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POSTNOTE
Number 435 June 2013
Environmental Impact of Tidal Energy Barrages
Overview
A tidal energy barrage across the Severn Estuary
could produce up to 5% of the UK’s electricity
demand. It would help meet renewable energy
targets but would have significant environmental
impacts. This POSTnote summarises evidence
on environmental impacts associated with the
operation of tidal energy barrages and the
effectiveness of compensatory measures.
Background
Energy Extraction
Tidal energy barrages are a form of low-carbon marine
renewable energy referred to as tidal range technology.
Tidal range technologies harness the energy of an incoming
or outgoing tide. There are various marine energy
technologies (such as offshore wind turbines, tidal stream
turbines), but tidal energy barrages offer the most
predictable and consistent supply of electricity.1 Tidal
energy barrages are dam-like structures consisting of
turbines and sluice gates (Figure 1) and are typically located
across the width of an estuary. There are two kinds:
„ Ebb Generation: the basin behind the barrage is filled on
the incoming tide. Once the tide has reached its highest
point, the sluice gates are shut for 1-2 hours while the tide
falls on the outside of the barrage. The sluice gates are
then opened allowing the water in the basin to pass out
through the turbines for 4-5 hours.
„ Ebb-Flood Generation: electricity is produced on both
the incoming and outgoing tides as the water passes
through the turbines.
„ Tidal barrages are a source of low-carbon
energy that can significantly reduce
dependence on fossil fuels.
„ Tidal barrages impact water movement and
the amount of suspended sediment, resulting
in loss of intertidal habitat.
„ It is uncertain how changes in water quality
will impact biodiversity, but there is likely to
be a change in the type of animals and
plants that colonise estuaries.
„ It is uncertain whether turbines designed to
lower fish mortality would allow for the
maintenance of fish populations, as existing
studies on their efficacy are limited.
„ In addition, a barrage may increase levels of
fish mortality due to predation, disease,
habitat loss and disruption to movement.
„ There is limited evidence on the efficacy of
compensatory and mitigation measures.
way to meeting UK renewable energy targets (POSTnote
324). There has been interest in constructing a barrage
across the Severn Estuary because of its large tidal range;3
a barrage from Cardiff to Weston-super-Mare could meet up
to 5% of the UK’s electricity demand. In 2010, the
Department of Energy and Climate Change (DECC)
published results of a feasibility study that investigated the
impact of an ebb-only tidal energy barrage across the
Severn Estuary.1 In respect of environmental impacts it
concluded that “the scale and impact of a scheme would be
unprecedented... and there is significant uncertainty on how
the regulatory framework would apply to it”. There has been
no further development of proposals for other UK barrages,
but in 2012, Hafren Power submitted a proposal to the
Government for an ebb-flood tidal energy barrage across
the Severn Estuary.
Figure 1: Cross-section of an ebb-generating tidal barrage.
Tidal Power in the UK
Tidal energy barrages have been proposed for a number of
UK estuaries including the Mersey, Humber, Duddon, Wyre,
the Wash and the Severn. UK Barrages could contribute up
to 15% of total energy production,2 which would go some
The Parliamentary Office of Science and Technology, 7 Millbank, London SW1P 3JA T 020 7219 2840 E [email protected] www.parliament.uk/post
POSTnote Number 435 June 2013 Environmental Impact of Tidal Energy Barrages
Box 1. Sediment Dynamics at Existing Barrages
There are a number of tidal energy barrages which include: two in
China (Jiangxia and Jindo Uldolmok), and one each in South Korea
(Sihwa), Canada (Annapolis Royal), France (La Rance) and Russia
(Kislaya Guba). However, there is no known published information on
environmental impacts at the Korean and Chinese barrages. The
Eastern Scheldt Storm Surge Barrier in the Netherlands is also
commonly referred to when discussing barrage impacts: though not a
barrage, it is likened to a barrage as it allows for the free passage of
water like an ebb-flood barrage.8,9
„ La Rance differs from many UK estuaries in that it has a very low
level of sediment.4 Observations since construction of the barrage
include diverse but altered fish, invertebrate and bird populations.5
However, because of insufficient data prior to construction it is
uncertain how species composition, abundance and distribution
has changed. There is no information on rates of habitat loss and
gain, or fish mortality.
„ The Eastern Scheldt was experiencing erosion prior to
construction of the barrier and other dams,6 but this was amplified
post-construction.7-9 There was a change in erosion patterns from
the erosion of channels to the erosion of intertidal habitat8,9 with the
rate of loss of intertidal habitat exceeding that expected due to sealevel rise.7
„ Previously stable habitats at Annapolis Royal are said to be now
undergoing erosion. However, because of a lack of data it is not
possible to say that the system has undergone a net loss of
habitat. In addition, erosion pressure is amplified by particularly
complex water currents and ice, factors which do not affect the
Severn Estuary.10-12
The increased rate of erosion downstream of Annapolis Royal and
upstream of the Eastern Scheldt may be caused by the structures
restricting sediment movement into these areas thus impeding habitat
development.13 The Severn Estuary is unusual in that it receives
relatively little new sediment from either rivers (fluvial) or marine
sources; the majority of sediment enters the system from the on-going
erosion of intertidal habitat.
Existing Barrages
The extent to which existing barrages (Box 1) and other
similar structures can inform future developments is
determined by similarity in characteristics and the availability
of data from before and after construction. For these
reasons there is limited data from La Rance, with a little
more known on Annapolis Royal and the Eastern Scheldt.
Predicting Impacts
To predict the impact of a tidal barrage, first requires an
understanding of the factors (for example, water currents
and sediment load) that influence estuary characteristics.
Prediction commonly starts with simplification of how an
estuary works and how a barrage may influence a
characteristic using data from similar estuaries or from the
estuary of interest. This is referred to as conceptual
modelling. Computer models are then used to simulate the
conceptual model and provide a measure of the likelihood
and scale of a particular impact occurring. Confidence in
computer models depends on the quality and availability of
data used to build the model, the type of model and whether
the prediction can be validated. Impact prediction of a
Severn Barrage suffers from two problems:
„ a lack of real data on certain factors to build computer
models
„ a lack of validating computer models due to a lack of
similar estuaries and similar barrages.
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Impacts
Most research on the impact of barrage construction has
focused on water movement (hydrodynamics) and physical
structure of habitat. This research has implications for
factors such as biodiversity and water quality.
Hydrodynamic
Hydrodynamic response to a tidal barrage has been
explored using computer models.14, 15 The following
hydrodynamic responses are predicted to occur following
construction of a Severn Barrage, and have been observed
at La Rance, Annapolis Royal and the Eastern Scheldt:
„ reduction in tidal range with loss of intertidal habitat area
(see Figure 2)14
„ reduction in water speed, resulting in reduced suspended
sediment.14
Figure 2: Estuary Habitat Types
Habitat Structure
Studies of habitat response are limited to conceptual models
only.16, 17 A range of habitats will be affected by a tidal
barrage including:
„ salt marsh: an important habitat for birds, fish, and plants.
Salt marsh depends on sediment transported and
deposited by the incoming tide for growth at a rate that
exceeds sea-level rise.
„ mud flats: an intertidal habitat that is vitally important for
migratory birds, and species of invertebrates and fish.
„ sand flats: an intertidal habitat important for many
invertebrate species.
Two conceptual models have been developed to try to
explain long-term change in habitat structure in the Severn
Estuary.16, 17 Both predict that there will be an initial loss of
intertidal habitat (mud flat and salt marsh) due to reduced
tidal range. However, the first predicts that new habitat will
be established almost immediately,16 while the second
predicts there will be no gain in habitat because of
increased erosion pressure.17 In an attempt to validate
model scenarios, efforts have been made to match the
Severn to La Rance, the Eastern Scheldt and Annapolis
Royal.4,7,13 Both La Rance and Annapolis Royal are of
limited use because of a lack of data quantifying the impact
of barrages on hydrodynamic and habitat structural
response (Box 1). The Eastern Scheldt provides some
useful information on the possible impact of a barrage.
However, the difference in the source and transport of
sediment between the Eastern Scheldt and the Severn
makes it difficult to conclude the same erosion patterns will
occur. Intertidal habitat development has not been predicted
using computer models for an ebb-flood barrage. In the
2010 DECC study,15 computer modelling of an ebb-only
barrage resulted in a number of impacts including:
„ loss of more than 36-63% of intertidal habitat
POSTnote Number 435 June 2013 Environmental Impact of Tidal Energy Barrages
„ reduction in peak suspended sediment load by a factor of
2-3 and rapid accumulation of up to 2m of sediment
upstream of the Severn Bridge
„ erosion of 1.3-3.1m at the Mid Severn flats and 0.3-1.4m
accretion over the Outer Severn channel.15
Water Quality
Hydrodynamics and suspended sediment have important
implications for water quality. There are factors which may
result in both a positive and negative effect on the
concentration of metals, nutrients, and pathogens within the
estuary.18 The propensity of an estuary to develop waterquality problems is dependent on two key factors:
„ the amount of time water spends in the estuary, which is
increased by an ebb-only barrage
„ the existing level of nutrients, pathogens and metals.
Predictions of impacts to water quality have been based on
conceptual and computer models. The 2010 DECC study19
found an ebb-only barrage in the Severn had no significant
impact on water quality, except for an increase in nutrient
levels. This, in combination with reduced suspended
sediment, would increase the level of algae and plants.
However, this increased productivity is likely to be less
pronounced under an ebb-flood barrage.
Biodiversity
The term biodiversity is used to describe the variety of life
(including plants, animals and bacteria) on earth.20 A tidal
energy barrage may impact biodiversity because of either a
loss, gain or a change in species. A simple gain in the
number of species may not represent a gain in biodiversity if
it results in the loss of rare or unique species that are
replaced with common species.21 While some studies
suggest that a Severn Barrage will increase biodiversity, 4,17
these are not based on studies in which the number and
type of species were evaluated before and after barrage
construction. For there to be an overall gain in biodiversity,
an increase in the number of some species (for example
invertebrates) needs to offset species loss due to habitat
loss, fish mortality in turbines, bird displacement and the
loss of rare or unique species. To date, no studies at
existing barrages have assessed overall change in species
diversity and abundance.
Invertebrates
Invertebrates (animals without a backbone) make up a
significant majority of the biodiversity in an estuary and
include Sabellaria, brown shrimp, and mussels. Invertebrate
diversity and abundance is predicted to increase following
barrage construction,4,17 though this depends on the extent
and type of habitat lost following barrage construction. Such
gains would likely be caused by a reduction in water velocity
and suspended sediment, and changes in sediment type.
Increased invertebrate abundance would likely be more
pronounced under an ebb only barrage scheme.
Birds
Species of waders appear to be particularly sensitive to
barrage operation, because of intertidal habitat loss and
displacement. In 1999, a barrage was constructed in Cardiff
Bay as part of a regeneration project. Studies in the bay
have revealed displacement of a number of species to
neighbouring sites. However displaced birds have exhibited
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reduced survival rates compared to non-displaced birds.22 In
both the Eastern Scheldt and Cardiff Bay, there have been
declines or local extinctions of wader species.7, 22
It is often reported that the loss of intertidal habitat and
subsequent decline in birds will be offset by the increase in
available food within the remaining intertidal habitat.
However, the DECC 2010 study concluded that the scale of
habitat loss for an ebb only barrage would far outweigh any
gain in habitat quality.36
Fish Mortality
There are two categories of threats to fish:
„ Direct: injury and mortality due to blade strike and water
conditions (for example water pressure) resulting in
damage or disorientation.
„ Indirect: loss and degradation of habitat which may be
important for feeding and spawning; and disruption to
movement (such as completion of migration).
Mortality due to blade strike is the most studied source of
fish mortality. Estimates depend on the type and operation
of a turbine, and species of fish.23 In order to reduce levels
of blade strike, the Oak Ridge National Laboratory (ORNL)24
produced a set of criteria for the design of ‘fish-friendly’
turbines. These criteria are based on an extensive literature
review of studies on single fish passage through turbines at
hydro-power schemes. In recent years, levels of survival
greater than 90% have been achieved.25-28 While high
survivability is possible, no field trials of turbines in an
estuarine environment have been carried out. In addition,
most studies of fish mortality have been carried out on
small-sized salmon, a notoriously robust species; therefore
reports of ‘negligible’ mortality levels may not be applicable
to the diversity of fish, crustacean and invertebrate species
common in estuaries29-31 such as adult salmon, shad,
flounder, brown shrimp. In addition, estimates of fish
mortality are based on fish making a single pass through a
turbine: fish living in an estuary may make multiple passes
in a day, increasing their risk of mortality.
Less studied is mortality due to sub-lethal injuries, predation
or indirect impacts. During turbine passage, blade strike and
hydraulic conditions can result in injuries (for example scale
loss, eye loss or abrasions) which may not cause immediate
mortality, but will reduce survival through disease or
decreased fitness. Fish commonly suffer disorientation
during turbine passage which increases predation risk by
other fish, fish-eating birds and aquatic mammals;23 this has
been observed at La Rance.32 In a recent study,33 mortality
because of predation of juvenile salmon was found to
account for between 46-70% of total mortality, indicating it is
potentially a considerable source of mortality. However, at
present studies on mortality levels due to predation pressure
and sub-lethal injuries are limited.
Very Low-Head Turbines
Very Low-Head (VLH) turbines are a new technology that
reportedly has a smaller impact on fish than existing
technologies.34 A recent trial on a VLH prototype achieved
100% survivability of both large and small eels.34 However,
very few trials of VLH turbines have been undertaken. In
response to growing interest in VLH turbines, the Canadian
government published guidelines for the testing of these
POSTnote Number 435 June 2013 Environmental Impact of Tidal Energy Barrages
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turbines. The report comments that “mortality, although
easy to define and measure, is simply one way to evaluate
the biological effectiveness of a turbine yet the majority of
studies focus only on mortality as an endpoint”. It suggests
that a “suite of endpoints should be examined which
incorporate relevant metrics that have the potential to
influence long-term survival, health, condition and fitness”.
Compensation and Mitigation
Tidal energy barrages may impact on features (such as
habitat types, birds, fish and invertebrates) protected under
a number of EU Directives (Box 2). Under development of a
Severn Barrage, impacts to protected features include:
„ habitat loss, for example salt marsh and Sabelleria reefs,
due to reduced tidal range and water speed
„ loss of intertidal habitat which supports designated bird
species such as Dunlin and Bewick's Swan
„ loss of designated fish, for example, Twaite Shad, River
Lamprey and Atlantic Salmon, due to degradation of
habitat, blocking of migration path and direct mortality
„ decline in the ecological status of a water body.
Compensation for Habitats and Birds
Compensation for designated intertidal habitats is typically
carried out by recreating the lost features (such as Atlantic
salt meadows, mudflats), referred to as ‘Like for Like’. The
most commonly used technique is managed re-alignment in
which “sea walls are deliberately breached to allow the
coastline to move inland” (POSTnote 342). While this
method has been successfully used to compensate for lost
habitat on port developments, which are relatively small,
there are concerns over its efficacy for the much larger
Severn Barrage scheme. The success of managed realignment is well documented for salt marsh habitats, but
less so for sand flats and mud flats.43 No compensatory
measures are available for the loss of subtidal habitats that
support Sabellaria reefs.36 Compensatory measures would
need to be implemented prior to construction allowing
sufficient time for monitoring to ensure sustainability of the
Box 2. EU Policy Affecting UK Estuaries
Many UK estuaries fall under the following EU directives:
„ The EU Habitats Directive seeks to maintain or restore habitats
and species listed under Annex I and II respectively and
designation of sites as Special Areas of Conservation (SACs).
„ The EU Birds Directive promotes the conservation of wild birds,
with identification of Special Protected Areas (SPAs) required for
vulnerable species listed in Annex I and migratory species.
„ The EU Water Framework Directive requires European Union
member states to achieve good status of surface water bodies
(including marine waters up to one nautical mile from shore) by
2027 (some types of water body are exempt, POSTnote 320).
The network of SACs and SPAs is referred to as the Natura 2000
network. Where a development impacts the Natura 2000 network, it
must first assess whether there will be a negative impact of
development (Article 6(3)). If there is, the development may only be
permitted to proceed (Article 6(4)):
„ if there is no “alternative solution” that is less damaging
„ that there are “Imperative Reasons of Overriding Public Interest”
(IROPI) for the development
„ that all compensatory measures necessary to ensure the overall
coherence of Natura 2000 is protected are undertaken.
Page 4
newly created features.
Compensation for Fish
A range of methods has been proposed for compensating
for the loss of fish populations and their habitats (Box 3).
However, research into these methods has focused on
design and implementation for a limited number of species
and at much smaller scales. For estuaries as large as the
Severn, most compensation measures would be required on
an unprecedented scale. The 2010 DECC study concluded
that “fish compensation measures are unlikely to completely
offset all negative effects”. 43
Equal Value
It is recognised that it may be difficult to recreate designated
features on a ‘Like for Like’ basis. The Sustainable
Development Commission (SDC)37 led an investigation into
the case of compensating with habitat of ‘Equal Value’
rather than ‘Like for Like’. ‘Equal Value’ could be an
alternative and adaptive form of compensation for
developments where it is not possible to recreate ‘Like for
Like’ or where there is a lack of available land for recreating
lost habitat. However, at present ‘Like for Like’ is an
untested method. The SDC investigation concluded that it
represented “an unprecedented challenge” for a Severn
Barrage scheme.
Conclusions
Tidal energy barrages have a number of environmental
impacts that vary in scale depending on the type of barrage.
There is insufficient evidence to determine the efficacy of
compensation and mitigation measures for a Severn
Barrage scheme. Some academics and NGOs are in favour
of an incremental approach and step by step development
of tidal power technologies and compensation on a smaller
scale. 38-42
Box 3. Compensatory Measures for Fish
The following possible compensation measures were investigated in
the 2010 DECC study:
„ Exclusion and diversion from turbines: while there is evidence
for their effectiveness, they potentially have contradictory effects on
different species.43
„ Bypasses: most research into the design of bypasses has focused
on freshwater species, such as salmon, trout, eels and shad. There
has been limited research on marine species.43
„ Herding and translocation: this measure involves the collection,
transportation and release of fish from downstream of the barrage,
to upstream. It is not an established measure, with research limited
to only a few species.43
„ Predator control: effective methods are limited to birds, with no
known effective methods for fish.43
„ Re-stocking: this measure supports population numbers, however,
it reduces genetic diversity of a population thereby reducing the
potential of a species to adapt and evolve.43, 44
„ Habitat creation and enhancement: an established method for
many estuarine and marine species, but there is less evidence for
its efficacy for migratory species.43
Endnotes
For references, please see:
http://www.parliament.uk/documents/POST/postpn435_Environmental-Impact-ofTidal-Energy-Barragesreferences.pdf
POST is an office of both Houses of Parliament, charged with providing independent and balanced analysis of policy issues that have a basis in science and technology.
POST is grateful to Nadia Richman for researching this briefing, to NERC for funding her parliamentary fellowship, and to all contributors and reviewers. For further
information on this subject, please contact the co-author, Dr Jonathan Wentworth. Parliamentary Copyright 2013. Image copyright Wildfowl and Wetland Trust.