affected primarily by salt water intrusions. They occur primarily when the lock is
operated for navigation (Fig. 6.6). They are usually handled by a system of syphons
that automatically evacuates salt water from the reservoir to the estuary. Because
the most significant inputs of salt water to the reservoir occur when boats pass
through the existing lock of the dam, navigation can be restricted during the
summer. Management rules with restrictions on the time schedule and number of
lock openings can thus generate conflicts, especially during the summer high season
for recreational sailing.
Today, the Arzal dam consists of five elements: an unsinkable dike, a set of five
sluices, a lock, a syphon and a fish passage.
6.3.2.3 Pilot Measures Implemented Within the DROP Project
The Arzal dam and its reservoir are managed by IAV, the practice partner of the
DROP project for the Vilaine case study. IAV is an EPTB (see Sect. 6.2.1), which
means that it is a French public organization for the cooperation of local authorities
(regions, departments, municipalities, etc.) towards the development and management
of rivers in the geographical context of a river catchment or sub-catchment. IAV faces
challenges related to salt water intrusion and reservoir management during the
low-flow season, when both water quantity and water quality constraints apply on the
management of the dam. As mentioned earlier, salt intrusions in the reservoir mainly
occur when boats pass through the lock at the dam. When water inflows tend to be the
lowest, recreational activities, including sailing, are generally at their highest level
(because it involves the summer period), and lead to a peak of salt water intrusions in
the reservoir, which can affect freshwater quality and, thus, quantity. To prevent salt
intrusions, syphons have been installed upstream of the dam to pump the contaminated
water from the reservoir back to the Atlantic Ocean. However, this system causes a
significant loss of resources: approximately, 300,000 m
3 of freshwater are evacuated
every day from the reservoir, compared to an average of only 10,000 m
3 of salt
intrusions. In addition, the syphons are not 100 % efficient: some brackish water still
enters the reservoir, and salinity peaks are regularly observed in late summer each
year. Moreover, this system leads to losses of substantial quantities of freshwater,
which, during prolonged periods of drought, may aggravate the problem of multiple
uses of the reservoir and impact the freshwater supply.
Currently, the only way to limit salt water intrusions and, consequently, water
losses from pumping is to place restrictions on the use of the lock in summer. The
two principal strategic management objectives during low-flow periods are (i) to
ensure a level in the Arzal reservoir that is adequate to permit all uses and (ii) to
keep saltwater out of the Arzal reservoir as much as possible to preserve freshwater
quality and guarantee the drinking water supply. However, these management
objectives are not always satisfactory because they may result in restrictions in the
use of water, which may not be fully accepted by other water users in the catchment: a preventive rise in water levels to supply summer water needs may result in
flooding the wetlands during the hay harvest, forcing IAV to pay financial
6 The Governance Context of Drought Policy and Pilot Measures …
121
operated for navigation (Fig. 6.6). They are usually handled by a system of syphons
that automatically evacuates salt water from the reservoir to the estuary. Because
the most significant inputs of salt water to the reservoir occur when boats pass
through the existing lock of the dam, navigation can be restricted during the
summer. Management rules with restrictions on the time schedule and number of
lock openings can thus generate conflicts, especially during the summer high season
for recreational sailing.
Today, the Arzal dam consists of five elements: an unsinkable dike, a set of five
sluices, a lock, a syphon and a fish passage.
6.3.2.3 Pilot Measures Implemented Within the DROP Project
The Arzal dam and its reservoir are managed by IAV, the practice partner of the
DROP project for the Vilaine case study. IAV is an EPTB (see Sect. 6.2.1), which
means that it is a French public organization for the cooperation of local authorities
(regions, departments, municipalities, etc.) towards the development and management
of rivers in the geographical context of a river catchment or sub-catchment. IAV faces
challenges related to salt water intrusion and reservoir management during the
low-flow season, when both water quantity and water quality constraints apply on the
management of the dam. As mentioned earlier, salt intrusions in the reservoir mainly
occur when boats pass through the lock at the dam. When water inflows tend to be the
lowest, recreational activities, including sailing, are generally at their highest level
(because it involves the summer period), and lead to a peak of salt water intrusions in
the reservoir, which can affect freshwater quality and, thus, quantity. To prevent salt
intrusions, syphons have been installed upstream of the dam to pump the contaminated
water from the reservoir back to the Atlantic Ocean. However, this system causes a
significant loss of resources: approximately, 300,000 m
3 of freshwater are evacuated
every day from the reservoir, compared to an average of only 10,000 m
3 of salt
intrusions. In addition, the syphons are not 100 % efficient: some brackish water still
enters the reservoir, and salinity peaks are regularly observed in late summer each
year. Moreover, this system leads to losses of substantial quantities of freshwater,
which, during prolonged periods of drought, may aggravate the problem of multiple
uses of the reservoir and impact the freshwater supply.
Currently, the only way to limit salt water intrusions and, consequently, water
losses from pumping is to place restrictions on the use of the lock in summer. The
two principal strategic management objectives during low-flow periods are (i) to
ensure a level in the Arzal reservoir that is adequate to permit all uses and (ii) to
keep saltwater out of the Arzal reservoir as much as possible to preserve freshwater
quality and guarantee the drinking water supply. However, these management
objectives are not always satisfactory because they may result in restrictions in the
use of water, which may not be fully accepted by other water users in the catchment: a preventive rise in water levels to supply summer water needs may result in
flooding the wetlands during the hay harvest, forcing IAV to pay financial
6 The Governance Context of Drought Policy and Pilot Measures …
121