Deep-sea design

6th June 2024


While biodiversity net gain is now making inroads, marine net gain is still in its infancy. Ed Walker explores the balance between enabling development and safeguarding our marine environment

Almost half of UK coasts are within some form of designation, including new highly protected marine areas. Biodiversity net gain (BNG) became mandatory in England this year, and proposals for marine net gain (MNG) are emerging. Against this context, our seas have never been busier, owing to industrial growth and the UK’s decarbonisation journey.

The UK has a diverse range of marine activity. Delivering new infrastructure projects requires substantial amounts of materials, such as concrete – an industry that relies heavily on aggregates extracted from the seabed. As we experience the effects of climate change, such as a rise in sea level and more storms, we are also witnessing accelerated deployment of coastal defence infrastructure.

Making waves

As an island nation, we rely on a network of ports and harbours for movement of commodities and people. As part of commitments to tackling climate change, the UK and Scottish governments have set legally binding targets to become net zero in greenhouse gases by 2050 and 2045 respectively. Addressing the pressing climate emergency and meeting net zero will also require a huge growth in low-carbon and renewable technology, much of which will be developed within our seas.

Multiple coastal generating stations are dotted around our coasts, using a range of technology, such as energy from waste, combined cycle gas turbines and nuclear (both legacy assets and a fleet of nuclear new build, including Hinkley Point C in Somerset and the sister station at Sizewell C in Suffolk). Wave and tidal energy is also gaining momentum, as is carbon capture, usage and storage, with Net Zero Teesside and Keadby 3 in north Lincolnshire now granted development consent. The UK is also one of the world’s largest markets for offshore wind, with approximately 15GW of installed capacity.

“Given the sheer scale of marine development, both now and in the future, it is vital that we get it right”


Our energy system is continuing to change in terms of transmission and interconnectors. In December 2023, National Grid switched on Viking Link (a new 1.4GW interconnector linking the UK and Denmark). In June 2021, the final cable of the North Sea Link was installed between England and Norway, while IFA2 began offering full flow of electricity between France and England in January 2021. To help provide greater North-South transmission capacity domestically, Scottish and English transmission owners are also in the process of developing Eastern Green Link 1 and 2 (two new high-voltage direct current transmission links, representing a combined 4GW of capacity between Scotland and England).

Development within the marine environment will typically lead to a range of marine impacts during construction and (if applicable) during operation, maintenance and decommissioning. While there is a need for a multitude of marine development, we must ensure it is managed appropriately.

Marine development is generally managed by Marine Scotland, the Marine Management Organisation and Natural Resources Wales in Scotland, England and Wales respectively. The Planning Inspectorate is also a key organisation as the government agency responsible for examining nationally significant infrastructure projects, many of which are marine in nature.

As part of the marine consenting process, the potential environmental impacts associated with development are thoroughly assessed, managed and, where required, mitigated. The environmental impact assessment (EIA) process is instrumental in supporting this process.

Management of interactions

Given the sheer scale of marine development, both now and in the future, it is vital that we get it right – but how?

Consideration of the key interactions between infrastructure and the marine environment at the earliest possible opportunity is vital. Through careful site selection, developers can help to avoid, reduce or minimise adverse impacts on the marine environment at the outset. This is rarely easy – our seas are becoming ever busier, so avoidance is not always straightforward (see Transform, Apr/May 2024, p30), and there is also a careful balancing act between environmental, commercial and technical factors.

As proposals evolve, meaningful discussions with relevant stakeholders are equally important. This helps to identify areas of concern, interest and also opportunity. For best results, this should take place before the formal EIA process starts, noting also that EIA scoping is a great opportunity to shape development.

As the life cycle of a project evolves, it is vital for developers to work with regulators and key stakeholders to refine the design of an infrastructure project. This process does not end at the point of consent – collaboration throughout the construction stage of a project is also important to deliver the best-performing development.

Equally important is willingness from regulators and stakeholders alike to participate in proactive discussions to find the best way forward to facilitate vital development while also managing marine impacts appropriately and proportionately. Management of impacts on the marine environment is the ‘base case’, but this is rapidly evolving.

Terrestrial BNG – state of play

In England, terrestrial BNG became mandatory in February 2024. BNG is described by the government as an approach to development to ensure that habitats are left in a measurably improved state following development. Under Schedule 7A of the Town and Country Planning Act 1990, developers must ensure a 10% BNG.

For BNG, biodiversity value is measured in ‘units’ – to determine the presence and volume of units, habitat is assessed based on several key factors, such as size, quality, location and type. Under BNG, developers must undertake an appraisal of habitat pre-development to determine how many units are present. This must be accompanied by a calculation of how many units are needed to replace those lost, and those required to achieve 10% BNG.

In England, this process is calculated and structured by a ‘statutory biodiversity metric tool’ – managed by Natural England, this has been through several iterations. If biodiversity cannot be increased on site, developers can purchase off-site BNG units from landowners via a private market, which Natural England will manage through a new digital register. As a last resort, developers can purchase statutory biodiversity credits directly from the government. However, early indications suggest that this will be an extremely costly option, thereby discouraging a coarse approach to BNG, where this is used as a quick fix.

In Wales, net benefit for biodiversity (NBB) has a similar intent to BNG, although there are some notable differences. Importantly, NBB does not follow a metric-based approach. In Scotland, proposals for a more holistic way of ensuring that the country is ‘nature positive’ by 2030 are emerging. Scotland has also published the Scottish Biodiversity Strategy to 2045 to help tackle the ‘nature emergency’.

Marine net gain – state of play

As described above, we can expect to see substantial infrastructure developed within the marine environment. This will interact with both marine habitats and species (whereas terrestrial BNG is purely based on habitat interactions).

While terrestrial BNG does include intertidal habitats, it does not extend further below mean low water springs and into the broader marine environment. In 2022, the Department for Environment, Food & Rural Affairs (Defra) undertook consultation on the principles of a new MNG system (see below). MNG is similar in principle to BNG – in essence, it is about leaving the marine environment in a measurably better condition post-development.

However, the marine environment is highly dynamic – in various ways, this will dictate a different approach to MNG implementation compared with BNG.

Looking ahead

Development within the marine environment is all about balance. To meet net zero and tackle the climate emergency, we – as an industry, regulator, developer and stakeholder – must work together to find that balance. Beyond this, if managed well by all parties, MNG is a real opportunity for mutual benefit.

The government is currently developing the policy of MNG – clarity is needed urgently on what the assessment framework for MNG will look like, and how this will be managed practically. As noted above, interactions with seabed habitats could potentially be managed in a broadly similar way to BNG through some form of metric. However, net gain in relation to species is completely new. Our seas are species-rich and this will therefore be vital to the success of MNG.

We also need guidance from key stakeholders (namely the statutory nature conservation bodies) on the delivery of MNG – this will presumably be based on a combination of site-based solutions and procurement of MNG units, with optionality for government-owned and managed credits, too. However, for MNG, owing to the complexities of the marine environment and often the sheer scale of development offshore, additional delivery mechanisms are likely to be needed. There will probably be a requirement for a combination of site-based activity and more strategic initiatives, depending on the project, scale, location and marine interactions.

Eye to the future

As custodians of our seas, it falls on all of us to safeguard our sensitive marine environment, now and in the future – that is the very definition of sustainable development. Equally, vital industry and the race to net zero mean that the marine environment is a critical resource. This is a tough balance to get right, but close collaboration between regulator and developer is critical to success.

As proposals for MNG start to become clearer, I encourage members of the IEMA community to embrace MNG, learn the intricacies of how it operates and then work to proactively embed MNG. If we work together to do this well, we can leave a legacy of infrastructure that helps support the UK’s decarbonisation journey while also delivering genuine benefits for our marine environment.



Defra consultation on the principles of marine net gain

  • Defra’s consultation sought views on a range of elements associated with MNG, with some headline findings emerging as follows:
  • MNG should measure habitats and species (89% yes and 11% no)
  • MNG assessments should not include potentially incidental impacts where benefits are uncertain (52% yes, 42% no, 6% uncertain)
  • MNG requirements should be proportionate to the scale and type of development (51% yes, 34% no, 15% ‘mixed’)
  • On the different types of pressure reduction to deliver MNG, 38% identified ‘other environmental measures’, 35% fishing pressure reduction, 11% restoration, 11% industry pressure reduction and 6% suggested funding schemes
  • An overwhelming 95% of respondents supported the notion that MNG will encourage the delivery of strategic interventions in addition to meaningful site-based interventions
  • MNG will allow for improvements to designated and non-designated features of marine protected areas (MPAs) to qualify as MNG (81% yes, 10% no, 9% mixed)

Ed Walker MEI FIEMA MIMarEST MCIWEM CEnv CMarTech C.WEM is an environmental specialist (coastal energy) working on a range of marine-power projects for global energy consultancy Xodus

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