Flood Risk Assessment in Urban Areas: The Historic City Centre of Aveiro as a Case Study

Abstract

Floods are among the most frequent and widespread natural hazards worldwide, with historic buildings proving to be particularly vulnerable. This work focuses on the large-scale flood risk assessment of the Historic City Center of Aveiro in Portugal. Due to the system of canals passing through the center and the extreme proximity to a lagoon, Aveiro is extremely prone to flooding. Furthermore, considering the great historical and artistic value the city center retains, its selection as a case study for flood risk assessment is fully supported. The work implements a recently developed methodology, combining hazard and physical vulnerability indicators to classify risk and define intervention priorities. Subsequent to an extensive survey and evaluation of almost 500 buildings, the raw data collected was classified using the Geographic Information System (GIS) tool. Through the macro-scale risk assessment, an essential insight is provided into the way each building is affected by flood events and, based on this knowledge, strategic rehabilitation interventions can be prioritized. To validate results, the work proposes a comparison with an analogous case study, and finally reflects upon the effective risk management and mitigation proposals as well as possible adaptations of the methodology for future applications.

1. Introduction

Floods are among the most widespread climate-related [1] and frequently occurring natural hazards. In the past decade alone, the annual reported economic losses from floods have reached tens of billions of US dollars, with their repercussions affecting thousands of people. More specifically, historic city centers and heritage buildings are particularly vulnerable to the destruction caused by flooding. This can be attributed to the fact that ancient settlements tend to be located within the vicinities of water sources, attesting to the important role water plays in the development of these communities. Unfortunately, this also means that the locations of present-day heritage structures and buildings are primarily within flood-prone areas, giving them a preexisting disadvantage of being exposed to hazardous conditions.

All these reasons demonstrate the necessity to develop further studies and methods to understand and manage the flood risk, especially for historic sites and structures. They have the propensity to present certain singularities, such as construction typologies and traditional building materials, that can lead to an increase in their vulnerability to flooding hazards. The potential loss of buildings with high and irreplaceable cultural value, as well as the consequent economic impacts from which a city could suffer, stresses the urgency to develop systems of indicators for managing the risk to heritage assets. These systems aim to provide valuable information for the assessment of the impact of natural hazards on cultural, social, economic, and environmental conditions. As such, historic centers are critical areas of study and deserve special attention.

To this end, the present study discusses the application of a recently developed flood risk-assessment approach that combines flood hazard and building vulnerability indicators to identify and classify risk while also defining intervention priorities. This methodology [1] is implemented in the Historic City Center of Aveiro, Portugal, particularly in the Beira Mar neighborhood, one of the most picturesque of the city. Due to its proximity to the Ria de Aveiro and the system of canals throughout the historic center, the city is highly prone to flooding. The selection of Aveiro’s historic center as the subject of this large-scale risk assessment is further supported by the city’s unique history and architecture, widely recognized to be of high heritage value and significance.

This study involves the survey and evaluation of 495 buildings, completed on-site as well as through remote survey methods, which are usually the go-to methods for the vulnerability assessment of post-earthquake events [2, 3, 4, 5]. The results were integrated into a Geographic Information System (GIS) tool, alongside prepared flood hazard data, for subsequent analysis of the respective flood vulnerability and risk. Through this macro-scale risk assessment, it is hopeful that valuable insight is generated into the manner in which each building is affected by the flood events. Discussions also include proposals of effective risk management and mitigation strategies, comparison of results with an existing case study, as well as adaptions of the methodology for utilization in different contexts.

2. Methodology

The analysis presented herein has been carried out using the methodology proposed by Miranda and Ferreira [1]. This particular methodology is adjusted so as to be implemented as a micro-scale assessment. In this case, the elements of interest are the historical buildings and the losses, which are mainly defined in terms of physical damage caused to the building fabric, namely, the direct tangible damage. For historic buildings, this damage is deemed the most severe. Moreover, to deal with historic sites, the method addresses direct intangible damage, in terms of losses to the cultural value of heritage buildings.

The vulnerability in this methodology refers to the intrinsic characteristics of the building. Other factors define the social, historical, and economic value of the asset and its interaction with the water. The results can be seen as a synthetic vulnerability function as they are not derived from damage data after flood events. The vulnerability index is a relative measure of the expected severity of the physical impact of the flood on the building in comparison to other assets ranked. The value of the vulnerability index does not account for the hazard and only partially accounts for the exposure, following an approach that is becoming common in flood risk assessment [6, 7].

As shown in Figure 1, the methodology defines flood vulnerability (FV) as the product of the exposure component (EC) and the sensitivity component (SC). Five parameters are considered to define the sensitivity component, each one of them containing four attributes. The exposure parameter refers to the exposure of the building to the flood, which encompasses aspects like the orientation of the most exposed façade to the water flow or the location and dimension of the openings—doors and windows [1].

Figure 1.

Diagram of methodology [1].

2.1 Flood vulnerability

The goal of vulnerability assessment is to understand how a system will be affected by floods. Even though there are different classes of vulnerability, physical vulnerability is the most commonly assessed because it is the easiest way to characterize it. Furthermore, the nonphysical vulnerability is usually evaluated in terms of economic loss, which is related to the interruption of various activities and is often much more challenging to estimate.

According to Balica [8], flood vulnerability can be determined by a physical approach, where hydrological models are used to estimate flood hazard and economic consequences. Moreover, Balica also mentions an empirical approach related to quantitative or qualitative indicators to rate the vulnerability without inputting the hazard intensity. Regarding the methodology adopted for the present study, no attribute has been assigned the value 0, since it is considered that every historic-building feature measured indicates some degree of vulnerability to the hazard. However, for all the descriptors to contribute equally, a lower boundary of 10 is applied to the rating scale for them, except for the land slope [9].

Based on the estimated individual parameters, the flood vulnerability is computed according to Eq. (1).

$$\text{Flood Vulnerability}\left(\mathrm{FV}\right)=\text{Exposure Component}\left(\mathrm{EC}\right)\times \text{Sensitivity Component}\left(\mathrm{SC}\right)$$E1

2.2 Exposure component

Exposure analysis aims to examine the economic assets and activities influenced by the flood. Exposure can be determined by geospatial mapping to identify the location of the assets of interest related to the flood hazard. In addition to this, the exposure contemplates some intrinsic characteristics of the elements analyzed.

2.2.1 Wall orientation

Wall orientation assesses the position of the asset with respect to the expected water flow. This parameter considers that the wall in a plane perpendicular to the water stream direction (fully exposed) presents a bigger pressure load than the ones that are simply immersed. It also considers the presence of openings reachable by the water. The characterization of the buildings as fully or partially exposed without openings is attributed to the ones where the windows and doors are not facing the flow of the water [10]. The assessment is based on the orientation and the characteristics of the main façade. In the case of buildings located in low-lying areas, the class is increased to C or to D for partially and fully exposed buildings.

2.3 Sensitivity component

2.3.1 Material

This parameter considers the lateral capacity of the structure, the resilience to ground movement, and the absorption capacity, factors that contribute to the risk of collapse in the short term and degradation in the longer term. In the case of degradation, it depends on two factors: the consequences of improper drying and on the contaminants and salts present in the flow [11].

Among materials, earth is considered the most vulnerable. The materials’ porosity determines their absorption capacity, but it can also promote their resilience, as it allows them to tolerate heavy wetting. It should be mentioned that the unbaked earthen structures are the most likely to present disintegration caused by the wetting and water pressure [12].

Masonry buildings are less prone to suffer collapse. However, they can present moisture in the elements and different types of decay such as spalling or exfoliation induced by the presence of salts and contaminants. Previous experiences demonstrated reduction up to 50% of the capacity in dry condition. Therefore, masonry structures should be properly inspected post-event [10].

Modern materials such as steel and concrete are less vulnerable. Moreover, the reparation and replacement of traditional materials may be more expensive. Timber elements are prone to biological attacks, such as fungi and insects. After the flood event, if the element is properly dried, serious damage is not expected. Moreover, saturated elements undergo significant deformation.

2.3.2 Number of stories

It is argued that single-story buildings are the most vulnerable, since they do not possess a flood-free story to facilitate the evacuation [13]. Nonetheless, in terms of stability, higher buildings with the same footprint and superficial foundations are more susceptible to the effects of the flood on the subsoil, such as differential settlements [5]. Therefore, for the present study, the vulnerability is considered as increasing with increasing number of stories.

2.3.3 Damage and cracking

Damage can lead to higher water absorption and lower strength in the materials, such as in the case of wood. Indeed, water ingress can easily occur through cracks, and once inside the building, water can damage movable assets and valuable nonstructural elements, contributing significantly to the economic losses. Therefore, restoration and maintenance measures are essential to prevent water penetration. This indicator includes an assessment of the state and quality of the interventions that can be carried out [6].

2.3.4 Age

This parameter may represent more valuable assets. This is valid for countries where historic structures have a higher real-estate value, being thus inhabited by higher-income residents [14]. Furthermore, older buildings are likely more prone to suffer flood damage. It is worth noting that the four levels of the age indicator may require an adaptation to the local evolution over time of the investigated building stock.

2.3.5 Heritage status

The heritage status is directly related to the socioeconomic impact that the preservation of these assets could have. For this reason, a higher value implies higher exposure of the asset. Moreover, the buildings that are listed are often subjected to more requirements, which results in higher intervention costs. In these cases, it is also important to contemplate the nonstructural value of artworks and other movable assets that are hosted in the building, a fact that will increase their vulnerability [15].

3. Case study: the Historic City Centre of Aveiro

The city of Aveiro is located on the shore of the Atlantic Ocean, in the Central region of Portugal. The territory is administratively designated as Beira Litoral, with Aveiro being the capital of the district and the name of the municipality (Figure 2a). The city has a population of 80,880 and a total area of 197.58 km².

Figure 2.

Aveiro’s city centre (a) and entrance of the Barra da Ria de Aveiro (b). Sources: Getty images/Google Earth.

The city’s location (Figure 2b) has unique characteristics: while to the east it is confined by mountains, to the north, south, and west spreads the immense lagoon to which the city gives its name—Ria de Aveiro—and that distinguishes the surrounding scenery stretching out to the sea. In an area geographically and historically isolated from the mainland communication routes, the estuary of the numerous waterways that flow into it has, over the centuries, been the determining factor in the movement of people and goods. In particular, the river Vouga, the largest of the watercourses, presents an important communication route. The existence of the Ria de Aveiro is essentially linked to the one of the city. Today, it is composed of countless islands, lagoons, and channels; however, throughout the centuries, the natural substrate has undergone profound changes, which has eventually led to the formation of the estuary.

3.1 History and development of Aveiro

Nowadays, Aveiro is one of the most populated cities in Central Portugal, grown around its thriving port. Nevertheless, over the centuries, the process of consolidation and growth of Aveiro has gone through different phases, each reflecting the living conditions in the region and its relationship with the lagoon and the sea.

Initially known as Avarium, documentation seems to place the origin of the city at least back to the 10th century. The first centuries of Aveiro’s existence are essentially marked by a period of consolidation of the primitive village, developed around the main church of St. Miguel, which assumed the status of a city only in the 13th century. Always linked to economic activities, Aveiro invested in the salt production and in the naval commerce as its most valuable assets, which constituted the main economic activities that accompanied its growth until the end of the 14th century [16].

Following the centuries, the building of the walls around the urban center and the development of the salt industry, agriculture, and fishing established for Aveiro a period of prosperity. However, the sedimentation in the “Barra” of the Ria de Aveiro (the link between the lagoon and the ocean) progressively led to a period of crisis that struck Aveiro until the 18th century, when the closure of the Barra became permanent. The construction of Barra Nova in 1808, along with the development of transport, especially the passage of the railway line Lisbon-Oporto, marked the beginning of a new era for the city [16].

This revival also coincided with the beginning of the industrialization process. During this phase, the city’s expansion took place mainly in the northern part of the canal, with the formation of the Beira-mar district. From the last half of the 20th century onward, the population of Aveiro continued to grow steadily, with the creation of the university being a contributing factor.

3.2 Building typologies and materials

The predominant building typology in Aveiro is the so-called gothic-mercantile that, commonly, features living and dining rooms facing the street and kitchens placed at the rear of the house. All the rooms are connected by a long corridor perpendicular to the street. The buildings tend to have a single structural span, and they are inserted in narrow and deep lots attached to one another [17]. The height can vary from one story to a maximum of four, but the majority possesses two or three stories.

The influence of Art Nouveau is evident in the decoration of a significant number of constructions since the beginning of the 20th century. In the city of Aveiro, Art Nouveau was partially adapted to local traditions, creating a peculiar combination characterized by outstanding decorated façades and conservative interiors often built with poor local materials.

Indeed, the dominant construction technique relies on natural materials. Masonry with lime mortar was used for various buildings. Until the early 20th century, the Eirol stone was largely used for towers and aqueducts, whereas the Ança stone was commonly used for foundations, basements, and decorative elements [18].

Nevertheless, most of the buildings observed are characterized by the use of adobe blocks in the construction of the external walls. It has been estimated that around 25% of the existing buildings of Aveiro are made of earthen blocks, of which 40% are located in the city center, including most of the Art-Nouveau-style buildings. This is an extraordinary and distinctive feature of the city but also one of its main problems when it comes to conservation, with adobe being very vulnerable to water and considerably less resistant than stone. Only around the 1950s was adobe progressively replaced by reinforced concrete [19].

Along with adobe, the presence of pinewood can be observed in abundance, mainly for roofs, floors, and window frames of the buildings. It was implemented mainly with the tabique technique, one of the most traditional building techniques in Portugal. Less frequent but still present is the usage of brick, especially for public buildings [19].

3.3 Flooding history

The estuary area of the Ria de Aveiro is spread over about 11,000 hectares, more than 6000 of which is permanently covered with water. The surrounding area has low altitude and orographic relief, lacking natural protections against flooding. Figure 3 shows the chronological distribution of the main urban-flood events occurred in the city of Aveiro.

Figure 3.

Timeline of the major floods in Aveiro during the 20th century [20].

Concerning the events before the 20th century, the documentation is scarce; therefore, fewer urban floods are recorded. However, it is known that a particularly great event occurred in 1739. Another one, in 1774, had terrible consequences as the Barra gave no outlet to the dammed waters and caused an epidemic that decimated the population.

In the 20th century, several flood episodes occurred. The most notable in terms of losses affected mainly the lower part of the city, with the historical city center being completely covered by water in 1937. Finally, severe floods struck Aveiro in 1955, 1957, 1964, and 1966, following strong storms and rise of water levels, and, more recently, in 1993, when the entire lower part of the city was inundated [20].

4. Site inspection and fieldwork

The present study focuses on the Bairro da Beira-mar, one of the most central and picturesque neighborhoods in the Historic City Center of Aveiro, characterized by the presence of its distinctive adobe Art Nouveau buildings.

4.1 Survey area

The investigated area is approximately 118.000 m² and consists of 495 buildings, mostly low-rise, which are divided into regular and some more irregular blocks forming a grid system. The buildings are commonly characterized by attached, deep lots with a very narrow front (mostly less than 7.5 meters) and mainly two or three stories. Various significant buildings, including the listed heritage, are located in the area.

4.2 Site inspection and fieldwork

The study area was divided, for ease of survey, into five specific zones (Figure 4) according to the main streets. The survey of the zones was completed in two phases. Phase 1 comprises the on-site surveys of Zone A, B, and C (242 buildings). Phase 2 consists of remote surveys for Zone D and E (253 buildings), complemented with on-site validation of the buildings not accessible remotely via Google Earth and Street View, as provided by Google and Maxar Technologies.

Figure 4.

Site survey zones.

The building survey was conducted using a checklist hosted on Google Forms. The building checklist includes the inputs for all the building parameters required (e.g., number of stories, construction material, heritage status, etc.) in order to fulfill the methodology, as well as additional information (such as number of occupants, function of building, etc.) that could be helpful for further analyses.

The first on-site inspection was carried out on 14 and 15 November 2021 by a team of four persons, working in pairs. The team completed the survey of Zone A, B, and C during the first on-site visit, covering 242 buildings.

The remaining Zone D and E were surveyed remotely using Google Earth and Street View. Having visited the site in Phase 1 prior to conducting the remote survey, the team was well-acquainted and familiar with the urban morphology and building features to be able to replicate an equally objective judgment when conducting the survey remotely.

Due to the limitations of Google Street View, several streets were visually inaccessible. Thus, no information was available for the inspection of a handful of buildings, amounting to approximately less than 10% of Zone D and Zone E combined. These buildings were surveyed in a separate on-site visit on 15 March 2022.

During this on-site survey, thermal imaging using an FLIR T540 thermal camera was conducted to ascertain the construction materials of the selected buildings. In some cases, the thermography provided inconclusive results, depending on the façade coverings. Indeed, thick plastering and finishing with different materials and colors hindered the interpretation of the structural elements beneath. Nonetheless, in several instances, reinforced concrete structural members (Figure 5a) as well as masonry units (Figure 5b), which would not have been visible to the naked eye otherwise, were clearly identified.

Figure 5.

Thermographic survey showing (a) reinforced concrete structure and (b) masonry units with 16 cm ruler as indicated by an asterisk.

5. Flood risk assessment

5.1 Vulnerability module

The on-site and remote surveys allowed for the classification of the parameters affecting flood vulnerability, according to the methodology described in Section 2. Figures 6a, b and 7a, b show the frequency distribution and the spatial distribution concerning, respectively, two of the most relevant parameters that compose the flood vulnerability index: the wall orientation, which determines the exposure component of the vulnerability, and the condition of the building, which falls under the sensitivity component. Regarding the former, all the investigated buildings fall into two categories, namely, “partial exposure with openings” or “full exposure with openings”, with most of the buildings that present high level of exposure (class D) being located closer to the canal.

Figure 6.

Frequency distribution of (a) wall orientation and (b) building condition parameters.

Figure 7.

Spatial distribution of (a) wall orientation and (b) building condition parameters.

For what concerns the buildings’ condition, their conservation state can be deemed as satisfactory. Almost 86% of the buildings assessed either are in a good condition (about 45%) or present minor conservation issues, mainly related with small cracks and moisture issues. The remaining 13% of structures face more serious degradation such as significant cracking, material loss, or settlement.

Regarding the heritage status, even though a great amount of the present building complexes are of architectural interest, only 3% of them are listed heritage with local/municipal interest. To consider the intangible value of such assets, the methodology was modified, without altering its underpinning rationale, to include within category B the status of significant local heritage that was not included in the original formulation.

However, uncertainties arose for several buildings in the assessment of the remaining parameters, namely, construction material and age. This is due to the recent finishing that does not expose the original construction materials beneath and a lack of documentary evidence on the evolution of the building stock in the neighborhood. To address these uncertainties, several assumptions were made. Most of the buildings with uncertain age were assumed to be built between 15th and 19th centuries, that is, category C. This option not only corresponds to the largest time range and, consequently, the highest probability for a building to be built during it but also is in accordance with the period of evolution of the Beira Mar neighborhood, completed during the 20th century (category B), which would be a less conservative assumption. On the other hand, buildings dating back to before the 15th century (category D) as well as to the 21st century (category A) were easily identifiable, due to their morphological and structural characteristics. Based on the aforementioned assumption, 81% of the buildings were classified as being built between the fifteenth and twentieth centuries.

Regarding the material, masonry and earthen constructions were often hardly distinguishable. Therefore, two cases were conceived. Case 1 has all uncertain construction materials classified as B (Masonry), while Case 2 has all uncertain construction materials graded as D (Earth). For the flood vulnerability assessment, Case 1 is selected, as it is more realistic based on the results obtained from the on-site survey.

Upon the classification, a clear predominance of masonry buildings emerged, accounting for 67% of the assets, assuming them as made of stone or bricks in the first scenario and of adobe in the second one. Nineteen percent of the buildings made of reinforced concrete are newer constructions.

Following this, the geographical distribution of the index is presented in Figure 8. The highest FVI value obtained for both the scenarios is 64.6. While this value is not negligible, in both cases, a very small percentage of buildings present a medium or high vulnerability.

Figure 8.

Flood vulnerability index results: Distribution over the study area Case 1 (a) and Case 2 (b).

Comparing the assessment of two different scenarios, it can be observed that Case 2, as expected, results in a higher vulnerability. Nonetheless, the overall vulnerability is slightly sensitive to the uncertainty in the classification of the material, as demonstrated by the very similar results both in terms of FVI values and distribution. Therefore, only Case 1 is considered for the risk assessment hereafter. The vulnerability is rather affected by the exposure component, with the most concerning buildings located along the canals.

Finally, the buildings are divided into three categories, namely, low, moderate, and high vulnerability (Figure 9). Index values of 20 and 40 are set as thresholds [14].

Figure 9.

Distribution of flood vulnerability levels over the study area (a) and histogram with respect to the number of buildings (b).

5.2 Hazard levels

The hazard component of the flood risk assessment was analyzed in collaboration with the University of the West of England, which provided flood extents, velocities, and water depth maps associated with return periods of 20 and 100 years—that is, frequent and rare flooding events, as per the Portuguese Decree-law no. 115/2010 of 22 October. From these maps, the flood depth and velocity affecting each building were estimated, transforming the overlaid data (Figure 10a) into building-specific data (Figure 10b and c).

Figure 10.

Hazard levels: (a) flood depth hazard map for 100 years return period; (b) flood depth per building for 20 years return period; (c) flood depth per building for 100 years return period.

Within the investigated area, the estimated flood extent for 20- and 100-year return period is the same. Similarly, the flood velocity is equal in both cases, corresponding to 0.05 m/s. The flood depth, instead, significantly changes in the two scenarios. Indeed, for the 20-year return period, the buildings will be exposed to an average depth of 0.93 m and for the 100-year scenario, to an average depth of 0.98 m. Out of the 228 buildings affected by the flood in the two scenarios, a water height between 1.5 and 2.5 m is expected for 26 building under the 20-year return period and 28 under the 100-year return period. This is a significant value, but it represents a low percentage of buildings (5%) in comparison to the total amount.

Water height (y) and velocity (v) are used to calculate a single hazard indicator as follows:

$$\mathrm{H}=\mathrm{y}\left(\mathrm{v}+0.5\right)$$E2

Based on the classes of hazard, defined in the Portuguese Floods Directive, five hazard levels are considered, namely, negligible, low, moderate, high, and extreme as shown in Table 1.

Hazard level	Hazard indicator range (m/s)	20 years (%)	100 years (%)
Negligible	0.0–0.5	81	80
Low	0.5–1.0	14	15
Moderate	1.0–1.5	5	6
High	1.5–2.0	0	0
Extreme	2.0–2.5	0	0

Table 1.

Distribution of hazard levels for Aveiro.

According to the criterion adopted in this matrix-based analysis, the level of flood hazard throughout the study area does not alter much passing from the 20-year to the 100-year peak flow scenario. By observing the distribution of percentages, the results can be deemed as satisfactory, portraying a low level of danger. In terms of spatial distribution (Figure 11a and b), it is possible to identify two blocks that can be particularly affected: one in the northern part of the city center and one in the southern, always in the flood-prone areas near the canal.

Figure 11.

Spatial distribution of flood hazard levels (a) and values for the adopted 20-year peak flow scenario (b).

5.3 Risk analysis

By deploying the aforementioned levels of vulnerability and hazard, the flood risk matrix is obtained, as reported in Table 2, where the numbers represent the level of risk on a four-point scale.

Flood risk matrix
Flood risk		Hazard
		Negligible	Low	Moderate	High	Extreme
Vulnerability	High	2	3	3	4	4
	Moderate	1	2	3	3	4
	Low	1	1	2	3	3

Table 2.

Flood risk matrix.

Figure 12 shows the risk maps for the two return periods. The flood risks for the 20-year return period (Figure 12a) and 100-year return period (Figure 12b) are remarkably similar. In a total of 495 surveyed buildings, 77.37% of them present low flood risk; 12.53 and 12.73% for 100 and 20 years, respectively, moderate; and 10.10 and 9.90% for 100 and 20 years, respectively, high. It is noted that for the 20-year return period, the moderate risk is slightly higher, raising more alarm for a dangerous situation in the near future.

Figure 12.

Spatial distribution of the flood risk results for 20 years (a) and 100 years (b).

It is observed that the buildings with a significant risk are located closer to the canal; they can be again distinguished into two blocks, and also, a significant number of them are located along one of the main streets, R. João Mendonça. Indeed, the buildings likely affected by the flood hazard, in the two return-period scenarios, are also the most vulnerable within the investigated area (Figure 13).

Figure 13.

Flood risk per building for a 20-year return period.

Overall, even though a percentage of about 10% of the buildings can be identified as being at high risk in the next decades, it is concluded that the flood risk for the Beira Mar neighborhood is relatively low, and in view of future mitigation strategies, the effort can be concentrated on the buildings located within the identified flood-prone area.

6. Comparison with the Historic City Center of Guimarães

For the purpose of providing a deeper understanding of the integrated approach presented for assessing flood risk in historic city centers, the authors deemed appropriate the development of a comparison between similar case studies. The example selected is the Historic City Center of Guimarães, a UNESCO Heritage Site located in the northern region of Portugal (Figure 14a). The case study presented is of particular relevance as it was chosen by Ferreira and Santos in 2020 as a pilot case [21], encompassing 9 blocks with 116 buildings, to implement the same methodology adopted in the present work. The area taken into consideration for the flood risk assessment is located in the “buffer zone” of the declared World Heritage Site area, as shown in Figure 14b.

Figure 14.

The Historic City Centre of Guimarães (a) and identification of the study area (b) [7].

According to Miranda and Ferreira [1], the city of Guimarães has been subjected to strong anthropogenic pressure due to increasing urban and industrial occupation, which originated the current environmental degradation of the Couros river basin as well as the substantial rise in severity of its flooding events. Hereafter, the Historic City Center of Aveiro and Historic City Center of Guimarães case studies will be addressed respectively as “Case A” and “Case B”.

For what concerns the exposure and sensitivity components, the comparison between the two case studies is shown in Figures 15 and 16. The main difference lies in the exposure component. Case A shows a sharp distinction in the distribution of the classes in comparison to Case B, where the values are more homogeneously arranged. This depends on the wall orientation indicator and reflects the canals’ location within the area. The sensitivity, on the other hand, presents a relatively uniform distribution for both cases, never exceeding the range of 40–70, approximately.

Figure 15.

Comparison between exposure components of Case A (a) and Case B (b) [21].

Figure 16.

Comparison between sensitivity components of Case A (a) and Case B (b) [21].

Furthermore, the overall flood vulnerability maps are shown for both case studies in Figure 17. The range of values for the vulnerability index in Case A never exceeds the 60–70 band, slightly higher than that in Case B, where the maximum registered values range from 50 to 60. However, the main difference lies in the percentage of buildings with recorded vulnerability higher than 30. The quantity is considerably greater in Case B, reaching 40% of the 116 evaluated buildings, compared to Case A where the most vulnerable ones only represent 22% of the study area.

Figure 17.

Comparison between flood vulnerability of Case A (a) and Case B (b) [21].

As seen, this discrepancy depends on the exposure component: in both cases, the most exposed buildings are also the most vulnerable ones. Even though in Aveiro there appears to be a great number of buildings presenting a higher exposure to the water flow, also considering that the canals almost completely surround the analyzed building stock, in Guimarães the Couros river basin passes through the investigated area. As a consequence, because several of the most sensitive buildings are also located in the central part of the study area, which has a great proximity to the water flow, the final values of vulnerability greatly increase. On the contrary, in Aveiro, the most sensitive buildings are not directly facing the canals.

Hence, it is possible to conclude that when applying the flood vulnerability index to a desired case study, the exposure is indeed a fundamental factor in determining the final flood vulnerability results, but so is the matching cross analysis with the sensitivity component.

7. Conclusions

The present report introduces a thorough study on the flood risk assessment performed in the Historic City Center of Aveiro (Portugal). The study was carried out applying a methodology elaborated by Miranda and Ferreira in 2019 [1], which has been proved to be a useful tool to evaluate the flood vulnerability of built aggregates, with a specific focus on areas distinguished by their heritage value.

A total of about 500 buildings were assessed, including 8 listed heritage assets of local interest. The data collection consisted of an extensive field survey and photographic documentation and was complemented by a remote survey. The spreadsheets of the raw data were post-processed to obtain the vulnerability results, which were defined by combining the exposure component (based on wall orientation) and the sensitivity component (based on heritage status, age, number of stories, condition, and material of buildings). During this process, it was observed that the most sensitive buildings were the ones located along the length of the canals. It is interesting to note that the most sensitive buildings were also revealed to be the most exposed ones.

Following this, the hazard scenario defined for the 20- and 100-year flood was deployed in order to obtain the outputs for flood extent, depth, and velocity. The results that were obtained from the hazard-level classification were deemed as satisfactory, portraying a low level of danger. In the end, the above outputs allowed to perform the final flood risk analysis by merging the hazard scenario with the previously assessed flood vulnerability and thus to acquire an insight on the impact of a future flooding event on the city center. It was concluded that the flood risk for the inspected area remains relatively low, with only 10% of the buildings presenting a high risk in the next decades. The high-risk buildings are located closer to the canal, thus within the identified flood-prone area, and also along one of the main streets of the city center.

Moreover, a comparison was conducted with a portion of the Historic City Center of Guimarães, where a similar approach for the assessment of flood risk was performed. Irrespective of the differences between the two case studies, the influence of the exposure indicator on the vulnerability index of the assets was confirmed.

Finally, it is stressed that the methodology allowed a prompt assessment of the flood risk. Nonetheless, uncertainties arose in the estimation of some parameters, calling for further calibration of the levels and investigations in the study area. Upon this more detailed analysis, the results of the flood risk assessment will be integrated into a decision-making framework for risk mitigation and improvement of the city’s resilience.

Acknowledgments

This work was partly financed by FCT/MCTES through national funds (PIDDAC) under the R&D Unit Institute for Sustainability and Innovation in Structural Engineering (ISISE), under reference UIDB/04029/2020. Authors acknowledge all the technical and financial support provided within the framework of the International SAHC Masters Course (www.msc-sahc.org).