A seconda delle cause che le determinano, le alluvioni si possono generalmente suddividere in tre gruppi:
- Primo gruppo – alluvioni causate da piogge intense o prolungate o dalla rapida fusione di neve, ghiaccio/ghiacciai;
- Secondo gruppo – alluvioni causate da forti venti. Si possono verificare lungo la costa ed in corrispondenza dello sbocco dei fiumi in mare;
- Terzo gruppo – alluvioni causate da terremoti sottomarini e, meno frequentemente, da eruzioni di vulcani sottomarini od insulari. I terremoti e le eruzioni sottomarine possono causare onde giganti – maremoti, che solitamente producono alluvioni nella fascia costiera .
Si possono ulteriormente distinguere attraverso le seguenti tipologie:
- Alluvioni fluviali
Avvengono in prossimità degli alvei fluviali. Sono le più frequenti e ne esistono di diverse tipologie. (vai a 2.1) - Alluvioni legate al drenaggio locale od all’innalzamento dei livelli di falda
In questa tipologia rientrano le alluvioni causate dall’inofficiosità del drenaggio locale e quelle causate dall’innalzamento dei livelli di falda che si sviluppano in aree urbane (Fig.2). Questa tipologia di alluvioni è principalmente causata dalla cattiva manutenzione, dalla mancanza di controllo o dalla pessima condizione dei sistemi idraulici in aree urbane. Queste alluvioni determinano il peggioramento delle normali condizioni di vita ed è causa di perdite economiche. Oltre che in corrispondenza delle piane alluvionali e dei principali assi di drenaggio, precipitazioni locali molto intense possono generare alluvioni anche in aree diverse. Se le condizioni locali non sono sufficienti a sostenere le intense precipitazioni, grazie all’infiltrazione ed al deflusso superficiale, le acque possono accumularsi e determinare un’alluvione. Anche l’innalzamento dei livelli di falda può essere fonte di preoccupazione e può produrre effetti negativi anche in assenza di eventi alluvionali. I piani interrati delle case sono particolarmente suscettibili agli alti livelli della falda. - Alluvioni legate all’oscillazione dei livelli dei laghi
Questa tipologia di alluvioni interessa, solitamente, ampie aree pianeggianti in prossimità delle coste lacustri ed ha una persistenza prolungata nel tempo. I cambiamenti climatici influiscono sulla frequenza e sull’estensione di questa tipologia di alluvioni, poiché il ciclo idrologico viene interrotto. Il livello dei laghi può avere fluttuazioni di breve termine, stagionali, o di lungo termine, di mesi od anni. Precipitazioni intense, stagionali, possono determinare l’innalzamento del livello dei laghi in brevissimo tempo, mentre la fusione del manto nevoso può incrementare il deflusso dalle sorgenti. Le fluttuazioni di lungo termine sono un fenomeno meno conosciuto, ma che può determinare alti livelli d’acqua e relativi problemi di alluvionamento che possono permanere per anni o addirittura per decenni. - Alluvioni (Inondazioni) costiere
Le alluvioni costiere, in combinazione con altre tipologie di alluvioni (in particolare quelle fluviali), possono produrre enormi perdite. (vai a 2.2)
Il termine ‘’alluvione devastante” viene usato per indicare tutte le tipologie sopra indicate, in grado di produrre enormi perdite in vite umane ed economiche.
Riverine flooding includes:
– Overflow from river channel or river floods
This kind of floods are with highest frequency. They are caused by bank overtopping when the flow capacity of rivers is exceeded locally. (Fig. 2.1 a)
The activity of the local authorities for the prevention of flood risk is most important to reduce the consequences.
The general factors affecting the river floods are the intensity and the duration of the rainfall, the possibility of rivers and steams to let pass the rising waters, the condition of the land surface – soil and vegetation, topography, etc. The river floods occur at the higher scale of the rainfall system, where the permeability capabilities of the multitude of smaller tributaries are gradually filled up and the water peak is usually gradually collected and transported to the larger river.
The lengthy rivers flow through flat terrain and the water movement speed is low. The same applies in the occurrence of a flood; the water peak is reached very slowly and afterwards it fades down very slowly over the course of a few days to a few weeks. Usually the reason for the flood is intensive and prolonged rainfalls occurring in other regions, often in other countries within the fluvial catchment area. The situation becomes even more complex when the heavy rainfall continues along the river. These floods are predictable as they are usually of seasonal character and this allows for the proper organization of actions aiming to decrease the consequences.
– Flash floods
There are floods caused by heavy rainfall taking place within a short timeline.
During the last decade, the frequency of these kinds of floods has increased with climate change.
Such floods are harder to predict and they can occur in dry places where there is no running river. Such floods can prove to be very dangerous since they start suddenly and develop very rapidly.
In the case of flash floods, the reaction is mostly hindered by the very short time between their origination/continuous heavy rainfall/ and the reaching of the water peak in a short time.
The flash flood is characterized by the rapid increase in the water level, the high speed and large quantity of debris in water, giving the flood a high destructive power. The main factors causing the origination of a flash flood are the intensive and continuous rainfall and the steep water sheds.
In certain regions the flash floods often result from isolated high-speed local rainfalls, rendering the warning and protection of the population a very difficult task. In other regions such floods occur annually in one and the same river, where warning the population is possible, but advance organization is needed, since the reaction time is quite short. Flash floods are capable of tearing out trees, undermining buildings and bridges and scouping new chanels.
During the month of April 2020, Yemen has experienced heavy rains which caused flash floods in different areas of the country. At least 150,000 people were affected and according to the health authorities seven people died. More recently, on 5th august 2020, at least 17 people, including 8 children, have been killed in flash floods in Yemen’s northern region.
On 9th august 2020, heavy rains and thunderstorms have caused flash flooding on the on Greek Island of Evia, killing at least seven people including a baby (See Fig. 2.1 b). The news was posted on 10th august 2020 also in the https://www.facebook.com/BeSafeNet.
– Alluvial fan floods
This type of flooding is typical for mountain and volcanic areas and rare for other regions.
Alluvial fan floods which adversely affect people lives usually occur when the threatened areas are at the passage between the mountain and flat areas or mountain and seacoast. When the requirements for the building works are not respected, there is a lack of control and the prevention plans are a formality. The consequences are always very destructive and with many victims e.g., in Venezuela in 1999, 19 000 people died.
The common torrents pass through the main riverbed and most of them have a stone-muddy structure. They are particularly dangerous since they often pass through urbanized areas (Fig.2.1 c). Their most dangerous characteristics are the suddenness, the straightforwardness of movement and the transportation of large quantities of hard materials, which pile up on, or destroy along their way settlements, roads, railway tracks, engineering equipment and agricultural lands.
The destructive forces of the torrent are defined by the increased capability to transport hard materials, the interruptibility of the movement, the high velocity of up to 15km per hour.
The human activity, related to the intensive use of mountain slopes leads to the increase in the volume and frequency of this type of flood, as well as to the increase in the number of locations where they can occur.
Alluvial fan floods. Their name/fan/ corresponds to the shape of the spreading mass. In substance they are the same as the above described but the solid mass prevails and they are more similar to the landslide.
Alluvial fan floods can cause greater damage than typical riverine flooding.
– Ice-jam floods
Floods caused by fast melting of ice/glacier. This kind of flooding in most cases can be predicted and this allows for organizing and undertaking measures for lowering of the consequences.
Serious damage is caused by river floods owing to the combination between rising waters and ice movement. The ice movement is usually accompanied with jamming/filling up of the riverbed with ice or by the accumulation of intra-water ice, forming an icy barrier, which in turn causes an additional increase in the water level and the flooding of new territory upstream. Furthermore, in case of breach of the ice barrier a powerful wave is created that could suddenly flood the territories located downstream.
The jamming of the river by ice occurs most often in rivers flowing from south to north such as the Oder, Wisla, Ob, Yenisei, Lena, Yukon and others.
This is explained by the fact that the southern parts of the river free themselves earlier than the northern sections, in the course of which the moving water, mixed with ice, meets an obstacle of fully frozen sections. Ice barriers can also be created at the beginning of the winter prior to the complete freeze, when there are still sections of the river that are not frozen yet. Sometimes this type of flood can occur in lengthy rivers flowing from west to east e.g., the Danube, and Amur.
The origination of such floods depends on the climate and the physical condition of the river valleys. The freeze is lesser where the waterside is higher and steeper. The freeze is usually more severe in dams, in engineering equipment, at the turns and bridges and at places where the river has low depth over a long distance.
For such type of floods, the estimated reaction time is rather short, requiring an obligatory and serious advance preparation. Since the locations of the ice jams will be known in advance thanks to long years of experience, precaution measures can be taken long time before the start of the ice movement in the river.
The elimination of the ice barriers is done through detonations or by using an ice-breaker, if possible.
– Dam-break floods
They happen very rarely but cause a high number of casualties. The permanent control of the hydro technical installations and organization of the announcement and protection of the threatened population are of great importance.
It is worth noting that the big flooding that occurred during the last two decades in most cases are the result of a combination of different kinds of floods, usually among the river flood and other kinds of flood as: provoked by a string wind, intensive and long lasting rainfalls, Monsoon, sea inflows, increase of the level of the underground waters or destruction of the functions of the drainage channels and other.
The breaking of a dam wall, especially of large dams, is rather rare and can hardly be foreseen. The overflow of water from medium- and small-seized dams is an event occurring more often as well as the breaking of the of land type walls of the small dams.
The floods caused by break of walls of large dams can be very dangerous since they can affect the life of a lot of people.
The breaking of a dam wall can occur in the case of a breach in the construction as a result of progressive erosion in the wall, destruction of the embankments /dikes. A construction breach can also occur in the event of a particularly severe flood. The dam walls are designed to withstand a powerful earthquake, but even so, an earthquake can extremely weaken the wall.
The floods caused by a dam wall break, due to their unexpectedness and high water speed, can inflict casualties and destruction comparable only to a natural disaster. This is the reason why the large dam walls are placed under constant control and surveillance with special equipment installed inside the wall, monitoring for any deviation in the facility’s parameters. Furthermore, the dam walls have inbuilt automated systems for warning the population about danger.
These are cases of floods at the riverbed after the dam wall which is not broken. Such a flood happened in Italy in 1963 when a huge landslide into Vaiont Dam pushed out the dam water which spilled over the dam wall without breaking it but causing catastrophic flooding in which 3000 people died.
The local authorities obligatorily have to include all dams in the protection plans.
In most cases, the smaller dams used for watering and other activities cause major problems. Usually their walls are of land type and require permanent maintenance and control of installations. When such walls are poorly maintained, they become very dangerous during intensive rainfalls or intensive snow melt. The local authorities have to keep records of such dams, to control permanently and to make organization for risk prevention.
For more information please click
http://www.besafenet.net/en-gb/technological-hazards-dam-failures#faq199
Of all river floods this is the one that is the most frequent.
The general factors affecting the river floods are: the intensity and the duration of the rainfall, the possibility of rivers and steams to let pass the rising waters, the condition of the land surface – soil and vegetation, topography and others. The river floods occur at the higher scale of rainfall system, where the permeability capabilities of the multitude of smaller tributaries are gradually filled up and the water peak is usually gradually collected and transported to the larger river.
These floods can be divided into two groups, depending on the form of the water-collection area and the length of the river:
- The first group includes floods occurring in lengthy rivers /Nile, Danube, Euphrates, Mississippi, Rhine, and others.
- The second group includes floods occurring in rivers with round shape of the water-collection basin /Congo/.
The lengthy rivers flow through flat terrain and the water movement speed is low. The same applies for the occurrence of a flood, the water peak is reached very slowly and afterwards it fades down very slowly over the course of few days to few weeks. Usually the reason for the flood is intensive and prolonged rainfalls occurring in other regions, often in other countries. The situation becomes even more complex when the heavy rainfall continues along the river. These floods can be predicted, they are usually of seasonal character and this allows establishing a proper organization to decrease the consequences.
In the case of rivers with round water-collection basin, after heavy rainfall the water peak reaches its highest value quickly and quickly decreases. Such floods usually happen in small and average sized rivers. Usually they are much more destructive because the water moves with higher speed.
The above examples refer to smaller rivers with similar characteristics.
Flash floods are often associated with isolated and localised intense rainfall. But Flash floods may result from the failure of a dam or the sudden breack-up of ice jam. Flash floods in urban areas is serios problem too.
The change in the climate has increased the floods of this type. Such floods are harder to predict, they can occur in dry places where there is no running river. Such floods can prove very dangerous since they start suddenly and develop very fast.
In the case of flash floods the reaction is mostly hindered by the very short time between their origination /continuous heavy rainfall/ and the reaching of the water peak in short time.
The flash flood is characterized by the fast increase in the water level, the high speed and large quantity of debris in water, giving the flood a high destructive power. The main factors causing the origination of a flash flood are: the intensive and continuous rainfall and the steep water sheds.
In certain regions the flash floods often result from isolated high-speed local rainfalls, rendering the warning and protection of the population a very difficult task. In other regions such floods occur annually in one and the same river, where warning the population is possible, but advance organization is needed, since the reaction time is quite short. Flash floods are capable of tearing out trees, undermining buildings and bridges and scouping new chanels.
Mountain torrents occur at intensive rains for long and short time in the mountain areas. This happens because the river bed slop is big, the water runs fast and carry with itself dispersive mass.
The improvement of the forecast systems already gives opportunity for announcement and information of the population at the threatened zones in most cases.
The above mentioned floods appear mostly at the rivers or dry river valleys they are included in the group of the river flooding.
Alluvial fan floods is a temporary spring formed in the beds of the mountain rivers, characterized in the sharp increase in the water level, which has a high content /10 to 75%/ hard particles /soil products and rock decomposition products /.
This kind of flood originate as a result of intensive and continuous heavy rainfall, swift melting of the seasonal winter cover or of glaciers, as well as due to the landslide of loose, cracked rock material /at terrain inclination of no less than 8 – 10%/.
Depending on the content of the torrent mass we can specify the following torrents: stone-muddy, water-stone and water-gravel. Besides that as a physical phenomenon they can be free or latent. The transporting medium of the free torrents is water with hard admixtures /Mud-steam/, and for the latent ones the medium is the soil mixture where the main water mass is connected with finely dispersed particles (Mud-flow, Debris flow) Mud-rock flow and Alluvial fan floods – the contents of the mass is mostly soil and rocks/stone –mud/.
In contrast to the regular flows, flows of the mass by default do not move permanently, but in separate waves /pushes/. The volume of the one-time mixture, transported with high velocity, may vary from hundreds of thousands to millions of cubic liters. The front of the moving mixture can have a diameter of 3-4 meters, and mass of 100 – 200 tons. With their large mass and movement speed this mass destroy roads, equipment, agricultural land and others.
The causes for the origination of this kind of flood are mainly the heavy rainfall, the accumulation of masses of crumbly and cracked material, the increase of the intensity of the surface water outflow.
The practice has demonstrated that the centre of this kind of flood/accumulated soil and rock masses/ starts to move after overcoming the resistance of the terrain and other obstacles, that are destroyed by the rain waters, and the rock and soil mass finds a way under the form of separate flows, which join the main flow, moving with high velocity.
This kind of flood within the river basin can be local or common. The local torrents originate in the beds of the river tributaries or in separate gullies.
The common torrents pass through the main river bed and most of them have stone-muddy structure. They are particularly dangerous since they often pass through urbanized regions. Their most dangerous characteristics are the suddenness, the straightforwardness of movement and the transportation of large quantities of hard materials, which pile up on, or destroy along their way settlements, roads, railway tracks, engineering equipment and agricultural lands.
The destructive forces of the torrent are defined by the increased capability to transport hard materials, the interruptibility of the movement, the high velocity of up to 15km per hour.
The human activity, related to the intensive use of mountain slopes leads to the increase in the volume and frequency of this type of floods, as well as to the increase in the number of locations they can occur.
Alluvial fan floods. Their name/fan/ corresponds to the shape of the spread mass. In substance they are the same as the above described but the solid mass prevails and they are more similar to the landslide.
Alluvial fan floods can cause greater damage than typical riverine flooding.
Volcanic ashes and Mudflows. They arise as a result of the Pyroclastic flow – this is a flow of heat gasses over the volcano slopes /about 1 000 C/ which quickly melt the snow into worm water running with high speed over the steep volcano slopes and carrying with itself solid materials. This phenomena is extremely dangerous for the people living near by.
For more information please click HERE
Floods caused by swift melting of ice/glacier.
Serious damages are caused by river floods caused by the combination between rising waters and ice movement. The ice movement is usually accompanied with jamming/fill up of the river bed with ice / or by the accumulation of intra-water ice, forming an icy barrier, which in turn causes an additional increase in the water level and the flooding of new territory upstream. Furthermore, in case of breach of the ice barrier a powerful wave is created that could flood the suddenly flood territories located downstream.
The jamming /barring/ of the river by ice occurs most often in rivers flowing from south to north /Oder, Wisla, Ob, Yenisei, Lena, Yukon and others/.
This is explained with the fact that the southern parts of the river free themselves earlier than the northern sections, in the course of which the moving water, mixed with ice, meets an obstacle of fully frozen sections. Ice barriers can also be created at the beginning of the winter prior to the complete freeze, when there are still sections of the river that are not frozen yet. Sometimes this type of flood can occur in lengthy rivers flowing from west to east /Danube, Amur/.
The origination of such floods depends on the climate and the physical condition of the river valleys. The freeze is lesser where the waterside is higher and steeper. The freeze is usually more severe in dams, in engineering equipment, at the turns and bridges, at places where the river has low depth over a long distance.
For such type of floods the estimated reaction time is rather short, requiring an obligatory and serious advance preparation. Since the locations of the ice jams will be known in advance thanks to long years of experience, precaution measures can be taken long time before the start of the ice movement in the river.
The elimination of the ice barriers is done through detonations or by using an ice-breaker, if possible.
The breaking of a dam wall, especially of large dams, is rather rare and can hardly be foreseen. The overflow of water from medium- and small-seized dams is an event occurring more often as well as the breaking of the of land type walls of the small dams.
The floods caused by break of walls of large dams can be very dangerous since they can affect the life of a lot of people.
The breaking of a dam wall can occur in case of breach in the construction as a result of progressive erosion in the wall, destruction of the embankments /dikes/. A construction breach can also occur in the case of a particularly severe flood. The dam walls are designed to withstand a powerful earthquake, but even so the earthquake can extremely weaken the wall.
The floods caused by dam wall-break, due to their unexpectedness and high water speed, can inflict casualties and destruction comparable only to a natural disaster. This is the reason why the large dam walls are placed under constant control and surveillance with special equipment installed inside the wall, monitoring for any deviation in the facility’s parameters. Furthermore, the dam walls have automated systems for warning the population of danger.
These are cases of flood at the river bed after the dam wall which is not broken. Such a flood happened in Italy in 1963 when a huge landslide into Vaiont Dam pushed out the dam water which spilled over the dam wall without breaking it but causing catastrophic flooding / 3000 people died/.
The local authorities obligatorily have to include all dams in the protection plans.
In most cases the smaller dams used for watering and other activities cause big problems. Usually their walls are of land type and require permanent maintenance and control of installation. When such walls are maintained badly they become very dangerous at intensive rainfalls or intensive snow melt. The local authorities have to keep records of such dams, to control permanently and to make organization for risk prevention.
For more information please click HERE
The following are the most important types of coastal floods:
– Storm Surge Floods
Often the hurricanes or the storm are in the sea. As a result, large waves are formed battering the coast and causing floods.
Storm surges commonly occur with coastal storms caused by massive low-pressure systems with cyclonic flows that are typical of tropical cyclones and severe winterstorms.
Factors influencing Storm surge intensity are: wind velosity, storm surge height, coastal shape, nature of coast and human activity.
Storm surges are controlled by four factors:
– The more intense storms have higher wind speeds which drive greater amounts of water across the shallow continental shelf, thereby increasing the volume and elevation of water pushed up against the coast. In areas with mild slopes and shallow depths, the resulting flooding can reach great heights.
– The low barometric pressure experienced during coastal storms can cause the water surface to rise, increasing the height of storm surges.
– Storms landfalling during peak astronomical tides have higher surge heights.
– Coastal shoreline configurations with concave features or narrowing bays create a resonance within the area as a result of the winds forcing in water, elevating the surface of the water higher than experienced along adjacent areas of open coast.
For more information please click
http://www.besafenet.net/en-gb/natural-hazards-hurricanes-and-storms-surges#faq315
– Estuarine floods
They are floods caused by the combination of strong winds and high tide.
This type of floods is typical for the Tropical and Equatorial belts and especially for islands e.g. Filipinas. With the change in climate they start manifesting themselves with increased intensity. Such floods in combination with other kind of floods like river ones can affect large areas within a short space of time.
– Floods caused by powerful pressing wind at the mouth of rivers, as a result of which water is retained in the river and the river water level rises.
– Floods resulting from tsunamis which are large seismic sea waves, impulsively generated by shallow – focus or high magnitude earthquakes which can cause disastrous flloding in coastal areas (Fig.2.2 a).
For more information please follow the link.
http://www.besafenet.net/en-gb/natural-hazards-tsunami#faq105)
Given that Coastal floods happen continuously in one and same region for most cases, they can be predicted precisely.
Usually the coastal floods are a combination of some of the above-mentioned floods including the river floods.
It is obligatory to elaborate and implement proper national and regional policies as well as national and regional plans to reduce the losses caused by floods.