„ POSTNOTE Number 435 June 2013 Environmental Impact of Tidal Energy Barrages

Overview „ 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 A tidal energy barrage across the Severn Estuary maintenance of fish populations, as existing could produce up to 5% of the UK’s electricity studies on their efficacy are limited. demand. It would help meet renewable energy „ In addition, a barrage may increase levels of targets but would have significant environmental fish mortality due to predation, disease, impacts. This POSTnote summarises evidence habitat loss and disruption to movement. on environmental impacts associated with the „ There is limited evidence on the efficacy of operation of tidal energy barrages and the compensatory and mitigation measures. effectiveness of compensatory measures.

Background way to meeting UK renewable energy targets (POSTnote Energy Extraction 324). There has been interest in constructing a barrage 3 Tidal energy barrages are a form of low-carbon marine across the Severn Estuary because of its large tidal range; renewable energy referred to as tidal range technology. a barrage from Cardiff to Weston-super-Mare could meet up Tidal range technologies harness the energy of an incoming to 5% of the UK’s electricity demand. In 2010, the or outgoing tide. There are various marine energy Department of Energy and Climate Change (DECC) technologies (such as offshore wind turbines, tidal stream published results of a feasibility study that investigated the turbines), but tidal energy barrages offer the most impact of an ebb-only tidal energy barrage across the 1 predictable and consistent supply of electricity.1 Tidal Severn Estuary. In respect of environmental impacts it energy barrages are dam-like structures consisting of concluded that “the scale and impact of a scheme would be turbines and sluice gates (Figure 1) and are typically located unprecedented... and there is significant uncertainty on how across the width of an estuary. There are two kinds: the regulatory framework would apply to it”. There has been „ Ebb Generation: the basin behind the barrage is filled on no further development of proposals for other UK barrages, the incoming tide. Once the tide has reached its highest but in 2012, Hafren Power submitted a proposal to the point, the sluice gates are shut for 1-2 hours while the tide Government for an ebb-flood tidal energy barrage across falls on the outside of the barrage. The sluice gates are the Severn Estuary. then opened allowing the water in the basin to pass out Figure 1: Cross-section of an ebb-generating tidal barrage. 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 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 2 to 15% of total energy production, 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 Page 2

Box 1. Sediment Dynamics at Existing Barrages Impacts There are a number of tidal energy barrages which include: two in Most research on the impact of barrage construction has China (Jiangxia and Jindo Uldolmok), and one each in South Korea focused on water movement (hydrodynamics) and physical (Sihwa), Canada (Annapolis Royal), France (La Rance) and Russia structure of habitat. This research has implications for (Kislaya Guba). However, there is no known published information on environmental impacts at the Korean and Chinese barrages. The factors such as biodiversity and water quality. Eastern Scheldt Storm Surge Barrier in the Netherlands is also Hydrodynamic 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 Hydrodynamic response to a tidal barrage has been .14, 15 water like an ebb-flood barrage.8,9 explored using computer models The following „ La Rance differs from many UK estuaries in that it has a very low hydrodynamic responses are predicted to occur following level of sediment.4 Observations since construction of the barrage construction of a Severn Barrage, and have been observed include diverse but altered fish, invertebrate and bird populations.5 at La Rance, Annapolis Royal and the Eastern Scheldt: However, because of insufficient data prior to construction it is „ reduction in tidal range with loss of intertidal habitat area uncertain how species composition, abundance and distribution 14 has changed. There is no information on rates of habitat loss and (see Figure 2) gain, or fish mortality. „ reduction in water speed, resulting in reduced suspended „ The Eastern Scheldt was experiencing erosion prior to sediment.14 construction of the barrier and other dams,6 but this was amplified post-construction.7-9 There was a change in erosion patterns from Figure 2: Estuary Habitat Types the erosion of channels to the erosion of intertidal habitat8,9 with the rate of loss of intertidal habitat exceeding that expected due to sea- level 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 Habitat Structure restricting sediment movement into these areas thus impeding habitat Studies of habitat response are limited to conceptual models 13 development. The Severn Estuary is unusual in that it receives only.16, 17 A range of habitats will be affected by a tidal relatively little new sediment from either rivers (fluvial) or marine barrage including: sources; the majority of sediment enters the system from the on-going erosion of intertidal habitat. „ salt marsh: an important habitat for birds, fish, and plants. Salt marsh depends on sediment transported and Existing Barrages deposited by the incoming tide for growth at a rate that The extent to which existing barrages (Box 1) and other exceeds sea-level rise. similar structures can inform future developments is „ mud flats: an intertidal habitat that is vitally important for determined by similarity in characteristics and the availability migratory birds, and species of invertebrates and fish. of data from before and after construction. For these „ sand flats: an intertidal habitat important for many reasons there is limited data from La Rance, with a little invertebrate species. more known on Annapolis Royal and the Eastern Scheldt. Two conceptual models have been developed to try to explain long-term change in habitat structure in the Severn Predicting Impacts Estuary.16, 17 Both predict that there will be an initial loss of To predict the impact of a tidal barrage, first requires an intertidal habitat (mud flat and salt marsh) due to reduced understanding of the factors (for example, water currents tidal range. However, the first predicts that new habitat will and sediment load) that influence estuary characteristics. be established almost immediately,16 while the second Prediction commonly starts with simplification of how an predicts there will be no gain in habitat because of estuary works and how a barrage may influence a increased erosion pressure.17 In an attempt to validate characteristic using data from similar estuaries or from the model scenarios, efforts have been made to match the estuary of interest. This is referred to as conceptual Severn to La Rance, the Eastern Scheldt and Annapolis modelling. Computer models are then used to simulate the Royal.4,7,13 Both La Rance and Annapolis Royal are of conceptual model and provide a measure of the likelihood limited use because of a lack of data quantifying the impact and scale of a particular impact occurring. Confidence in of barrages on hydrodynamic and habitat structural computer models depends on the quality and availability of response (Box 1). The Eastern Scheldt provides some data used to build the model, the type of model and whether useful information on the possible impact of a barrage. the prediction can be validated. Impact prediction of a However, the difference in the source and transport of Severn Barrage suffers from two problems: sediment between the Eastern Scheldt and the Severn „ a lack of real data on certain factors to build computer makes it difficult to conclude the same erosion patterns will models occur. Intertidal habitat development has not been predicted „ a lack of validating computer models due to a lack of using computer models for an ebb-flood barrage. In the similar estuaries and similar barrages. 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

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„ reduction in peak suspended sediment load by a factor of reduced survival rates compared to non-displaced birds.22 In 2-3 and rapid accumulation of up to 2m of sediment both the Eastern Scheldt and Cardiff Bay, there have been upstream of the Severn Bridge declines or local extinctions of wader species.7, 22 „ erosion of 1.3-3.1m at the Mid Severn flats and 0.3-1.4m It is often reported that the loss of intertidal habitat and accretion over the Outer Severn channel.15 subsequent decline in birds will be offset by the increase in Water Quality available food within the remaining intertidal habitat. Hydrodynamics and suspended sediment have important However, the DECC 2010 study concluded that the scale of implications for water quality. There are factors which may habitat loss for an ebb only barrage would far outweigh any 36 result in both a positive and negative effect on the gain in habitat quality. concentration of metals, nutrients, and pathogens within the Fish Mortality estuary.18 The propensity of an estuary to develop water- There are two categories of threats to fish: quality problems is dependent on two key factors: „ Direct: injury and mortality due to blade strike and water „ the amount of time water spends in the estuary, which is conditions (for example water pressure) resulting in increased by an ebb-only barrage damage or disorientation. „ the existing level of nutrients, pathogens and metals. „ Indirect: loss and degradation of habitat which may be Predictions of impacts to water quality have been based on important for feeding and spawning; and disruption to conceptual and computer models. The 2010 DECC study19 movement (such as completion of migration). found an ebb-only barrage in the Severn had no significant Mortality due to blade strike is the most studied source of impact on water quality, except for an increase in nutrient fish mortality. Estimates depend on the type and operation levels. This, in combination with reduced suspended of a turbine, and species of fish.23 In order to reduce levels sediment, would increase the level of algae and plants. of blade strike, the Oak Ridge National Laboratory (ORNL)24 However, this increased productivity is likely to be less produced a set of criteria for the design of ‘fish-friendly’ pronounced under an ebb-flood barrage. turbines. These criteria are based on an extensive literature Biodiversity review of studies on single fish passage through turbines at The term biodiversity is used to describe the variety of life hydro-power schemes. In recent years, levels of survival 25-28 (including plants, animals and bacteria) on earth.20 A tidal greater than 90% have been achieved. While high energy barrage may impact biodiversity because of either a survivability is possible, no field trials of turbines in an loss, gain or a change in species. A simple gain in the estuarine environment have been carried out. In addition, number of species may not represent a gain in biodiversity if most studies of fish mortality have been carried out on it results in the loss of rare or unique species that are small-sized salmon, a notoriously robust species; therefore replaced with common species.21 While some studies reports of ‘negligible’ mortality levels may not be applicable suggest that a Severn Barrage will increase biodiversity, 4,17 to the diversity of fish, crustacean and invertebrate species 29-31 these are not based on studies in which the number and common in estuaries such as adult salmon, shad, type of species were evaluated before and after barrage flounder, brown shrimp. In addition, estimates of fish construction. For there to be an overall gain in biodiversity, mortality are based on fish making a single pass through a an increase in the number of some species (for example turbine: fish living in an estuary may make multiple passes invertebrates) needs to offset species loss due to habitat in a day, increasing their risk of mortality. loss, fish mortality in turbines, bird displacement and the Less studied is mortality due to sub-lethal injuries, predation loss of rare or unique species. To date, no studies at or indirect impacts. During turbine passage, blade strike and existing barrages have assessed overall change in species hydraulic conditions can result in injuries (for example scale diversity and abundance. loss, eye loss or abrasions) which may not cause immediate Invertebrates mortality, but will reduce survival through disease or Invertebrates (animals without a backbone) make up a decreased fitness. Fish commonly suffer disorientation significant majority of the biodiversity in an estuary and during turbine passage which increases predation risk by 23 include Sabellaria, brown shrimp, and mussels. Invertebrate other fish, fish-eating birds and aquatic mammals; this has 32 33 diversity and abundance is predicted to increase following been observed at La Rance. In a recent study, mortality barrage construction,4,17 though this depends on the extent because of predation of juvenile salmon was found to and type of habitat lost following barrage construction. Such account for between 46-70% of total mortality, indicating it is gains would likely be caused by a reduction in water velocity potentially a considerable source of mortality. However, at and suspended sediment, and changes in sediment type. present studies on mortality levels due to predation pressure Increased invertebrate abundance would likely be more and sub-lethal injuries are limited. pronounced under an ebb only barrage scheme. Very Low-Head Turbines Birds Very Low-Head (VLH) turbines are a new technology that Species of waders appear to be particularly sensitive to reportedly has a smaller impact on fish than existing 34 barrage operation, because of intertidal habitat loss and technologies. A recent trial on a VLH prototype achieved 34 displacement. In 1999, a barrage was constructed in Cardiff 100% survivability of both large and small eels. However, Bay as part of a regeneration project. Studies in the bay very few trials of VLH turbines have been undertaken. In have revealed displacement of a number of species to response to growing interest in VLH turbines, the Canadian neighbouring sites. However displaced birds have exhibited government published guidelines for the testing of these

POSTnote Number 435 June 2013 Environmental Impact of Tidal Energy Barrages Page 4 turbines.35 The report comments that “mortality, although newly created features. easy to define and measure, is simply one way to evaluate Compensation for Fish the biological effectiveness of a turbine yet the majority of A range of methods has been proposed for compensating studies focus only on mortality as an endpoint”. It suggests for the loss of fish populations and their habitats (Box 3). that a “suite of endpoints should be examined which However, research into these methods has focused on incorporate relevant metrics that have the potential to design and implementation for a limited number of species influence long-term survival, health, condition and fitness”. and at much smaller scales. For estuaries as large as the Compensation and Mitigation Severn, most compensation measures would be required on Tidal energy barrages may impact on features (such as an unprecedented scale. The 2010 DECC study concluded habitat types, birds, fish and invertebrates) protected under that “fish compensation measures are unlikely to completely a number of EU Directives (Box 2). Under development of a offset all negative effects”. 43 Severn Barrage, impacts to protected features include: Equal Value „ habitat loss, for example salt marsh and Sabelleria reefs, It is recognised that it may be difficult to recreate designated due to reduced tidal range and water speed features on a ‘Like for Like’ basis. The Sustainable „ loss of intertidal habitat which supports designated bird Development Commission (SDC)37 led an investigation into species such as Dunlin and Bewick's Swan the case of compensating with habitat of ‘Equal Value’ „ loss of designated fish, for example, Twaite Shad, River rather than ‘Like for Like’. ‘Equal Value’ could be an Lamprey and Atlantic Salmon, due to degradation of alternative and adaptive form of compensation for habitat, blocking of migration path and direct mortality developments where it is not possible to recreate ‘Like for „ decline in the ecological status of a water body. Like’ or where there is a lack of available land for recreating Compensation for Habitats and Birds lost habitat. However, at present ‘Like for Like’ is an Compensation for designated intertidal habitats is typically untested method. The SDC investigation concluded that it carried out by recreating the lost features (such as Atlantic represented “an unprecedented challenge” for a Severn salt meadows, mudflats), referred to as ‘Like for Like’. The Barrage scheme. most commonly used technique is managed re-alignment in Conclusions which “sea walls are deliberately breached to allow the Tidal energy barrages have a number of environmental coastline to move inland” (POSTnote 342). While this impacts that vary in scale depending on the type of barrage. method has been successfully used to compensate for lost There is insufficient evidence to determine the efficacy of habitat on port developments, which are relatively small, compensation and mitigation measures for a Severn there are concerns over its efficacy for the much larger Barrage scheme. Some academics and NGOs are in favour Severn Barrage scheme. The success of managed re- of an incremental approach and step by step development alignment is well documented for salt marsh habitats, but of tidal power technologies and compensation on a smaller less so for sand flats and mud flats.43 No compensatory scale. 38-42 measures are available for the loss of subtidal habitats that support Sabellaria reefs.36 Compensatory measures would Box 3. Compensatory Measures for Fish need to be implemented prior to construction allowing The following possible compensation measures were investigated in sufficient time for monitoring to ensure sustainability of the the 2010 DECC study: „ Exclusion and diversion from turbines: while there is evidence Box 2. EU Policy Affecting UK Estuaries for their effectiveness, they potentially have contradictory effects on Many UK estuaries fall under the following EU directives: different species.43 „ The EU Habitats Directive seeks to maintain or restore habitats „ Bypasses: most research into the design of bypasses has focused and species listed under Annex I and II respectively and on freshwater species, such as salmon, trout, eels and shad. There designation of sites as Special Areas of Conservation (SACs). has been limited research on marine species.43 „ The EU Birds Directive promotes the conservation of wild birds, „ Herding and translocation: this measure involves the collection, with identification of Special Protected Areas (SPAs) required for transportation and release of fish from downstream of the barrage, vulnerable species listed in Annex I and migratory species. to upstream. It is not an established measure, with research limited „ The EU Water Framework Directive requires European Union to only a few species.43 member states to achieve good status of surface water bodies „ Predator control: effective methods are limited to birds, with no (including marine waters up to one nautical mile from shore) by known effective methods for fish.43 2027 (some types of water body are exempt, POSTnote 320). „ Re-stocking: this measure supports population numbers, however, The network of SACs and SPAs is referred to as the Natura 2000 it reduces genetic diversity of a population thereby reducing the network. Where a development impacts the Natura 2000 network, it potential of a species to adapt and evolve.43, 44 must first assess whether there will be a negative impact of „ Habitat creation and enhancement: an established method for development (Article 6(3)). If there is, the development may only be many estuarine and marine species, but there is less evidence for permitted to proceed (Article 6(4)): its efficacy for migratory species.43 „ if there is no “alternative solution” that is less damaging „ that there are “Imperative Reasons of Overriding Public Interest” Endnotes (IROPI) for the development For references, please see: „ that all compensatory measures necessary to ensure the overall http://www.parliament.uk/documents/POST/postpn435_Environmental-Impact-of- coherence of Natura 2000 is protected are undertaken. Tidal-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.