5 Towards Developing Guidelines
41
Although a substantial number of predictive studies have been undertaken with
regard to the likely effects of barrages, relatively little information has yet been forthcoming on the actual impact of implemented schemes; a situation which somewhat
constrains debate on the likely effects of proposed developments. However, a change
from a brackish estuary to a freshwater lake will have many obvious changes on the
ecology! The present account draws primarily on predictive studies and is, therefore,
largely theoretical, but some information on the actual impact of the Tawe (partial
exclusion) and Tees (total exclusion) Barrages is presented in the guidelines (see Fig. 5.1).
The environmental policy of zero change in ecology is an impossible target and
efforts must be directed towards identifying the key issues and deciding what degree of
change is acceptable or beneficial and indeed sustainable. This will result in some mitigation measures and hopefully some enhancements in the development of a scheme.
The potential impact of barrage developments in tidal environments on fisheries and
nature conservation interests is of major concern to the Environment Agency because
of the Agency's fundamental responsibilities in both of these areas. It is also a concern
to anglers, fishery owners, conservation groups and the general public.
It is the responsibility of the barrage scheme promoters to undertake an environmental assessment of their scheme, should this be required, and to present the results
of their studies as an environmental statement. This document should be prepared in a
manner suitable to answer any questions posed by the Environment Agency. For their
part the Environment Agency must be aware of the likely impacts of barrage developments on fisheries and nature conservation and have a view on what constitutes reasonable good practice for the assessment of such impacts, i.e. defending the rights of
legitimate "users" of the environment. This latter point is of particular importance as
reliable methods to precisely predict the impact of barrage schemes on fisheries and
nature conservation interests do not exist.
5.4.2
Hydrodynamics
The construction of a barrage can alter the tidal hydraulic regime in a number of ways.
It is important to considers the effect a barrage is likely to have on the main hydrodynamic processes that control levels, flows, mixing, flushing and transport of dissolved
and suspended matter in the separated water bodies upstream and downstream of the
new structure. The issues relevant to hydrodynamics can be summarised as follows:
• The truncation of an estuary by a barrage can radically alter the tidal regime downstream specially if the estuary is close to resonance.
• The pattern of saline intrusion, stratification and gravitational circulation, pollution, transport and deposition of fine sediments in a deep estuary can be sensitive
to the changes in tidal flows caused by the construction of a partial or complete
barrage and by the extraction or rerouteing of non-saline inflows.
• A poorly located barrage in a shallow estuary (where tidal range approximately equals
depth) can cause major siltation problems downstream.
• Half tide barrages which are designed to hold up low-water levels to prevent the
exposure of unsightly mud banks and barrages with navigation locks, which allow
the intrusion of seawater, give rise to a whole class of issues in the upstream pool.
41
Although a substantial number of predictive studies have been undertaken with
regard to the likely effects of barrages, relatively little information has yet been forthcoming on the actual impact of implemented schemes; a situation which somewhat
constrains debate on the likely effects of proposed developments. However, a change
from a brackish estuary to a freshwater lake will have many obvious changes on the
ecology! The present account draws primarily on predictive studies and is, therefore,
largely theoretical, but some information on the actual impact of the Tawe (partial
exclusion) and Tees (total exclusion) Barrages is presented in the guidelines (see Fig. 5.1).
The environmental policy of zero change in ecology is an impossible target and
efforts must be directed towards identifying the key issues and deciding what degree of
change is acceptable or beneficial and indeed sustainable. This will result in some mitigation measures and hopefully some enhancements in the development of a scheme.
The potential impact of barrage developments in tidal environments on fisheries and
nature conservation interests is of major concern to the Environment Agency because
of the Agency's fundamental responsibilities in both of these areas. It is also a concern
to anglers, fishery owners, conservation groups and the general public.
It is the responsibility of the barrage scheme promoters to undertake an environmental assessment of their scheme, should this be required, and to present the results
of their studies as an environmental statement. This document should be prepared in a
manner suitable to answer any questions posed by the Environment Agency. For their
part the Environment Agency must be aware of the likely impacts of barrage developments on fisheries and nature conservation and have a view on what constitutes reasonable good practice for the assessment of such impacts, i.e. defending the rights of
legitimate "users" of the environment. This latter point is of particular importance as
reliable methods to precisely predict the impact of barrage schemes on fisheries and
nature conservation interests do not exist.
5.4.2
Hydrodynamics
The construction of a barrage can alter the tidal hydraulic regime in a number of ways.
It is important to considers the effect a barrage is likely to have on the main hydrodynamic processes that control levels, flows, mixing, flushing and transport of dissolved
and suspended matter in the separated water bodies upstream and downstream of the
new structure. The issues relevant to hydrodynamics can be summarised as follows:
• The truncation of an estuary by a barrage can radically alter the tidal regime downstream specially if the estuary is close to resonance.
• The pattern of saline intrusion, stratification and gravitational circulation, pollution, transport and deposition of fine sediments in a deep estuary can be sensitive
to the changes in tidal flows caused by the construction of a partial or complete
barrage and by the extraction or rerouteing of non-saline inflows.
• A poorly located barrage in a shallow estuary (where tidal range approximately equals
depth) can cause major siltation problems downstream.
• Half tide barrages which are designed to hold up low-water levels to prevent the
exposure of unsightly mud banks and barrages with navigation locks, which allow
the intrusion of seawater, give rise to a whole class of issues in the upstream pool.