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What is Flood
A flood is a phenomenon concerning the flooding in a short time (from hours to days) of a well-defined and commonly subaerial, by a body of water.
This may be a natural phenomenon as the overflowing of rivers, from their usual bed or basin, in a violent and devastating flooding or for combined action of high tide and typhoons in coastal areas, the arrival of a tsunami on of a coast, or even sudden melting of glaciers and snowfields of natural causes (typical of those by subglacial volcanic eruptions in Iceland). When the flood is caused by overflowing rivers swollen by rain is also talk of high flood, which may also be related erosion and change in morphology of areas involved in the phenomenon.
Time Of Occurrence
A fundamental characteristic of risk assessment Flooding / flood is called the time of recurrence, which is the statistical frequency with which an event of given intensity floodable returns over time.
The estimated time of recurrence (which depends on geomorphological, climatological, hydrological and, increasingly, anthropogenic) is carried out through analysis of time series of flood events (possibly over a period of centuries), integrated with the comprehensive assessment geographical and geomorphological land on which they insist the auction or the river basin of reference (rainfall data, set-up of river beds, embankments and structure of the state, for any compensation funds, the presence and functionality of any dams, etc.).. The time of occurrence is expressed as "Y range": for example, a time of occurrence Y <20 (Years <20) means that the risk floodable at that distance (alluvionabilità area) is less than 20 years, or that - on average - more than once every 20 years the area can be flooded (risk greater than 5% / year). For events of increasing magnitude, of course extends the range Y, then Y are growing larger and potentially destructive phenomena. An event such as Y500 or Y1000 is an event floodable a magnitude as to be able to submit - statistically - only once every 500 or 1000 years (risk of 0.2% or 0.1% / year).
The figure for the time of recurrence is useful both in engineering, which at actuarial and insurance: in engineering, construction carried out in areas with alluvial Y below certain parameters (usually 20 or 50, or in areas of alluvial river ) must meet a series of safety standards much more stringent, for insurance also are usually required insurance premiums against the risk floodable that are much higher in areas Y50 or Y100, and in some cases, insurance does not ensure even the houses in areas Y <20 (very high premiums or ask for it).
The term flood or water damage is often used in the case of the phenomenon of high water in Venice, where it is caused by the combined effect of particularly high tide and low atmospheric pressure. On a smaller scale, some cities have developed on the edge of small lakes may be subject to flooding to overflow of lake water as a result of excessive supply of the lake when its tributary is swollen by heavy rain, as often happens in Como.
In geologic time there is evidence of some sudden flooding of vast areas, arid remained below sea level because of the geological evolution of the region, an example is the height of the Mediterranean at the end of the Messinian salinity crisis.
There are also instances of flooding caused deliberately by humans, such as the flooding of the Dutch land, put in place to slow the advancing German armies, and the inundation, or flooding of valleys upstream of the construction of dams.
Many rivers that flow over relatively flat land across vast floodplains. When heavy rains or melting snow causes the increased flow of river water over the banks and vast stretches of shallow water covering the territory of the river basin. The deposition of fertile sediments in this process increases the fertility of the land.
Flooding from the sea, as a result (even combined) of storms or tides, or those caused by a tsunami, can overwhelm the natural defenses or artificial coastlines, such as dunes or dams built by man. These represent the greatest threat to people living near the coast, especially if the area can easily be flooded because it placed at the same level or lower level of the waves.
During human history, a fertile soil and a navigable route of communication have attracted agriculture and other human activities. To defend cities and farms from some rivers prone to flooding, have been studied extensive and elaborate systems of dams and canals along the coasts and around the city. Unfortunately, containing large amounts of water, dams can cause more flooding (or flooding) should they give up. Another reason to build artificial barriers to the waves is the human desire to steal land from the sea, as was the case in the Netherlands and Louisiana.
There may also be flooding caused by man, even though the events on a smaller scale - usually not traumatic - it speaks more appropriately of flooding.
In the Netherlands, has been used to delay or stop the flooding of all the advance of enemy troops, the last time was in 1944 when the German occupiers use these tactics to block the Allied advance.
During the Kingdom of Savoy, the flooding of lands adjacent to rice fields on the enemy's path has led to significant tactical advantages.
After he built a dam, a valley or a wider area may also be completely filled by water retained in the reservoir. The dams are constructed in this case to ensure water supplies or for the generation of hydroelectric power.
In ancient times many civilizations worked to try to cushion the effects of flooding and use it for their benefit. First of all peoples were allocated near the Tigris and Euphrates (known in antiquity as the Idiglat and Purattu), as the Sumerians, who created a network of irrigation channels aimed at containing artificial flood of two rivers.
Another population that engages in this work was the Egyptian, who tried to channel the lands around the Nile River. The Nile each year, following the long rains season in 'area of its southern basin, was characterized by a flood, the Egyptians believed to be of divine origin, resulting in overflow of water and flooding of farmland along its course on earth Egypt. Following the withdrawal of water from the flooded land, the river deposited silt on the ground, which allowed Egypt rich harvests for thousands of years thanks to a legendary fertility. The construction of the Aswan Dam, with the consequent adjustment of the water course of 'water, blocked the recurrence of this natural phenomenon.
In Western countries, the rivers are more likely to cause flooding or floods are managed carefully. They are used as embankments of the defenses, containments, basins and closed to prevent the river exceeds its banks. The coastal flooding have been operated in Europe with coastal defenses, such as dams and sea defense of the beaches eroded by the sea.
London is protected from flooding by a huge mechanical barrier across the Thames barrier which is raised when the sea level reaches a certain point (see the Thames Barrier).
Venice has similar protections, although they are not able to protect the lagoon of Venice from high waves, and that will become increasingly less appropriate if there will be the anticipated rise in sea level.
The larger and more elaborate defenses can be found in the Netherlands, where they are called the Delta Project, the dam dell'Oosterschelde as the crowning work. These works were undertaken in response to the flooding of the North Sea in 1953 that struck the southwestern part of the country. The Dutch had already built one of the largest dams in the world in the north, the Afsluitdijk, completed in 1932, in response to a previous flood in 1916.
The areas subject to flooding are typically the same high-density housing, as part of the countries Bases, England, New Orleans and the Mississippi Delta. Local laws have been enacted to prevent the construction of buildings in areas prone to flooding, in some cases the pressure of manufacturers has caused the erosion of the checks, a growing number of new areas that need the protection of artificial defenses.
The Bangladesh has not been subject to marine flooding since 1995, but the country relies heavily on foreign support and technology to prevent further disasters. The United States of America has provided refuge from the hurricanes, and India provides the government weather forecasts to enable planning of responses to tropical cyclones. Over the years small cults have formed the so-called "patrols of the flood", composed mainly of young people who travel the rural roads in search of the next channel or marsh to cross. They risk their lives to bring back some of the best photographs available on the floods.