5 Towards Developing Guidelines
45
face waters. However, these exchanges do occur and their importance under certain
conditions, such as those which could result from the construction of a barrage, may
not be negligible.
Tidal barriers and barrages affect the water level in an estuary and since there is
interaction with the aquifer they can affect the groundwater levels. Tidal surge barriers
have little impact on the mean water levels in the estuary and hence little impact on
groundwater. Power-generating tidal barrages will probably have some, if limited, impact, but water storage tidal barrages and multi-functional tidal barrages will result in
an increase in mean water level in the estuary, primarily landward of the impounding
structure, and hence an increase in groundwater level. The increase in groundwater
level could be widespread, but, except for immediately adjacent to the estuary, it could
take a considerable time to develop. The rises in mean water level in the estuary, landward of the impounding barrage structure, are likely to result in conditions which mean
there will always be a flow of groundwater away from the estuary.
As well as affecting groundwater flow, water storage tidal barrages and multi-functional tidal barrages can result in changes in groundwater quality. The shallower water
table will result in surface pollutants reaching the groundwater in a shorter time. Flow
from the estuary can result in groundwater pollution from the surface water and also
the spread of pollution away from the estuary from pollution sources such as waste
disposal sites and contaminated land. Saline intrusion into aquifers may occur if high
levels of saline water are retained upstream of barrages. The changes in salinity will
result in a small change in density of the groundwater. This change could affect the
load-bearing capacity of the soils, but any affect is likely to be significantly smaller
than that which would occur as a result of increases to groundwater level due to the
barrage.
5.4.6
Water Quality
Both upstream and downstream water quality may be affected by the introduction of a
tidal control structure. Tidal flushing will be altered and salinity variations will change.
The scale of change will depend on the location, design and mode of operation of the
barrage or barrier. A flood protection barrier which is operated infrequently for a relatively short period of time is unlikely to have a significant impact on water quality. A
fully tide-excluding barrage will have an obvious impact upstream through exclusion
of seawater and the cessation of tidal flushing. A tidal overtopping barrage can result in
saline stratification which could have the most serious impact on dissolved oxygen.
Significant reductions in tidal flushing upstream of the barrage will result in longer
residence times and will tend to increase the risk of algal blooms (see Fig. 5.3). Any
deterioration in water quality in the impoundment, from whatever cause, is likely to
impact on the behaviour of migratory fish. Changes in salinity may impact on the overall ecosystem. A change from saline or brackish water to a freshwater environment will
have obvious impacts on the ecosystem. However, sudden changes in salinity caused by
intermittent ingress of saltwater into an otherwise freshwater environment may have a
serious consequence and prevent the development of a stable ecosystem.
Any significant reduction in the volume of water moving upstream of the proposed
site (i.e. the tidal volume) will result in lower tidal velocities downstream. In deep estu-
45
face waters. However, these exchanges do occur and their importance under certain
conditions, such as those which could result from the construction of a barrage, may
not be negligible.
Tidal barriers and barrages affect the water level in an estuary and since there is
interaction with the aquifer they can affect the groundwater levels. Tidal surge barriers
have little impact on the mean water levels in the estuary and hence little impact on
groundwater. Power-generating tidal barrages will probably have some, if limited, impact, but water storage tidal barrages and multi-functional tidal barrages will result in
an increase in mean water level in the estuary, primarily landward of the impounding
structure, and hence an increase in groundwater level. The increase in groundwater
level could be widespread, but, except for immediately adjacent to the estuary, it could
take a considerable time to develop. The rises in mean water level in the estuary, landward of the impounding barrage structure, are likely to result in conditions which mean
there will always be a flow of groundwater away from the estuary.
As well as affecting groundwater flow, water storage tidal barrages and multi-functional tidal barrages can result in changes in groundwater quality. The shallower water
table will result in surface pollutants reaching the groundwater in a shorter time. Flow
from the estuary can result in groundwater pollution from the surface water and also
the spread of pollution away from the estuary from pollution sources such as waste
disposal sites and contaminated land. Saline intrusion into aquifers may occur if high
levels of saline water are retained upstream of barrages. The changes in salinity will
result in a small change in density of the groundwater. This change could affect the
load-bearing capacity of the soils, but any affect is likely to be significantly smaller
than that which would occur as a result of increases to groundwater level due to the
barrage.
5.4.6
Water Quality
Both upstream and downstream water quality may be affected by the introduction of a
tidal control structure. Tidal flushing will be altered and salinity variations will change.
The scale of change will depend on the location, design and mode of operation of the
barrage or barrier. A flood protection barrier which is operated infrequently for a relatively short period of time is unlikely to have a significant impact on water quality. A
fully tide-excluding barrage will have an obvious impact upstream through exclusion
of seawater and the cessation of tidal flushing. A tidal overtopping barrage can result in
saline stratification which could have the most serious impact on dissolved oxygen.
Significant reductions in tidal flushing upstream of the barrage will result in longer
residence times and will tend to increase the risk of algal blooms (see Fig. 5.3). Any
deterioration in water quality in the impoundment, from whatever cause, is likely to
impact on the behaviour of migratory fish. Changes in salinity may impact on the overall ecosystem. A change from saline or brackish water to a freshwater environment will
have obvious impacts on the ecosystem. However, sudden changes in salinity caused by
intermittent ingress of saltwater into an otherwise freshwater environment may have a
serious consequence and prevent the development of a stable ecosystem.
Any significant reduction in the volume of water moving upstream of the proposed
site (i.e. the tidal volume) will result in lower tidal velocities downstream. In deep estu-