Var

Var catchment presentation and flood events


1/ Geographic presentation

The Var is a french river in the south-east wich originates in Estenc at 1 790 meters altitude. The var flow through 114 kilometers before flowing in the Mediterranean sea near Nice. Its 5 main tributary are the Cians, the Tinée, the Vésubie, the Coulomp and The Estéron. The Var through 31 city including Carros, Nice and Saint Laurent du Var.

Its catchment area is of 2 820 square kilometers.


 


Summarize of Var catchment characteristics :
    · Altitude : 0-3000 m
    · Area : 2820 km²
    · Length of the Var River : 125 km


2/Topographic and hydrologic presentation

The Var River flows in an area with a variable topography. The elevation goes from 3000 m in the mountains to 0 m at the mouth to the Mediterranean Sea.     

              


A big part of the catchment is mountainous, thus it follows a nival regime (melt of snow during spring). For the same reason, in average the slope is important even in the plains and until the outlet in Nice.

The Var flow is in average 50 m3/s with a really low river discharge during summer (sometimes less than 10 m3/s) due to small amount of rainfall. But during autumn and winter, intense rainfall can occur, creating important and quick floods for the Var and its tributaries

 

3/ 1994 flood event

    a) Description of the flood event

On November 1994 the Var peak discharge during 1994 flood was 3 500 m3/s. The return period of this event is about 100 years. 


The principal reasons of this exceptional flood are : 

-      The event happened during the middle of autumn which is a humid season in France. So in addition with thin soil, soil saturation was fast and the runoff became particularly important.  

-     The average cumulative precipitation was about 350 mm in 72 h. But this water height was falling on 2/3 of Var catchment. In the lower part the rain was up to more than 700 mm but for example in Carros rainfall didn’t exceed 100 mm.

It was not a very intense rainfall but it happened in a large area. That is why the runoff volume of this flood was that huge.

This flow of 3 500 m3/s during the event exceed the previous limit flow prescribed of 3000 m3/s which be considered as an event wich occure every millenium.

Suddenness of this event was not predicted by local weather report, however we observed a rapid devolopment of the flood : 1500 m3/s was measured at 15h and 3000 m3/s at 17h.

    b) Other possible causes

  • Meteorological hazards
  • Abusive extractions of river's gravel by companies in the Building and Public work sector
  • Forest fire on the Esteron mountain
  • Agricultural decline 
  • nature of bassin soil prepared by a wet summer
  • development of weirs in the downstream the river for electricity production
  • the containment of the river 
  • urbanization of Nice and Saint-Laurent-du-Var

4/ History of  progressive Var urbanization

Extension of Nice urbanization since 1900

  • 1847 - 1865 : Construction of a flood barrier on the left side (Les Sardes)
  • 1890 - 1974 : Construction of a flood barrier on the right side 
    • flood prevention and agricultural development
    • Concentrated the flow that lead to a linear erosion
Reduction of the stream bed was accompanied by bridge building (1860) and Railway line (1912) which limited the possible connected flows.
  • 1964 - 1980 : Extraction of 1.9 millions tonnes of gravel per year (or 300 000 m3/year) that drive to falling levels of groundwater and weaken flood barriers structures. This event demonstrated the important urbanization without government control.
  • Reduction of estuary space by the construction of a commercial center : Cap 3000 and the extension of the Nice airport since 1950.
  • 1970 : Beginning of construction project, 16 weirs have been established every 1 km in order to rise level of ground water table.
  • 1984 - 1989 : Building of several micro-hydroelectric plants which reduce again the useful width of river and permit sediment accumulation and lead to the outbreak of alluvium blocks.
So before 1850, the water streamed along a valley of 1 km wide, today the width is on average of 250 meters.

Progressive Var artificialization

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