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Severn estuary outer tidal reef proposal (early version)
Up-to-date website with pdf downloads: Severn Tidal Reef Project
A tidal barrage for the Severn Estuary designed from an environmental perspective.
Author: Rupert Armstrong Evans, Armstrong Evans & Associates * June 2008

BackgroundTidal Reef SystemEngineering IssuesEnvironmental IssuesEconomic IssuesSummary of all Issues

BACKGROUND TO THIS PROPOSAL
The debate as to whether to build a barrage across the Severn Estuary to generate electricity has gone on for well over 100 years and has once again come to the fore with the Government announcement of proposals to build additional nuclear plants and/or the promotion of 'green generation' including from tidal power. But within the environmental lobby a major split has opened up between the promoters of a 'Big Barrage' (the Severn Tidal Power Group) on the one hand, and the wildlife interests (WWF, RSPB, FOE and local councils) on the other. Fear of increased flood risks, damage to wildlife habitats and compromised navigation are but some of the many and complex issues.

In this proposal I am presenting a radically different concept for tidal power generation that is driven by the need to address environmental issues before rather than after the project has been formulated. Most of the studies into tidal generation have been approached from the perspective of known hydraulic and civil engineering practices. I am suggesting that a consensus of 'environmental imperatives' are used to define these requirements, in much the same way as the 'North Sea' defined the requirements for building oil platforms, and not the other way around. I suggest that it is the requirements of the large low-head water turbine manufacturers that have defined the previous proposals, and many of the resulting problems are then being addressed as mitigation measures.

This proposal is for a barrage 'system' and is not at this stage tied to one particular turbine design or barrage location, though initial investigations point strongly towards an 'Outer Barrage' between Minehead and Aberthaw, with bidirectional generation. I am seeking a reasonable compromise between the environmental constraints and a technological solution that some turbine manufacturers might wish to avoid because it opens up the market to many more engineering competitors, with a consequential lowering of the potential profit margins.

The 'Big Barrage' between Cardiff and Weston (Brean and Lavernock), is designed to provide the necessary hydraulic head for conventional low-head water turbines, and this involves delaying the tides by many hours to produce this head difference. It is this long delay in the tidal cycle that causes most of the potential problems. Massive disruption to the levels and flow patterns within the estuary will result from the high localised discharges from the 40MW turbines. The impacts of such disruption are many, complex and may be very detrimental to the environment as well as navigation. The S.T.P.G barrage comprises a lot of 'hard engineering structures' in the form of powerhouses, sluices and ( ship locks which will have a significant visual impact. It is also designed with an adequate freeboard to protect the powerhouse and roadway from flooding and damage. Such a structure is very expensive on account of the head differential and wave action that it is required to withstand and the quantity of material requires for its construction is very large.

THE SEVERN TIDAL REEF SYSTEM
This new system offers a radically different approach to other proposed systems because it is based on a much more modest soft-engineered structure. It is intended to be almost totally environmentally benign and to take account of the ecological constraints from the outset. The impounding structure is much smaller than a conventional barrage but it still has the potential to generate as much electricity as a full height barrage. It will require significantly smaller quantities of material during its construction, so the cost and environmental impacts on land or through dredging operations will also be much reduced.

The 'Tidal Reef' would typically be comprised of 20 km of turbine caissons located astride a seabed causeway of pre-cast concrete foundation units anchored with piles and 'armour stone'. Over 1000 turbines of around 10 metres diameter would produce 5000MW, but for about twice the generation period of the proposed Cardiff-Weston barrage. The annual generation could therefore be in the order of 20 TWh because the area enclosed by the 'Reef is so much greater than for the Cardiff-Weston barrage.

The operation of the reef/barrage requires only a modest alteration to the tidal cycle amounting to a delay of around two hours as opposed to six hours for the large barrages. The key to this design is that it uses a large number of simple fixed flow turbines that work on a very low constant head difference across the structure. Floating caissons would contain the turbines or a movable 'crest gate' reaching to the surface of the water would divert the flow into fixed caissons sitting on the foundation 'causeway'. This arrangement allows the structure to 'track the changing tide level' and maintain the small head difference irrespective of the stage of the tide so that the turbines that are operating run at near constant power and efficiency.

The electricity generation can therefore spread over a longer period and the turbines and civil structures do not have to be designed for high intermittent stresses caused by using higher heads. It is a much more modest structure which is also less intrusive and damaging to the environment. Water turbines for very low heads are large, but need not be expensive, as the materials required for their construction can be significantly cheaper than the single regulated Kaplan turbines proposed for the other barrage projects.

The arguments in favour of this proposal are outlined in this proposal, but if any of the many arguments are substantiated, there is in my opinion, a godd case for this proposal to be considered in much more detail to bring it into a similar frame of reference as the other competing proposals, particularly as the environmental impact of the other proposals is causing great concern.

ENGINEERING ISSUES
A barrage located much further West near Minehead would offer, according to the Black & Veatch study of 2007, around 20Twh/annum of generation, which is about 50% higher electricity production than the proposed Cardiff-Weston barrage. The lower mean tidal range at this location of around 9 metres, as opposed to 11 metres at Cardiff, should not in itself be a disadvantage with the proposed 'Reef system.

Location of proposed tidal reef, compared with barrage

The longer generation period would make it easier to synchronise the generation period with periods of peak demand. Furthermore, a relatively small increase of head of around half a metre over the nominal operating head of 2 metres, will enable the scheme to operate at an overload capacity to meet peaks for short periods without causing a significant increase environmental problems.

The greater storage capacity offered by the outer barrage location further enhances this feature of the project because the percentage change in water level/tidal phase is much less significant and within the range variations that vou would exoect from factors such wind direction.

The cost to build the grid connections will be considerable but for the Somerset end of a reef barrage near Minehead there is an advantage in being much closer to the existing Hinkley Point 400 KV grid lines than for a barrage at the Cardiff-Weston site.

The core of the reef system is a simple causeway structure that is low enough so that it can be overtopped by storm surges and exposed at low tides without sustaining damage. Pre-cast cellular foundation modules could be placed with land based tracked cranes or gantries being much easier and cheaper to operate than crane barges and jack-up platforms.

The turbine modules would be installed along pre-cast foundation modules using the foundation causeway for access, which would considerably increase the 'operating window' when compared to marine access that is severely limited by the state of the tide and prevailing weather conditions.

Crest gates incorporated into the structure would prevent the tidal flow from passing over the turbine modules if they were sitting on the foundation causeway. If the turbine modules are buoyant, they could be installed so that could move up and down over the causeway foundation. It will be more economic to build than a conventional barrage that is designed to counter the waves and support a higher differential pressure of at least 8 metres.

The reef is almost entirely made up of turbine modules, which are comprised largely of water passages and voids, so in addition to the low aspect ratio of a caisson when compared to an embankment, much less material required. The height of the barrage is also reduced by not having to protect a roadway, having to counter storm surge levels or to take the higher operating head necessary for the 'big barrage's Kaplan turbines.

Over 10 million tons of rock fill would be saved by not having to dredge a new shipping channel and further dredging would be saved by not having a relatively small number of very large turbine caissons that need a greater installation depth. The Southern landfall is adjacent to the Somerset Steam Railway which is experiencing severe coastal erosion problems and has been used to transport armour stone from a nearby quarry, for this and the recent Minehead sea defence work.

Much of the construction can be achieved using land based construction machinery such as tracked cranes, as opposed to floating crane barges that are considerably more expensive to operate. By providing a series of intermediate 'island refuges' it will not be necessary to remove construction equipment to the shore at high tide.

The construction period for the large barrage is estimated at 15 years, whereas Mulberry Harbour the pre-fabricated floating dock built for the D-day landing in Normandy took only six months to build in secret around the UK before being installed in Normandy under enemy fire. It comprised 9km of cast concrete caissons, not dissimilar to my current proposal. A time scale of 15 years before revenue earning would make the big project uneconomic from the start.

Revenue earning could start within a couple of years of work commencing on the 'Reef Project' and well before the barrage is completed. This is possible because of the low differential head. So although the water will tend to flow around the completed sections, generation should be possible at reduced efficiency. A total construction period of five or six years would make the project much more attractive financially.

Navigation requires a depth of over 20 metres to accommodate the existing generation of cargo ships. The Cardiff-Weston project incorporates a new dredged shipping channel to Avonmouth requiring the removal of around 10 million cubic metres of material and a ship lock capable of passing the largest ships at all stages of the tide. A prime consideration for the existing and planned ports, is maintaining the high water levels. With the 'Reef Project' there is a small head differential across the barrage and near high water when the head is no longer adequate for power generation, the whole barrage is opened to allow the remaining water to continue up the estuary. The reduction in the peak tide level would thus the minimal but in the event of a storm surge the maximum levels could be controlled by delaying the opening of the barrage.

Energy lost before high water can in part be recaptured by allowing generation to start earlier on the falling tide. Ships could pass through the barrage unhindered during a period of about two hours before high and low water. A single 'flash gate' could be used at any stage of the tide up to the capability of a particular ship and depending on whether it was passing with or against the tide. A 'raceway' or very low lift ship loch for two metres of differential head would also be required for smaller and less powerful vessels, as opposed to 10 metres of differential head for the 'big barrage' scheme. The low lift lock could be a completely pre-cast structure that is floated into place like a floating dry dock.

Land drainage outfalls at a number of locations are compromised by the Cardiff-Weston barrage 'holding' the high water for several hours in order to obtain the necessary operating head for the turbines. The 'Reef Project' requires only a short delay, which could be as little as one hour, to allow a fall of around 1500mm before generation can start.

Large Kaplan turbines of high specific speed have good full flow efficiency at rated head and R.P.M. but these ideal conditions are only met twice in every tidal cycle, so all other stages of the tide are a compromise, giving lower efficiency and output. By contrast the 'reef system uses many smaller turbines, that will have slightly lower full flow efficiency but which run under 'ideal design conditions' for almost the whole of the operating period. The water turbines used in a barrage such as the Ranee scheme, are usually unidirectional, so a significant portion of the civil engineering cost is associated with sluices and by-pass channels to let the tide in. Bi-directional turbines, whilst requiring a more ingenious hydraulic design, save on these associated structures such as sluices.

Simple fixed-flow turbines can be built by many more sub-contractors around the country, so the price per kW will probably be the same or even lower than the large turbines that can only be built by a handful of international companies. The use of smaller and more innovative turbine designs is possible because of the modular design and more companies have the physical capability to build them. By limiting the operating head of the turbines to less than two metres, the differential pressure exerted on the structure in much lower than for a barrage, making the construction easier and less sophisticated so pre-cast unanchored concrete caissons similar to those used in the construction of Mulberry Harbour for the D-day landing, would allow the construction to proceed rapidly during the 'windows of opportunity' when the tides and weather are favourable.

Significant engineering design challenges will need to be addressed; these include the size, number, type and layout of the turbines, the effect and method of controlling marine growth, the systems for installing and maintaining the turbine modules and the behaviour of the caissons under storm conditions.

The routing of service and power cabling through the foundation causeway to the islands and the flexible umbilical connections will all need development, but much of the technology already exists and is used in the offshore oil industry.
The modelling of the flows through such a structure, how it impacts on tide levels, estua-rine currents and potential generation and operation, will be the first step in examining the merits of this proposal.

The number of man-hours required for offshore operations will have to be kept to a minimum if the cost are to be contained, reducing the risks of engineering failures and environmental disturbance if the concept is kept simple and the turbine modules are relatively small, though numerous.

ENVIRONMENTAL ISSUES
The 'Reef System' is designed with the environmental constraints such as Natura 2000 to the fore, as opposed to mitigation measures after the engineering has been designed. The phase change in the tides and the alteration of the natural regime will be significantly less than for a conventional barrage, resulting in minimal disruption to migrating fish, bird life and the mudfiat ecology. The 'Reef is in effect one continuous line of small turbines, typically 3MW each, so the tidal flow is not diverted across to a smaller number of large turbines of over 30 MW each that would radically alter the water flow patterns within the estuary, with the possible consequences of erosion or silting.

Migratory fish can pass safely through the 'Reef by incorporating slower running, low specific speed turbines of fixed geometry. This type of turbine need not be more expensive to build on account of the simplicity of design, the lighter construction and the use of lower cost materials, all made possible because the operating head is both low and constant. For fish to pass through the turbines without injury it is necessary to have wide clearances, smooth surface finishes and the lack of pinch points. A differential operating head of around two metres will not adversely affect fish by rupturing their swim bladders, which might occur if the head difference was greater. The choice of low specific speed turbines with a small number of blades with wide openings is feasible on this very low head, which should allow the safe passage of salmon and sea mammals.

Loss or altered habitat resulting from the altered tidal range is largely avoided with the 'Reef System' simply because the working head is so much less and the resulting changes very much smaller. Flooding and peak tides should be reduced upstream of the reef and only marginally altered on the seaward side because the delay in the tide cycle is so short. Difficulties with land drainage outfalls should also be reduced, simply because the deviation from the natural tidal range will fall within what is experienced naturally, even if the pattern is altered marginally.

Dredging and quarrying will be much reduced with the 'Reef Project' when compared to the 18 million mt" required for the big barrage. On-shore infrastructure environmental impact will be lower for the 'Reef because the more constant generation will make better use of the electrical infrastructure and 'hard engineering' structures will be physically smaller. Distributing the construction of caissons between several shipyards around the country will cause less disruption to the sensitive local environment and possibly improve employment in locations that do not have the capability to build the large 'big barrage' caissons'.

The aesthetics of the estuary and surrounding landscape, while being altered, will not be altered as much as that required by a 'big barrage'. The 'Reef will be almost totally submerged at most stages of the tide, while the 'big barrage' will tower ten or fifteen metres above low water level, and four or five above average high water levels.

ECONOMIC ISSUES
The material required for construction the 'Reef Project' will be significantly less than that required for a 'big barrage' because it is lower and only has to withstand two metres of head difference, and there is no need to have expensive 'non-revenue generating' technology such as sluice gates. No road is incorporated, because the cost of providing the extra height and protection from waves is not the most economic way of providing a road crossing. For each metre increase in height you have to provide about five metres of width. So a structure half the height of the 'big barrage' uses about a quarter of the material. No conventional ship lock would be needed as a single or double gate flush lock will allow ships to pass through at any stage of the tide, and dredging a new shipping channel would also be avoided.

The generation profile of the 'Reef Scheme' would be flatter, generating less power but for a longer period. This requires smaller generator and electrical transmission capacity, and a corresponding reduction in the intrusion on the landscape as well as improved electrical efficiency and utilization. Returns from the electricity generated will be seen well before completion of the project using the 'Reef System' improving the early economic returns significantly.

The simple fixed-flow turbines for a 'Reef System' can be built by many more subcontractors around the country allowing local manufacture, so the price per kW will probably be the same or even lower than the large turbines that can only be built by a handful of international companies. For maintenance the complete sealed turbine units could be lifted out and replaced with a serviced unit. Unlike the 'big barrage' turbines that are far too heavy to lift out once installed and require conventional dry powerhouses with personnel inside them to be protected from the elements.

A mixed public/private partnership would be much easier to implement with the 'Reef, because the main infrastructure including the underwater foundations could be public funded, with secondary power developers bidding for space to install a range of competing turbine devices along the barrage (similar to the planned 'Wave Power Hub' off the North Cornwall coast)

The Risks during installation of a 'Reef are reduced because the technology is kept simple and 'mammoth' operations are kept to a minimum or eliminated all together. Because of the nature of the working environment with strong tides and poor weather, the installation of the huge caissons for the 'big barrage' would be very risky. Building the scheme in smaller steps reduces risk. The long-term performance of the project will be more secure if several technologies and competing developers install many turbines. The chances of catastrophic failure are much reduced with the 'Reef Concept'. The Unknowns relate mainly to the environmental impacts and large scale marine operations, such as placing the caissons. The smaller the barrage and alteration of the tidal regime the lower the environmental risks and the cost of mitigation measures. The smaller the caissons are, the less the risk there is of an expensive accident.

SUMMARY:
The engineering issues

This proposal addresses the hydraulic engineering concepts that are necessary to meet the perceived and stated environmental concerns that are known to the author. It does not set out to define locations, technologies or operating systems but to present a logical direction in which the desien process can move while taking into account a wide spectrum of environmental concerns.

Limiting the operating head of the turbines to less than two metres, the differential pressure exerted on the structure in much lower than for a conventional big barrage'. The construction is then easier, less sophisticated and cheaper. The construction can also proceed more quickly during the 'windows of opportunity' when the tides and weather are favourable.

Navigation requires a depth of over 20 metres to accommodate the existing generation of cargo ships. Dredging operations would be far more modest for the 'Reef System' as ships could pass through a single 'flush lock gate' or a ship lock for only two metres of differential head as opposed to 10 metres of differential head for the 'big barrage' scheme.

Large Kaplan turbines of high specific speed have good full flow efficiency at rated head and R.P.M. By contrast the 'reef system uses many smaller turbines that will have slightly lower full flow efficiency but which run under 'ideal design conditions' for almost the whole of the operating period.

The environmental issues
The 'Reef System' is designed with the environmental constraints such as Natura 2000 to the fore, and is in effect one continuous line of small turbines, typically 3MW each, so the tidal flow is not diverted across to a smaller number of large turbines of over 30 MW each that would radically alter the water flow patterns within the estuary, with the possible consequences of erosion or silting.

Migratory fish can pass safely through the 'Reef by incorporating slower running, low specific speed turbines of fixed geometry. A differential operating head of around two metres will not adversely affect fish by rupturing their swim bladders or injure sea mammals.

Loss or altered habitat resulting from the altered tidal range is largely avoided with the 'Reef System' simply because the working head is so much less and the resulting changes very much smaller. Flooding and difficulties with land drainage outfalls should also be reduced.

Dredging and quarrying will be much reduced with the 'Reef Project', on-shore infrastructure will be less and distributed construction of caissons will cause less disruption to the sensitive local environment.

The aesthetics of the estuary and surrounding landscape, while being altered, will not be altered as much as that required by a 'big barrage'.

The economic issues
The material required for construction the 'Reef Project' will be significantly less than that required for a 'big barrage' because it is lower and only has to withstand two metres of head difference, no road is incorporated, no conventional ship lock would be needed, and dredging a new shipping channel would also be avoided.

The generation profile of the 'Reef Scheme' would be flatter, generating less power but for a longer period. Returns from the electricity generated will be seen well before completion of the project using the 'Reef System' improving the early economic returns significantly.

The simple fixed-flow turbines for a 'Reef System' can be built by many more subcontractors around the country allowing local manufacture, so the price per kW will probably be the same or even lower than the large turbines that can only be built by a handful of international companies.

A public/private partnership is well suited to build the 'Reef Project'. The main infrastructure including the underwater foundations would be public owned and power developers would finance and build the turbine systems.

The Risks during installation of a 'Reef are reduced because the technology is kept simple and 'mammoth' operations are kept to a minimum. The long-term performance of the project will be more secure if there are several technologies and competing developers.

Version 2.1 (25th June 2008)
later pdf version
Rupert Armstrong Evans
Armstrong Evans & Associates
www.evans-engineering.co.uk

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