2.3 WEIR AND EXPLOITATION

Weir

Along the Var there are 11 fixed thresholds (weirs) see figure 1. The overexploitation of the alluvial resource, linked to regional development, led in the 1960s to a lowering of the piezometric level of the alluvial water table, which was not compatible with the agricultural exploitation of the plain at that time. The necessity of maintaining this alluvial water table led to the creation of cross-sectional thresholds and the construction of a concrete riprap, built between 1972 and 1985.
Figure 1: Locations of the thresholds on the river Var

The threshold numbers 2 to 10 were originally constructed with a drop of the order of 5m. Another threshold, known as threshold no. 1, with a small drop (1.5 meters initially), lies below the Pont Napoléon III at the mouth of the river and its objective is to prevent the intrusion of a salt water in to fresh water and to protect the Napoléon III bridge.

The project also planned for the construction of other thresholds further upstream of No. 10. However these were not carried out, with the exception, however, of threshold No. 16 (figure 3).

During the 1994 flood, threshold 2 and threshold 3 were broken with threshold 4 destabilizing (figure 4), posing the risk of breaking.

Each threshold - n ° 4 to 16 - was equipped with a hydroelectric power station with a power initially varying between 1,094 kW (threshold n ° 7) and 2,425 kW (Threshold n ° 2). The annual production of hydroelectric power stations is 152 502 MW, covering the energy needs of 10% of the population of towns in the Var. All hydroelectric power stations are on the left bank, except for the threshold n ° 16.

 

 The construction of the thresholds caused a profound change in the flow of water from the Var. The river has gone from an original aspect of a river in a braid on a bed of pebbles with rare vegetation, to a succession of bodies of water with a central channel of flow and terraces of silts, with installed alluvial forests. These silts clog the bottom of the bed: the thresholds have thus caused the increase in the level of the bed and therefore the risk of overflow during heavy precipitation. In addition, since the cessation of the extraction of sand and pebbles upstream, alluvial deposits are present. This alluvial storage resulted in the stopping of two microcentrals which can no longer function due to a lack of waterfall.

The thresholds, by clogging the bottom of the bed, has the effect of reducing the infiltration of the water that supplies the water table by the river, from which the water supplying the population is drawn. For this reason, SAGE Var (water management and management plan) advocated the gradual lowering of the thresholds from upstream to downstream to return to the Mediterranean aspect of the Var, to reduce the risk Flooding and restore biological continuity for fish. To date, two thresholds have been lowered by the General Council (thresholds 9 and 10).                

Exploitation

Development of the Var river.

At the border between France and the city of Nice, the plain of the Var did not populate until late. There was a need to benefit from the development of the Var and its advantage of being near a water source. Whilst also seeking protection against the natural phenomena of flooding and development within the floodplain. Therefore, the men of the lower valley of the Var began successive arrangements of the bed of the river.

At the beginning of the 20th century, the development of a farm on both sides of the river "Le var" due to the availability of contaminants. This resulted in the exploitation of the alluvial deposits of the river, which met the need for infrastructures of an increasing population (figure 1).


At the end of the Second World War, agriculture was still the major development in the plain thanks to extremely fertile lands, well fed 
by floods in the Var that deposited silt. In addition, the shallow water table provides large amounts of water. Following the need
for reconstruction and the rapid economic development of the Côte d'Azur, the Var bed was the main source of crushed materials
in the Alpes-Maritimes department for decades. Moreover, the aggregates extracted had the advantage of being of very good quality
and of a granulometry quite favorable to these numerous works.
It is with the expansion projects of the city of Nice and neighboring
towns that this activity really takes off and allows the major works of construction of the current communication way.

However, this low cost has sometimes led to the very frequent use of these untreated materials for the construction of embankment.

Due to the massive exploitation of these aggregates, there was a significant reduction in the low water line, which had significant result incidents such as:

-          The lowering of the piezometric level of the water table

-          The scouring of the structures (bridges, dam ...)

                     -           The condemnation of the gravity feeding of canals, derived from the river.

Nevertheless, technically and economically, the lowering of the water table was also advantageous. It allowed for the rehabilitation of the still marshy agricultural land and delayed the extensive planning and protection work that had been envisaged.

The gradual containment of the Var has resulted in a slight increase in the amount of deposits by gravel. The first extractions therefore appear as a remedy for this increase. But the overexploitation of the gravels slowly leads to the tilting of the bed and the lowering of the water table. In total, nearly 150 years of natural input are extracted in 40 years (50 million cubic meters of material).

Chronology of Var river exploitation

-          1947: Start of exploitation of gravel and sand in the river bed

-          1947-1967: Extraction of approximately 30 million cubic meters of aggregates, mainly downstream from Nice St Isidore

-          1968-1983: Extraction in the river bed of about 20 million cubic meter

                     -          1983: A definitive ban on the exploitation of the river "Var" in the public domain of the river, from the confluence of Vésubie to the mouth of the river
Comments