5 Towards Developing Guidelines
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2. Tidal surge protection barrages (barriers) are constructed to reduce the risk of flooding from tidal surges. They may be permanent or only operated occasionally. Where
they are operated occasionally they would not be expected to have any significant longterm effect upon tidal propagation or the overall hydraulic regime of the estuary. However, the highest tides will never be allowed through so the absolute tidal limit moves
downstream, with possible effects on the tidal limit ecology. It is also probable that
there will be a temporary local impact which could change the current velocity or bed
sediment distribution. Within their design life it is also probable that the barrage
will be used with increasing frequency to combat the predicted rise in mean sea level.
3. Tidal power generation barrages are constructed to provide electricity generation
from the tidal movements and are usually constructed in areas of high tidal range
(>4 m). This type of structure deliberately modifies tidal range and propagation to
extract energy.
4. Water storage barrages are constructed to provide a freshwater lake reserve for abstraction. All tides are totally excluded. Some are to provide a minimum depth for
cooling water abstraction (e.g. original Clyde Weir).
5. Improved water quality barrages aim to improve the flushing characteristics within
a multi-outlet estuary to improve water quality, for example in the case of the Po
River Delta in the northern Adriatic.
6. Silt exclusion. A particular problem sometimes occurs in the upper tidal reaches of
estuaries with high concentrations of silt. During periods of low fresh water flow, silt
migrates upstream and deposits, causing a reduction in the cross-sectional area of
the estuary and its capacity to deal with a fluvial flood.
7. Traffic management. The City of Hull has a particular problem in that the River Hull
is used for commercial vessels but is a barrier to road traffic. The main road bridges
are low level and such is the nature of the tides that vessels requiring maximum
depth have to navigate the river at the same time of day as peak road traffic occurs.
This requires the bridge to be lifted causing severe delays to commercial traffic. One
option being considered is a barrage to maintain a navigable depth at all times,
allowing flexibility of traffic management.
S. Multi-function barrages. Most barrages aim to achieve more than just one of the
above objectives. However, the majority of barrages have their primary aim as satisfying only one of the above objectives. Where the primary aim of the barrage construction consists of more than one of these objectives then the barrage may be
described as a "multi-function barrage". Each promoter will have different objectives for a barrage. The main promoters of barrages in England and Wales are:
- Development corporations, e.g. Cardiff Bay Development Corporation (Cardiff
Bay Barrage)
- Local government, e.g. Greater London Council (Thames Barrier), Swansea City
Council (Tawe Barrage), Kingston upon Hull City Council (Hull Barrage)
- Private developers, e.g. Hayle Harbour barrage developers (Hayle Harbour Barrage)
- Power generation companies and consortiums, e.g. Nova Scotia Power Corporation (Annapolis Royal Tidal Power Station) and The Severn Tidal Power Group
(Severn Barrage)
A barrage will become a permanent feature, so the long-term ownership and management issues are as important as the construction itself. This is especially true in the
37
2. Tidal surge protection barrages (barriers) are constructed to reduce the risk of flooding from tidal surges. They may be permanent or only operated occasionally. Where
they are operated occasionally they would not be expected to have any significant longterm effect upon tidal propagation or the overall hydraulic regime of the estuary. However, the highest tides will never be allowed through so the absolute tidal limit moves
downstream, with possible effects on the tidal limit ecology. It is also probable that
there will be a temporary local impact which could change the current velocity or bed
sediment distribution. Within their design life it is also probable that the barrage
will be used with increasing frequency to combat the predicted rise in mean sea level.
3. Tidal power generation barrages are constructed to provide electricity generation
from the tidal movements and are usually constructed in areas of high tidal range
(>4 m). This type of structure deliberately modifies tidal range and propagation to
extract energy.
4. Water storage barrages are constructed to provide a freshwater lake reserve for abstraction. All tides are totally excluded. Some are to provide a minimum depth for
cooling water abstraction (e.g. original Clyde Weir).
5. Improved water quality barrages aim to improve the flushing characteristics within
a multi-outlet estuary to improve water quality, for example in the case of the Po
River Delta in the northern Adriatic.
6. Silt exclusion. A particular problem sometimes occurs in the upper tidal reaches of
estuaries with high concentrations of silt. During periods of low fresh water flow, silt
migrates upstream and deposits, causing a reduction in the cross-sectional area of
the estuary and its capacity to deal with a fluvial flood.
7. Traffic management. The City of Hull has a particular problem in that the River Hull
is used for commercial vessels but is a barrier to road traffic. The main road bridges
are low level and such is the nature of the tides that vessels requiring maximum
depth have to navigate the river at the same time of day as peak road traffic occurs.
This requires the bridge to be lifted causing severe delays to commercial traffic. One
option being considered is a barrage to maintain a navigable depth at all times,
allowing flexibility of traffic management.
S. Multi-function barrages. Most barrages aim to achieve more than just one of the
above objectives. However, the majority of barrages have their primary aim as satisfying only one of the above objectives. Where the primary aim of the barrage construction consists of more than one of these objectives then the barrage may be
described as a "multi-function barrage". Each promoter will have different objectives for a barrage. The main promoters of barrages in England and Wales are:
- Development corporations, e.g. Cardiff Bay Development Corporation (Cardiff
Bay Barrage)
- Local government, e.g. Greater London Council (Thames Barrier), Swansea City
Council (Tawe Barrage), Kingston upon Hull City Council (Hull Barrage)
- Private developers, e.g. Hayle Harbour barrage developers (Hayle Harbour Barrage)
- Power generation companies and consortiums, e.g. Nova Scotia Power Corporation (Annapolis Royal Tidal Power Station) and The Severn Tidal Power Group
(Severn Barrage)
A barrage will become a permanent feature, so the long-term ownership and management issues are as important as the construction itself. This is especially true in the