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Environmental Impact of Tidal Barrages

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Environmental Impact of Tidal Barrages 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
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