Meteo-hydro risk. The phenomenon

Fenomeno Rischio Meteo-Idro

In Italy the meteo-hydrogeological and hydraulic risk is widespread and presents itself in different ways depending on the geomorphological structure of the territory.

The principal meteo-hydrogeological and hydraulic phenomena are listed below.

 

Floods are among the most typical manifestations of hydrogeological instability, and they occur when the waters of a river are not contained by the banks and flow into the surrounding area, damaging buildings, industrial settlements, roads, and agricultural areas. 

The most significant floods that have affected Italy and resulted in a heavy toll both in terms of human loss and damage were the floods of the Po River in Polesine (1951), the Arno River (1966), and the Po River in northern Italy (1994 and 2000). However, floods occurring in small basins are common in Italy due to heavy and localized rainfalls that are difficult to predict. Such basins, primarily present in Liguria and Calabria, are characterized by a few hours of flood development that result in hazardous floods that often cause casualties and damage to the environment and can severely impair the economic development of the affected areas. 

Floods are natural phenomena; however, among the causes of the increased frequency of floods are undoubtedly high levels of human activities and widespread soil sealing, which, by preventing the infiltration of rain into the ground, increases the quantities and rates of water flowing to rivers. In addition, other causes are the insufficient cleaning of rivers and the presence of debris or vegetation that undermines the ordinary flow of water. 

It is possible to reduce the risks of adverse consequences from floods through structural interventions, such as banks, retention reservoirs, spillway channels, and water drains, and non-structural interventions, such as land management or emergency management. In the latter case, the preparation of the warning system, the drafting of emergency plans, and the implementation of an efficient coordination system of the activities planned in these plans are essential. 

An efficient warning system based on forecasting models linked to a monitoring network is crucial for alerting institutional bodies in the area as early as possible and reducing people's exposure to events and damage to the territory through the implementation of real-time prevention measures. These include the activities of territorial hydraulic protection and the regulation of outflows from reservoirs to laminate the flood.

Cyclones can form in the Mediterranean with similar characteristics to tropical systems called TLC - Tropical Like Cyclone or Medicane - Mediterranean Hurricane. These are low-pressure systems that have a dynamic evolution similar to that of tropical cyclones, although they are generally less extensive and shorter in duration. These phenomena can occur with a frequency of about one event per year.  

These systems are part of the cyclones that occur at mid-latitudes, including European and Mediterranean latitudes and are called extratropical cyclones. Tropical systems have a completely different genesis and dynamics from extratropical ones. In tropical systems, the energy comes from the condensation of water vapor sucked by the system above the sea surface. In contrast, in extratropical ones, the energy comes from the temperature difference between the warm (typically subtropical) and cold (typically subpolar, Arctic) air mass from whose collision the system originates. Questi sistemi fanno parte dei cicloni che si verificano invece alle medie latitudini, comprese quelle europee e mediterranee, e che per questo sono chiamati extratropicali (extratropical cyclones).  

This means that the tropical-like cyclones generated in the Mediterranean have a mixed genesis, meaning that to form, they still need the contribution of a disturbed situation already present on the weather scene. This happens because the surface temperature and the extent of the Mediterranean are considerably lower than those of a tropical ocean. The autumn period results statistically among the likeliest for such phenomena, although they can still occur in the winter period.   

The tropical-like cyclone appears as a spiral of clouds with a well-delineated eye. It is marked by persistent, heavy rainfall with predominantly thunderstorms, showers, and severe winds, which can exceed 50 to 80 knots (100 to 150 km/h), with intensity up to storm or hurricane that can generate stormy sea conditions. On the coast, the winds produce storm surges of particular severity, generally resulting in a sudden rise in sea level. As a result of the impact on land, the cyclone tends to lose strength. 

A meteorological-climatic situation characterized by a generalized reduction in rainfall has been created over the last few decades. Prolonged periods of low rainfall have, in particular, been registered over the previous few years, causing water emergencies in the more significant part of the domestic territory, aggravating situations already in a state of emergency. 

Among the factors contributing to the occurrence of water crises is the inadequacy of the water supply network, which in Italy has a loss of water intake of 27 %, with peaks as high as 40 %. 

Emergencies. The worst water emergencies in Italy in recent times were recorded in the summer of 2002 (particularly in the central-southern regions) and in the summers of 2003 and 2006 (particularly in the northern regions). Under these circumstances, the lack of water created significant limitations in the civil, farming, and industrial sectors. The Civil Protection Department took action, in agreement with the competent Ministries and Regions involved, upon the declaration of a state of emergency by the cabinet and through ordinances granting the Regional presidents nominated Extraordinary Commissioners with the powers and means required to deal with the emergency in the water supply sector and integrated waterworks service. 

During the summer of 2003 water crisis involving the entire Po Basin, with the idea of preventing further emergencies and using the ordinary instruments put at disposal by the legislation in force, the Civil Protection Department acted as the promoter to an agreement stipulated with the Basin Authority, the Valle D’Aosta, Piedmont, Lombardy, Veneto, Emilia-Romagna Regions, the Inter-regional River Po Agency (AIPO), the National Grid Distribution Operator (GRTN), Consortia regulating lakes, the National Reclamation, Irrigation and Improved Land Association (ANBI) and power producing companies located in the basin. 

Prevention. To avoid the worsening of the water crisis, a series of measures, including individual measures, should be adopted to preserve and manage the national water heritage appropriately. These measures involve cautious, rational management of the aquifers, reduced consumption, water pipe repairs or upgrades, and dual supply and distribution networks, using pure water for potable purposes and purified water for compatible usage. 

In recent decades, due to indiscriminate gravel and sand extractions along the riverbeds of many Italian rivers, the contribution of fluvial solid transport delivered to the beaches has decreased. For this reason, in many coastlines the shoreline is noticeably retracted and increasingly close to road infrastructure, buildings, industrial settlements, threatening their safety and sometimes forcing the population to evacuate the area. 

The problem is aggravated by storm surges that with variable frequency hit the coasts and change, even substantially, the morphology of the coastline. 

To counter the phenomenon of coastal erosion, numerous defense works have been implemented, including transverse to the shore (groynes), longitudinal (breakwaters), and grazing (bank walls, bulkheads, etc.). Where the retreat has been substantial enough to erode a large part of the beach, more radical interventions have been implemented, such as artificial nourishment, which consists of feeding a beach with landfill extracted from borrow pits.

Fog, in larger or smaller and more or less compact banks, forms when the air in the lower layers of the atmosphere is particularly stagnant and humidity condenses into tiny water droplets.

These particular weather conditions appear especially in autumn and winter in low or depressed areas (plains, valleys, basins), and they are naturally promoted close to humidity-rich regions, such as those in the proximity of streams or dense green areas. 

The times most at risk for fog formation are typically the coldest, i.e., the night and early morning hours; during the day, the sun can guarantee the gradual lifting or at least partial thinning of fog in most cases, but in some weather conditions, the phenomenon persists for most of the day.

Fog absorbs and disperses light, decreases contrast and color difference, and reduces objects' visibility. Ultimately, it dramatically reduces horizontal visibility and thus constitutes a grave danger to traffic.

In fact, hundreds of people die each year as a result of imprudent driving with fog, often in rear-end collisions, but also in the running off the road, impacts with trees, poles, bridge abutments, or in head-on collisions, due to failure or delayed perception of curves, fixed obstacles, or other vehicles. 

A landslide is described as the "movement of a mass of rock, soil, or debris along a slope." 

The causes of these destabilization processes are complex and often interconnected. Besides the amount of water or snow, deforestation, and fires are significant causes of landslides: on wooded slopes, tree roots consolidate the soil and absorb excessive water. The country's alpine and Apennine territories and the coastal territories are generally at risk of landslides, given the nature of the rocks and the slope, which give the slope a certain instability. In addition, climatic characteristics and the annual rainfall distribution contribute to increasing the area's vulnerability. 

Also, human action on the territory can cause landslides. Intense land transformation performed by human activities, often without criteria and respect for the environment (construction of buildings or roads at the foot of a slope or mid-slope, ski slopes, etc.), can cause land subsidence. 

Landslides present different dangerous conditions based on the mass and velocity of the landslide body: in fact, landslides with low danger are characterized by a low mass and constant, low velocity over long periods; on the other hand, other landslides present a greater risk since they suddenly increase in velocity and are characterized by a substantial mass. 

For prevention purposes, it is not easy to define precursors and thresholds, which are understood as the amount of rainfall capable of triggering the landslide movement and as ground displacements/deformations, past which the collapse of unstable masses could occur. 

Effective landslide defense can be achieved through nonstructural interventions, such as protection standards on risk areas, monitoring systems and emergency plans, and structural interventions, such as retaining walls, anchors, micropiles, concrete injections, rockfall nets, sprayed concrete layers, etc. 

When temperatures in the lower layers of the atmosphere get close to zero, rainfall assumes the form of snow, and depending on the intensity and persistence of the phenomenon, it can accumulate substantially on the ground, impeding traffic flow. The phenomenon can also affect vast areas, involving people and activities.

In addition, following snowfall, in some situations, temperatures fall well below zero, resulting in dangerous ice sheets on roads and sidewalks, posing an even greater risk than the snowpack for the stability and grip of vehicles and the balance of people. 

Heatwaves are extreme weather conditions during the summer, characterized by high temperatures above usual values that can last for days or weeks. 

The World Meteorological Organization (WMO) has not made any standard definition of a heatwave, and in several countries, the definition is based on the exceeding of threshold temperature values defined by identifying the highest values observed in the historical series of data recorded in a specific area. 

A heatwave is determined by the climatic conditions of a specific area. Therefore, defining a temperature-threshold risk valid for all latitudes is impossible. 

In addition to temperature and relative humidity values, heatwaves are defined by their length. Indeed, it has been proven that prolonged periods of extreme weather conditions have a more significant health impact than isolated days with the same weather conditions. 

Heatwave bulletins. For 2012, the Ministry of Health is coordinating the National forecasting and warning system to prevent the effects of heatwaves on the population. Heatwave bulletins for the 27 cities and information on how to protect yourself from the health effects of heat are available in the heatwave section of the Ministry of Health website. 

Active from May 15 to Sept. 15, the system is deployed in 27 Italian cities (Ancona, Bari, Bologna, Bolzano, Brescia, Cagliari, Campobasso, Catania, Civitavecchia, Firenze, Frosinone, Genova, Latina, Messina, Milano, Napoli, Palermo, Perugia, Pescara, Reggio Calabria, Rieti, Roma, Torino, Trieste, Venezia, Verona, Viterbo). From 2004, the activation date, to 2011, the Civil Protection Department coordinated the system. 

The rainfall associated with a thunderstorm is characterized by rapid and considerable variations in intensity in space and time. Concentrating substantial amounts of water in a short time over relatively small areas can result in heavy showers that occur extremely irregularly and discontinuously over the territory. 

Risks connected with rain showers. The typically impulsive nature makes rain showers a significant risk, first and foremost in terms of the immediate and sudden repercussions they can have on the territory, undermining the stability of slopes, triggering surface landslides, mudflows, and landslips that can even involve the roadway, and rapidly swelling streams and minor watercourses, which, especially in the summer season, can go from a dry state to a flooded state in a very short time, without prior notice. The bed of a stream in a lean state (or even in a dry state, looking like a barren expanse of rocks) can suddenly turn into a rushing stream of water, capable of dragging things and people with it, as a result of a storm that perhaps developed in the upstream area, without necessarily involving the area where the river is located and thus making the event even more unexpected. 

Hail showers. Under special conditions, when the temperature difference between the ground and the upper layers of the atmosphere is very high, thunderstorm clouds result in hail showers, i.e., the downfall of ice grains, which in some cases can reach considerable size, enough to damage the sheet metal of a car and endanger the safety of people. 

This phenomenon, which can involve territories of variable extent, is generally caused by geological causes. Still, in recent decades, it has been locally worsened by human action and reached more significant dimensions than those of natural origin. 

Subsidence. Produced or aggravated by human actions, they can be caused by groundwater withdrawal, gas or oil extraction, loading of significant artifacts, solids extraction, etc. In this case, the total values can be up to a few meters. Natural subsidence is caused by several factors: tectonic movements, cooling of magmas within the earth's crust, compaction of sediments, etc.; natural-type vertical movements can reach values of a few millimeters per year. 

Sinkholes. Sinkholes are a problem that only resembles subsidence to a certain extent, but contrary to the former, it has essential repercussions on civil protection. These phenomena are caused by natural cavities in the subsurface and cavities created by man since ancient times (underground quarries, places used for various purposes, deposits, waterworks, sewage pipes, drains, etc.). 

In Italy. Progressive subsidence phenomena have occurred along the Adriatic coastline from Rimini to Venice in our country, especially around the Po estuary, but also around urban agglomerates such as Milan, Bologna, and Modena: above all in the latter areas due to extracting water from the subsurface. More recent cases have been reported in Puglia, Piana di Sibari, and Pianura Pontina. Moreover, there are frequent instability problems caused by underground cavities, often causing considerable material damage and, in many cases, even the loss of human lives in Italy. 

Prevention. Underground cavity-related risk is prevalent in large built-up areas where the action of human beings has resulted in creating holes in the subsurface, mostly long lost and forgotten, above all due to the uncontrolled growth of built-up areas over the last few decades. Preventive-related measures to be implemented consist of the correct management of water resources, avoiding excessive withdrawal from the aquifers, and disciplined planning of the extractive activities. 

Thunderstorm refers to a set of phenomena that develop, typically concurrently, in massive, swollen-looking, vertically developing storm clouds called cumulonimbus clouds. These phenomena occur in relatively small areas, with rapid and sudden development and almost always significant intensity, often with violence. These features, along with the high degree of unpredictability of this type of phenomena and the lack of possibility of determining their location and timing of evolution, make thunderstorms a danger involving multiple risks, some of extreme importance. 

Thunderstorm-related dangers. These can be attributed to the three types of weather phenomena associated with thunderstorm clouds: 

•  Lightning strikes, which are sudden electrical discharges from the cloud reaching the ground, accompanied by the luminous manifestation of lightning and followed in our perception by the rumble of thunder; 
•  Gusts, i.e., brief intensifications of wind speed on the ground that occur impulsively and suddenly;
•  Showers, i.e., intense rainfall generally of short duration, are characterized by an often sudden beginning and end and rapid and considerable intensity changes. Depending on thermodynamic conditions, the showers may be rain, hail, or snow. 

In particular meteorological and environmental situations, the thunderstorm is the site of the formation of a tornado. This phenomenon is as short-lived and localized as it is intense and destructive. It is recognizable by the funnel-shaped cloud descending from the cumulonimbus to the ground and can activate extreme instant wind intensities. When a similar vortex is triggered on the sea surface, we speak of a waterspout, a shorter-duration phenomenon that can reach the coastline with equally dangerous effects. 

Lightning is the most fearsome danger associated with thunderstorms. Most lightning accidents occur outdoors: mountains are the most at risk, and so are all exposed places, particularly in the presence of water, such as beaches, piers, docks, and outdoor swimming pools. There is some risk associated with lightning, even indoors. A thundercloud can result in lightning even without necessarily bringing precipitation. 

Avalanches are a critical event due to a sudden loss of stability in the snow on a slope and subsequent sliding of a portion of the snowpack involved by the fracture, valley-wards. 

Detachment may either be spontaneous or induced. In the first case, the avalanche may be caused by factors such as the weight of fresh snow or a rise in temperatures. 

Induced detachment instead may be of two types: accidental, as happens with people walking or skiing on a slope covered with fresh snow who unintentionally cause an avalanche by their weight, or else programmed, occurring in skiing areas when dangerous slopes are cleared with explosives. The danger of avalanches is closely linked with tourists in the mountains and, therefore, with greater exposure of people, buildings, and facilities at risk. Classifying avalanches is not accessible due to the numerous variables coming into play: type of detachment, type of snow, the position of the slip plane, etc. 

Types of avalanches. We have a surface layer avalanche when a fracture occurs in the snowpack, while we talk about deep-seated avalanches when this happens at terrain level. Avalanches can also be skimming, i.e., in contact with the surface or forming clouds, usually of dry snow. 

Causes. Avalanches can either be spontaneous or induced. There are different causes, but in any case, it is referable to the low cohesion force of the snow mass, causing relevant detachment. A superficial layer of snow lasting for a long time, warmer spring temperatures, and the action of relatively heavy rainfalls all contribute towards detachment. 
Predicting an avalanche is no easy task, as often there are no forewarning signs before it starts to slide down. However, areas prone to avalanches are known with particular precision and hazardous situations are reported through the so-called “avalanche bulletins.” 

Prevention. The measures to implement in the event of risk of an avalanche mainly consist of knowing which areas are involved by these phenomena: they are almost always in the same places: high mountain areas with steep bare rock faces, between 2,000 and 3,000 meters, moreover not covered by vegetation.
It is crucial to avoid these areas where these detachments are predicted, which are frequent in early spring when an increase in temperatures could suffice to cause the mass of snow to start melting suddenly. 

In particular meteorological situations, in atmospheric layers close to the ground, intense currents are activated, which may persist more or less for a long time, sometimes for as long as 24 or 48 hours, over vast areas of the national territory, giving rise to strong winds on land and the simultaneous increase of wave motion over the seas. 

In addition, when a particular area is affected by thunderclouds, intense vertical currents are activated within them, both in an ascending and descending direction; when these currents reach the ground, they spread out horizontally, following the conformation of the ground, giving rise to sudden shifts in the surrounding air mass, activating intense wind gusts. This is why, during thunderstorms, the wind blows irregularly and discontinuously, in gusts, intensifying suddenly and striking generally with intermittent and short-lasting stretches. 

In the case of strong winds, additional sudden and impulsive gusts that are generally irregular and discontinuous may occur for intermittent stretches of shorter or longer duration. Indirect effects typically represent the most severe dangers with intense winds if one is struck by objects suddenly hurled down by gusts (branches, roof tiles, vases, light poles, road signs, billboards, scaffolding, etc. ), which, depending on the intensity, can move larger or smaller and heavier objects, even to the point of knocking down entire trees or uncovering entire roofs in the most severe cases.