while they are significantly lower from July to October, with an average of 15 m
3
/s.
In the Vilaine catchment, the low-flow period runs from May/June to October.
6.3.1.2 Drought Threats and Water Scarcity in the Vilaine Catchment
Traditionally considered to enjoy water in abundance, the northwestern regions of
France do not always have drought policy and management plans formulated at the
river basin scale. In the Vilaine River, however, provisions have been taken to create
frameworks for some main water-related issues, notably for drinking water distribution
and low-flow management in some sensitive parts of the catchment. In fact, the
Vilaine catchment is heterogeneous regarding drought threats. The eastern part of the
catchment is classified as more sensitive to water scarcity because of its exposure and
vulnerability, as irrigated crops are rooted in these areas. Since 2011 (Regional Decree
of 24 January 2011), the entire Vilaine catchment is no longer classified as a
water-sensitive area (in France, ZRE, zone de repartition des eaux). In France, a
‘water-sensitive area’ refers to an area where water resources are insufficient compared
to water uses, and the area is thus considered to require special protection regarding
low-flow water levels. In water-sensitive areas, abstractions other than for drinking
water are limited between 1 April and 30 October at their current level. Small dams
may be permitted only if they have no impact on low-flow rates. Because the Vilaine
catchment is no longer considered a ‘water-sensitive area’, it is no longer possible to
constrain water uses permanently in the Vilaine area (although water use restrictions
may be put in place during dry years, i.e. in below mean situations). Recent water
management measures in the Vilaine claim that the catchment should again be registered as a ‘water sensitive area’ (see Sect. 6.3.3 for a detailed description of water
management in the catchment).
6.3.2 The Arzal Dam
6.3.2.1 One Initial Objective: Regulating the Risk of Floods
The Arzal dam (Fig. 6.3) is located at the mouth of the Vilaine catchment, just above
the outlet of the river to the Atlantic Ocean (Fig. 6.2). It was built in 1970 to isolate the
lower reaches of the river basin from the ocean and block the tidal flow, which
previously travelled inland as far as the city of Redon and could aggravate flooding in
the valley when floods in the Vilaine River occurred simultaneously with a high tide.
6.3.2.2 An Opportunity: A Reservoir with Multiple Uses
In addition to regulating the risk of floods, the dam also allowed the creation of a
reservoir of 50 Mm
3 of freshwater, stretching up to 80 km. A water supply
118
I. La Jeunesse et al.
3
/s.
In the Vilaine catchment, the low-flow period runs from May/June to October.
6.3.1.2 Drought Threats and Water Scarcity in the Vilaine Catchment
Traditionally considered to enjoy water in abundance, the northwestern regions of
France do not always have drought policy and management plans formulated at the
river basin scale. In the Vilaine River, however, provisions have been taken to create
frameworks for some main water-related issues, notably for drinking water distribution
and low-flow management in some sensitive parts of the catchment. In fact, the
Vilaine catchment is heterogeneous regarding drought threats. The eastern part of the
catchment is classified as more sensitive to water scarcity because of its exposure and
vulnerability, as irrigated crops are rooted in these areas. Since 2011 (Regional Decree
of 24 January 2011), the entire Vilaine catchment is no longer classified as a
water-sensitive area (in France, ZRE, zone de repartition des eaux). In France, a
‘water-sensitive area’ refers to an area where water resources are insufficient compared
to water uses, and the area is thus considered to require special protection regarding
low-flow water levels. In water-sensitive areas, abstractions other than for drinking
water are limited between 1 April and 30 October at their current level. Small dams
may be permitted only if they have no impact on low-flow rates. Because the Vilaine
catchment is no longer considered a ‘water-sensitive area’, it is no longer possible to
constrain water uses permanently in the Vilaine area (although water use restrictions
may be put in place during dry years, i.e. in below mean situations). Recent water
management measures in the Vilaine claim that the catchment should again be registered as a ‘water sensitive area’ (see Sect. 6.3.3 for a detailed description of water
management in the catchment).
6.3.2 The Arzal Dam
6.3.2.1 One Initial Objective: Regulating the Risk of Floods
The Arzal dam (Fig. 6.3) is located at the mouth of the Vilaine catchment, just above
the outlet of the river to the Atlantic Ocean (Fig. 6.2). It was built in 1970 to isolate the
lower reaches of the river basin from the ocean and block the tidal flow, which
previously travelled inland as far as the city of Redon and could aggravate flooding in
the valley when floods in the Vilaine River occurred simultaneously with a high tide.
6.3.2.2 An Opportunity: A Reservoir with Multiple Uses
In addition to regulating the risk of floods, the dam also allowed the creation of a
reservoir of 50 Mm
3 of freshwater, stretching up to 80 km. A water supply
118
I. La Jeunesse et al.