Tidal lagoon environmental interactions: regulator perspective, solution options and industry challenges

Renewable energy technologies are being developed and deployed globally to tackle climate change. Marine energy, comprising wave and tidal resources, is currently un-tapped in the UK to a significant degree.

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Jul 19, 2017
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Authors: Kathryn Mackinnon; Helen C. M. Smith; Francesca Moore 

Abstract

Tidal range energy is an attractive renewable energy option, particularly in areas of high tidal range, such as the UK. Historically one of the main barriers to tidal range developments in the UK, specifically tidal barrages, has been regulatory environmental concerns and uncertainty surrounding environmental impacts. Tidal lagoons are often suggested as a means of reducing the environmental impact of barrage options. Recent developments in the lagoon sector mean it is now more important than ever to further consider the environmental impacts arising from tidal lagoons and the potential constraints these impacts may pose to the industry's future growth. Environmental impacts and their interactions are complex, often making them difficult to understand and manage. Here, the authors develop a conceptual framework to categorise impacts, present results from consultation with regulatory and policy organisations and discuss potential impact and enhancement solution options. This study includes a number of case studies to present lessons learnt, opportunities, cautions and successful implementation of past solutions. In the absence of operational tidal lagoons, these case studies are based on barrages and other relevant developments.

Introduction

Renewable energy technologies are being developed and deployed globally to tackle climate change. Marine energy, comprising wave and tidal resources, is currently un-tapped in the UK to a significant degree. Tidal energy consists of two energy extraction types: tidal stream and tidal range. Tidal range energy can utilise both barrage and lagoon technologies. Fig.  shows a basic breakdown of this categorisation.

Fig 1: Basic categorisation of tidal lagoons within marine energy

Tidal barrages contain turbines within a barrier that extends between the banks of an estuary or river. Tidal lagoons differ as they contain bodies of water in a basin, which is either constructed along one side of an estuary or river, or located completely offshore [].

Both tidal barrages and lagoons extract energy from the tides by creating an artificial difference in water levels, or head. Higher water levels are constrained by the lagoon walls and sluice gates; when these are opened, the flow of water drives turbines to generate electricity.

In the UK alone, tidal lagoon energy could contribute up to 8% of the current electricity demand, assuming six tidal lagoons are constructed [ ]. Tidal lagoons have additional advantages such as a 100-year life span, reduced uncertainty through the use of proven technology, a high level of predictability and the opportunity to phase shift energy generation around a coastline. These advantages alone make tidal lagoons an attractive renewable energy option for the UK.

However, despite the advantages, there are currently no operational man-made, energy generating tidal lagoons in the world. The reasons for this include the lack of serious project proposals in the past, in addition to concerns regarding reduced energy output when compared with barrage systems.

Recent developments, including the awarding of a development consent order to Tidal Lagoon Swansea Bay in June 2015 [ ] and the announcement of the government review into the feasibility of tidal lagoons for the UK [ ], mean that it is now more important than ever to consider lagoons as a key player in the energy market.

Given the recent developments in the industry it is vital that the interactions of a lagoon with the environment are further understood and solutions to address impacts are sought. Further understanding will allow impacts to be effectively managed, reducing the chance of environmental constraints hindering the industry's growth in the future.

Environmental interactions of tidal lagoons are discussed below, given the absence of any operational tidal lagoons all case studies presented in the paper are based around tidal barrage developments or other relevant projects.

Lagoon environmental interactions

Assessing the environmental impacts of a tidal lagoon is a complex activity. Ecosystems are a detailed web of interactions between the physical, or abiotic, environment and the living, or biotic, environment. Further complexity is added through the site- and design-specific nature of any interactions, the potential cumulative impacts and the fact that these interactions are in a constant state of flux. As a result of this, any environmental impacts of a lagoon will be site specific, will have knock-on implications through the ecosystem, society and local economy, and are likely to alter over the course of a lagoon's 100-year life span.

Rather than attempting to list all of the potential environmental impacts of tidal lagoons, this paper proposes a conceptual framework (Fig.  ) and provides examples of impacts to illustrate this framework. Fig.  shows the grouping of environmental impacts in the framework and how these are linked to the lagoon development, society and the economy. The arrows in Fig.  describe the links, often two-way, between each of the groups, for example, the abiotic environmental impacts will have a two-way interaction with the biotic impacts. These interactions are discussed further below.

Fig 2: Grouping of environmental impacts and interactions with tidal lagoon developments, society and economy

Abiotic interactions

Research to date has focused on abiotic impacts including the alteration of hydrodynamics and tidal range resource [ – 10 ], morphodynamics [ 11 – 15 ] and water quality [ 16 17 ]. Abiotic impacts are strongly linked to each other, for example changing hydrodynamics is a driver for changing sediment regime. Abiotic impacts also significantly influence biotic impacts such as how water quality might influence the marine biodiversity. In addition to this, abiotic impacts have the potential to impact the lagoon itself, such as sediment build up influencing energy extraction performance or maintenance strategies.

Case study – La Rance siltation: The La Rance barrage, built in 1966 has a capacity of 240 MW and stretches 750 m across the Rance River in Brittany, France [ 18 ]. Since its construction and operation, changes to the sediment regime have been observed, with around 30,000 m 3 of silt added to the marine basin each year [ 19 ]. This is thought to have been caused by the increase in slack water times and therefore a reduced current [ 19 ]. The La Rance operation now includes maintenance dredging, particularly in the navigational channels towards the estuary head [ 20 ]. This abiotic change is therefore increasing the barrage's operation and maintenance activities, increasing the amount of dredging required and thereby increasing operational costs.

Biotic interactions

Biotic impacts of tidal lagoons, sometimes referred to as ecological impacts, are less well understood. A few papers have considered the ecological interactions and linked these with society [ 21 22 ], but Hooper and Austen's [ 22 ] paper was the only one found to comprehensively review the impacts of tidal range schemes along with discussing societal and economic factors related to them. Whilst it focused on tidal barrages, some of the impacts will be transferable to tidal lagoons.

As with any food chain interaction, biotic impacts will influence a web of other biotic parameters across all the trophic levels of an ecosystem, such as how overfishing of sand eels has led to a decline in their predator populations of seabirds the Arctic Tern and Puffin [ 23 ]. Perhaps a less obvious interaction is that of biotic impacts influencing abiotic parameters. An example here might be biofouling altering the hydrodynamics around a structure, or biotic waste from fish farming altering the local water quality [ 24 ]. These interactions are likely to result in a chain of effects, where a change in the abiotic environment will lead to a biotic change, which in turn results in a secondary abiotic change and so on. It is therefore important to consider the wider knock-on effects of any environmental impact that a development may have.

Case study – Sihwa Barrage impact chains: Sihwa Barrage in South Korea was built as a dam in 1984 [ 25 ]. The dam blocked the natural flow of the river and altered its sediment regime. This abiotic change led to a deterioration in water quality, including excessive phytoplankton growth (eutrophication) [ 26 ], a biotic impact which potentially had knock-on implications for the abiotic environment in deeper water, for example decreased sunlight penetration and temperature changes. In 2012, the dam was opened as a retrofitted tidal barrage allowing the reintroduction of sea water through turbines, reinstating the flow of water, improving the ecological water quality and generating renewable energy [ 27 ].

Environmental benefits

With all changes to the environment there will be winners and losers. Often overlooked in the industry are the potential environmental benefits and, as such, beneficiaries (people, society and the environment). As with all other interactions, these may be a consequence of positive abiotic or biotic impacts.

There will be a number of benefits of lagoon developments; the most obvious of these is the new habitats presented for species colonisation. This assumes a net gain in habitat area and condition over and above any initial habitat loss.

A well-known example of an abiotic positive impact is that of reduced local coastal erosion. Marine sediment transportation is sensitive to changes in the coastline form; introduction of a lagoon may reduce coastal erosion. It should not go unmentioned that it could also exacerbate coastal erosion if the local processes are not fully understood and incorporated into early lagoon planning and design.

Flood protection could also be a potential abiotic benefit provided by tidal lagoons, again, this is a complex issue as it assumes positive manipulation of a sensitive environmental process. A lagoon that is not implemented well could increase flood risk to a local area through siltation of inflows and a raised water table. It is therefore essential to fully understand the local processes prior to assessing the true benefits that could be provided by a lagoon.

Case study – Thames Barrier flood protection: The Thames Barrier crosses 520 m of the River Thames and has the sole purpose of providing flood protection to central London [ 28 ], similar to the protection which could be provided by a tidal lagoon. It has 10 steel gates that can be raised across the river preventing tidal surges from the sea, or lowered to relieve the pressure of river flooding within the catchment [ 28 ]. Lowering the gates also allows for tidal current flow through the barrier and access for marine traffic. Careful management of the Thames Barrier therefore allows for flood protection and control of the catchment services which could be provided by the tidal lagoon industry.

Key impacts – the regulator perspective

Identifying the ‘most significant’ impacts is subjective and, as already mentioned, complex. Interestingly the regulators’ perspective on what they believe to be the key outcomes, impacts and benefits may differ from the tidal industry and societal perspective. Marrying these different perspectives is part of the challenge of realising the successful deployment of tidal lagoon projects in the UK, especially given that one of the key barriers to tidal barrage developments has historically been regulatory environmental concerns [ 29 ]. In this section, we present a snapshot of the environmental issues that are at the forefront of regulators’ minds.

A short online questionnaire was sent to regulatory, policy and conservation organisations. This was targeted at participants in decision-making roles within relevant organisations. The main aims were to determine what regulators perceived to be the most desired outcomes of future tidal lagoon developments, the key impacts (positive and negative) and how they believed developers should focus on improving the environmental status of future tidal lagoons. Four key questions were asked as follows:

  • What outcomes do you believe to be the most important for future tidal lagoon developments?’
  • What do you consider to be the most significant environmental impact of tidal lagoon developments?
  • Other than low carbon energy and direct economic benefits, what are the priority opportunities a lagoon could provide?
  • Where should developers be focusing their efforts to enhance the environmental status of tidal lagoons?

The questionnaire received a 51% response rate with a total of 21 different organisations contributing (Fig ).

Fig 3: Organisations participating in the questionnaire

Participants believed the most desired outcomes for future tidal lagoon developments to be a ‘cost competitiveness’ and a ‘good environmental status’ (Fig.  ). Good environmental status here was defined as reducing negative impacts and enhancing positive impacts where possible. This is not linked to the marine strategy framework directive which defines ‘good environmental status’ differently [ 30 ].

Fig 4: Response to question: ‘What outcomes do you believe to be the most important for future tidal lagoon developments?’

Participants deemed the three most significant environmental impacts to be ‘sediment regime alterations’, ‘changing hydrodynamics’ and ‘restricted passage and migration’, as shown in Fig 5. It is important to remember here that these impacts will interact with each other, the wider biotic and abiotic environment, society, economy and even the lagoon itself.

Fig 5: Response to question: ‘What do you consider to be the most significant environmental impact of tidal lagoon developments?’

Benefits can be provided ‘naturally’ from the lagoon or by providing solutions to address impacts that go over and above regulatory drivers. The questionnaire asked participants about the potential benefits that could arise from both of these sources. Flood defence and control along with leisure and recreation featured highly in the ‘natural’ benefits mentioned by participants. Of the additional benefits mentioned by participants, habitat protection, creation and enhancement of biodiversity were mentioned most frequently.

These questions provided an idea of the outcomes, impacts and benefits in the minds of the participants at the time of taking the questionnaire. The next step was to determine where regulators believed developers should focus their efforts in terms of improving the environmental status of future lagoons. A variety of responses to this question were given. However, there was a key theme throughout the majority of responses and that was that there should be a focus on site selection to avoid impacts in the first instance.

Range of solution options

Whilst avoiding impacts should be a priority there are a range of other solution options to be aware of. Typically, negative impacts are addressed and positive enhancements sought working down the mitigation hierarchy. The mitigation hierarchy is a strategy for addressing impacts; it works to first avoid impacts, then reduce and then finally compensate or offset. There is also potential to harness benefits in this way, by delivering enhancements at each stage of the hierarchy to offset the negative impacts. Within the hierarchy, strategies for impact solutions such as site selection, engineering design and technology, biodiversity offsetting by restoration, creation of habitat and catchment-based measures such as payment for ecosystem services (PES) schemes can be categorised (Fig.  ).

Fig 6: Mitigation hierarchy and potential strategies at each level

Case study – Venturi-enhanced turbine technology (VETT) turbines and impact avoidance: A key concern with tidal energy schemes is the impact of fish mortality. The first stage of the mitigation hierarchy is to avoid impacts, with one strategy being through careful technology choice. The VETT being developed by VerdErg is based on the Bernoulli principle and exposes no moving parts to the water channel [ 31 ]. VerdErg have achieved excellent results in recent tests, showing no impact on fish mortality with passage through the VETT's primary flow path [ 32 ]. This is therefore an example of a potential technology choice which could reduce the impact of a tidal energy development on fish mortality.

Case study: river Fowey PES scheme: An example of a catchment-led approach to addressing environmental problems is that of the River Fowey PES auction. The River Fowey had issues with water pollution as a result of cumulative agricultural run-offs [ 33 ]. The West Country Rivers Trust, as part of the ‘Upstream Thinking’ initiative, distributed funding to farmers via a competitive bidding auction to deliver land-based measures which in turn improved the river's water quality [ 33 ]. Similarly, PES has the potential to help reduce nutrient levels in coastal waters e.g. nitrogen levels in Poole Harbour as per the Defra PES pilot project [ 34 ]. This type of approach could be applied to not only offset any negative environmental impacts which may arise due to tidal lagoon development, but to help to deliver an overall net gain in the environmental impact of a lagoon development.

Achieving a good environmental status is not only about addressing negative impacts but also about enhancing positive impacts wherever possible. This allows a project to work towards an environmental net gain. Environmental enhancements can be developed within the lagoon design itself or through later compensatory or offsetting stages of the hierarchy.

Case studies – built-in enhancements, Sydney Harbour: Adding new infrastructure in the sea will be a driver for change in the marine environment [ 35 ]. Building in enhancements early on in the engineering design can increase the positive environmental impacts of a development, contributing towards achieving environmental net gain. An example here is the intertidal habitats created in seawalls in Sydney Harbour. The designs do not compromise the engineering requirements or cost but do increase the diversity of species able to live on the sea walls [ 35 ]. Fig.  shows a photograph of their construction.

Fig 7: Built-in rock pools in Sydney Harbour walls. Source: [ 35 ]

Remaining industry challenges

Whilst environmental impacts are a prominent challenge for consideration, it is important not to lose sight of balancing these with a lagoon's primary goals. As a business model a lagoon is dependent on its ability to generate and sell energy. Therefore, any strategies to address impacts need to ensure that a lagoon's energy efficiency and power generation are not compromised.

A key challenge for lagoons, and one that is currently under review, is cost [ ]. There needs to be a balance between enhancing the environmental status of a development and keeping a project cost efficient. Any mitigation or enhancement strategies need to be environmentally worthwhile in order to balance the cost implications they present.

Another point for consideration is the overarching benefit of a lagoon as a means of displacing fossil fuel power stations. Analysing the local environmental impacts is vital in order not to undo the environmental, economic and societal benefits created by increasing renewable energy capacity as a means of combating climate change.

Every tidal lagoon will have environmental impacts, both positive and negative. If the positive impacts outweigh the negative impacts then an environmental ‘net gain’ can be achieved. This allows a development to have an overall positive impact on the environment.

The ecosystem services assessment framework [ 36 37 ] offers a way to assess, quantify and value environmental changes and determine what they mean for societal and economic wellbeing. When incorporated into the established environmental impact assessment (EIA) process, the framework provides a means to identify potential environmental mitigation and enhancement options. The eftec study in 2010 made some progress in this area, by valuing habitat loss in the context of tidal range developments [ 38 ]. Despite this progresses, multiple challenges still remain in developing a fully integrated cost–benefit analysis, to include economic, societal and environmental interactions.

There would be benefit in allowing environmental information to be incorporated into economic appraisals to represent environmental costs and benefits within developments from an early stage; arguably it is market failure not to do so. This would allow developers to find cost-effective means (financially and environmentally) to achieve an environmental net gain in a way that goes over and above the regulatory drivers.

As it stands, lagoons are suggested as environmentally advantageous alternatives to tidal barrage developments [ 22 ]. Individual lagoons are typically much smaller scale than individual barrage options and so their environmental impacts are limited by comparison. However, multiple lagoons would be required to provide energy generation at a strategic level, as such the cumulative environmental impacts of multiple lagoons will be a remaining challenge for consideration. The UK Strategic Environmental Assessment, including cumulative impact assessments of all project EIAs will go some way into managing this concern. However, knowledge surrounding the issue is still limited; hence the government's review into tidal lagoons is currently seeking evidence to address it [ ].

Dealing with uncertainty

One of the key challenges is identifying the impacts and dealing with the uncertainty associated with them. Here we can consider impacts as ‘knowns’, ‘known unknowns’ and ‘unknown unknowns’.

  • The knowns are impacts we are aware of and can therefore work towards developing solutions for. Whilst these are understood, there are no operational lagoons and so there is still a level of uncertainty surrounding the extent of these impacts.
  • The known unknowns are impacts we know that we have little knowledge about. These can be managed through targeted research and survey work to reduce their uncertainty. In this way, they can move to the ‘knowns’ category and solutions can then be developed to address them.
  • The unknown unknowns will only come to light when there is an operational tidal lagoon. At present these are not likely to present a regulatory barrier given that the regulators will not know what they are in order to regulate them. This does not mean that they are not an industry concern. Early stage monitoring of lagoon developments will be required here to move these impacts into the ‘known unknowns’ category and eventually the ‘knowns’ category.

Dealing with uncertainty surrounding impacts is a challenge, with ongoing industry attempts to address the key research gaps. An example of this is the Offshore Renewables Joint Industry Programme's Forward look for the wave and tidal sectors [ 39 ]. Developing solutions to ‘knowns’, investigating further the ‘known unknowns’ and careful monitoring of the ‘unknown unknowns’ will allow the industry to manage environmental uncertainty and push the growth of the sector forward.

Conclusions

Tidal lagoons are an attractive marine renewable energy option for the UK. Recent developments in the sector mean it is now more important than ever to further understand and manage impacts in order to reduce the potential constraints they may pose on the industry. A lagoon's interaction with the environment is complex. Abiotic and biotic environmental impacts (positive and negative) will interact with each other, the wider ecosystem, the lagoon itself, society and the economy. Environmental impacts can also accumulate, have knock-on impacts and can change significantly in extent and type over time. There is usefulness in using a conceptual framework to understand and manage environmental impacts and their interactions.

Regulator consultation revealed that environmental status and cost competitiveness are at the forefront of regulators minds for tidal lagoon's future outcomes. The key negative impacts were believed to be ‘hydrodynamic changes’, ‘sediment regime alterations’ and ‘restricted passage and migration’. Potential environmental benefits noted by the consultation include ‘flood defence and control’, ‘leisure and recreation’ and ‘habitat protection or enhancement of biodiversity’. The consultation provided a snapshot of the impacts and benefits currently taking priority in the regulatory industry.

Whilst avoiding impacts should be the first point of call, there are a number of different solution and enhancement options working down the mitigation hierarchy. A number of these solution options have already been successfully demonstrated in other industries and could be translated to the future lagoon industry. The remaining industry challenges are around creating project environmental net gain, balancing environmental concerns with project cost and energy efficiency and dealing with impact uncertainties.

Acknowledgments

This paper was written based on work conducted for an EngD, sponsored by Black & Veatch at the Industrial Doctoral Centre for Offshore Renewable Energy (IDCORE) [ 40 ] a consortium of the University of Exeter, University of Edinburgh and University of Strathclyde. IDCORE is funded by both the Energy Technologies Institute and the Research Councils Energy Programme (grant number EP/J500847). Some of the content in this paper was presented at the All Energy Conference in Glasgow 2016 [ 41 ].

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Kathryn MacKinnon

Research Engineer, Black & Veatch

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