42
T.N. Burt . I.C. Cruickshank· M.A. Littlewood
• The intruding seawater forms a stable two layer system with little or no vertical
turbulent exchange. Mud and polluted particulate matter tend to be trapped in the
lower layer causing major water quality problems. There can also be significant chemical exchange across the sediment/water interface.
• It is difficult and expensive to design and operate gate control and selective withdrawal strategies which are effective at flushing the landward extremities of the lower
polluted layer without nullifying the benefits of the barrage.
• Construction works can have significant adverse effects in terms of erosion and
deposition of marine muds.
• A barrage may damage the fauna and flora, by whatever cause, both upstream and
downstream of a barrage and this is a main issue to be considered during construction and operation. There is particular concern for the effect of reduced tidal flows
and regulation of freshwater inflows on mixing and flushing of dissolve4 and suspended pollutants. Another area of concern is the potential changes in the patterns
of erosion and deposition of fine sediments. Changes in the hydraulic regime will
also impact on the behaviour of migratory fish.
5.4.3
Morphology
The morphology, or shape, of an estuary depends upon the fluvial and tidal flow within
the estuary and the fluvially and tidally borne sediments. In a natural estuary, the morphology results from a balance between the flow and sediment movement. The river
brings fluvial sediment into the estuary while tidal flows introduce tidally borne sediments from the sea. The construction of a barrage disturbs this natural equilibrium.
The action of a barrage is to affect the flow that is responsible for the movement of the
sediment and also to directly affect the movement of sediment at the barrage site itself.
The impact on the flow may be local to the barrage, leading possibly to local scour, or it
may be general, as, for example, when the barrage maintains a permanently high water
level upstream. In this latter case the impact of the barrage on the morphology of the
river may extend for some distance upstream. By modifying the flow in an estuary the
impact on the morphology may also extend a significant distance downstream.
The barrage usually increases water levels upstream. By excluding tidal flows, a further impact of a barrage is usually to reduce the discharge at the barrage itself. These
effects reduce the hydraulic gradient upstream and hence reduce the sediment-transporting capacity of the channel upstream of the barrage. The sediment production
from the catchment remains the same as in the pre-barrage conditions and so the balance between sediment production and transport is disturbed. The reduced transport
capacity upstream leads to sediment deposition upstream of the barrage. This can adversely affect flood levels and can affect other users of the estuary. Any reduction in
flows at the barrage itself may lead to sedimentation in the channel downstream. This
can have an impact on other activities in the estuary and adversely affect flood levels.
By retaining water upstream, a barrage normally reduces tidal discharges downstream. This disturbs the balance between channel size and shape and the discharge and
so a barrage induces morphological changes downstream. A barrage may also act as a
physical barrier to the movement of coarse sediment, reducing the movement of fluvial
T.N. Burt . I.C. Cruickshank· M.A. Littlewood
• The intruding seawater forms a stable two layer system with little or no vertical
turbulent exchange. Mud and polluted particulate matter tend to be trapped in the
lower layer causing major water quality problems. There can also be significant chemical exchange across the sediment/water interface.
• It is difficult and expensive to design and operate gate control and selective withdrawal strategies which are effective at flushing the landward extremities of the lower
polluted layer without nullifying the benefits of the barrage.
• Construction works can have significant adverse effects in terms of erosion and
deposition of marine muds.
• A barrage may damage the fauna and flora, by whatever cause, both upstream and
downstream of a barrage and this is a main issue to be considered during construction and operation. There is particular concern for the effect of reduced tidal flows
and regulation of freshwater inflows on mixing and flushing of dissolve4 and suspended pollutants. Another area of concern is the potential changes in the patterns
of erosion and deposition of fine sediments. Changes in the hydraulic regime will
also impact on the behaviour of migratory fish.
5.4.3
Morphology
The morphology, or shape, of an estuary depends upon the fluvial and tidal flow within
the estuary and the fluvially and tidally borne sediments. In a natural estuary, the morphology results from a balance between the flow and sediment movement. The river
brings fluvial sediment into the estuary while tidal flows introduce tidally borne sediments from the sea. The construction of a barrage disturbs this natural equilibrium.
The action of a barrage is to affect the flow that is responsible for the movement of the
sediment and also to directly affect the movement of sediment at the barrage site itself.
The impact on the flow may be local to the barrage, leading possibly to local scour, or it
may be general, as, for example, when the barrage maintains a permanently high water
level upstream. In this latter case the impact of the barrage on the morphology of the
river may extend for some distance upstream. By modifying the flow in an estuary the
impact on the morphology may also extend a significant distance downstream.
The barrage usually increases water levels upstream. By excluding tidal flows, a further impact of a barrage is usually to reduce the discharge at the barrage itself. These
effects reduce the hydraulic gradient upstream and hence reduce the sediment-transporting capacity of the channel upstream of the barrage. The sediment production
from the catchment remains the same as in the pre-barrage conditions and so the balance between sediment production and transport is disturbed. The reduced transport
capacity upstream leads to sediment deposition upstream of the barrage. This can adversely affect flood levels and can affect other users of the estuary. Any reduction in
flows at the barrage itself may lead to sedimentation in the channel downstream. This
can have an impact on other activities in the estuary and adversely affect flood levels.
By retaining water upstream, a barrage normally reduces tidal discharges downstream. This disturbs the balance between channel size and shape and the discharge and
so a barrage induces morphological changes downstream. A barrage may also act as a
physical barrier to the movement of coarse sediment, reducing the movement of fluvial