44
T.N. Burt . I.e. Cruickshank· M.A. Littlewood
• Change in flood regime upstream (tidal to fluvial dominance, peak flow to flood
volume as critical factor)
• Flood forecasting and control of the structure
• Groundwater seepage from higher impounded levels
• Downstream reflected wave - and operational procedures to minimise this
• Changes to the discharge control of surface water drainage systems
• Changes in the river bed level resulting from the deposition of sediments
• Anticipated rises in sea level
Flood defence is one of the main functions of river authorities including the Environment Agency, which is responsible for the main rivers in England and Wales. The
Agency is likely to "require developers to undertake mitigation works at their own cost
as a pre-condition to agreement and seek to ensure that development does not proceed
until such works are implemented". At all stages of a barrage project from the initial
pre-feasibility studies through to the construction and operation, flood defence is one
of the key issues that has to be considered.
The first issue to consider is that the regime will change as the relative influence of
tide and river (fluvial) flows change. Upriver of a tide-excluding barrage, especially one
built near the mouth of a river, the natural river cross-section is generally large because
of its previous need to accommodate both tidal and river discharge. The channel itself
is therefore likely to have more than adequate capacity to pass the design fluvial flood.
The flood risk is therefore almost entirely dependent on the ability of the structure
itself to pass the design flood. It is therefore essential that adequate discharge capacity
is provided within the structure. Of course fluvial flows can only be passed when the
tide level is below the impounded level (i.e. when the barrage is not "tide locked") and
the available head for discharge may be limited by the tide level.
It may be possible to drain down the water ahead of any anticipated flood to maximise the storage capacity and minimise the risk of flooding. In many cases it may also be
considered undesirable to drain down an impounded river/estuary because of the effect on structures, moored vessels, etc. If a drain down procedure is to be used a good
warning system will have to be adopted. Closing a tidal surge barrier at low water can
be of great advantage to reduce the tidal restriction on the evaluation of fluvial
floodwaters if the structure is far enough downriver to provide a sufficient storage area
to accommodate the fluvial flood discharge until the barrier can be re-opened on the
following falling ebb tide. The Hull tidal surge barrier, for example, has been used in
this way, closing the barrier at low water, thereby providing the whole tidal volume as
storage for the flood volume. Drains upriver of a proposed barrage will, in many cases,
be controlled by tide flaps, impounding drainage water during tide-locked periods and
discharging at low tide. If a permanent high water level is created by a barrage, this
method will cease to work and pumping or diversion will be necessary.
5.4.5
Groundwater
Barrages affect the water levels in the estuaries they cross. Historically, levels in surface
water channels have been studied by hydrologists and sub-surface flows by hydrogeologists without any real concern for the exchange between surface water and sub-sur-
T.N. Burt . I.e. Cruickshank· M.A. Littlewood
• Change in flood regime upstream (tidal to fluvial dominance, peak flow to flood
volume as critical factor)
• Flood forecasting and control of the structure
• Groundwater seepage from higher impounded levels
• Downstream reflected wave - and operational procedures to minimise this
• Changes to the discharge control of surface water drainage systems
• Changes in the river bed level resulting from the deposition of sediments
• Anticipated rises in sea level
Flood defence is one of the main functions of river authorities including the Environment Agency, which is responsible for the main rivers in England and Wales. The
Agency is likely to "require developers to undertake mitigation works at their own cost
as a pre-condition to agreement and seek to ensure that development does not proceed
until such works are implemented". At all stages of a barrage project from the initial
pre-feasibility studies through to the construction and operation, flood defence is one
of the key issues that has to be considered.
The first issue to consider is that the regime will change as the relative influence of
tide and river (fluvial) flows change. Upriver of a tide-excluding barrage, especially one
built near the mouth of a river, the natural river cross-section is generally large because
of its previous need to accommodate both tidal and river discharge. The channel itself
is therefore likely to have more than adequate capacity to pass the design fluvial flood.
The flood risk is therefore almost entirely dependent on the ability of the structure
itself to pass the design flood. It is therefore essential that adequate discharge capacity
is provided within the structure. Of course fluvial flows can only be passed when the
tide level is below the impounded level (i.e. when the barrage is not "tide locked") and
the available head for discharge may be limited by the tide level.
It may be possible to drain down the water ahead of any anticipated flood to maximise the storage capacity and minimise the risk of flooding. In many cases it may also be
considered undesirable to drain down an impounded river/estuary because of the effect on structures, moored vessels, etc. If a drain down procedure is to be used a good
warning system will have to be adopted. Closing a tidal surge barrier at low water can
be of great advantage to reduce the tidal restriction on the evaluation of fluvial
floodwaters if the structure is far enough downriver to provide a sufficient storage area
to accommodate the fluvial flood discharge until the barrier can be re-opened on the
following falling ebb tide. The Hull tidal surge barrier, for example, has been used in
this way, closing the barrier at low water, thereby providing the whole tidal volume as
storage for the flood volume. Drains upriver of a proposed barrage will, in many cases,
be controlled by tide flaps, impounding drainage water during tide-locked periods and
discharging at low tide. If a permanent high water level is created by a barrage, this
method will cease to work and pumping or diversion will be necessary.
5.4.5
Groundwater
Barrages affect the water levels in the estuaries they cross. Historically, levels in surface
water channels have been studied by hydrologists and sub-surface flows by hydrogeologists without any real concern for the exchange between surface water and sub-sur-