**4. Results**

#### *4.1. Rainfall Events*

With respect to the event of 4 October 2010 on the Ligurian coast, from the analysis of rainfall data recorded in the basins considered (Table 1) it can be highlighted that for the Genoa Sestri Ponente area precipitation resulted from the formation of intense self-regenerating systems (MCS) due to a configuration favorable to a strong convergence between South and South-East, which insisted on the center of the region and in particular on the border between the provinces of Genoa and Savona. Around midnight on 4 October 2010, a stormy event of strong intensity occurred in the area of the Ligurian coast, enhanced by the orographic barrier of the Alpine–Apennine chain and favored by the high sea temperature due to the concomitant presence of an anticyclonic front in the Mediterranean Sea.

After about six hours, a violent weather-pluviometric system reached the town of Varazze with rainfall of about 100 mm/1 h and 220 mm in 3 h. Between 9 p.m. and 12 p.m., the storm cells that had hit the Riviera di Ponente moved towards Genova Sestri Ponente, which was about 20 km to the east. Here, the rainfall recorded at the Mt. Gazzo station (OMIRL-ARPAL hydrological Service) reached 124 mm/1 h, 243 mm/3 h, 360 mm/6 h and 411 mm/12 h (Figures 4–6), compared to annual averages of about 1100 mm. In the areas surrounding Sestri Ponente, high intensities of rain were recorded: 98 mm/1 h peak and a cumulative rainfall of 377 mm/12 h in Pegli (west), while at the Bolzaneto station (north) cumulative rainfall was recorded at 73 mm/1 h and 295 mm/12 h. In Genoa and east of Sestri Ponente, the peak intensity was 40 mm/1 h and the cumulative was 100 mm/12 h (Figure 7).

From 1945 to present day, in which at least ten events with damaging effects on the ground occurred, the event of 4 October 2010 in Liguria the sixth highest for rainfall intensity within a time period of 12 h.

**Figure 4.** Cumulative rainfall map for a time period of 3 h: (**A**) Genova Sestri Ponente, 4 October 2010; (**B**) Olbia, 18 November 2013; (**C**) Livorno, 9 September 2017. Rain gauge stations: (GP) Genova Pegli; (MG) Monte Gazzo; (PO) Pontedecimo; (BO) Bolzaneto; (LM) Livorno Mareografo; (ST) Stagno; (QU) Quercianella; (VB) Valle Benedetta; (CO) Collesalvetti; (SI) Siberia; (SA) Santermo; (SL) Santa Luce; (SO) Solvay; (PU) Putzolu; (MO) Monti; (MP) Monte Petrosu; (SP) Sa Pianedda.

**Figure 5.** Cumulative rainfall map for a time period of 6 h: (**A**) Genova Sestri Ponente, 4 October 2010; (**B**) Olbia, 18 November 2013; (**C**) Livorno, 9 September 2017. Rain gauge stations: (GP) Genova Pegli; (MG) Monte Gazzo; (PO) Pontedecimo; (BO) Bolzaneto; (LM) Livorno Mareografo; (ST) Stagno; (QU) Quercianella; (VB) Valle Benedetta; (CO) Collesalvetti; (SI) Siberia; (SA) Santermo; (SL) Santa Luce; (SO) Solvay; (PU) Putzolu; (MO) Monti; (MP) Monte Petrosu; (SP) Sa Pianedda.

**Figure 6.** Cumulative rainfall map for a time period of 12 h: (**A**) Genova Sestri Ponente, 4 October 2010; (**B**) Olbia, 18 November 2013; (**C**) Livorno, 9 September 2017. Rain gauge stations: (GP) Genova Pegli; (MG) Monte Gazzo; (PO) Pontedecimo; (BO) Bolzaneto; (LM) Livorno Mareografo; (ST) Stagno; (QU) Quercianella; (VB) Valle Benedetta; (CO) Collesalvetti; (SI) Siberia; (SA) Santermo; (SL) Santa Luce; (SO) Solvay; (PU) Putzolu; (MO) Monti; (MP) Monte Petrosu; (SP) Sa Pianedda.

> The meteorological event that affected the Olbia area on 18 November 2013 is among the most serious in recent years that have extensively affected the entire region of Sardinia

in a paroxysmal manner. The set of meteoclimatic conditions resulted in the formation of strong "V-shaped" self-healing marine storms on the eastern sector [28] and is linked to the Cleopatra perturbation with consequent effects on the ground, such as flash floods.

The violent thunderstorms, produced by the convergence line of winds from the west and south-east close to the internal reliefs in the upper part of the river basins, have given rise to cumulative rainfall exceeding 230 mm in the area; an exception to this is Olbia, which hovered around the 120 mm mark with a maximum hourly intensity of 61 mm (Sa Pianedda station) [49]. With respect to the rainfall trend, reference was made above all the rain gauges of Olbia and Putzolu (ARDIS hydrological network): the latter, less than 10 km as the crow flies west of Olbia, is more representative of the hydrological conditions persisting in the interior of the hydrographic basins along which the most damaging effects occurred. The set of data available, however, allowed a better spatial reconstruction of the meteoric event.

At the Olbia rain gauge, the maximum intensity was 28 mm/h (between 6 p.m. and 7 p.m.) and it was preceded by at least another 8 h of precipitation with variable intensity up to 18 mm/h. At the Putzolu rain gauge, higher rainfall heights were recorded, with a daily cumulative of 175.2 mm compared to that of Olbia with 117.6 mm. The maximum hourly intensities in Putzolu reached 45 mm/h between 3 p.m. and 4 p.m., while at the Monte Petrosu rain gauge, the intensity over 15 min was 26.4 mm along the coast. The station that recorded the highest cumulative precipitation was that of Sa Pianedda, which is close to the first hills south of Olbia, with values equal to 150 mm/3 h, 167.2 mm/6 h and 247.4 mm/12 h. The estimated return periods are about 200 years (up to 12 h) for the Putzolu station and about 50 years (up to 6 h on the data) for Olbia [49] (Figures 4–6). Over 80% of the recorded rainfall was concentrated in just over six hours, but persisted for over 12 h (Figure 7); due to their continuity, they were sufficient to cause maximum flows not only in the smallest basins but also in the terminal sections of the largest hydrographic basins.

During the night between 9 and 10 September 2017, the flood event that affected the town of Livorno was anticipated by several storms. During the first, which mainly affected the coastal areas between the territories of Livorno city and Marina di Pisa, maximum cumulative rains of 63.4 mm/1 h was recorded over Livorno (between 8:45 p.m. and 9:45 p.m.) and 65.6 mm/1 h in Marina di Pisa (Bocca d'Arno station). In latter area the rainfall continued to intensify, but the rains practically stopped in Livorno after 9.45 p.m.

Starting from 2:00–2:30 a.m. on Sunday, a new and strong thunderstorm, which then turned out to be the most violent, mainly affected the areas between the southern area of Livorno city and Rosignano town [43,50–52]. In these areas, the values of rainfall reached, which on short durations are really extreme, had peaks higher than 42.4 mm/15', 121.8 mm/1 h (Rp > 200 y), 210 mm/2 h and 230 mm/3 h [44,50] (Figure 7). There is a clear difference between the maximum data recorded in these hours in the different time intervals of the durations 1, 2 and 3 h by the stations of Quercianella and Valle Benedetta compared to the stations located slightly further south or more inland (such as Castellina Marittima and Santa Luce) (Table 4) or further north (as Livorno Mareografo); this difference highlights the strong localization of the thunderstorm phenomenon that locally discharged over 200 mm of rainfall in 2 h (Figures 4–6). Estimated return periods for 1 h and 3 h of rainfall that were recorded during this event are more than 200 years.

**Figure 7.** Hyetograms and hydrograms for the events of Genoa (**A**), Olbia (**B**) and Livorno (**C**). Legend: 1. Hourly intensity (mm/h), 2. Cumulated rainfall (mm), 3. Discharges (m3/s); 4. Discharges for return period = 50 years (m3/s); 5. Discharges for return period = 200 years (m3/s).


**Table 4.** Maximum cumulative rainfall values over short durations (from 15 min to 3 h) recorded for the stations of Quercianella, Valle Benedetta, Santa Luce and Castellina Marittima [48].

#### *4.2. Ground Effects of the Flash Floods*

With respect to the 2010 event in Genoa, the effects on the ground were determined by rainfalls produced by the convergence between the northern currents coming from the colder Po basin and the warm humid current coming from the sea. In the face of the quantities of rain received from the Sestri Ponente area, the water level in the streams increased in a short time: the Varenna Stream in Pegli reached a peak of 2.62 m with a rapid increase of 2.08 m in one hour (Pegli hydrometer). The Molinassi, Chiaravagna, Ruscarolo and Cantarena streams flooded the areas close to their beds and the Genoese quarter of Sestri Ponente (Figure 8A). Along the streets, the water level reached variable heights from 20 cm to over 150 cm; the estimated discharge rates for the Chiaravagna and Cantarena streams were comparable to those calculated for a Return time (Rt) of 50 years, while for the flood of the Molinassi Stream it was estimated at Rt = 200 years.

The response times of the meteorological-hydrological event were extremely short: Testimonies and amateur images document that after less than half an hour from the peak of flow, the flood of the Chiaravagna stream occurred and it poured along the roads as the water leaking from the streambed. The rainfall caused a rapid increase in flooding along with the transport of suspended materials and floating shrubs and trees that were eroded along unprotected embankments. The speed of the process made it impossible to implement interventions, unless the event was almost concluded. People were taken by surprise along the streets: water spread into residential areas and businesses and caused extensive damage (Figure 8B).

Along the slopes of the Chiaravagna (11 km2) and Molinassi (2 km2) basins, many shallow landslides have been triggered: they interrupted the access roads to the small inhabited areas placed on the hills.

Many streets were flooded and the settlements on the adjacent hills were isolated. The flow of mobilized debris was quickly channeled along the river beds and lower areas which caused critical hydraulic conditions in the secondary hydrographic network and also because the canals that pass culverted under the roads and the inhabited area were not able to dispose the relevant discharges and were quickly clogged.

With respect to the 2013 event, the area affected by the event was estimated at around 1500 km<sup>2</sup> and includes three main basins: the Cedrino and Posada basins and the catchment upstream of Olbia [53]. The city of Olbia was the most affected city, with eleven victims; much of the downtown area was inundated by the flood waters of the San Nicola and Seligheddu streams in the mouth area. Witnesses claim to have seen the hydrometric levels increase by about 3 m and this would be confirmed by the simulations carried out by [54] and associated with flow velocities higher than 3.2 m/s in the upstream sectors along the hydrographic lines. The railway embankment and the various bridges upon the arrival of the flood wave along the aforementioned canals had a dam effect and caused the flooding of the streets and the first floors of the houses. The most acute phase of the flood event was observed between 5:00 p.m. and 7:00 p.m., with more evident manifestations at around 6:00 p.m. on the Rio Seligheddu Stream and at around 6:30 p.m. on the urbanized stretch of Rio Gadduresu Stream which is its left tributary. Around 9:00 p.m. the flood had subsided with evident manifestations in the sectors surrounding the watercourses of Seligheddu, San Nicola, Zozò, Paule Longa and to a lesser extent the Pasana. In some

sectors (former Artillery area), with variable tie rods up to about 2 m, the effects of the flood of Rio Gadduresu Stream from the north and east overlapped with those of Rio Seligheddu Stream from the south, which is a condition favored by the existing artifacts which represented temporary structures that are damming to the outflow of flood waters (Figure 9A–C) [55].

**Figure 8.** (**A**) Chiaravagna Stream flooded the ground floors, where there are many business shops and in some cases the water and mud depending on the preferential outlet flow found that entered the shops from the rear part, as can be seen in a sports shop in the Aprosio Square [56]. (**B**) Image taken from a movie. Chiaravagna Stream during the paroxistic phase of flooding: the black line indicates the submerged left bank wall; the red asterisk represents the bridge of Chiaravagna Street. The building from which the photograph was taken was built just on the riverbed in the 1960s. The October 2010 event (the last one of a long list) was recognized as one of the causes of the flooding of the river: it was demolished a few years later [56]).

**Figure 9.** (**A**) Olbia: Although about 15 h had passed since the paroxysmal phase, in some morphologically depressed areas of the city the water remained well above 1.3–1.4 m in height. Some cars were totally submerged in one of the streets most affected by the flood in the Baratta quarter (courtesy of private citizen). (**B**) Olbia, left bank of the Rio Seligheddu Stream, canalized stretch (foto Luino). Water level left a marker on the wall, which is evidenced by white line. The height of the waters is evidenced by the bed dragged and placed over the roof of the ground floor (white arrow). (**C**) Olbia: very common situation along the areas close to the streambeds (foto Luino). In this image, the Rio San Nicola (red arrow) waters exceeded the natural bank that was devoid of embankments or retaining walls and easily reached the houses located on the left bank, which was a short distance away. In many houses, the underground garages have been foolishly built and had been totally flooded (black arrow indicates the level), with serious damage inducted to parked cars. The extracted cars are so saturated with fine material (silt and clay) that they had to be demolished (image in the left corner).

> During the 2017 event, watercourses flooded the surrounding areas in the hilly sector, where some bridges were damaged and many residents remained isolated. Towards the valley, in the area of the Rio Ardenza and Rio Maggiore mouths, the effects were even more serious with extensive flooding and entire neighborhoods invaded by water and mud. The major effects on the territory (floods, overflows and transport of debris material) were caused by the minor hydrographic network that originates from the Livorno hinterland and flows directly into the sea, as in the case of the Rio Maggiore and Rio Ardenza streams. The culverted stretches, often having insufficient section and occluded by detrital, vegetal and urban material carried by the waters, were bypassed by the floodwaters that retraced the ancient surface river paths. In particular, the waters of the Rio Maggiore Stream, despite the presence of retention basins, managed to flow freely in the area of the Stadio Ardenza District, in the neighboring streets and in Barriera Margherita. The Rio Maggiore Stream escaped from the culvert and poured with high speed into a fenced courtyard that was morphologically depressed compared to the nearby streets. The ground-floor flat in Sauro Street (Figure 10A,B) was flooded within minutes and four people drowned in it. Due to

the overflow of the Ardenza Stream and its tributary Forcone stream, four other people lost their lives. The flood event caused in total eight victims in the Livorno area.

In addition to the Rio Ardenza and Rio Maggiore streams, the Ugione, Quercianella and the Chioma streams inundated large areas (Figure 10C). A bridge adjacent to a provincial road collapsed along the Ardenza. The total damage was estimated at 180 million EUR [57].

**Figure 10.** Livorno. (**A**) aerial photograph of the Ardenza Stadium district. In the foreground isthe house where the four victims drowned (white asterisk). The yellow line highlights the culverted streambed of the Rio Maggiore: its flood waters were the cause of the rapid flooding of the courtyard (evidenced by red lines) and of the ground-floor flat [58]. (**B**) Livorno: the house where four people from the same family drowned (courtesy of Il Tirreno). Their ground-floor flat is located at the end of a large courtyard, below street level on Rodocanacchi Street (area of the Ardenza football stadium). The red arrow indicates the level reached by the floodwaters in a few minutes. (**C**) Livorno: the Stagno district that was largely flooded by the Rio Ugione floodwaters with the Via Aurelia (yellow line) and the refineries in the background [58].

#### *4.3. Urban Geomorphology*

The case studies examined are characterized by a similar geomorphological structure and recent evolution, which has profoundly changed almost all Mediterranean urban areas [22,29]. Sestri Ponente, Olbia and Livorno are in fact three cities built on a coastal floodplain at the mouth of small hydrographic basins (Figure 11A–C).

**Figure 11.** Hydrographic networks: culverted stretches are indicated with red lines. (**A**) Genoa Sestri Ponente; (**B**) Olbia; (**C**) Livorno. Streams: (a) Varenna; (b) Molinassi; (c) Cantarena; (d) Chiaravagna; (e) Ruscarolo; (f) Ugione; (g) Maggiore; (h) Ardenza; (i) Forcone; (j) Quercianella; (k) Chioma; (l) San Nicola; (m) Canale Zazà; (n) Gadduresu; (o) Seligheddu; (p) Paule Longa.

In this morphological situation, it is therefore possible to identify the processes related to the shapes of the landscape: the sea wave action along the coastal strip and the river dynamics in the rear belt, up to the hill base. The footprint of the anthropic landforms dominates the current urban landscape. If natural landforms were almost exclusive until the 18th–19th centuries, then in the last 150 years there has been a gradual increase in anthropogenic activities in the area with consequent major changes [59]. Two historical moments demonstrate grea<sup>t</sup> changes in the urban landscape due to anthropic impact: the first around the middle of the nineteenth century (industrial revolution) and a second in the second half of the twentieth century [60], after the conclusion of the Second World War (greater well-being, population growth and finally tourism). At present time, in all the cases examined it is therefore possible to identify modified natural landforms, anthropic landforms and disappearing (or vanished) landforms. The most common modified natural form is the main and secondary riverbed: in all the cases analyzed, the riverbed in the coastal floodplains was narrowed, channeled, rectified, often culverted and sometimes diverted.

In Sestri Ponente, the Chiaravagna Stream is channeled in its 2 km terminal, while the Ruscarolo Stream, originally an autonomous basin, flows entirely into the Chiaravagna. The other four streams of the plain are invisible, that is, culverted practically throughout the terminal stretch and flow within the urban fabric. A multitemporal cartographic comparison allows us to evaluate a narrowing of the riverbed in the last 200 years between 25% and 50% [34].

In Olbia, the Seligheddu Stream to the south and the San Nicola Stream to the north are represented today by artificial canals cemented with an inverted trapezoidal section, with many deviated and culverted sections, and affects the town with a multi-kilometric development [49].

The coastal stretch between Livorno and Antignano is characterized by three watercourses: to the south, the Rio Ardenza Stream is channeled in its final stretch for at least 2 km, with conspicuous narrowing and rectification of the section of the riverbed; on the other hand, the Rio Maggiore Stream and the Fosso Botro canal have been transformed into culverts [43].

The anthropogenic shapes dominate the urban landscape and can be traced back to forms of accumulation: in Sestri Ponente, Olbia and Livorno, among the most significant and common are the embankments, the reclaimed land at sea and the defenses along the coast, while urbanization has involved remodeling of the existing topographic surface with diffuse fillings.

Sestri Ponente is dominated by the sea fill on which the shipyards and the Genoese airport are built on: the railway line runs on an embankment and, further upstream, there is the motorway embankment. The fluvio-coastal plain is practically fully urbanized and the most significant phases of expansion were those of the second half of the 19th century and in the second half of the 20th century.

Olbia has a deeply modified coastal strip for the construction of the maritime station by the filling of the sea; the road and railway embankments emerge in the strip behind the historic core. Particularly significant is the construction of artificial canals built in the 1920s, while urbanization appears significant from the second half of the 20th century, but continued into the third millennium.

The urban coastal stretch south of Livorno has sea fills that are almost continuous, but they are less deep than the ones in Sestri Ponente and are protected by sea defenses. In the strip immediately behind it, there are the railway and motorway embankments; urbanization is continuous, with the exception of some sports facilities, and can be traced back to the second half of the 20th century. The waterways have been channeled and retention basins have been built along the final stretch of the Maggiore Stream.

Lastly, the disappeared forms, i.e., those dismantled or covered by anthropic activities, deserve a mention: In the case of Sestri Ponente, in addition to the consumption of land on the entire coastal floodplain there is the disappearance of the beach that occupied the entire stretch of sea in proximity to the coast for a length of about 2.5 km. In the Livorno coastal

strip, the small cliffs modeled in the cemented sandstones alternating with small beaches have been incorporated into the sea fillings built from the second half of the 19th century, while in the coastal plain behind them marshes and coastal dunes have disappeared. In the case of Olbia, even if the imprint of the rias coast remains evident, the disappearance of the marshes has been noted, which characterized much of the coastal strip north and south of Olbia (Terranova Pausania) and into which the hydrographic network used to flow, as well as the salt flats near the historic center and small beaches bordered by cliffs modeled in granite between the Roman Port in the north and the Lepre Island in the south.

## **5. Discussion**

The case-studies presented in this article show some geomorphological similarities: the cities arose on alluvial plains, typical of the coastal strip [22,29,56,61] of the Ligurian-Tyrrhenian Sea. They extend for several kilometers along the coastline (2.6–11.5), occupying variable areas (8–30 km2) with high hills behind (330–580 m) and some kilometers away (3.8–6). Many streams cross the cities and some of them possess relevant areas (up to 38 km2): they can reach relevant discharges during the violent rainfall events and in proportion to the basin areas (up to 8.6 m3/s/km2) (Table 5).

With respect to the weather-hydrological aspect, it should be emphasized that all the considered events occurred during the autumn season (from September to November), in conditions of a storm system triggered by cyclogenesis at the Gulf of Genoa (Liguria and Tuscany) or by the extra-tropical cyclone Cleopatra (Sardinia).

**Table 5.** Geomorphological features of the three cases analyzed. City Develop, development of the city along the coastline; Hill Height, height of the hills located behind the city; Distance, distance between the coastline and closest hills; Basin Area, area of the largest hydrographic basin behind the city; Max Discharge, maximum discharge for a 200-years return period.


Despite the trend in the number of rainy days being negative and the progressively decreasing annual cumulative rainfall, perturbations capable of generating intense rainfall are increasingly frequent in the Mediterranean area [10], with a growth corresponding to intense geo-hydrological events. A sentence that summarizes this concept is currently widely used: "it rains less, but worse".

If we add the progressive increase in temperatures to this [62], it is possible to confirm the data on climate change underway [63,64] with recent evidence of events studied in the geo-hydrological field: For example, the event of Lavagna-Genoa in 2002 [30], autumn 2011 in Liguria [65] and event of October 2014 in Liguria [33,66]. These data are also supported by studies in other disciplines [67,68].

The events considered had rainfall that reached values between 10.4–15.2% of the annual total in 1 h; between 22% and 25.8% in 3h (minimum difference between the minimum and the maximum); between 26.5% and 32.7% in 6 h; finally, values between 27.2% and 42% in 12 h (Table 6).

**Table 6.** Rainfall of the three events (for 1,3,6 and 12 h) compared with the mean annual precipitation, MAP (%).


The percentages obtained actually indicate the rainfall characteristics of each individual event: the Livorno event was more concentrated on 1–3 h, the Sestri Ponente on 6 h and the Olbia event on 12 h. The intense precipitations occurred in correspondence with strong winds and storm surges, showing hourly cumulative rainfall values comparable to half-yearly or annual averages. The precipitations sent the hydrographic system into crisis: the levels of the streams grew rapidly, reaching and exceeding the alert levels.

The responses of river basins to intense and short rainfalls depended on several factors: (i) land use, (ii) bedrock permeability, (iii) thickness of the eluvium-colluvial cover and (iv) initial content of soil moisture. If, in normal conditions, the Mediterranean catchment areas possess a runoff coefficient of 0.4–0.6, in the conditions of saturated or impermeable soils, runoff coefficients close to 1 can be achieved [69,70].

The streambeds in the cities, over the years, have been gradually narrowed to conquer more urban areas and often the beds have been culverted for long stretches and flow under roads and buildings. It is therefore natural that the streambeds were not able to dispose of the huge discharge of the streams in which water shrubs, large trees (uprooted upstream or in the riverbed), garbage cans and vehicles are often transported.

The increase in urbanized areas has amplified this problem, leading to an increase in waterproofed surfaces; this has caused the irreversible loss of soil and consequent impact on the flow of water, which when unable to infiltrate the soil, is dispersed by surface flow. The latter, increasing in terms of drained volumes and transit speed, is responsible for problems in the control of surface waters, in particular during particularly intense rainfall phenomena. The growth in waterproofed surfaces, in fact, involves an increase in the runoff coefficients and a reduction in the run-off times, making it necessary to construct structures for containment and disposal (bypasses) of exceptional flood events.

In the last 150 years, the Sestri Ponente, Olbia and Livorno cities have suffered remarkable transformations, especially in the coastal areas (Figure 12). This has led to the growth of urbanized areas along the plain, with partial or total impairment of the areas of fluvial pertinence. The waterways have undergone riverbed narrowing, containment and lateral and bottom constraints, which has resulted in the unsuitability for the disposal of significant flood flows.

The serious damage recorded for the cases treated, primarily the 27 overall casualties of the three events described, are linked to the vulnerability of the numerous elements present in the fluvio-coastal plains of Genoa, Olbia and Livorno. Among the anthropogenic forcings that have influenced the hydro-geomorphological dynamics and which have determined the increase in risk conditions, the following are counted [71,72] (Figures 13 and 14):


**Figure 12.** Hydrographic networks, urbanized areas and land use of the basins of Genoa Sestri Ponente (**A**), Olbia (**B**) and Livorno (**C**). Land cover: (1.1) urban fabric; (1.2) industrial; (1.3) mine, dump and construction sites; (1.4) artificial non-agricultural vegetated areas; (2.1) arable land; (2.4) agricultural areas; (3.1) forests; (3.2) shrub and/or herbaceous vegetation associations; (3.3) open spaces with little or no vegetation.

**Figure 13.** Urban evolution, evidenced by different colors for Genoa Sestri Ponente (**A**), Olbia (**B**) and Livorno (**C**) since the end of the 19th century. In the legend: (H.N.) Hydrographical network; (C) Culvert; (F.A.) Flooded area.

A very important aspect that characterized all three events was the lack of communication with the population before and during the culminating phase of the event [73–75]. In all three cities, the inhabitants were surprised by the flooding of the streams and the

rapid growth of water in the city: this made it impossible, in some cases, to escape to safety (15 drowned victims) or to bring their own goods (especially vehicles).

A last and very important topic concerns the historical sources. Moreover, in this study, a careful historical reconstruction has made it possible to detect how much the anthropic changes may have influenced the bed of the watercourses year after year, decisively conditioning the floods during the paroxysmal phase of the event and creating casualties and a lot of damage. A correct utilization of the historical sources could save lives and goods.

**Figure 14.** Urban development in the coastal plain of: (**A**) Genoa Sestri Ponente, compared with the percentage of urbanization involved over time by a flood of comparable size in terms of volume and area to that occurred 10 April 2010; (**B**) Olbia, compared with the percentage of urban areas involved over time by a flood of a comparable extent in terms of volume and area to that of 18 November 2013; (**C**) Livorno, compared with the percentage of urbanization involved over time by a flood of comparable magnitude in terms of volume and area to that of 10 September 2017.

**Figure 15.** The importance of the historical sources is underlined by this 1824 cadastral map of Livorno. The Rio Maggiore Stream flowed naturally and was free from anthropogenic influences (blue stroke on the map). Its course has been diverted over the years and, above all, the stream has been culverted: it now flows invisibly between the houses of the city (orange stroke). The house where the victims drowned in September 2017, which will be built afterwards in the early 20th century, is highlighted in red ([76], modified).
