Solution of choice

Photo by Lesley Lawrence

Consideration of options

A number of schemes for generating electricity have been proposed for the Severn Estuary. The main ones are captured on the Figure below. These include barrages as well as lagoons, whether attached to land or completely surrounded by water. Over the years, many have supported lagoons as these were perceived to be a way of developing the tidal resource incrementally. Such views have not necessarily been based on a detailed understanding of the respective benefits and drawbacks of barrages and lagoons.

The aim of the Team is to develop a solution which best meets needs both in the South West of England and in Wales as well as for future generations.

The purpose of this part of the website is to demonstrate why the proposal for a barrage is preferred over other options.

Lagoons

Unlike the Swansea Bay Tidal Lagoon, which would be located in the bay between the Gower/Swansea and the coast from Neath to Port Talbot, lagoons proposed for the Severn Estuary would reduce the flow area in the Estuary.

On an incoming tide there would be a head difference of up to 2m between the level of main flow and the water within any proposed lagoon. This is unavoidable since this is how tidal range generation works – it needs a head difference across turbines to generate electricity.

Tides entering the estuary will do so at the same volumetric flow rates as before the lagoon gets built with the result that the flow velocities past the lagoon will be greater.

This will have two effects:

• Greater erosion on the opposite bank of the estuary unless this is protected with a concrete wall or other hard revetment.

• The water level outside the lagoon could also rise as a result and lead to increased flooding during spring tides/winter storms.

Apart from these effects, the capital cost per unit of energy produced (MWh) is likely to be greater than for a barrage. The reason is because there is a proportionately larger amount of embankment relative to the amount of energy captured. In other words, the cost of the electricity produced will be higher.

A lock would need to be factored into the capital cost to allow dredging craft to enter the lagoon. Even though a lock would also be needed for leisure craft, its size is likely to be dictated by the larger size of the dredging craft and the additional cost is unlikely to be offset by regular larger vessel use.

The only way to counter this price differential is by claiming other benefits such as a realised increase in land values as a result of building the lagoon and by protecting that land against flooding. For the latter, this gain in land value is usually only possible where the area protected is already either built up or allocated for development

Any protection from floods or against erosion would be limited to the area contained by the lagoon. Indeed, because of the change in flow patterns around the lagoon, there is also the risk of increased flooding and/or erosion due to eddying currents outside the embankment where it meets the original coastline. This is in addition to the potential erosion on the opposite bank.

Because of its size, a lagoon would have a lower generating potential than a barrage.

Stand-alone lagoons, with water all around, have been considered but have the exact same problems of increasing flow velocities in the main channel except that in this case it would affect both sides of the estuary.

Barrages

The barrage that has been promoted most is from Brean Down in Somerset to Lavernock Point in South Wales.

This alignment has been modified by the present team for two reasons:

• To bring the Somerset Levels to within the flood protection afforded by the barrage

• The landing in the south west was within a Designated area for bird protection which is better avoided

The change in alignment will add to the cost of the barrage. However, the additional cost will be offset by the increased generation potential. This is estimated to be at least 20% although the exact figures will be calculated during the next stage of design development.

There will need to be locks in the barrage to allow commercial sea-going vessels as well as leisure craft to traverse the barrage.

The barrage will be shared by both the South West and Wales as will its economic benefits.

The barrage would allow for estuary-wide improvements to the environment including re-establishing water’s edge vegetation such as salt marshes and reducing the incidences of coastal erosion and accompanying rockfalls. In addition, it will protect the Somerset and Gwent Levels, Bristol, Cardiff, Newport and the whole estuary against further damaging and expensive tidal flooding.

There is also the possibility of adopting a barrage further down the estuary – from Minehead to Aberthaw. This has already been looked at by others and, whilst it has merit, it was considered too expensive at the time. With the change in priorities and the urgent need for a source of reliable renewable energy to meet the 2050 carbon Net Zero target, the capital costs may not be as important provided that the cost of generation remains competitive.

The proposed Great Western Power Barrage will provide up to 10% of UK’s supply needs and has estuary-wide benefits and so is our solution of choice.

Mixing lagoon(s) with a barrage

Superficially, this appears to be step-wise solution for developing the Severn Estuary and, according to some analysts, a mix may yield a similar amount of energy overall.

The caveat has to be whether lagoon(s), which would almost certainly be built first, would deflect the main flowlines in the estuary to make it very difficult to capture the bulk of the energy at an upstream barrage. This is likely to apply to any lagoons along the upper estuary as far down as Porlock in the South West where the estuary widens.

There is a line of reasoning which says that there should start with the smaller-scale lagoons as an incremental approach to allow tidal power technologies to be fully developed and environmental impacts assessed, before a large scale barrage is attempted. However, as already discussed, lagoons are not suited for this location. If an incremental approach is favoured, a better approach would be to building a smaller prototype barrage at another estuary. Consideration could be given to either the Wyre Estuary in Fleetwood (£300m) or the Mersey Estuary (£3.5bn).

Since a barrage would appear to be the only practicable solution to maximise the potential of the Severn Estuary, the even larger barrage from Minehead to Aberthaw might also be considered to capture the greatest energy possible at the earliest stage.

Evaluating the solution

The decisions we make now for our energy supply will have long-term implications, and therefore we need to be doing everything we can to ensure they are robust in the face of an uncertain future.

Increasingly, climate change demonstrates the need for resilient infrastructure that is able to withstand future weather conditions that may include significant increases in storms, flooding and heat. Our energy supply similarly has its own set of uncertainties, including the unavailability of nuclear power or negative carbon technologies; unreliable supply of gas from abroad, or other capacity market shocks as were experienced in December 2021.

It is necessary that projects shaping our future energy supply are tested under a range of future energy scenarios to ensure that it performs under a wide range of conditions. Proposals that affect the UK energy supply should be tested, not only against the four National Grid Future Energy Scenarios, which are largely intended for short-term decision making, but also against a broader range of core and non-core scenarios, that will reflect the relevant uncertainties over long-term timescales and encompass deep uncertainties.

The Great Western Power Barrage will add resilience to any future energy mix and physical resilience through flood prevention. It is our expectation that this will be borne out through rigorous qualitative and quantitative analysis of uncertainty as the scheme moves through the phases of development.