The term flood traditionally refers to the overflow of a River, generally in Crue, which submerges the close grounds. It is often extended to the overflows of artificial works such as reserves or networks of Assainissement.
According to the International institute of the water of Stockholm (SIWI), over the period 1996-2005, approximately 80% of the natural disasters were of weather or hydraulic origin, and the floods would have at the time of this decade affected on average 66 million people per annum between 1973 and 1997. In fact the natural disasters produce the most damage.
CausesThey can have many causes, cumulable;
- natural causes, dependant on the weather climatic risks and phenomena awaited (Monsoon in India for example) or on a natural event (landslide) which prevents the usual flow of water;
- anthropic causes; the drainage, the irrigation, the imperméabilisaiton of the grounds, some practical agricultural intensive can accelerate the streaming of water and limit the infiltration of it.
TypologiesThe floods can be
- slow; like the risings of the the Rhone in 2003, making following one rainy long period.
- brutal: after a violent storm or one or two days of strong rains on dry ground, certain floods can violently damage the fields, villages and cities, as well as many infrastructure, as with Florence in 1966 or water invaded many districts of the city, prestigious buildings and to destroy many works of art stored in the basements of the Offices.
They can be torrential as in France with Nimes in 1988 or with Vaison-the-Roman in 1992; they are accompanied then often by flow mud. The floods result from a certain number of weather conditions. One distinguishes two great types from them: the wide risings flash and floods. Each one has an origin, characteristics and a duration different.
Some raw flash are short and very localized. They are generally due to short but intense stormy rains, which do not manage to disperse by infiltration, streaming or flow. The most frequent cause of these floods is a storm which moves slowly and can pour enormous quantities of water on a zone limited in very little time. The storms which move more quickly are less awkward in this respect, because they give rain on a wider zone. The raw flash often ennoient in valleys or throats. When the humid air is thorough towards the mountain, it rises, and can cause a storm accompanied by torrential rains. If the wind maintains the storm stationary, water can stream on the slopes of the mountain and go down until the bottom from the valley. The throats are as funnels which accelerate the flow of the water, whose force carries all on its passage.
Fight plans against the floodsIt is based on an evaluation of flows (Atlas of the floodplains) and a double approach; preventive, and curative.
Préventivement, the communities and individuals can seek to restore zones of sufficient expansion of rising. Since there is inadequacy between the quantity of water to evacuate and the capacities hydraulic S, the management of the floods also aims to:
- to restore or improve of the capacities of flow (maintenance of the banks, elimination of the Embâcle S, clearing out…),
- to limit the Proofing grounds in urban environment (infiltration of water of roofs and streaming (after depollution if necessary), creation of valleys and green areas likely to be used as buffer zone). In rural environment, more adapted cultivation methods and a management anticipatoire of the Streaming aiming at storing water as of the top of the catchment area, and by slowing down it and infiltrating it better via a network of slope, hedges, valleys, meadows and meadows easily flooded and ditches makes it possible not to enlarge the floods downstream.
In certain cases, the flood brings into play hydrological mechanisms more complex, like the risings of the Somme in 2001 due essentially to an increase of the Ground water. This one would have contributed up to 80% of the flow of the river.
The innondation are the objects of modeling according to their period of return (decennial risings, centennial, etc But the rain will remain a random phenomenon, in a dubious and too complex climatic context so that calculations can all envisage. The documents of Town planning, RAINED or SCOT must thus integrate this constraint, the principle of prevention and precaution, and regulate the right to build. In the countries known as developed, in the event of important Risk, the Risk of damage to the goods and the people is more or less covered by the insurances, and it must be in France for example taken into account by a prevention plan of the risks of flood (PR).
Various steps are in hand. Thus, in area Paris ienne, one evaluated the consequences of a centennial Crue. In spite of the hydraulic work carried out upstream, on the the Seine and its affluents, its consequences would be as catastrophic as in 1910. Important precipitations of the beginning of the year 2002, combined at the still high level of the ground water, led the specialists to launch an alarm to the beginning of the year 2002.
It allowed an awakening of the brittleness of certain underground equipment (subway and trains, electric transformers, etc) as well as many companies or administrations. Plans of intervention were elaborate (obturation of certain galleries of subway for example) and of the files were put at the shelter (the reserves of several museums are in basement).
ConsequencesThe floods, in particular in the poor countries often do many victims and property damage. They are responsible for 40% of the accidents fatal due to the natural disasters in the world. But, in certain areas of the sphere, the floods belong to the natural cycle of the seasons. During more than two millenia, the risings of the the Nile made thrive Egyptian civilization and of many tropical zones are still dependant on the annual risings, and monsoons which fertilize and irrigate the cultures, by reconstituting water reserves for the dry season.
Means of fight
They are preventive and curative and at the same time buildings and to build on a basin slopes scale.
They include more and more operations of Renaturation (ex: restoration of Wetlands, reintroduction of the beaver, fight against the streaming and the urban proofing, plantation of enherbées Bands, afforestation or Forest of protection, restoration of zones of expansion of raw upstream, as of the top of the Area catchment, etc
The documents and payment of town planning and occupation of the ground theoretically make it possible to prohibit, even locally to destroy construction in floodplain. Certain urban payments (ex: Device ADOPTED, developed in area Nord-Pas-de-Calais around Douai in the north of France, in zone of mining Affaissement, particularly vulnerable) impose that the new roads and construction are conceived so that the rain water is stored and infiltrated on the spot, as much as it would be the case in the absence of construction. It is also one of the targets of HQE. Certain areas financed “atlas of the floodplains” , by catchment area (ex in the Nord-Pas-de-Calais), like document of carried to knowledge to help the communes not to authorize construction in floodplain more.
The average curatives are limited. They are primarily the firemen or teams of civil security who pump out the cellars and help the population or the companies.
More global solutions necessary, are encouraged in Europe by the Parent directive on water, and the emergence of poles of assistance and council. The floods could be exacerbated with the cast iron of the glaciers and the rise in the sea level. Various work of Prospective and Modélisation S (See for example the European programs “ PESETA ” and “ PRUDENCE ”) is in hand within the framework of the adaptation to the climate changes, which can help the communities with better preparing
See tooForecast of raw the catastrophic Floods Hydraulic urban Hydrology Sustainable development environmental High-quality (HQE) Cevennes Storm enherbées Bands Adopted
- ADOPTED (Site dedicated to the experimentation and the promotion of alternative means of fight against the floods and the urban streaming)
- the floods in France, photographs and explanations
- Rapport of the mission on the floods of September 2002 in the south-east of the ministry for Health
- Floodplains in Languedoc-Roussillon
- Carte of the floodplains in the Midday-Pyrenees
- Web site of fresh water, Canadian governmental site, documented well (in French)
- Cartes of the floodplains of the Walloon region, in Belgium
- the floods in Belgium on Wikinations.be
- http://www.epama.fr/, the comprehensive strategy of reduction of the floods is declined in the Ardennes in a program
- site dedicated in support of the communes for the prevention of the risks: ''' safety common information '''
- Directing of Forecast of Believed of the basin Loire-Bretagne '''
- Service of Forecast of Believed in France '''
- History of the fight against the risings in Switzerland (209 pages), by the ASSETS
Simple: Flood Zh-yue: 水浸
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