**1. Introduction**

Climatic changes, including the increase in extreme temperatures and number of heavy rainfall events, have been recorded since around 1950 in different areas of the world [1]. Floods are already the most frequent in European coastal areas [2–4] and among the costliest and deadliest natural processes in the Mediterranean area [5].

Based on an international disaster database [6], 200 billion Euros in damages related to various calamities since 1900 have occurred in the countries surrounding the Mediterranean Sea out of which 85 billion are related to river flooding [7,8].

The observed variability of flood frequency and discharge magnitude is therefore the result of a complex interaction between rainfall history and the factors that control the response of the river basins, in particular, run-off modes. In the Mediterranean environment, the global warming process manifests itself with an increase in the average annual air temperature and with a variation in the rainfall regime [9,10]. Some regions such as Liguria, Tuscany and Sardinia in Italy; Provence-Alpes-Cote d'Azur and Corse in France; and Catalonia and the Valencia area in Spain are particularly exposed to flash floods [7,11] for which rainfall peaks and the flood peaks of the watercourses are very close in time. This particular pattern is the result of the interplay between the dominant atmospheric low level

**Citation:** Faccini, F.; Luino, F.; Paliaga, G.; Roccati, A.; Turconi, L. Flash Flood Events along the West Mediterranean Coasts: Inundations of Urbanized Areas Conditioned by Anthropic Impacts. *Land* **2021**, *10*, 620. https://doi.org/10.3390/land 10060620

Academic Editors: Andrea Petroselli, Raffaele Pelorosso and Matej Vojtek

Received: 9 May 2021 Accepted: 7 June 2021 Published: 9 June 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

flow circulation patterns and the relief and orientations of the northern Mediterranean coasts, which forces convergence and triggers convection.

The magnitude and impact of extreme floods vary significantly over the Mediterranean region with a significative difference between the West and East parts [12]. The western part of the region is much more prone to high impact and high magnitude events [13–16]. This is probably due to: (a) The proximity of the Atlantic Ocean and oceanic climatic influences at latitudes where eastward atmospheric flows dominate [7,17]; (b) the reliefs surrounding the West Mediterranean Sea forces the convergence of low-level atmospheric flows and the uplift of warm wet air masses that drift from the sea to the coasts, thereby triggering active convection and consequently short intense bursts of rainfall.

The coasts of the Mediterranean Sea are characterized by short and intense rainfalls [13,17], which in recent decades have shown an increasing frequency [18,19]. Furthermore, for Italy, the statistics seem to indicate an increase in geo-hydrological phenomena in small basins for the last 30–50 years [20]. These rainfalls, especially if concentrated on reliefs not far from the coasts, can generate the phenomenon of flash floods along the short streambeds that have significant slopes [21–23]. Such precipitations are characterized by convective events, typically with 100 mm or more cumulated rainfalls over a few hours. The affected areas are often limited to a few hundred square kilometers, with rapid hydrological responses, e.g., less than 6 h delay between the peak rainfall intensity and peak water discharge downstream [8].

The coastal urban flooding is a complex phenomenon which may occur in various forms such as: Urban flooding due to high intensity rainfall (pluvial flooding); urban flooding due to inadequate drainage; flooding caused by overtopping in the channels or streams/rivers. In coastal urban cities such as Genoa, Olbia and Livorno, severe flood scenarios mostly take place due to combination of surface flooding and stream overtopping. Urban flooding is one of the most severe problem in numerous parts of the world because they affect goods and can cause casualties. Urban flood, being a natural disaster, cannot be avoided; however, the losses incurred due to flooding can be reduced by proper flood mitigation planning. As such, it is necessary to have a proper estimation of flood extent and flood risk for the different flow conditions so that proper flood evacuation and disaster managemen<sup>t</sup> plan can be prepared in advance.

The flash flood consequences and the ground effects are amplified if the floodwaters spread to densely urbanized areas [24–27]. They are usually crossed by canalized streams that are often culverted for long stretches. These streambeds, which have been narrowed year after year to acquire new urban spaces [28,29], are often surmounted by bridges that are inadequate, with spans that are clearly insufficient for the discharge of flood waters.

Flash floods characterized by severe ground effects are generally triggered by:


during the events of 21–22 October 2019 and 23–24 November 2019 [35]. These events generally cover a larger area from hundreds to thousands of km2.

Along the Italian coasts, during the period from September to November, the socalled "Meteorological Fall" is the main season for flash floods that cause severe damage and often casualties due to their suddenness. This is particularly the case of mesoscale convective systems producing long lasting and stationary rainfall events that lead to strong responses by the watersheds concerned (i.e., high runoff rates due to soil saturation) and substantial coincidence between the peak rainfall and flood peak in small hydrographic basins (<250 km2).

The north-western coasts of Italy are historically subject to flash floods: Ligurian coasts, along with the Tuscan coasts and coasts in Sardinia. There are at least 46 damaging flood events that have been recorded in the last 30 years, practically one every 7.8 months (Table 1): 9 events occurred in Tuscany, while 19 cases occurred in Liguria and Sardinia.

**Table 1.** Severe meteorological events during the period 2000–2020 that occurred in Sardinia and along the coasts of Liguria and Tuscany with severe consequences (flooding/flash floods) for urban areas and inhabitants. The content of the brackets displays the victim numbers. The three events described in this article are bolded.


On the one hand we can affirm that flash floods are geo-hydrological processes linked to particular hydro-meteorological conditions and that their behaviors are significantly affected by climate change [36], but on the other hand we cannot omit the fundamental role played by wild urbanization [37], which has affected many towns on the western Mediterranean coast. This uncontrolled expansion occurred after the Second World War and appears to have been particularly significant in the already notoriously hazardous areas that have undergone and imparted important changes to the hydrographic network over time.

There are some fundamental reasons that constitute the basis of the decision to write this paper. First of all, the fact that no author has so far compared the events that took place in these regions, which are located adjacent to one another along the coast around the Upper Tyrrhenian Sea, in the national and international body of literature and research. The second reason is to compare the dynamics of three similar flood events, both in terms of meteorological and hydrological characteristics, and in terms of effects on the ground, with particular attention to the identification of any anthropogenic factors. In detail, three flash flood events were chronologically analyzed (Figure 1): (1) Genoa Sestri Ponente (Liguria), which occurred on 4 October 2010; (2) Olbia flood (Sardinia) on 18 November 2013 triggered by cyclone Cleopatra; (3) Livorno flood (Tuscany) on 9 September 2017 triggered by the cyclone Genoa Low.

**Figure 1.** Geographic location of the case-studies: Genoa Sestri Ponente (Liguria), Livorno (Tuscany) and Olbia (Sassari, Sardinia). Other places named in the text: (1) Cagliari; (2) Savona; (3) Versilia; (4) Garfagnana; (5) Imperia; (6) Massa and Carrara; (7) Villanova Strisaili; (8) Capoterra; (9) Bordighera, Vallecrosia, Ventimiglia; (10) Cinque Terre; (11) Elba Island; (12) Grosseto, Maremma; (13) Recco, Camogli, Chiavari; (14) Bitti; (15) Orbetello.

#### **2. Study Area**

#### *2.1. Genoa Sestri Ponente City*

Sestri Ponente is a district of the Genoa Metropolitan City, the regional chief town of Liguria (northern Italy) (Figures 1 and 2A); it extends over an area of about 8 km2, with a population of 45,000 inhabitants. Until 1926, Sestri Ponente was an autonomous municipality until it was incorporated, together with other municipalities of the Genoa neighborhood, called "genovesato", into the "Big Genova" City. The historic core of Sestri Ponente is represented by a narrow coastal strip, about 1 km long, running parallel to the shoreline and included in the floodplain between the Chiaravagna stream to the east and the Cantarena stream to the west. The industrial expansion in the 1920s and 1930s developed in the alluvial plain around the historic core, while the subsequent urban sprawl from the 1960s to date has continued upstream and occupies the hill slopes behind (Figure 2A). Now the city is developed for 2.6 km in length along the coastline.

**Figure 2.** Elevation maps and catchments of the case studies: Genoa Sestri Ponente (**A**), Olbia (**B**) and Livorno (**C**). Numbers refer to catchments in Table 2.

From the geomorphological point of view, Sestri Ponente stands on a narrow alluvial plain that is 2 km length and less than 500 m wide and is genetically linked to the action of several watercourses and significantly modified by anthropogenic landforms. Drainage networks are well developed with a torrential hydrological regime. Catchments are very small in size (generally <10 km2) and characterized by high energy relief (Table 2) due to elevation exceeding 700 m a.s.l. at a few kilometers from the coast and high slope gradient locally higher than 60% (Figure 3A): Rio Chiaravagna Stream (11 km2, with an estimated maximum full-flow rate of 213 m3/s for a 50-years return period), Rio Cantarena Stream (1.58 km2, 52 m3/s), Rio Molinassi Stream (2.00 km2, 66 m3/s) and Rio Marotto Creek (0.67 km2, 22 m3/s) [38]. The final stretches of these waterways are generally canalized and drained.

**Table 2.** Morphometric parameters of the basins analyzed (the numbers refer to Figure 2). H Max, maximum altitude; H Med, mean altitude; G med, mean gradient; Sup, surface; Ss, soil sealing; L rn, river network total length; Dd, Drainage density.


Land use in the studied catchments mainly consists of artificial surfaces (32.5%) in the lower part and forests and seminatural areas (45.0%) in the upper part of the basins; subordinately are agricultural areas (7.0%) (Supplementary Materials Table S1)

The climate of Sestri Ponente is typically Mediterranean, with hot summers and relatively mild winters: the average annual temperature is about 15 ◦C with a significant upward trend. The mean annual rainfall during the period from 1960 to 2019 is about 1100 mm and the annual rainy days are 71 with an average rainfall rate of 14 mm/d, which is also increasing [21]. The episodes of intense and short-lived rain are frequent, especially in the autumn months (October and November), when typically small-sized and quasistationary V-shaped convective systems are generated [31]. These systems result in extreme flood-causing rainfalls, for which ground effects are often disastrous. In the post-war period alone, flood events occurred 10 times in 69 years, which on average amounts to 1 event every 6.9 years (Supplementary Materials Table S2).

#### *2.2. Olbia City*

The Olbia city is one of the major cities of Sardinia and the most important link with the mainland and other Mediterranean countries due to its tourist and commercial port (Figures 1 and 2B). It extends for about 40 km<sup>2</sup> (including the more peripheral districts) with a population of about 60,000 inhabitants, while during the summer it can reach 100,000 inhabitants.

The whole coastal sector has undergone important changes since the early 1900s as a result of managemen<sup>t</sup> and sanitation. In the first decades of the 20th century, large swampy and brackish areas were reclaimed. Then, beginning from the 1960s Olbia experienced a demographic increase related to the tourist development in the neighboring Costa Smeralda, which is world-famous for its sea landscape and resorts. In 50 years, the town has tripled its population, transforming the socio-economic fabric and the population of inhabitants increased from 17,800 inhabitants (1961) to 60,000 today.

The plain of Olbia occupies a structural depression resulting from the kinematics of the Sardinian-Mediterranean block related to the formation of the Western Tyrrhenian Sea [39]. Due to the irregular and indented coastal morphology, with parallel hollow rias separated by prominent ridges featured by typical plateau and sierras, drainage patterns are mainly dendritic and slightly developed; the upper part of catchments locally exceeds 700 m a.s.l. and the watercourses that result are generally steep (Figure 3B). As a consequence of the progressive urbanization of the plain, streams crossing the urban area are strongly modified due to canalization and flow regulation. The main watercourses (Table 2) are the Rio Seligheddu Stream (38.4 km2, with a maximum capacity of 330 m3/s for a 200 year return period), the Rio San Nicola Stream (30 km2, 170 m3/s) and the Rio Gadduresu Stream (7 km2, 55 m3/s) as well as other minor ones (<5 km2), resulting in an overall flow rate exceeding 600 m3/s [40,41].

Land use in the studied catchments consists mainly in agricultural use (63.9%); artificial surfaces are present in suborder (12.9%) and forests and seminatural areas (22.7%) (Supplementary Materials Table S1).

The climate is typically Mediterranean, with mild and humid winters and hot and dry summers. The average annual temperature is about 16 ◦C. Sardinia is located almost at the center of a low-pressure area that determines the convergence of different air masses and the formation of self-regenerating convective systems, especially during the winter period, resulting in strong "V-shaped" marine thunderstorms. The mean annual precipitation ranges between 600 and 900 mm and the annual rainy days are 60 (rainfall rate 10–15 mm/d): rainfall is more abundant in the autumn and winter months (from October to December), while a minimum rainfall peak occurs in summer [41]. The urban development of Olbia is relatively recent. As a consequence, only documents of floods post Second World War have been discovered and considered: at least 16 flash floods in the last 69 years (on average 1 event every 4.3 years) (Supplementary Materials Table S2).

#### *2.3. Livorno City*

Livorno city extends for just over 100 km<sup>2</sup> of surface along the Tyrrhenian coast and represents one of the most important trade and industrial centers in Tuscany (central Italy) (Figures 1 and 2C): the urban area is subdivided into numerous districts and a total of approximately 158,000 inhabitants are counted. The city has already developed in historical times around the port area, with progressively more important enlargements since the beginning of 1900 linked to the development of communications and industrial activities. Urbanization has resulted in the growing occupation of natural drainage areas and floodplains [29,42]. Most expansions relate to the period after Second World War.

Livorno town stands on a flat sector (in the north-side and along the coast) which corresponds to a polycyclic marine terrace characterized by the presence of three orders of sea terraces that are, at least, aged between the middle Pleistocene and the upper Pleistocene. They are crossed by a hydrographic network that consists of several streams. The alluvial plain is, in fact, a low coastal terrace located north of the city and is shaped by the course of the River Arno in the mouth area.

Catchments are small (Table 2), with a size of generally <30 km2; they do not reach particularly high elevation (about 400 m a.s.l.) in their upper sectors; they possess mildly steep terrain and gentle hilly slopes (Figure 3C). Drainage networks are well developed; they possess watercourses of limited length; and a well-defined grid of small tributaries: the major ones are the Rio Ugione Stream (33.2 km<sup>2</sup> with a maximum evaluated discharge of 137 m3/s for a 200-years return period), the Rio Ardenza Stream (21.2 km2, 284 m3/s) and the Rio Maggiore Stream (8 km2, 100 m3/s) [43].

**Figure 3.** Slope maps and catchments of the case-studies: Genoa Sestri Ponente (**A**), Olbia (**B**) and Livorno (**C**).

Land use in the studied catchments mainly consists of forests and seminatural areas (50.0%); agricultural are present in suborder (28.5%) and artificial surfaces (20.4%) (Supplementary Materials Table S1).

During the period 1969–2018, the mean annual rainfall recorded in Livorno is about 800 mm that is mainly concentrated during the autumn, with an average of 74 rainy

days (rainfall rate 10.7 mm/d) [44]. The climate is Mediterranean, characterized by warm summers, mitigated by the presence of sea breezes and not particularly cold winters. The average annual temperature is about 15.8 ◦C.

Damaging effects on the ground in Livorno are associated with both flooding of water streams and pluvial floods due to intense rainfall events: before the last event, they occurred at least 16 times in 71 years. On average there is 1 event every 4.4 years (Supplementary Materials Table S2).

#### **3. Materials and Methods**

#### *3.1. Research Methodology*

The applied methodology has been developed in different phases. First of all, a close bibliographic and cartographic research has been carried out, with the aim of cataloguing recent and historical floods that occurred in the three study areas (Genoa, Olbia and Livorno) and correlate regions. Flood and damage information have been derived from different sources: (i) newspapers articles and chronicles notes from local media, (ii) inedited documents and technical and event reports collected in archives of local municipalities, (iii) books and scientific papers gathered in the libraries and (iv) interviews with local inhabitants.

We obtained rainfall and hydrological data about recent and past events from Hydrological Annals edited by SIMN ("National hydrographic and Tidal Service"), as well as technical and weather-hydrological reports compiled by territorial agencies and regional databases.

For Liguria, information from "Reports of the weather-hydrological events (2003– 2019)" by ARPAL and from "Pluviometrical regional database" by OMIRL was mainly used. For Sardinia, information from "Report of the weather-hydrological events (2013)" by ARPAS was mainly used. For Tuscany, information from "Report of the weatherhydrological events (2009–2019)" by CFR and LaMMA Association was mainly used.

Subsequently, a closer pluviometrical examination has been performed for the most recent and damaging flood events: (i) 4 October 2010 in Sestri Ponente [45], (ii) 18 November 2013 in Olbia [46] and (iii) 9 September 2017 in Livorno [47,48].

Flooded areas in the Sestri Ponente, Olbia and Livorno cities during the three considered events have been surveyed and provided by the regional cartographic online database of Liguria, Sardinia, Tuscany and also by the Livorno Municipality. Culverted streams and canals in the Olbia urban area have been surveyed from aerial photointerpretation, technical surveys and the analysis of technical reports for the construction of the drainage canals.

A multi-temporal comparison has been performed using historical and current topographical and cartographical maps in order to identify anthropogenic landforms and to reconstruct the urban evolution for each case study. In addition, field observations have been carried out to evaluate geomorphological and hydrological aspects of the urbanized areas involved in the flood events. All maps and cartographic data used in the research are listed in Table 3.

We integrated all georeferenced data in a Geographical Information System: using QGIS, we derived new original thematic maps which are useful to provide the immediate identification of both flooded areas in relation to urbanization and the main anthropogenic landforms within each study area.

**Table 3.** Vector and raster data used in the research. Name: DTM, Digital Terrain Model; CORINE, Coordination of information on the environment (European Environment Agency, 1995); TMI, Topographical Map of Italy. Source: GE, Google Earth; IGM, Italian Military Geographical Institute; ISPRA, Higher Institute for Environmental Research and Protection; LR, Liguria Region; PGRA, Flood risk managemen<sup>t</sup> plan, SR, Sardinia Region; TR, Tuscany Region. Type: R, raster; V, vector.


#### *3.2. Hydro-Meteorological Data of the Last Flood Events*

Liguria, Sardinia and Tuscany overlook the Ligurian-Tyrrhenian Sea and are three regions that are very prone to violent atmospheric phenomena (see Table 1). Genoa Sestri Ponente, Olbia and Livorno, which are coastal cities that arose at the mouth of rivers, naturally have a long and troubled history of floods that are usually flash floods: their degree of damage has increased year after year in proportion to the degree of urbanization reached by the cities. The areas of river pertinence, that is, those closest to the riverbeds which were once occupied by fields and pastures, have gradually been invaded by buildings; consequently, every time a watercourse overflows during present times, damages result.

This finding stems precisely from the review of the latest cases in the three cities examined. For Genova Sestri Ponente, the last flood of 4 October 2010 was undoubtedly the most serious in terms of damage. The storm cell formed on the first morning of 4 October 2010 stabilized in the neighborhoods of the western city between Pegli, Sestri Ponente and Val Polcevera where it unleashed all its power and caused a true flash flood. In about five hours, between 8 a.m. and 1 p.m., over 400 mm of water fell on the hills behind Sestri Ponente [45]. All the streams reached rapidly exceptional discharges: the waters violently flooded shops, garages, basements, squares, streets, washed parked cars away and also resulted in a victim.

Olbia has also suffered many flood events in the past: 14 events in the period 1946– 2010. However, the severest flood was the last flash flood that occurred on 18 November 2013. Six provinces out of the eight existing on the island were affected: the total damage

amounted to about 660 million EUR [46] and 18 were casualties; some were drowned at home and others were dragged by the fury of the streamflows while driving their cars. The Cyclone Cleopatra hit the interior of Sardinia with cumulative rainfall greater than 400 mm: the Olbia rain gauge station recorded a value of 117.6 mm, while the Putzolu rain gauge station, in a village close to Olbia, recorded 175.2 mm in 24 h [47]. The waters of the canals and streams crossing the town overflowed with heights greater than two meters on the countryside level. In the town if Olbia, 11 were the casualties and 40 people were hospitalized for symptoms of asphyxia and hypothermia after having been at the mercy of freezing water for hours. Over 2000 displaced people resulted and some hundreds of millions of EURO in damages were incurred.

Livorno also has a long list of flood events; there is at least 14 between 1946 and 2004, but the most recent event was certainly the most serious of its history. On the evening of 9 September 2017 a violent cloudburst hit the Livorno area: 242 mm of rainfall (74.8 mm in 30 min and 210.2 mm in 2 h) was recorded on the hilly areas just east of the city, on the upper basins of the Rio Maggiore and Rio Ardenza streams [48]. The Rio Maggiore stream, culverted in the 1980s, overflooded at the beginning of the culvert and invaded a large area of the city. In total, over 4.3 km<sup>2</sup> were flooded and a large part of this was an urbanized area. The damage to structures and infrastructures was very serious (6.6 million euros), with 8 casualties and a dozen injured in the city alone.
