PRECIPITATIONS AND FLOODINGS

PRECIPITATIONS TYPES OF PRECIPITATIONS HOW PRECIPITATIONS FORM ? THE DEPRESSIONS - COLD FRONT - WARM FRONT - OCCLUDED FRONT THE AVERAGE PRECIPITATIONS - IN THE WORLD - IN FRANCE THE FLOODS DEFINITIONS TYPES OF FLOODS THE CAUSES WHERE ARE THE MOST FREQUENT ? IN FRANCE IN THE WORLD MONSSOON CONSEQUENCES THE FLOOD OF PARIS IN 1910

FLOODS

DEFINITIONS

A flood is the submersion of a zone (fast or slow) can be caused in several ways, by rains important in duration and (or) intensity.

A watercourse presents :

- The riverbed where water remains pernamently ;

- The mid riverbed, river basin which is ordinarily occupied by the riparian forest (dense and varied végetation borderng the river), which flows over periods of 1 to 10 years on average. The mid riverbed is subjected to frequent risk of flooding, the water velocity and transport of sediment during floods are important ;

- The floodplain is occupied by a river during floods. Outside the floodplain,river flood risk is zero ( which does not exclude the risk of flooding by stormwater runoff, especially in urban areas.

THE TYPES OF FLOODS

There are three main types of floods :

Direct overflow : A flood can occur when a river
overflows and stream out of his riverbed to occupy
her major bed then he invades entire valleys.

 

By accumulation of water runoff : when there is insufficient capacity
of infiltration, evacuation of soils or drainage system during the abnormal
rainfall. These floods can occur in built-up areas, outside the bed of the watercourse
itself, when impede the normal flow of intense rainfall, soil sealing and design
of urbanization and of networks sanitation (thunderstorms, especially of cevenol type).

 

Indirect overflow : water date back to through
the alluvial groundwater in sewer systems
at low points located... by siphoning

 

But a flood can result in other more or less current phenomena :

-during a storm, a hurricane or a tsunami there is often a flooding of coastal zones ;

- during the destruction of a work (dams) ;

- during sudden floods ;

Left, in Saskatchewan, dry creek Indianhead the07/17/1996.
Right, same creek in flood the 04/10/1996.

THE CAUSES

MANY FACTORS INVOLVED IN EUROPE :

- Often the three areas of watercourses (minor riverbed, mid riverbed and floodplain) are transformed by human activity or climate change. So with a more favorable soil to runoff there is a decrease in the permeability of the entire basin due to deforestation, forest fires, the multiplications of coated surfaces (roads, houses, ...) and then makes more brutal floods.

- The distribution and intensity of rainfall in the watershed (which is the place where the rivers collect water from a territory more or less large) and duration of precipitations.

- In Europe at the end of the spring and early summer during warm spell, heat waves or situation of the foehn wind that causes a significant melting of snow.

- In spring, the ice break-up of the rivers in the mountains (Alps, Pyrenees), or in countries at high latitudes as Canada which are dams when it freezes in winter can also cause flooding.

- Severe thunderstorms in summer and spring which cause violent rainfalls.

- In autumn stormy rainfall, especially on the Mediterranean coast have effects that can be felt throughout the southern half of the country France.

- In northern and western France oceanic rains cause flooding, especially in winter and spring.

- The importance of absorption by the grounds and infiltration in the basement which feeds the groundwater affects flooding. Saturated soil by recent precipitations doesn't absorb, which could cause flooding with more precipitations.

 

THE MAIN CAUSES FLOOD IN OTHER COUNTRIES ARE :

- Monsoons in India, Bangladesh and other countries resulting in heavy rainfall.

- Storms or cyclones followed by heavy rains cause flashfloods, landslides, mudflows and flooding. This is often followed by a phenomenon called "storm tide".

- During El Niño or La Niña there is an increase in precipitation in some countries around the Pacific causing floods see stronger monsoons.

Flooding in Texas 07/06/2002

Flooding in India during the monsoon

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