Natural and Environmental Disasters

University College LondonDepartment of Civil, Environmental & Geomatic Engineering

Natural and Environmental Disasters(CEGEG030/CEGEM030)Student: Carmine Russo 14103106

Table of contents1Introduction22Thunderstorms anatomy32.1Multicellular thunderstorms32.2Mesoscale Convective System (MCS)42.3The V-shape52.4Self-regenerating thunderstorms53The sequence of events63.1Meteorological analysis synoptic scale63.2Mesoscale meteorological analysis114Aftermath and hydrometric analysis144.1Evolution and recorded data154.2Analysis hydrometric: hyetograph and flow rates184.3Situation in Brugnato (Vara River)204.4Situation in Pontremoli (Magra River)214.5Situation at the confluence between Magra and Vara River234.6Description of the event on the coastal rivers (Monterosso, Vernazza, Levanto and Bonassola)244.7Consequences254.8Economic impact295Conclusions and lessons learned30References:33

1 IntroductionAutumn is the rainiest season over much of the Italian territory, since cyclones from the Atlantic, in their motion to the east, run through trajectories more southern and there are easier conditions for the formation of cyclones downwind to the Alps, which originate usually from the Gulf of Genoa and then migrate eastward. The waters of the Mediterranean Sea, still warm, accelerate the growth of the cyclone downwind (Buzzi and Tibaldi 1978; Buzzi et al. 1987) and in particular attract hot and humid air from Africa to Italy, associated with heavy rainfall in the areas exposed to the sirocco like the Tyrrhenian regions and the southern side of the Alps.

Fig. 1.1 - Precipitation events in October on Italy's north depicted as a percentage (%) of events with precipitation intensity to the ground more than 20 mm in the period 1971-1990. (Frei and Schr 1998)Note the maximum on the East Liguria (also known as "Levante Ligure") and the High Tuscany.

The rainfalls can stop over for long time if the natural movement eastward is hindered by the presence of an anticyclone in southeastern Europe, as often happens during the transition between summer and fall (in fact, the self-regenerating convective storms are quite common in Italy in the fall season). The formation and development of these systems, which can sometimes show a mesoscale organization, depends on the meteorological characteristic configuration at the level of synoptic scale and on particular dynamics and thermodynamic conditions at the level of mesoscale. It may then generate conditions for a quasi-stationary and long lasting convection often favored by interaction with the complex orography of southern Europe on the Mediterranean Sea (Davolio et al. 2009).In 2011, after a stable and very warm September, the seasonal change occurred at the end of October with the entry of two troughs (two cold fronts) from the Atlantic; the topography of northern Italy and in particular of Liguria has fostered the formation of organized stormy systems who have self-regenerated in the same areas for several hours, discharging heavy rainfall and causing flash floods in Eastern Liguria and Tuscany Italy (October 25) and in the city Genoa (November 4), with extensive damage and loss of human life.In particular the first of this series of meteorological events, the 25th October 2011, caused death and destruction in a strip of land between the coast of eastern Liguria and Lunigiana. The self-regenerating storm system, after classified as Mesoscale Convective System V-Shape (MCS V-shape), affected approximately 700 km of land, on which have been recorded extraordinary rainfall: over 300 mm in 6 hours, with a peak of just under 500 mm in the same time frame (540mm/24h in Brugnato, 455mm/24h in Calice al Cornoviglio, 382mm/24h in Monterosso), with huge rainfall rate (153mm/h in Brugnato, 129mm/h in Calice al Cornoviglio, 111mm /h in Levanto).

2 Thunderstorms anatomy Thunderstorms are composed of one or more convective circulation cells, formed by an area with updrafts of warm moist air that originates and feeds the cell, and a region with currents descendants (downdraft) characterized by strong rainfall and gusts of wind. According to the intensity of the vertical wind shear, thunderstorms are classified as: single cell (characterized by small size and weak vertical wind shear), multicellular (characterized by large size and strong vertical wind shear), supercell (a single thunderstorm cell which has a rotating updraft generally associated with a mesocyclone, that thanks to the strong vertical wind shear interacts only partially with the downdraft, allowing the cell to self-sustained and long-lasting). The life cycle of a thunderstorm cell (shown in figure 3.1) is divided into three stages that depend on the intensity and direction of the vertical motions.

Fig. 2.1 Model of generation and evolution of a thunderstorm cell. (Byers and Braham 1949) 1) Cumulus humilis: the cloud is characterized by the updraft in the entire cell due to the ascent of hot air from the soil (thermal currents) that usually reaches the saturation the forced condensation level (LCL); from this point the "thermal current" continues its rise and, if it reaches the level of free convection (LFC), it will continue to accelerate upwards thanks to the buoyancy force. The cumulus grows and in special cases of instability passes to the stage of "cumulus congestus", reaching heights of around 6 km. 2) Mature stage: characterized by the development of the cumulonimbus cloud that can reach heights of 10-12 km or more; in this stage are present updraft and downdraft also of strong intensity. The circulation of downdraft is produced by the drag force induced by drops on the air and by the evaporative cooling of the air. 3) Dissipation stage: the entire cell is affected by a slight downdraft; the top of the cloud can reach the Tropopause, spreading horizontally, and giving rise to the anvil of the thunderstorm (cumulonimbus incus). With the decay of the updraft and the consequent termination of the rain, the downdraft becomes weak, and finally dissipates completely, leaving behind a residue of cloudy air.

2.1 Multicellular thunderstormsThe multicellular systems are composed of multiple storm cells, each of which follows its own evolution, promoting the development of new cells. In case of strong vertical wind shear, the various storm cells can be so integrated that lose their identity, giving rise to an organized system on a larger scale and of longer duration.A fundamental characteristic of these thunderstorms is the gust front: when the cold air downdraft reaches the ground it expands horizontally, creating strong linear gusts; the gust front is denser than the hot and humid air existing, and for this reason it wedges under it, causing a forced lifting of the air mass, which creates new storm cells.

Fig. 2.2 - Structure of a multicellular(a) vertical profile of potential equivalent temperature ;(b) the vertical profile of the horizontal component of the wind u.One notes that the new convective cells are formed where the air is raised by the gust front; When this air mass reaches the LFC, spontaneously begins to rise due to buoyancy force. The water vapor condenses to form cloud droplets and ice particles in the updraft, which subsequently will form a new downdraft. The dry air of the surrounding environment with lower potential equivalent temperature (), enter in the cell at the average levels, in its rear part.

Multicellular thunderstorms can organize in their turn in several ways: multicellular cluster, multicellular line (squall line), Mesoscale Convective System (MCS) and Mesoscale Convective Complex (MCC).

2.2 Mesoscale Convective System (MCS)According to Zipser (1982), an MCS is an atmospheric system that shows convective cells embedded in a circulation at mesoscale that is at least partly driven by convective processes; This general definition includes a wide variety of mesoscale phenomena from multicellular thunderstorm cluster, which have a short-term, up to a well-organized systems, such as squall lines, MCC and even hurricanes.Usually the study of MCS tends to consider all the phenomena that occur at the mesoscale (which have a Rossby number, ratio of inertial to Coriolis force, close to 1), and systems which have a radius comparable to the Rossby radius of deformation (length scale at which rotational effects become as important as buoyancy or gravity wave effects in the evolution of the flow) (Schubert et al. 1980; Cotton et al., 1989). The Rossby radius of deformation can be expressed as:

Where: is the phase speed of an inertial gravity wave; is the vertical component of the relative vorticity; f is the Coriolis parameter; V is the tangential component of the wind at the radius of curvature R;crudely identifies the scale at which rotational influences or inertial stability of a system become important.For MCS, in the middle latitudes, is about 300 km and the influence due to the effect of Coriolis become significant in times of 3-6 hours.The MCS can be divided into two types, depending on the dynamic mechanism that gives rise to the convection and to the formation of large cloud systems that characterize these storms: The events of type 1 occurs when an extensive air mass potentially unstable at low levels is forced to climb into a frontal zone or region baroclinic (i.e. a region in which fronts separate warmer from colder air, hence distinct air mass regions exist); the type 2 event occur in an atmosphere more barotropic (i.e. uniform temperature distribution) and depend on production, by the downdraft, of a "cold pool" and its interaction with the vertical wind shear present, which can produce very large updraft. Because of that, type 2 event derive more from the characteristics and processes imposed by convection itself.2.3 The V-shapeSome violent storms show a distinct "warm spot" at the top of the system (Mills and Astling 1977), with colder areas adjacent, that are organized in a V-shape, called enhanced V-shape ; the V opens in the direction of expansion of the anvil of the storm. The characteristic V-shape develops when a strong updraft penetrates the lower stratosphere, creating an overshooting on top that blocks the wind at higher levels, forcing the flow to diverge around it; the flow erodes the summit of the updraft and carries cloud's residues downwind (McCann 1983). The transport of these residues can be seen by IR satellite images in areas at lower temperature, which are organized in a V shape; the cooler area, which is near the apex of the V, is associated to the adiabatic expansion due to the rise of air in the updraft at the time when it reaches the tropopause (Heymsfield and Blackmer 1988).

Fig. 2.3 - IR satellite images (color and black and white) of a temporal enhanced-V shaped. The presence of the V-shape in a temporal indicates a high wind shear in the troposphere and a vigorous updraft, which leads to phenomena such as intense flash floods.

2.4 Self-regenerating thunderstormsThe self-regenerating thunderstorms, better defined as a clusters of multicellular with regeneration upwind, are fearsome storm structures because of their persistence on a given area, where they can bring violent storms and flash floods; these systems can sometimes be classified as MCS and may have the characteristic V-shape.The self-regenerating thunderstorms develop in an atmosphere unstable and need huge amounts of energy to keep active, energy that is present on the sea in the form of steam and heat available; for this reason the updraft of the storm uses the sea as source. It has been noted that their formation is more frequent at the "shallows" because in areas of shallow sea water temperature is higher.Fig. 2.4 - Structure of a self-regenerating thunderstorm: for the development of these systems, it is necessary that the convective cells are hooked by a strong jet at high altitude (10-12 km) which creates a kind of "suction" from the bottom to the top and favors the ascent of air masses hot and humid. From the satellite the storm cells of this type appear highly elongated, with the anvil which also extends for hundreds of kilometers along the jet stream. The appellation self-regenerating is given because once formed the first cumulonimbus, and after the first thunderstorm, this will be swept away by the strong currents at high altitude, but will be quickly replaced by a new cumulonimbus that will drift, and its place will be taken by a third party, and so on.

3 The sequence of eventsTo understand the mechanisms that led to the formation of a stationary thunderstorm self-regenerating that affected the Levante Ligure and high Tuscany, is necessary to perform a meteorological analysis at synoptic scale and mesoscale.

3.1 Meteorological analysis synoptic scaleSince October 24th, it was visible a very large trough (Fig. 3.1) in the vicinity of the Atlantic, with main axis extended in southeastern direction from Iceland to Morocco (the evolution is illustrated and explained in the next pictures , from fig. 3.2 to fig. 3.9)

Figure 3.1 Geopotential height at 500 hPa at 0000 UTC on 24 October.

Fig. 3.2 - Pressure at sea level at 0000 UTC on 24 October.The trough was associated with a deep low to the ground (976 hPa at 0000 UTC on October 24th,) centered near the west coast of Ireland, while the Central and Eastern Europe was affected by an anticyclonic promontory with maximum pressure (1033 hPa) located close to the Baltic countries.

Figure 3.3. As in Fig. 3.1, but at 0000 UTC on 25 October.During the day of 24 October, the trough has deepened gradually turning its axis counterclockwise by entering the Mediterranean Sea; the minimum main pressure, in the other hand, moved backward towards the northwest going to position on the south of Iceland.

Figure 3.4. As in Fig. 3.2, but at 0000 UTC on 25 October.The deepening of the trough, has triggered a cyclogenesis leeward to the Alps near the Gulf of Lion, already visible in the early hours of October 25, with the formation on the western Mediterranean of a wide cold front from the Gulf of Lion pushed up north Africa, and a warm front just north of Corsica, over the Ligurian Sea (see also fig. 3.5)

Figure 3.5. Analysis of fronts related to the hours of 25 October 1200 UTC (elaboration of UK Met Office).

Figure 3.6. Wind height 700 hPa at 1200 UTC on 25 October.The so formed synoptic configuration, has fostered an intense advection of warm-moist air and subtropical unstable on the basin of the Ligurian Sea in the day of 25th October, extended from the lower and middle layers of the atmosphere up to the high troposphere, ), from West Africa and led by the trough at altitude (see also next two images)The reinforcement of the high pressure on southern Italy and the Balkans in the same day has slowed the natural motion of the cold front eastward favoring a considerable accumulation of moist air over the Ligurian Sea, given the persistence of hot conditions for 18-24 hours.

Figure 3.7. Relative humidity (%) at height of 700 [hPa] at 1200 UTC on 25 October. Note the peak (between 80 and 100% in the Liguria area), note also the blue strip of moist air coming from north Africa (see also fig. 3.8)

Figure 3.8. Image sensor SEVIRI satellite MSG channel WV 6.2 m reported at 0600 UTC on 25 October. In red, is highlighted the corridor of moist air that goes from Africa to the Ligurian Sea, led by the trough in altitude.

Fig. 3.9 - In the image taken from the European satellite to 16:30 h local 25.10.2011, the Mesoscale Convective System responsible of the flash flood (immersed in the a extensive disruption). It can be noted (red box) the top of the violent thunderstorm cell in the form of "V".

3.2 Mesoscale meteorological analysisTo understand the mechanisms that have allowed the formation of a self-regenerating thunderstorm it is necessary to analyze the phenomenon to a more small scale.

A key role has been played by the orography of northern Italy and in particular of Liguria; the particular position of the point of high pressure and low pressure, created two completely different wind regimes: the west ("Ponente Ligure") affected by strong mistral winds with a significant drop in temperatures, while the East ("Levante Ligure") exposed to the southern warm currents with temperatures close to 20C (above the averages of the period).On the sea of La Spezia, has come to create a convergence zone (Fig. 3.11) between the stream that flowed along the humid southern Tyrrhenian and the strong winds of the mistral (relatively dry);

Figure 3.10. Synoptic map the ground referred to the 11:00 UTC 25th October (meteocentre.com).The formation of an anticyclonic promontory in the Po Valley associated with presence of a minimum on the Gulf of Lions has resulted in a high pressure gradient over the Ligurian Sea, recalling from Po Valley cold air in the lower layers (from North-East to South-West direction)

On the sea of La Spezia, has come to create a convergence zone (Fig. 3.11) between the stream that flowed along the humid southern Tyrrhenian and the strong winds of the mistral (relatively dry);

Fig. 3.11 - Direction and intensity of the surface wind at 09:00, 12:00, 15:00 and 18:00 UTC on 25 October (Moloch model). It shows the area where there is convergence between the winds of the mistral and sirocco. Note the stationarity of the convergence line (from 09:00 to 18:00 UTC didnt change position).

Fig. 3.12 - Winds to the ground (ground level) at 13 UTC on 25 October 2011 (ARW model, consortium LAMMA). It is evident the horizontal wind shear in correspondence of the Cinque Terre, between the lively flow of sirocco which blows upward from the Tyrrhenian and the northern ventilation (downward) over the Ligurian Sea.

The contrast between the two flows is evident by analyzing the map (Fig. 3.13) of at height of 950 hPa, 12:00 UTC on 25th October. The result was the formation of a quasi-stationary front line which favored strong ascending motions and triggered the storm system.

Fig. 3.13 Equivalent Potential Temperature 950 hPa at 1200 UTC on 25 October (Moloch model). Note the strong thermal contrast over the Ligurian Sea caused by the convergence of two air masses of different origin.

4 Aftermath and hydrometric analysisThe exceptional rainfall, were those typically associated with the transit of cumulonimbus and have continued with the greatest intensity for about 6 hours flooding the soil surface along a strip, about 10 km wide on average, with an estimated volume of water of about 200 million cubic meters; outside this strip the precipitations have been progressively less heavy.

Fig 4.1 Area affected by MCS V-shape (image source Google earth elaboration Carmine Russo)In areas most exposed, precipitations of such intensity produced the overflow of rivers and torrents of the Tyrrhenian coast between Levanto and Vernazza, of the Magra River and its tributaries at different points, which was, unfortunately, followed by the loss 13 human lives. The rain fallen on steep slopes, made of a substrate with a covering of land altered and not anchored (such as those that characterize Cinque Terre), have resulted in an accentuated and widespread erosion, that triggered mudslides. Mudslides have been accumulated very quickly in the watercourses, or they have slid downstream, increasing their volume with the debris of the river bed and the debris of previous landslides. The devastating effects of floods, landslides and mudslides on the Ligurian Riviera, have resulted in significant damage to infrastructure (the collapse of bridges, interruption of the provincial and municipal roads, as well as some sections of the motorway and railway, with temporary suspension of essential services such as water, gas and telecommunications).

Fig. 4.2 Key: Area affected by huge rainfall; path followed by cumulonimbus; direction of the runoff and area where occurred the main debris flows; hydrometric station of Fornola: slightly downstream of the confluence between the river Magra and the Vara river. The maximum flow rate of the river Magra exceeded (image source Google earth elaboration Carmine Russo)4.1 Evolution and recorded dataIn the early morning (first 6 hours) of 25th October, there are heavy precipitation on the East ("Levante Ligure") and Lunigiana (30-40 mm in 6 hours) with the formation of the first thunderstorm announcing the passage of the warm front.Between the hours 0600 and 1200 UTC a storm system develops, triggered by the convergence to the ground between the south winds and mistral; the storm soon becomes self-regenerating, assuming a mesoscale organized structure (0800 UTC), and also showing a V configuration (from 1000 UTC) stretched from the sea to the north-eastern inland of La Spezia (Fig. 4.3 and 4.4). At this time the thunderstorm activity intensifies significantly (Fig. 4.5 and 4.6), as well as the rains (Fig. 4.7 and 4.8) with accumulations up to 220-230 mm in 6 hours on the Eastern Liguria and 120 mm in 6 hours in Lunigiana (territory of Italy, within the provinces of La Spezia and Massa Carrara), with the first inconvenience: landslides in La Spezia.

Fig. 4.3 - False color image sensor MODIS of NASA's Terra satellite - 10:15 UTC on 25 October. Initial phase of the storm system self-regenerating V-shaped within the sector indicated by the blue arrow. (NASA 2011)

Fig. 4.4 - Surface Rainfall Intensity (SRI) in at 09:30 and 10:45 UTC on 25 October. Note the stationarity of the storm system. (Image from radar of Dipartimento della Protezione Civile, DPC - 2011)

Fig 4.5 - Satellite image MSG channel HRV referring to 1200 UTC on October 25 (ARPAL) and satellite image MSG channel HRV refers to the 1345 UTC on 25 October (EUMETSAT). Note the storm system to V within the sector indicated by the blue arrow.Between 1200 and 1800 UTC the storm system reaches maximum intensity, keeping the V configuration (Fig. 3.19). The thunderstorm activity intensifies further as demonstrated by the marked lightning (Fig. 3.20); in these hours thunderstorms are still on the same areas, as shown by the radar (Fig. 3.21), with accumulations of 220 mm in Lunigiana and up to 270-280 mm on Levante Ligure (in 6 hours).

Fig. 4.6 Lightning strikes detected between 1300 and 1900 UTC on October 25. After 1700 UTC we see the shift to the east of the storm system. (Image LAMMA consortium)Figure 4.7 - SRI (mm h -1) at 1245, 1345, 1445 and 1615 UTC on 25 October. Note the stationarity of the storm system until 1445 UTC, while the image of the 1615 UTC, is possible to note a shift eastwards of the entire system. (Image from Dipartimento Protezione Civile)

Fig. 4.8 Cumulative rainfall from 0200 UTC of 25th October to 0200 UTC of 26th October (24h) in the whole Liguria region (Source ARPAL).

In the last 6 hours of the day, the strorm unlocks and a heavy rainfall moves toward the southeast, forcefully striking the Versilia (maximum accumulations of 170-190 mm in 6 hours), the Apuan Alps and the Apennines of Lucca and Pistoia.

At the end of the day of 25 October, there are accumulated (Fig. 4.8) up to 540 mm in 24 hours in the Levante Ligure and up to 370 mm in 24 hours in Lunigiana. The most significant rainfall data (some of which are derived from the pluviograms on fig. 4.9, 4.10, 4.11, 4.12, 4.13) are shown below and confirm the intensity and persistence of the storm system:

18 mm fell in 5 minutes Brugnato (SP), 143 mm in one hour in Brugnato (SP), 121 mm in one hour in Calice al Cornoviglio (SP), 101 mm in one hour in Levanto (SP), 472 mm in 6 hours at Brugnato (SP), which is the new Italian record, the previous dated 8 October 1970, when 447 mm fell in six hours in Genoa-Bolzaneto, 365 mm in 6 hours in Calice al Cornoviglio (SP), 349 mm in 6 hours in Monterosso (SP), 511 mm in 12 hours in Brugnato (SP), 539 mm in 24 hours in Brugnato (SP), 371 mm in 24 hours in Pontremoli (MS), a record for this location since 1920

Fig. 4.9 Hyetograph, hourly rainfall rate [mm/h] (on the left) and cumulative rainfall [mm] (on the right), measured by the pluviometer of Brugnato - Borghetto Vara (image source ARPAL). Note the growing intensity of precipitation dawn, reaching the 143 [mm/h] recorded in mid-afternoon. The total of the event is of 542 [mm], equal to approximately one third of the amount of rain that normally falls on the area in a year. Over an interval of 6 hours it has been recorded a value of about 470 [mm], reasonably one of the intensity maxima ever detected in Italy on that period of time.

4.2 Analysis hydrometric: hyetograph and flow ratesBelow (from Figure 4.10 to Figure 4.13) relevant hyetographs, relative to some pluviometric stations, where the maximum values were observed.Fig. 4.10 Hyetograph, hourly rainfall rate [mm/h] (on the left) and cumulative rainfall [mm] (on the right), measured by the pluviometer of Monterosso (image source ARPAL)

Fig. 4.11 Hyetograph, hourly rainfall rate [mm/h] (on the left) and cumulative rainfall [mm] (on the right), measured by the pluviometer of Pontremoli Santa Giustina (image source ARPAL)

Fig. 4.12 Hyetograph, hourly rainfall rate [mm/h] (on the left) and cumulative rainfall [mm] (on the right), measured by the pluviometer of Levanto - San Gottardo (image source ARPAL)

Fig. 4.13 Hyetograph, hourly rainfall rate [mm/h] (on the left) and cumulative rainfall [mm] (on the right), measured by the pluviometer of Calice al Cornoviglio (image source ARPAL)4.3 Situation in Brugnato (Vara River)Regarding the Vara basin, the portion most affected by the event is from Brugnato towards the valley: in this stretch also all the main tributaries of the river have overflowed, causing severe flooding in the towns of Brugnato (torrent Cravegnola), in Borghetto Vara (river Pogliaschina), Pignone (stream Pignone). Progressing towards the valley, the river Vara occupied everywhere all the riverside areas, coming to affect with local erosion also the highway embankment.

Fig. 4.14 - Evolution of the hydrometric height of the River Vara at Piana Battolla, SP (image source ARPA Liguria): note the impressive and sudden flood wave which occurred between the afternoon and evening of 25 October 2011 (increase of 7 m in a few hours)

Fig. 4.15 Rainfall rate [mm/h] in Brugnato and evolution of the level, in [m], of the River Vara in Borghetto di Vara, SP (image source ARPA Liguria). The picture shows rain and level observed on the same site for stations located on the mid-mountain basin (Vara - Brugnato - about); it can be seen how the time lag between the moments of greatest intensity of rain and the passage of the respective flood peaks is quite reduced.4.4 Situation in Pontremoli (Magra River)From the side of Lunigiana, the height of water of the river Magra caused flooding in Pontremoli, Villafranca Lunigiana and the flooding of the entire city of Aulla; progressing downstream, erosion has caused the collapse of a section of the supporting wall of the Autostrada A15 on the right bank, the flooding of the hydrometric station of Calamazza and the collapse of the bridge Stadano Bonaparte.

Fig. 4.16 Rainfall rate [mm/h] in Pontremoli and evolution of the level height, in [m], of the River Magra in Santa Giustina, SP (image source ARPA Liguria). The picture shows rain and level observed on the same site for stations located on the mid-mountain basin (Magra - Pontremoli - about 150 km^2); it can be seen how the time lag between the moments of greatest intensity of rain and the passage of the respective flood peaks is quite reduced.

Fig. 4.17 - Evolution of the hydrometric height of the River Magra in Santa Giustina - Pontremoli, SP (image source ARPA Liguria)

4.5 Situation at the confluence between Magra and Vara RiverEven in the stretch downstream of the confluence Vara / Magra, all riverside areas were flooded and the flood wave caused the collapse of about two-thirds of the Colombiera Bridge, located about 1 km upstream of the river mouth.In the images below are compared the same hyetographs seen previously (for Pontremoli and Brugnato) with the hydrograph registered by the hydrometer of Fornola, downstream of the confluence Vara / Magra.We can note that the height of full here is passed a few hours (6-8) after the most intense phase of the precipitation upstream.

Fig. 4.18 Rainfall rate [mm/h] in Pontremoli and evolution of the height, in [m], in Fornola, SP after the confluence between the rivers Vara and Magra (image source ARPA Liguria).

Fig. 4.19 Rainfall rate [mm/h] in Brugnato and evolution of the height, in [m], in Fornola, SP after the confluence between the rivers Vara and Magra (image source ARPA Liguria).

Fig. 4.20 - Evolution of the height of the Magra River in Fornola, SP (ARPA Liguria): note the impressive and sudden flood wave which occurred between the afternoon and evening of 25 October 2011 (increase of 7 m in a few hours) due the exceptional rainfalls recorded in the basins of the tributaries rivers .4.6 Description of the event on the coastal rivers (Monterosso, Vernazza, Levanto and Bonassola)As mentioned, were also interested municipalities of the coastal strip between Bonassola and Vernazza.Some of the smaller rivers are not monitored by the hydrometric sensor of regional network; However, from information gathered in the area, has been rebuilt what happened.

At Bonassola the channel where the San Giorgio creek, underneath the roadway, went pressurized and the center of the village was inundated;At Levanto, there was the flooding of the river Ghiararo, which exceeded the level embankment in several places, flooding the roadway and the areas around the river, residential and agricultural (see fig. 4.12); At Monterosso and Vernazza (see fig. 4.10 and 4.21) the culverts have gone in pressure of the respective streams, in town centers (stream Buranco in Monterosso, Vernazza creek in Vernazza) and in the hamlet of Monterosso, Fegina (Fegina creek, Molinelli creek). In both villages, the intensity of rainfall has resulted in a very significant number of landslides that resulted in the contribution of massive volumes of solid material.

Fig 4.21 The cleaning of the culverts in Monterosso after the 25th October 2011 (image from Il Tirreno Newspaper)

4.7 ConsequencesAt the end of the event, in both countries, the deposition of sediments reached the first floors of homes in the valley. The muddy water flows in the area of Vernazza began to affect the town between 14.00 and 14.30 (about five hours after the exceptional rains started) getting worse rapidly with a maximum flow rates of hundreds of cubic meters/sec, transforming in rapid debris flows.These powerful flows have dragged dozens of vehicles, transporting them to the valley or to the floodplains (as happened in Vernazza). After reaching the end part of the floodplains, due to the decreasing inclination, the debris flows deposited lots of sediments and tree trunks, causing the clogging of the floodplains and of the various water channels. The debris flows and muddy-detrital, therefore, had to slide along the roads built over the covered floodplains (floodplains road), i.e. waterbed that flow underneath the roadway or culvert).

Fig. 4.22 Watercourse and main road in Vernazza: in the image on the left is possible to see one of the points where the watercourse is being channeled under the road; in the image on the right is possible to see the beginning of the short stretch where the river flows open again, immediately before of being channeled underneath the road again and to pass through the residential area Image from Google Earth and Google Street view elaboration Carmine Russo

Fig 4.23 - Image from Google Earth and Google Street view elaboration Carmine RussoDebris flows had channeled quickly, reaching the villages located at the valley of the basins that drain the slopes of Cinque Terre and those that flow in the valleys of the Magra and Vara causing widespread destruction of artifacts and the life losses of at least 9 people (until November 2, 2011). The range of the debris flows, tall trees trunks and big boulders was exceptional, reaching values of several hundred cubic meters/sec so they knocked out the existing hydraulic structures that were unable to dispose so powerful flows.Consequently they invaded urban areas wreaking destruction and victims. Based on the information available so far, it seems that most of the populated areas devastated by mudslides have been caught unprepared. This is surprising since, before the onset of rapid debris flows, there have been many minutes and hours of exceptional rain.It almost seems that the event, at the local level, it has not been perceived as an exceptional event that could cause destruction.

Fig. 4.24 Main road in Vernazza before and after the 25 October 2011 (source www.cinqueterre.it)

In the devastated area between Liguria and Tuscany there were many pluviometers functioning that were recording the rainy exceptional event: it seems that among those involved in the civil defense, no one had been able to understand the uniqueness of the phenomenon and that there was not a civil protection plan tested that could raise the alarm to the whole territory affected by the hydrological phenomenon.It is surprising that the area along the valley of the Vara and Magra was not alarmed, that areas would inevitably have been affected by the tumultuous and enormous amount of water that down-poured in the areas of the route of cumulus clouds.The hydrometric station of Fornola, located just downstream where the river Vara flows into the river Magra, recorded the progress of the water flow rates; the maximum water level of the Magra (+ 7m, never reached so far) was measured about 2 and a half hours after the climax of the event in the rainy basin of Vara (see Fig. 4.19)

Fig. 4.25 aerial view of Fornola the 26th October 2011 (image source Massimo Sestini Il Tirreno newspaper)Among the rainier area of Vara and the Fornola area, there are about 20 Km; there are about 30 km from the hydrometric station and the valley of Magra where there is the town of Pontremoli, Pontremoli was characterized by more than 300 mm of rainfall fallen in six hours when the event has reached the maximum intensity.Fig 4.26 - Pontremoli, bridge on the Magra river (image source Martina Saladino)Fig. 4.27 - Pontremoli , bridge on the Magra river during the evening of 25 October 2011 (image source www.ciaolunigiana.com)

It seems that many areas downstream of the most affected rains zone, have been reached by the wave of the flood, in the valley of the Magra and Vara, a few hours after the identification of the event, nevertheless institutions and population were unprepared. Among the rainier area of Vara and the Fornola area, there are about 20 Km; there are about 30 km from the hydrometric station and the valley of Magra where there is the town of Pontremoli, Pontremoli was characterized by more than 300 mm of rainfall in six hours when the event has reached the maximum intensity.

Fig. 4.28 - image source www.wikipedia.comIt seems that many areas downstream of the most affected rains zone, have been reached by the wave of flood, in the valley of the Magra and Vara, a few hours after, nevertheless taking unprepared institutions and population.

Fig 4.29 - Brugnato after (on the left) and during (on the right) the flash flood (image Massimo Sestini Il Tirreno newspaper)

A general problem that concerns the towns located in a narrow valley crossed by a river bed road, which has become the main street, is the presence of many vehicles parked above urban centers. Systematically vehicles are pulled by debris flows towards the town and partly layed on the beach. In Vernazza, some amateur videos show that numerous vehicles (including vans and pickup trucks), dragged from the water flow that partly had invaded the streets, were swallowed by the covered river bed at around 14.00; it is assumed that they have contributed to clogging the culvert of riverbed and then to flooding along the road above.

Fig. 4.30 A road of Fornola the 26th October 2011 (image source Massimo Sestini Il Tirreno newspaper)The flows of water, mud and debris, channeled along the town's roads with significant flow rates (measured between 120 and 240 cm/sec in Monterosso) and a height reached of 2-3 meters above the road surface, sowed destructions and damages to all the premises located at the ground floor by invading and accumulating debris and mud; that caused extensive damage to economic activities and to public and private properties (shops, post offices, banks, churches etc.). Even worse, some people died, dragged by rushing streams.

Fig. 4.31 - On the right the village of Monterosso in 1954-55 when the riverbed was still largely uncovered (IGM image, from Army Geographic Institute). At the center: the riverbed turned into channel (called Via Roma) which has been devastated by the flow-muddy debris that has re-appropriated the area of its competence (left images taken from internet: Secolo XIX newspaper and YOU reporter).

Fig. 4.32 Vernazza town center: height reached by the mud (Image from www.cinqueterre.it)

Fig. 4.33 destruction in Brugnato close to the Vara river (Image from Il Tirreno newspaper)

4.8 Economic impactIn total, 26 municipalities have been damaged by the flood in the area of La Spezia on 25 October 2011.The total damage amounted to 730 million euro (source government of Regione Liguria), of which 200 million urgent to restore roads, sewers and remove the debris. The total damage from the territory also includes those individuals and productive activities.

Fig. 4.34 the Colombiera Bridge on the Magra River, collapsed after the events of the 25th October 2011Until now, the Liguria Region has received 90 million euro from the Government as part of the funds allocated for the flood of Liguria and Tuscany. A sum which is not enough even to cover sums emergencies. Economic activities would need more than 200 million euro to restore the damage, but currently available only 30 million. The Region of Liguria has also distributed some 350,000 Euros to the families who have had a bereavement. This is not a compensation for the missing persons but of immediate help for the relatives. 5 Conclusions and lessons learnedImproving the environmental defense with the Immediate Hydrogeological Alarm. The current civil protection system does not work properly; it is not able to face the hydrogeological catastrophic events caused by cumulus clouds and it does not guarantee the security of citizens. The storm was promptly identified and that the area that would be affected was outlined. Nevertheless, it looks like the storm alarm diffusion put the institutions safe from lawsuits (their bureaucratic task finished with the alarm spread).

Fig. 5.1 Before (on the left ) and after (on the right) the mud invaded Vernazza

Fig. 5.2 - Before (on the left ) and after (on the right) the mud invaded Vernazza

Fig. 5.3 - Before (on the left ) and after (on the right) the mud invaded Vernazza

It seems that local governments (municipalities, provinces, regions) have assumed the alarm as one of many, and they have underestimated. Maybe they were not organized to predict what might happen in small basins with steep slopes (like those of the Cinque Terre) and along the main river courses and perhaps were not developed adequate civil protection plans.It 'clear that, in such cases, there must be a very close coordination between the institutional actors in charge of the defense of the territory and citizens, and a control room bringing together the weather and environmental data in real time and is able to activate an early warning system that must be given when a critical situation has been identified and the area that will be affected.There must also be an organization that can raise a hydrogeological alarm immediate, which must be activated in urban areas and in the territory covered by infrastructures of strategic importance (see the case of the collapse of Colombiera bridge), after a few minutes the various pluviometers distributed on the territory have started to record a typical of exceptional rain cumulonimbus.Moreover, urban areas located in the valleys (e.g. Monterosso, Vernazza) should have a civil protection plan that allows the evacuation of riverbeds-road and the safety of citizens, in a few minutes: after a few minutes that the pluviometers showed that the basin is affected by rains very intense (in the case of similar situations), the alarm should take along the riverbed road and side streets that can be invaded by streams of water, mud and debris , which may occur after a period ranging from about 15 minutes (if in the basins, there are parts of the vegetation destroyed by fire) or after several minutes how happened in Vernazza (about 5 hours).

From the point of view of mitigation, a problem of strategic importance is the need to retain the huge volume of debris that is carried downstream along the river beds causing, often, the total clogging of river beds themselves and of the streets in the living area. Along streams, upstream of, must be built adequate weirs, each capable of holding several thousand cubic meters of debris and tree trunks.

Fig. 5.4 Effects of the debris flow and mud flow in Vernazza (image source Il Tirreno newspaper)

Fig. 5.5 Effects of the debris flow of the Magra River on the coast on La Spezia the 26th October 2011 (image source Il Tirreno newspaper)Some of the interventions listed, have recently been approved and are being implemented in the control plan of the river Vara, which should be completed in about three years and providing for the raising of the banks between Brugnato and Piana Battolla (section affected by many torrents affluent), the construction of escape routes for water (to save the downstream countries), and an emergency plan.Unfortunately, given the configuration of the territory, the risk can never be reduced completely, and inhabitants must learn to live with those events.References:

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