Presentation on theme: "A flood occurs when there's an accumulation of river materials. "Flood" can be a catastrophic event caused by adverse weather conditions that cause torrential."— Presentation transcript:
A flood occurs when there's an accumulation of river materials. "Flood" can be a catastrophic event caused by adverse weather conditions that cause torrential rains for days or weeks. It is a partcularly devastating phenomenon and it is part of natural disasters, for its dramatic impact on the lives and works of human beings. A flood is an unexpected event, although it may be predictable, especially in those countries affected by the annual phenomenon of the monsoons and cyclones, followed in the most advanced nations with the most modern tools provided by modern meteorological science.
A flood carries large amounts of soil and debris torn from the force of water, causing further damage and making rescues more difficult. It is not uncommon that in a predominantly mountainous areas, and especially in those subjected to anauthorized building, a flood is accompanied by landslides or mudslides. The landslides of the land, beside being a danger in itself, may divert waterways or fill in ponds, causing damage and victims in greater quantities during low rainfall duration.
During a flood, in addition to the mass of water, large quantities of mud and other sediments are transported in the territories adjacent to the river bed. Throughout the ages this mechanism gives as the formation of alluvial plains.
CAUSES The main cause of the damages and deaths caused by flooding is not the geology or the time, but the distribution of populations near water. Since ancient times man has built towns by the sea or rivers for a faster and cheaper access to food reserves and to the routes of communication. The fertile soil of delta banks is regularly subject to the phenomenon of flooding for the normal variations of rainfall. In fact, while man tries to prevent flooding by levees, dams and pipes, he helps it with the devastating effect of deforestation and unauthorized building in places at risk.
Flooding by sea, for strom and tide effects, or those caused by a tsunami, can overwhelm the natural or artificial defenses of the coastlines, such as dunes or dams built by man. These represent the greatest threat for people living near the coast, especially if the area is situated at the same level or at a lower level of the waves.
Throughout human history, a fertile soil and a navigable route of communication has attracted agriculture and other human activities. To defend the cities and farms from some rivers predisposed to flooding, mankind has studied and developed vast systems of dams and canals along the coasts and around cities. Unfortunately, containing large quantities of water,dams could create greater flooding if the dam gives way. Another reason to build artificial reefs for waves is the human desire to steal land to the sea, as it has happened in the Netherlands and Louisiana.
FLOODS IN ITALY In Italy, the hydrological risk has spread extensively and it presentes itself in different ways: landslides, flooding and landslide morphological of torrential character, transport of mass along the conoid in mountainous and hilly zones, flooding and subsidence in hilly areas and plains. This is due to the geological and geomorphological conformation of territory characterized by raising ranges. Furthermore, the hydrological risk is increased by human presence in the territory since he makes changes increasing the possibility of the occurence of the phenomena.
The abandonment of mountain land, the unauthorized building, continuing deforestation, the use of farming techniques that don’t respect environment, the opening of loan quarries, the occupation of areas of river relevance, uncontrolled extraction of fluids (water and gas) from the underground, the improper removal of inerts from river beds, the lack of maintenance of slopes and water courses have certainly been worsened the geological instability and has highlighted the fragility of the Italian territory. The continuing occurrence of these episodes has led to a risk management policy that addresses the problem not only during emergencies. So we moved to a culture of anticipation and prevention, distributed at various levels, focusing on identifying conditions of risk and in the adoption of interventions aimed at minimizing the impact of events.
FIRST CASES OF FLOODS The first time in human history that we hear about floods occur in ancient Egyptian age. They waited for the flood of the River Nile for new fertile land to cultivate. The silt deposited by the flooding of the Nile was for them a source of livelihood, because this soft ground to work was rich of substances that were used by the plants to grow luxuriantly. So their wealth was due to these events, which scanned the changing of seasons. Going on in history we have learned about floods, exspecially in autumn, when many Italian areas are badly very damaged. So are floods something useful or harmful?
There's a flood event when the river water is not contained by the banks and spill into the surrounding countryside or a town. Generally, the old were familiar to the areas that were periodically affected by floods, and did not build their houses in these particularly unfortunate areas. When in Italy, the population began to grow quickly and cities lacked the space to build, these areas were also used, trying to reduce, at best, the effect of the flood raising embankments. But often this has not happened, and for that in Italy there are many areas that may be affected by the floods.
The main reasons that cause a flood are two: the rain and the neglect of the territory. If you can not control the natural element, since none of us is capable of deciding when it needs to rain, it is possible for man to do something to limit the effects of a very intense and prolonged rain.
But now we will see the dynamics of a flood event, that is what happens because a flood take place. The water of a river is the result of the rainfall that took place within its drainage basin. To get an idea of what a drainage basin may be, try to imagine a valley bounded by any mountain ridges and crossed by a river. All the rain that falls within this valley will reach the river, while one that falls beyond the mountain ridges will flow to another watercourse. This way you can understand that all the land is divided into catchment areas, because all the water that falls is concentrated in rivers.
Before reaching the river, the water runs on the land and the amount that ultimately reaches the river will be less than that which fell from the sky. It happens that the rainfall is intercepted by the leaves of the trees and bushes, by grass, while what remains, begins to slide on the ground. But even here, some water infiltrates into the subsoil and therefore the amount of water that reaches the river is reduced even more.
If the rain is very strong and prolonged, the water that is absorbed by plants and soil will be low compared to that which falls, and the river that gathers this' water begins to swell. At this point it becomes important to know how much of this water can be contained by the river. If the volume of water is excessive compared to what the banks of the river can hold, otherwise flooding occurs and the surrounding area is flooded.
HOW CAN WE SOLVE THIS PROBLEM? AND WHAT CAN WE CITIZENS DO? We need to reduce non-cultivated areas and special cultivation techniques, to try to prevent fires and keep clean the banks of the smallest waterways. It's also important to point out the forest fires promptly by calling the fire brigade or the offices of the State Forestry Corps. To reduce flood risk it is especially important that the design of buildings is made, as today, following a special study on the effects of a flood. As we have already said, there will always be those cases impossible to predict, but we must strive to reduce the effects of events that can be scheduled in advance. In this case, the flood of the rivers will not be a tragic event, but only a natural phenomenon.
FLOODS IN SICILY Even in Sicily, there have been cases of natural disasters which have resulted in victims and irreparable damages to the area. In 1907 and in 1925 Palermo had been severely tested by heavy rainfall which caused widespread destruction and flooding, in the previous century and had been hit twice, in 1851 and in 1862. So it deals about recurring events even with long periods of return and not occasional events if they have no longer verified since 1931 to today.
Palermo is situated on a plain, the famous Conca D'oro once planted with citrus, surrounded by mountains on 3 / 4 up to a height of 1333m and the remaining quarter overlooking the sea with a NE exposure. The morphology of the plain is not constant, but characterized by rises and depressions distributed in bits, there are at least four main river basins that cross it resulting in the Tirreno: Pass Channel Rigano, the Danisinni-Papireto Torrente, the Torrente Kemonia and the Oreto River often artificially diverted. These features together with the flow, huge of the phenomenon marked the disaster of 21-23 February 1931.
The overview of those days, as evidenced by the charts of the historical Wetterzentrale shows for the day 21st a state of blocking anticyclones over the Atlantic with the alimentation of a depressionary channel North-South from the North Sea to the Central Basin of Mediterranean and adjacent depression on Sicily. On days 22nd and 23rd that the cold currents fed depression revolve first from the north east then to the East North East for bridge link between the anticyclone of the Azores anticyclone and the Russian are according to what in slang is called Weikoff bridge. We have then a stationary semi-depression that appears on the Sicilian capital initially with ground currents from the North West, then North, finally by Northeast. They are cold currents that are generated in contrast to the cells of the Tyrrhenian basin storms laden with rain.
Rainfall which is then stimulated by Stau effect caused by the barrier at the back of the city. The result of this interaction was 50 hours of almost continuous rain with a combination of precipitation of 395mm, but if we take into account also the rain of the day 20th and 24th we arrive at a total of 618mm in 5 days.
The effects: total flooding of the city center and the suburban areas with water levels varying from 2m up to 6m in height. The relief to population came in several cases by boats and floating decks were also built for population's evacution. In addition to the rain a strong wind blew, probably a wind coming from the North that made the walls collapse and a 'huge crane for the construction of the building The Post. Incalculable damages to houses and infrastructure, and unfortunately 10 victims and 21 more wounded.
Since then the city has not suffered more events of this magnitude. In autumn and winter occurs occasional storms as the most recent of 24 September 2007 but with the quantities of rainfall much lower, only 25mm but it is half the average rainfall total for the month of September.
Made by: Di Clemente Mariaelena Travaglia Silvia Altavilla Grazia Maria Scarpulla Erminia With the collaboration of: Sturnio Sarah Bannò Mariolina Parisi Marina Livera Serena Popolo Silvia Giangreco Alessia