1917 and has experienced very low reconstruction rates after the 1978 earthquake [26
.It is worth noting that rubble from the 1978 seismic damage has contributed, along with other geomorphological factors, to the formation of current urban fills. Based on an analysis of the composition of Thessaloniki’s urban fills, Makedon et al. [16
] projected that a seismic event could still inflict substantial damage in the historic centre.
The performance of Thessaloniki’s building stock in case of an earthquake has been widely analysed in the literature, via models, risk assessments and scenario-based methodologies [24
]. It is beyond the scope of this article to analyse in detail the assessment methods used in the cited studies; however, their common conclusion is that the city, and particularly its historic centre, would be severely impacted by an earthquake similar to the one that occurred in 1978. Around 10‒12% of the current buildings located within the Municipality of Thessaloniki are expected to suffer severe losses, while the estimated costs of repair for direct damage reach approximately €1,500,000,000 [26
]. Moreover, indirect economic and cultural blows are expected to be inflicted by damage to historic monuments, which serve as tourist sites and preserve Thessaloniki’s urban identity.
In addition to building codes, urban design following the typical approach favoured by other Greek cities plays a key role in Thessaloniki’s exposure to catastrophic damage in case of seismic events. The historic centre and inner zones are characterised by high densities of buildings, a lack of public open spaces and green spaces, and insufficient parking spaces [28
]. Narrow streets and unregulated urban development constrain the ability of local and national authorities to design appropriate evacuation routes in case of emergency, impeding the role of civil defence in planning disaster management. The lack of open spaces (potential assembly areas) and the ageing building stock, as highlighted in the PRA, represent major resilience concerns for the city; we explore such concerns below, unaddressed by the RS, through our case study of the Acheiropoietos electoral ward.
2.2.3. Surface/Groundwater Flooding
The review of Greek national catalogues of natural hazard events and scientific literature on nation-wide flood hazard and risk assessment (see Table A1
) confirms that the whole prefecture of Thessaloniki is highly susceptible to flash floods. From a historical perspective, Diakakis et al. [29
] analysed the spatial distribution of the flood events recorded in 1880–2010 and found that the Metropolitan Area of Thessaloniki is second in Greece after Athens, for either total number of events per prefecture or number normalized per 100 km2
. The evidence for an increased clustering of flood events is also confirmed by the study by Papagiannaki et al. [30
], which found around the Thessaloniki area one of the highest frequencies of high-impact weather events, with a total of 27 events in the decade 2001–2011, of which 71% were flash floods. Indeed, across the Metropolitan Area of Thessaloniki, severe floods occurred on 8 October 2006 and 21 September 2011 (both events were also studied by Nikolaidou et al. [31
] with the aid of Earth Observation data from satellites), 16 July 2014, 7 September 2016 and 11 May 2018.
Flash floods normally occur as a result of substantial precipitation in areas with limited ground absorption capacity; they may also cause cascading urban geohazards, such as landslides, sinkholes, and erosion [32
]. Moreover, they are extremely complex and unexpected hazardous phenomena, which take place in short temporal periods and thus are very challenging to predict [33
]. As global climate change is expected to increase the frequency of extreme weather incidences, there are more compelling reasons for local authorities to focus on addressing their impact.
In Thessaloniki, geology can also favour surface flooding. Quaternary lower terrace alluvial deposits and red clay, especially in the southern and eastern parts (Figure 5
a,c), which tend to be impermeable, are susceptible to runoff water and inevitably to the enhancement of the potential for serious flooding downstream, during heavy rainfall events. This runoff water afterwards concentrates in ravines and streams, forms larger volumes of water and frequently joins in a fast-flowing mass of water and debris.