GB2510203A - Tidal power plant with membrane dam construction - Google Patents

Abstract

A tidal power plant has a dam comprising a rigid or flexible membrane damming e.g a sea inlet, in preference to massive dams of poured concrete or ballast. A flexible membrane h is supported by spaced tripods of props v, w. The top of the membrane is supported between props by a suspension cable g (see figure 3). The flexible membrane described is of a woven or net base overlain with an impermeable coating or a woven textile throughout. The membrane is fixed to the sea bed, and may have landward and seaward skirts l or inflated gaskets to prevent leakage around the sea bed. This dam may make tidal range schemes economically and environmentally feasible which otherwise had been discounted. The structure may be protected from wave damage by a linear array of wave powered electricity generators deployed to seaward.

Description

A marine dam by means of a thin membrane for tidal range electricity generation and for fresh water storage for water supply and flood control purposes.

Description

Background

it is now generally established that the earth's climate is warming, that mankind may have a part to play in this process and that reducing our carbon dioxide emissions may help to reduce this tendency. The British government has pledged that we as a nation will greatly reduce our national emission of carbon dioxide in the long term. To that end it is Is trying to increase our use of renewable sources for electricity generation. Increased ind.ependence from insecure foreign sources of energy would be an additional advantage.

Much attention has been given to wind generation but wind sometimes does not blow.

Water is much heavier than air (i.e. its specific gravity is greater) and its flow therefore involves that much greater kinetic energy which it may be possible to convert into electricity. Natural movements of sea water are an important source of energy that the government rightly wants to see utilised if it can be found feasible to do so.

Considerable effort has rightly been put into research concerning electricity generation from the kinetic energy of the flow' of sea water brought about by tidal movement under the influence of the moon and from ocean currents, such as the Gulf Stream, induced fundamentally by energy from the sun. These are referred to as tidal stream and ocean current systems of generation respectively.

Distinct from these energy sources, the present patent application concerns tidal range electricity generation. That is the generation of electricity from the hydrostatic pressure difference either side of a barrier across the mouth of a sea inlet subject to rising and falling tides under the gravitational influence of the moon.

Under this system the rise and fall of the tide is restrained by a dam which directs the tidal flow of water into and out of a sea inlet exclusively through turbines which generate electricity. The turbines are contained within ducts so that all the water passing in or out is used to drive the turbine rotors thus making efficient use of the stream of sea water.

This system has similarities to the use of the potential energy in hydroelectric schemes but the head of water is much less than is generally the case at a dam on a mountain river.

The barrier has to resist the pressure of a head of water from seawards at high tide and from the trapped lagoon (the landward side) at low tide. The hydrostatic head can be reduced by reducing the resistance to flow inherent in the chosen design of turbine system or by opening by-pass sluices so that only a lesser head of water is allowed to build up. One set of difficulties that such a system has to face that the dam of a land bascd hydroelectric scheme docs not face is thc harsh marine environment, but mcasurcs can be taken to reduce such harshness and to allow ease of repair and renewal.

A tidal range generating station has been in commission at La Rance on the north coast of France generating electricity for the French grid since 1966. It uses a substantial conventional concrete dam. Two large tidal range generation projects with substantial dams are in advanced stages of preparation at Sihwa Lake and Incheon Bay near the city of Seoul in the Republic of Korea (South Korea). Several other such projects function in various locations in the world including Annapolis Royal off the Bay of Fundy, Nova Scotia. Canada and sites in China and on the Barents Sea coast of Russia.

British governments of different political persuasions have presided over the planning stagcs of prjccts aimcd at harncssing the latent energy of the large tidal flow in and out is of the sea inlet called in its outer part the Bristol Channel and, further inland, the Severn Estuary. Thomas Fulljames Gloucestershire County Surveyor proposed such a scheme in 1849. The design of the p rojeet has been substantially developed since the 1980s.

in late 2010 the UK government shelved the plan.

The problem that the inventioii solves.

There were concerns about the cost of construction and the ecological impact.

A large part of that cost would be that of building a dam to enclose this large area of water. A dam enclosing the largest realistic volume of sea water and thus enabling the generation of the largest feasible proportion of British national electricity needs would stretch from approximately Hurtstone Point, Devon, west of Minehead to Nash Point west of Llantwit Major on the Welsh coast about 12 miles (about 19km) away. An average depth of water along this line is around 20 metres, the deepest point being 37 metres (122 feet) at mean sea level.

Making a dam along this line by conventional means using ballast or by casting concrete in situ is difficult and expensive especially in the deeper pads of the channel. A concrete dam as would be used for hydroelectric schemes in mountainous river systems (like the Hoover Dam in the United States) would be difficult to install in the sea since it is difficult to evacuate sea from the submarine site of setting of poured concrete. A conventional dam of concrete blocks, ballast or large rocks would have to have a very broad base and a very large volume. Sourcing the materials and installing them would be a massive undertaking.

The solid construction of the long Afsluitdijk in the Netherlands, built in the twentieth century to allow the reclamation of land under the Zuider Zee only has to cope with a local sea depth of around 5 metres. It needs to be substantial to protect against flooding of the inhabited areas and farmland protected by the dam. A breach of a dam across a sea inlet for tidal range generation would be undesirable but would be likely to be of less human consequence. It would cause a rise in water level but this would remain within the pie-existing tidal range (although possibly higher than the usual range of levels once the barrier is installed). If the new design of dam gave way suddenly in its entirety, a flood wave like a small tsunami could take place but the design would need to prevent this. A breech along part of the course of the dam would be unlikely to cause a substantial wave.

Finding a cheaper and simpler way to construct a marine dam without the extensive use of concrete or of ballast might allow a project currently deemed uneconomical to be revisited. The cost might be less in money terms and also in terms of the carbon dioxide emissions involved in sourcing and transporting the large quantities of hard core. The chemical process involved in the production of large amounts of cement would emit large amounts of CO 2. Extracting such a lot of hardcore from any site nearby would leave a large, unsightly quarry amid famously beautiful countryside.

is The environmental consequences of such a dam can only be a matter of conjecture until it is built. This makes it unappealing to undertake such a big, expensive project as a traditional ballast or concrete dam with unknown ecological side effects.

The local ecological consequences may not be as bad as feared since the waters of the Bristol Channel are currently very turbid and carry a lot of silt because of the vigour of the tidal flows at this particular site with its tidal range the second greatest in the world.

This prevents light from penetrating the water so it is largely sterile, bereft of organisms that need to photosynthesize. A dam that reduces the agitation of water in this inlet would probably allow silt to settle, light to penetrate, plankton to thrive and to support a kod chain of fish and sea birds and perhaps even a productive, locally-based fishing or fish farming industry contributing to food security for the UK population.

Domestically generated renewable electricity would tend to make us less dependent on fossil fuels and even on Uranium sourced from abroad. This would give us greater political independence in pursuit of our foreign policy objectives in the world.

Against the unknown but possible local ecological costs, including changes to wetland habitats and their bird populations, must be put the significant contribution that such a scheme would make to the ecological -of reducing national greenhouse gas emissions and those of other nations to which we could export the technology. The avenge area covered by such a sea inlet when functioning under this system for electricity would be similar to the pre-existing state. The tide would still rise and fall albeit possibly by a slightly smaller vertical range.

io A simpler, cheaper darn design might therefore solve several pmblems which hold the scheme back at present. The financial cost might become affordable. Less greenhouse gases may be emitted by manufacturing less cement and by sourcing and transporting less hardcore. The countryside would not have a big hole in it.

Pilot schemes involving membrane dams at smaller sea inlets could make it affordable to evaluate designs from an engineering point of view, to measure their ecological impact and to devise countermeasures to deal with any side effects. They would be easier to deconstruct and remove if necessary. Possible UK sites might include: Ernie Mouth, Devon; Abereiddy, Pembrokeshire; the head of Loch Striven near Glendauel, Argyll; the lough between Island Magee and the mainland of Co Antrim south of the straights at Magheramorne near Lame, County Antrim.

if successful, the system could be applied to other major British sea inlets such as the Wash, the Solway Firth, Strangford Lough and inner Sound east of Skye and other sea lochs on the vest coast of Scotland. The system could even be used on a much larger scale at north and south entrances to the Irish Sea if adequate locks were provided for access by shipping to the major Trish Sea ports including Liverpool, Belfast and Dun Laoghairc. The system could be sold on the world market for use at many locations.

Bermuda, for instance, seems to have a suitable location. Sea inlets where the tidal range is not as great as in the Bristol Channel and that are not currently considered economical sites for tidal range generation under pre-existing technology might become worth harnessing for generation.

The rising tide takes some time to sweep around the British Isles. High tide times vary around the British Isles in such a way that tidal range electricity could be supplied to the National Grid serially as the tidal wave progresses around our shores, thus sustaining supply for a greater proportion of the 24 hours of each day and relieving power stations of other generation modes for a longer period than if only a single tidal range generation site were in commission. Tides go through two complete cycles a day approximately.

They vary from place to place and with phases of the moon and of the year, but they are astronomically predicable with great precision in a way that wind-driven generation is not, despite so much national resource being invested in wind..

The present draft application is being offered, altruistically, as a gift from a UK citizen to the UK government, hoping that it can be technically usethl and that the ownership of this invention can be retained as part of the wealth of the nation in a contemporary environment in which our generation capacity has largely passed to the hands of foreign or multinational companies. It is offered with a desire that HM government, aided by British universities, develop its own facilities for research and development in this field rather than leaving such activity to commercial enterprises almost exclusively.

A subsidiary use of such technology would be to retain pools or lagoons of fresh water in lagoons at sea both in order to acts as reservoirs for fresh water for subsequent domestic, agricultural and industrial use and to accept flood water pumped from rivers as they reach the upstream margins of flood plains via conduits to relieve flooding of farmland and industrial and residential properties.

What the invention does.

The hydrostatic pressure of the head of water to be held back by a dam in this use is small compared to a dam for hydroelectricity generation in mountainous regions. The Oroville Dam in the United States, for instance, holds back an enormous head of water of 230 metres. Tt therefore has to resist huge hydrostatic pressure.

The present invention consists of the use of a strong but relatively thin membrane to act as the water retaining dam to facilitate tidal range generation. This invention does not concern the design of the turbines, turbine-housing, locks, sluices or mechanisms for fixing structures to the seabed. Two variants of membrane are proposed: one is a flexible membrane (see Figure 1) of which the contour changes with the phase of the tide from convex on the seaward surface as it restrains the outflow on the ebb tide, to convex on the landward side as it restrains the inflow on the flood tide. The other variant is a stiffer membrane composed of frames covered by marine-environment-resistant treated steel plate (Figure 2) or reinforced concrete prefabricated plate sections rather likc the prefabricated sections of the former Berlin Wall or the wall that has been erected between Israel and the West Bank. Systems using both types of dam structure are hero described.

I have not tested these plans experimentally and cannot vouch for their actual functioning or their safety. Experimental work and trials are needed to verify these.

Flexible membrane version.

The new invention consists of the use of a robust but flexible membrane to retain the rising flood tide on its seaward side and/or the falling ebb tide on its landward side in such a way as to direct the path of the sea water through a battery of turbines retained in a housing fixed rigidly to the sea bed (see Figures I and 3). A lock would also be provided for shipping to pass in and ont, (see layout in Fig 7) as would sluices to provide a bypass route for the sea water in the event of water passage through the turbines needing to be moderated or stopped or if, for other reasons, the tidal flow needed to be allowed to bypass both membrane barrier and turbines. Those solid structures would all be firmly and rigidly fixed to the rock of the seabed by methods that are already available, particularly in the oil industry and in the construction of arrays of off-shore wind turbines. At the landfall of the dam on either side of the inlet a similar robust fixed structure would hold the side margins of the membrane at a landfall embankment (Figure 8).

The flexible membrane would consist of a tough waterproof material (Figures 16 and 17) with a mesh reinforcement similar to the reinforced tarpaulin material used to cover scaffolding around buildings under construction or repair and to prevent material dropping from the worksite onto people below. It would be specially developed for the purpose described. The reinforcing mesh can be of great tensile strength and an occlusive surface would be overlaid on both sides so that it would be held against the mesh whether pressure was seaward or landward at low and high water respectively.

The surface of the membrane would have fixing points (Figures 16,17) of similar material moulded or cast into the integral structure to give firm and waterproof anchorage points for the tethering of adjacent sheets of the same material and for surrounding skirts to prevent leakage at joints in the membrane. The fixation point would be secured by a reinforcing cord to the fabric of the warp and weave of the underlying net. An alternative way to effect secure, watertight joints would be to develop -or to source from elsewhere -a technique for adhesive fixation of overlying layers of similar membrane to be fixed to both sides of the mesh. Doing this out of the water (during or after manufacture) would be simpler than under sea water but it may be possible to find or develop a technique to perform this procedure underwater.

The flexible membrane would be held all around its perimeter.

The bottom edge would be fixed (Figures 9 and 10) to fixings of one of a variety of already extant designs, or by either a tripod of treated steel rods with protruding shafts subsequently threaded and fitted with bolts, or by a tripod of long screws, drilled into the sea bed (not illustrated) or by techniques currently available to the oil industry and to installers of harbours and off-shore wind turbine arrays. The spacing of these would be decided based on prototype experience but 10 metre intervals may be suitable. The bottom edge of the membrane (Fig 9) would be reinforced by the incorporation of a bottom edge cable or tape of steel, polypropylene or similar material either incorporated into the mesh or sewn to it prior to coating with the occlusive membrane or the mesh and membrane would be folded around it as a seam and tethered back on itself Inflated gasket held in open-topped seabed tunneL An alternative fixation of the bottom edge of the membrane to the sea bed could be as follows. A small diameter tunnel (e.g. I to 2 metre diameter) could be bored a sufficient distance below the seabed to ensure there were robust rock structure between the roof of the tunnel and the seabed. Then a slot would be cut between the tunnel roof and the seabed to allow the bottom skirt of the membrane to enter through it and be secured to a tube or inflated "sausage" within the tunnel that could be inflated with air or water to press firmly against the wall of the tunnel to effect a water-tight junction and anchor the membrane against being avulsed by wave or hydrostatic forces on the membrane. Alternatively the membrane could pass from above sea level down through the sea, through the slot, around the sausage and back up out of the slot and up to above sea level as one unit without ajoin. The sausage, then, once inflated with air or water under pressure would thus secure the invaginated membrane in the tunnel and make a water-tight joint between the membrane and the sea bed. The system is rather like the cord resembling an electric flex trapped within the seam of the edge of a caravan awning being secured to a channel that it can be fed into running along the side wall of a caravan thus tethering the awning to the caravan and making a weather-tight join. See figure 22.

The top edge of the membrane (Figure 11) needs to rise above the recognized highest possible high spring tide sea level by a margin based on prototype experience, allowing for tidal surges and storms. As an alternative the top edge could be placed at the level of the highest high tide mark itself, and not higher, so as to allow waves to wash over it at high tide if there were a severe storm, thus allowing the kinetic energy of a large wave to be dissipated.

Trials of setting the surface of the membrane at an angle to the vertical so that the bottom edge is further seaward than the top edge may allow large waves to discharge their energy upwards instead of directly into the substance of the membrane fabric. This might need greater spacing of the seaward-from the landward legs of any tripod or tetrapod props (see text below).

The top edge of the membrane would be reinforced by a cable or tape as at the bottom edge or would be hitched to it by links like rock climbers' karabincrs (Figure II). That cable or tape is in turn suspended fmm props placed at intervals along the course of the membrane, which props stand on the seabed and protrude to a level above the water to suspend the entire membrane. The suspension is through the medium of a cable allowed to adopt a catenary curve as it hangs between fixation points at the top of successive props as does the main suspension cable of a typical suspension bridge..

These props have a number of possible designs (Figures 13,14,15). One option (figure 13) is tripods formed of protectively coated steel in the form of a strong cylindrical tube or an H or L cross-section beam. The feet would be tethered to the seabed by fixings of design analogous to those attaching the membrane base to the seabed but bigger. They would need to prevent both lateral movement and pulling out of the bases of these props (avulsion from the seabed). In this case the single (stronger) leg is on the seaward side of the membrane and the other two on the landward side, so that the force of waves would be resisted by two steel beams and not just one. This arrangement would be reversed if found desirable in the light of experience with a prototype, and the size and strength of the various beams could be adjusted in the light of trials.

An alternative but otherwise similar design is a four legged prop or tetrapod (Figures 14,15). The length of such legs would have to be tailor-made to each location to take into account any irregularity or angle of the seabed. This might not be inherently necessary in the case of tripod legs as a tripod can settle on any surface.

In either case a cable would be attached to the joint where the three or four legs are gathered together at the top or apex. (Figure 13) This cable would run the length of the dam adopting a catcnary curve bctwccn each prop (Figure 3b) . From this cable smaller cables would drop to the cable along the top edge of the membrane to suspend it. A cable between the tops of successive props without any cable hanging from it would steady the is props and resist any force causing the props to diverge. (Fig 3 a and b) Sufficient room would need to be allowed between the seaward and landward leg or legs to allow the membrane and its suspension cables to adopt a varying contour bowed seaward as the tide goes out (ebb tide) and landward as it comes in (the flood tide) (Figure 1). If at the computer modeling stage or prototype stage the interval were found insufficient to allow such excursion (plus a safety margin to allow for unusual waves) then longer tripod/tetrapod legs could be used or an elongatedjunction plate (Figure 13 c) could be incorporated at the top of the gathered legs to add distance between the seaward and landward leg or legs.

An alternative prop could. consist of a single wider cylindrical tube or stanchion of metal or concrete (Figure 15) more deeply and more robustly inserted into the seabed and of sufficient angular robustness of the whole structure and of its fixing to resist the seaward and landward stresses of wave impact and hydrostatic pressure. The single stanchion version should be located on the seaward side of the membrane to spread the impact of waves so that the membrane is not abraded by waves against the tower. An altemative system is for the membrane to be attached to the sides of the single tower in a similar way (Figure 12 a and c) as that described below for its junction with the various other "solid structures" along its route (turbine housing, sluices, lock, and landfall embankment, each of which would be fixed rigidly to the rock of the seabed or the coasts on either side of the inlet).

The membrane is fixed to the side of these solid structures (Figure 12 a for flexible membrane or c for rigid steel plate membrane) by a retaining bracket of two L cross-section strips of steel running along the course of the attachment with the retained edge of the membrane (reinforced by a tape or cable to prevent it pulling out) damped in between these two strips.

Shelter from wave impact should be provided by a linear array of wave powered electricity generation devices that are already available (Fig 7) deployed to seaward of the dam and parallel to it. They would absorb a lot of the energy of the waves leaving the barrier itself relatively protected.

Leaks around the bottom edge of the membrane (Figures 9 and 10) are controlled by a skirt of the membrane seamlessly continuing past the junction with the seabed and fanning out onto the seabed to landward and to seaward of the join by a requisite distance probably 10 metres. The peripheral edge of the membrane skin is fixed to the rock of the seabed by smaller but similar fixings to those used to secure the bottom margin of the body of the membrane and ballast is deposited on its upper surface to press it down onto the seabed firmly and prevent water from passing under the bottom margin in any quantity. Alternatively the same function could be achieved by heavy concrete bars with handling loops, lowered onto the upper surface to form a continuous line of heavy material pressing the skirt onto the mechanically graded and smoothed seabed rock.

is If currents of escaping water were detected (by ultrasonic probe from a maintenance barge) bypassing the dam at any point inwards during a rising tide or outwards during a falling tide, the entrance point of the escape channel could be closed off by dumping further ballast or concrete bars onto the surface of the membrane skirt.

Ballast could be topped up regularly by a dedicated suction dredger ship sourcing the ballast from elsewhere. This equipment would be used also in the construction phase to remove by suction any loose particulate deposit at the proposed site of fixing. It could also be used to remove ballast if repair or replacement of a section of membrane is required. If bars were used they could be removed by ship-mounted crane or submarine robot.

Junctions in the membrane (Figure t2a) are kept to a minimum particularly at sea surface level to resist stress from waves damaging the membrane. However, seams in the membrane would be secured by an overlapping junction. The reinforced edge of one section would insert between the landward and seaward skirts attached to the edge of the adjoining section. The skirts are then laid against the two surfaces of the adjacent section and tethered to them at tie fixation points to seal the joint to prevent water escaping through the joint in either direction. This fixation could use a robust version of domestic cable ties. Alternatively as shown in fig 12b the join of sheets of membrane could be made flush by the membrane sheets being clad on both sides by a patch or skirt tied to the surface fixing points already manufactured into the membrane.

In manufacture the membrane would have fixation points (Figures 16 and 1 7) stitched, welded or stuck with adhesive to both surfaces of the membrane at regular, frequent intervals to enable the skirts to be tethered to the surface without leaks at their fixation points. These points would link in to the reinforcing mesh of the substance of the membrane. Hydrostatic pressure presses the skirt on the respective high pressure side against the body of the membrane to eliminate leaks.

At the margins of the turbine housing, lock, sluices and landfall emplacements the membrane is held (Figure 12 a) by its reinforced edge being trapped between two L-cross-section holding strips bolted or otherwise fixed to the solid structure's vertical surface.

Rigid membrane of metal This alternative composition of thc membrane takes the form of sheet steel coated with a protective layer to prevent rusting (Figures 2,4,19,20). This layer is of zinc, plastic, powder coating or other material.

The sheet of steel is held all round its periphery and at intervals across its surface by a framework of sturdy steel beams to which it is welded on shore prior to being placed in position. Fig 19a. The bottom edge is pre-contoured to fit the predetermined contour of the seabed. The steel frame would be contoured by established pipe bending techniques.

The junction with the seabed (Fig I 9b) would be leak-proofed by the deposition of ballast is material by dredger directly onto the bottom edge as it meets the seabed or by means of a skirt as described for the flexible membrane which is held in place by fixings to the seabed and by ballast or concrete blocks with lifting loops.

The junction of the rigid membrane to the turbine housing, locks, etc would be sealed by pairs of L cross-section strips (Figure 12 c) with a shock absorbing system to avoid the impact of waves disturbing the integrity of the solid structure. These joints are protected against leaks by a system of skirts similar to the system with the flexible membrane.

The fabrication of this structure requires a considerable amount of steel, it may help in visualizing it to consider a large container ship the Emma Maersk whose hull is 30 metres deep, which depth would be sufficient to constitute the membrane over its informally estimated average depth. Over much of the course of any such barrier across the Bristol Channel the depth is 20 metres or less. The width of the Bristol Channel at this point is equivalent to 49 times the length of this ship. This may seem a lot but the membrane structure seems likely to be simpler to construct and this would be done on shore prior to installation. Thickness of steel plate and the intervals between steel support beams would need to be established in prototype studies.

The steel membrane and its supporting/reinforcing frame (Figures 2,4 and 19) need to be kept upright and in the correct position viewed in plan. The membrane and frame would either be in a straight line in sections joined by leak-proof joints or zigzag (as seen from above -not illustrated). The latter structure would have an inherent rigidity and resistance to being pushed over by strong waves and water pressure. This stability would be reinforced by props from seabed fixings some distance away on either side of the barrier and stretching up at an angle to reach the upper edge of the frame.

If the steel membrane and frame were in a straight line it would also be held by angled props on seaward or landward side or both from seabed fixings to the upper margin of the frame but the integrity of the shape would depend much more completely on the props so they need to be stronger.

The whole structure can be offered some protection (Figure 7) from damage by wave impact by placing an array of devices to generate clcctricity from the kinetic cncrgy of waves such as the sea-surface following devices already devised. This array is placed along the seaward side of the length of the membrane at a set distance informed by prototype experience) This arrangement neatly generates renewable electricity from waves while protecting the membrane dam from much of the destructive force of the waves' impact.

The seabed fixing could follow an adaptation of the slotted tunnel design described for the lower margin of the flexible version of the membrane. In this case an inflated gasket could be fitted between the bottom of the rigid frame of the steel membrane and a machined smooth path or trough on the seabed into which drilled fixings could hold the frame. Figure 23.

is A watertight seal could also be effected between adjacent sections of rigid membrane by a similar inflated gasket. Figure 24. The bottom edge of the membrane whether flexible or rigid can be held to the seabed and the junction sealed with an inflated gasket between the membrane edge and a smooth manufactured track to which it can be tethered thus compressing the gasket. In this instance the track can be made of material resistant to the marine environment.

Rigid membrane of prefabricated reinforced concrete.

The membrane could also be composed of pre-cast reinforced concrete panels shaped like an inverted T and held in a frame of coated steel contoured to the shape of the seabed and clad on both sides with a flexible occlusive membrane. (not illustrated). Alternatively a concrete wall resting on the sea bed could. employ the technology alread.y u.sed. for the construction of the Berlin Wall and, more recently, the security wall dividing Israel from the West Bank. This could be built in invert T cross-sectional sections at a coastal site and transported and lowered into position by special barges. In this case attention would be needed to the design of the joints between panels to eliminate leakage of seawater by closely fitting contour of the edges of adjacent sections or by cladding both sides of each joint as described above for joints in other modes of bather. The sections may need to be tailor-made to conform to the contour of the seabed at the specific location for which the section is destined. The seal with the seabed may be made more water tight by a skirt and ballast as described for the steel and flexible versions of the membrane dam.

Installation of the concrete version and possibly of the metal version could be effected by horizontal assembly on a row of temporary pontoons or barges and computer controlled simultaneous rotation of the entire structure from horizontal to yertical coinciding with its launch into the sea onto its prepared smoothed bed. This would eliminate the difficulty of a part-erected barrier allowing tidal flow around the margins of any remaining aperture in the partly installed barrier. The rapid installation as is mentioned here could be effected at a time of zero tidal flow at high or low water. The sealing of the joint between the bottom edge and the seabed could be rapidly effected by use of a suction dredger laying ballast or the laying of concrete slabs at least before many tidal phases had passed.

Resisting the potentially destructive energy of wave impact demands that the upper portion of the membrane and its supporting structures be of particularly great strength.

That of the lower portion needs to be able to resist the lesser hydrostatic pressure caused by the difference in water level on either side due to tidal range and some upwards tension whenever a wave hits the upper section but it would not have to resist direct wave impact forces below the spring tide low water level. The strength of the upper part probably needs to be greater than is required of the lower part. It may be practical to make the upper part of stronger material or to make the whole to the specification found to be required of the upper part. I ()

Alternatively angled baffles could be positioned to the seaward side of the membrane so as to deflcct the impact of waves upwards absorbing their kinetic energy. If found necessary a line of such baffles could be held by a similar row of props such as the tripods which have already been described as supports for the membrane. If a row of is wave driven generators were available they may absorb the wave energy to a degree that makes such baffles unnecessary.

Maintenance The flexible membrane may suffer tears from wave damage. It may suffer degradation from the marine environment causing the occlusive material to lose its plasticizer and become brittle. If this were found to be the case on prototype testing, a different material may be found to replace it such as a robust polyester as used in canvas fabric.

Alternatively if its useful life in the marine environment were moderate or better a scheme for timely renewal of the membrane could be devised. In this case, any baflast could be removed by the dredger or any concrete bars lifted by crane or robot. Skirt and bottom fixings or intermediate links thereto could be detached and a section or all sections could be replaced with new ones. Much of this work could be done by purpose built submarine robots.

Tears or degeneration limited to the surface of a single sheet of the membrane could be repaired by attaching a patch of similar material over the defect on both seaward and landward sides tethering the edges to the pre-manufactured fixing points by the fixings similar to tough cable ties perhaps by robot.

Most damage to the membrane whether flexible or stiff is likely to be sustained at sea surface level under the impact of waves. At this location repairs may be easier. It may be found possible to defer repairs for some time pending calmer weather and still water at high or low tide without severely diminishing generation capacity or altering lagoon sea levels. This would mean tolerating a temporary leak in the membrane and reduction of generating capacity which may be slight.

Steel membrane deterioration by rust could be delayed by available coatings at the fabrication stage on land. Rusted holes in the structure could be replaced by submarine welding or by patches fixed by suitable water-resistant bolts.

Marine flora and fauna could be scraped off the metal version's surface by machinery in situ if found necessary. Tt scorns likely that a srnooth shiny surface of the main body of the flexible membrane would not accept colonisation by such fauna or that such colonisation could be found acceptable but this could be the subject of prototype studies.

Staff access.

Staff access to the membrane and its structure could be by very light rail car on a line supported on brackets projecting on the landward side of the tripod, tetrapod or single stanchion supports of the flexible membrane version or the angled supports on the landward side of the steel membrane. The railcar could be protected against falling in the case of derailment by running in a tubular cage.

Alternative access methods would include a dedicated barge running along a physical cable or directed by a laser guidance system.

Shipping There is an inescapable cost to shipping access imposed by proceeding with any barrage like this. Avonmouth receives about four ships per day. The Bristol Channel is too shallow for them to pass at low tide. The time taken for a ship to transit the lock (about an hour) would be kept to a minimum by transit as near as possible to high tide so that the discrepancy of water level on the two sides would be at a minimum. In fact there would be a delay in the equalization ofthe levels either side of the membrane to slightly after high and low tide because of the inertia of flow in the system.

Consideration could be given to rnaking the support structures even more robust so that they could support a passenger rail link for passengers from Devon and Somerset to South Wales. This seems more attractive and more usethl if the barrage were sited further up the Bristol Channel between Brean Down near Weston super Mare and the Bristol conurbation on the English side, and Lavernoek Point near Cardiff See Fig 5. However a barrage at this site would capture a much smaller volume of seawater and therefore holds less potential for electricity generation. It seems confusing to amalgamate the two functions at loss of the main advantage for generation. It would also require a much more robust structure to bear a much greater load if public transport were to use such a structure.

Pontoons to hold the upper edge A subsidiary model (Figure 20) which would be attractive if detailed model studies showed it was safe, is to use floating pontoons on both landward and seaward sides of the top membrane edge to support the stmcture. This would do away with the need for supports largely or completely. The pontoon on the side having (for the time being) the higher water level would support the top edge of the membrane. The pontoon on the lower side would pay out a cable to allow the edge to rise up from it to the leading edge of the higher pontoon. The control of the reeling in/paying out of' the mooring cables and the cables from the top membrane edge to the pontoons would be under the control of computer operated winches programmed to follow the risc and fall of the tide closely at that location -data that could be precisely predicted although allowance would have to be made for sea level variations caused by wind blowing the sea into the Bristol Channel.

The unknown quantity of this method is not knowing whether the pressure of water would overcome the elevating pull of the floating pontoon. If the latter was insufficient, the membrane might be pushed down and the retained water would spill over the top of it which could be dangerous. The intervals between the pontoon pairs would need to be decided on the basis of prototype studies. It does need a properly scientific assessment because not having to build the props would offer a considerable saving although the cost of the pontoons and their management by computerised winches has to be put against that.

Application to other sites.

The great advantage of this design in all its possible variants is that it holds out the is possibility of damming other inlets, large and small, for the carbon neutral generation of electricity. Other types of site (Figure 6) would present slightly different technical issues.

Examples are deeper, narrower inlet mouths in Scotland's west coast, narrow inlet mouths with forccffil tidal stream as at thc entrance to thc na above Clcddau Bridge at Milford Haven (upstream of the oil refineries and dock) and the entrance to Strangford Lough, County Down.

At some Scottish locations the size of the retained body of water can be greatly increased by additional damming of small gaps between islands by a similar method, enabling the main membrane to capture a much bigger area. A main barrier at the north end of Inner Sound to the east of Raasay would capture the energy of a very large body of water if additional dams were added between Raasay and Rona, Raasay and Skye and between Skye and the mainland at Kylerhea.

Tidal lagoons not attached to land.

A further application is to tidal lagoons not attached to land. Electricity could be generated by the inflow of sea water with the tide and its subsequent outflow without the whole structure being in contact with land at all. Suitable locations could include areas of the North Sea off the east coast of the UK. Designs prepared so far by others focus on rubble bank dams and therefore favour very shallow waters. The shallower the sea at the site of the lagoon the greater the proportion of the tidal excursion to the height ofthc membrane required and thus the likely usefulness for generation by the entrapment of water at that site. In other words the deeper the sea at the site in question the more submarine portion of the membrane you have to install to use the portion at the top where the sea level fluctuates with the tide. The membrane technology described here would enable countries with shallow tidal seas but without indented coastlines to benefit so may be of interest to export customers.

Such a lagoon entirely deployed at sea could also function as a freshwater reservoir if the technological challenges of perfecting a water-tight junction at the foot of a flexible or ngid membrane dam can be achieved.

Such a lagoon was two additional possible functions other than electricity generation: first as freshwater storage for use as part of a national water distribution grid without the engineering costs and planning obstacles of making an on-land reservoir and secondly for storing water diverted along conduits from swollen rivers for flood relief which water could also subsequently be used as supply for a national water grid.

Patent Application by Richard Rodgers My ref RR2013 A Drawing recommended to accompany abstract: Figure 5 please Drawings (Figures): The figures numbered as below are referred to in the text and drawings.

I Cross section of flexible membrane barrage during (a) ebb (b) flood tide.

2 Cross section of rigid membrane barrage during (a) ebb and (b) flood tide.

3 Tmpression (a) and elevation (N of barrage: flexible membrane version.

0 4 Tmpression (a) and elevation (b) of barrage: rigid steel plate version.

Map of preferred Bristol Channel location.

6 Map of other potential sites for use.

7 Plan of whole installation.

8 Landfall embankment profile.

is 9 Cross section of flexible membrane bottom edge and seal.

Detail of cross section of bottom edge of flexible membrane.

11 Flexible membrane top edge.

Ct) 12 Membrane junctions (a) flexible to solid structures (turbine housing, lock, sluice, landfall embankment) (b) flexible to adjoining sheet of flexible membrane. (c) Rigid membrane to fixed stmctures. (d) Rigid membrane to adjoining rigid membrane sheet).

O 13 Prop: tripod version (a) from seaward. (b) from landward. (c) Elongated junction plate for apex of structure: impression and lateral view.

14 Prop: tetrapod version (a and b) ditto.

0 25 15 Prop: single stanchion version from land.ward..

16 Flexible membrane structure: face.

17 Flexible membrane structure: cross section.

18 Flexible membrane: top edge.

19 Rigid membrane: (a) cross section. (b) bottom edge.

20 Pontoon supports for membrane top edge.

21 Suspension Bridge Principle: Suspension of top edge of membrane dam.

22 Invaginated bottom fold of membrane held in slotted tunnel in rock of seabed.

23 Inflated gasket seals bottom of rigid membrane onto seabed.

24 Tnflatcd gasket seal between rigid membrane sections.

Drawings explained: 1. Cross section of flexible membrane barrage during (drawing a) ebb tide and (drawing b) flood tide. The landward legs of the tripod are actually not in the same plane as the seaward one. Key to drawing by indicator letters: (a) apex joint of the three poles of the tripod prop (b) water level on landward or lagoon side (c) water level on seaward side of membrane.(d) lagoon level during flood tide. (e) sea level during flood tide (IT) reinforced top edge of membrane (g suspension cord suspending membrane top edge from the apex joint and elsewhere from the catenary cable.(h) membrane (i) bottom edge of membrane (j)seabed fixation for foot of tripod leg (k) seabed fixation for bottom edge of membrane (1) skirt of membrane material. (s) seam or horizontal join between individual sheets of membrane.

2. Cross section of rigid membrane barrage during (a) ebb and (b) flood tide. Key: (b) ebb tide lagoon level (c) ebb tide sea level (d) flood tide lagoon level (e)flood tide sea level (j) seabed fixation for bottom of prop (m joint of top of membrane with props or single seaward or single landward prop (n) coated steel plate membrane (o) steel horizontal frame elements (p) seabed fixation for bottom of membrane (q) sea water (r lagoon water (x) seaward prop for steel plate membrane frame. (y) ditto landward prop. The design may not need both seaward and landward struts but both are shown here.

3. Impression (a) and elevation (b) of barrage: flexible membrane version. The view is from the southeast looking north towards Wales in a section without the fixed structures being visible except that the lock/turbine housing is depicted in the distance. The approximate shape is shown of the catenary cable from which suspension cords descend to the top edge of the flexible membrane. Key as for above figures plus: (t) top cable adding lateral stability to tops of successive props. (u) catenary cable (w) landward prop legs.

4. Impression (a) and elevation (b) of barrage: rigid steel plate version. The view (a) is C') oriented as for Fig 3a. In fig 3b the rectangular frame structure with vertical and horizontal rectangular cross-section tubular steel is shown. In the centre is an expansion/contraction joint to allow for changes in temperature and to allow some elasticity. These would be inserted at intervals. Key: as in above drawings plus (z) C vertical steel tube member of the steel frame for the steel plate membrane. (a') elastic material (or inflated gasket) included in the expansion joint between adjacent vertical frame members to give some expansionleontraetion capability.

O Map of preferred Bristol Channel location. The thick line from Llantwit Major to near Minehcad gives much more worthwhile generating capacity than the line from Brean Down BD to Lavernock La and very much more than at Aust just upstream from the motorway bridges.

6. Map of other potential sites for use; chiefly The Wash a wide shallow sea inlet, The Solway Firth and the Inner Sound near Skye. Lough Foyle is shared with the Irish Republic. There are many more smaller inlets which could be useful once the technology is available and for test sites as mentioned in the text.

7. Plan of whole installation. Sluices could be accommodated in the turbine housing.

The siting of the lock needs to allow the best shipping channel. More than one turbine battery may be needed to streamline the inflow and outflow to best generating effect.

8. Landfall embankment profile. The staff access facilities and power lines will need to be accommodated here also but are not shown. Key as above drawings plus (c') fixing of flexible or plate membrane edge to the concrete of the embankment.

9. Cross section of flexible membrane bottom edge and seal. Depicted is the flexible membrane with its rope core leading down to the joint with the seabed fixing and turning seaward. A skirt is attached on the landward side to the membrane and the seabed with ballast or concrete blocks or both keeping it pressed down.

10. Detail of cross section of bottom edge of flexible membrane. The landward skirt has been omitted for simplicity. Preferably a karabiner securing point is pre-fabricated into the membrane so the joint is water tight. Any deficiency can be covered with an extra skirt or patch as below.

11. Flexible membrane top edge. A tough (polyester or similar) tape is incorporated into the substance of the top edge with stitching securing it to the mesh strands. Key as previous drawings.

12. Membrane junctions (a) flexible to solid structures (turbine housing, lock, sluice, landfall embankment). The basis of this junction is the parallel faces of a pair of L cross-section steel brackets as a strip along the whole height of the join with bolts through the brackets capturing the reinforced membrane edge. Key: (b') bolt through L cross-section steel strip into solid fixed structure (e') L cross section steel strip the entire vertical height of the attachment (d') bolt to retain the reinforced edge of the flexible membrane (c') reinforced edge of flexible membrane (f') ropc mesh within the flexible membrane (g') occlusive plastic covering element of the membrane 11/Figure 12(b) flexible to adjoining sheet of flex ible membrane. The patch on the side depicted uppermost in the drawing has been omitted for simplicity. The edges or any patch would be secured to waterproof fixing points woven into the rope structure of the membrane. Key: as above plus (h') karabiner type link to hold the adjacent reinforced edges of adjacent sheets of flexible membrane (i') "welded on" fixing C') point on the membrane surface for attachment of the edge of the patch (j') patch of flexible membrane material. A patch is applied on both sides but here is only shown on one side. 1/ Figure 12 (c) Rigid membrane joint to fixed structures. All the metalwork deep to the skirt material is rounded off to prevent abrasion. The rubber O strip (1') adds some resilience. Key as above plus (k') bolt through L cross section retaining strip which holds edge member of rigid membrane frame (1') cushioning resilient strip to add elasticity il Figure 12 (d) Rigid membrane to adjoining rigid 0 membrane sheet. A degree of elasticity may need to be incorporated. (1'). An alternative would be a reinforcing section welded to the steel plate membrane itself to take the strain when connected to the adjacent plate by a karabiner-like link. Key: (m') lug welded to the steel edge member of supporting frame of membrane. (n') elastic or sprung link holding the edging members together by means of the lugs.

13. Prop: tripod version (a) from seaward. (b) from landward. These are tubes or H or L cross-section with suitable links to join to bottom-and top-fixings of pre-existing design which have been omitted from the figure for simplicity. (c) Elongated junction plate for apex of structure: impression and lateral view. This may be needed if the excursion of the flexible membrane and suspending system would otherwise hit the legs of the tripod or ofthe tetrapod and if it were impracticable to provide longer legs for these props so as to provide greater excursion at the level of maximum bulge of the membrane.

14. Prop: tetrapod version (a and b) ditto. These props have four legs each with one being rather hard to see in the drawing.

15. Prop: single stanchion version from landward. Key: (o') stabilising prop to add resistance to being pushed over by waves. (p') stanchion of prefabricated concrete or of steel.

16. Flexible membrane structure: face. Net technology already available would be used to incorporate the vertical and perpendicular cords of a strong net which is then overlaid on both faces by a tough impermeable layer of plastic material which itself could have a lattice of internal reinforcement. Fixing points are located at frequent intervals to allow attachment of patches and skirts to the surface without penetrating it. These are the circular features at some of the intersections of the net structure.

17. Flexible membrane structure: cross section. Key as above. A strong net of ropes intersecting at right angles covered (during manufacture) in an occlusive layer of plastic material with fixing points incorporated.

18. Flexible membrane: top edge. The stitching (r') of the strong attachment of the reinforcing edge tape (e') to the edge rope of the net (f).

19. Rigid membrane: Figure a: cross section. Steel plate is welded to tubular bar steel and Jo protectively coated. Figure b shows the fixing to the seabed.

20. Pontoon supports for membrane top edge. The precisely computer programmed winches at the cud of cach pontoon rcmotc from the membrane (v') to seaward and to landward control the elevation of each pontoon and thus control its position while allowing for changes in water level. The mooring ropes to the seabed are kept taut but is allow the pontoon to rise and fall with the water level. The winches at the membrane side (w') alternately reel in and pay out mooring cable of the edge of the membrane so that which ever pontoon is the upper pontoon holds the top edge of the flexible membrane above the level of the higher mass of water whether it then is the sea or the Y) lagoon. Pontoons would be arrayed along the course of the membrane dam with a frequency to be determined by prototype experience but probably every 100 metres.

The higher the freeboard from higher water level to pontoon winch the greater the safe distance between pontoons and the smaller the total number of pontoons C necessary.

21. Suspension bridge principle: suspension of top edge of membrane dam.

22. Invaginatcd bottom fold of membrane held in slotted tunnel in rock of seabed by C inflated, tube. Sec text page 7 line 1. Shown is the cross-section of a tunnel bored. by tunnel boring machine just below the seabed and a slot cut from the sea into the tunnel.

23. Inflated gasket seals bottom of rigid membrane onto seabed. The bottom of the rigid frame is held onto the seabed by fixings and the space between seabed and the frame is filled by an inflated tube gasket.

24. Inflated gasket seal between rigid membrane sections. An arrangement similar to previous fig 23 but deployed vertically between rigid membrane sections.

25. Flexible membrane fixed to manufactured bedding strip or base plate. This is a refinement of the fixation of the membrane (a) flexible variety and (b) rigid variety to the seabed aimed at securing a more predictable seal not depending from day to day on the natural and perhaps uneven texture and surface of the geologically variable material of the seabed. In this instance bedding and water tight seal is effected by a inflated gasket between the bottom edge of the membrane and a manufactured strip or base-plate of smooth marine resistant material such as stainless steel which remains permanently fixed to the seabed. The pemianent strip on the seabed is fixed to the seabed rock by glue or sealant that can be applied underwater such as epoxy glue. If the gasket needs changing a robot could do this if it can be made to release fixings securing the bottom margin of the membrane to the permanent base plate. Then a damaged gasket can be removed and replaced by a new one preferably at a phase of tide when there is little tidal flow.

Claims (20)

1. Claims 1. The use of a membrane to dam a sea inlet or at sea away from land for the purpose of generating electricity or for storage of water for supply or for flood control.
2. 2. The membrane concerned is either flexible, changing its contour with the phase of the tide, or rigid composed for metal, plastic or concrete.
3. 3. Whether flexible or rigid the membrane is fixed to the seabed and to the necessary fixed structures along its route such as a turbine housing and a lock.
4. 4. Junctions of the membrane with these structures and with the seabed arc protected against leakage of seawater by a skirt in continuity with the membrane on seaward and landward side held down by fixings to the seabed and by ballast or concrete bars deposited on it or by an inflated gasket held between to dam margin and the smoothed seabed.
5. 5. The top of the flexible version of such a membrane is held above water level by a tape or cable on its margin which is in turn suspended from a cable attached to supporting props.
6. 6. The props supporting the flexible membrane are either metal tripods, tetrapods or single stanchions of metal or concrete fixed into the seabed, with or without a stabilizing strut.
7. 7. Sections of flexible membrane are joined at their reinforced edges, the joint being overlaid by a skirt or patch on each side, the edge of which is tethered to the receiving surface by ties at pre-manufaetured fixation points on the membrane's surface.
8. 8. The material of the flexible membrane is of plastic or other manufactured material adherent to and overlying a strong mesh of cords on both sides with fixing points affixed to each surface in manufacture at which the watertight integrity of the membrane is not breached.
9. 9. An alternative material for the flexible membrane is a woven canvas of material resistant to the marine environment.
10. 10. The top and bottom and the side margins of the membrane are reinforced by tape or cable enfolded in the margin or incorporated into it allowing attachments to hold it securely.
11. 11. The rigid version of the membrane is of metal plate coated to protect it from the marine environment.
12. 12. The surface of the rigid version is held by a frame in jointed sections by welding, bolts or rivets.
13. 13. The rigid membrane is fixed to the seabed, the junction protected against leaks by skirts of flexible membrane similar to that described above, attached to the seabed by fixings and pressed down against the seabed by deposited ballast and/or concrete bars until no further leakage is detected.
14. 14. The rigid membrane is supported in the upright position by struts from the seabed to its top edge on either side or both sides with or without the additional support of a zigzag configuration when seen from above.
15. 15. Joints in the rigid version are protected by seals similar to the skids described above.
16. 16. Protection from the impact of waves is provided by a linear array of devices for the generation of electricity by the kinetic energy of waves, leaving a calmer band of sea to the lee of the array, on the membrane's seaward side.
17. 17. Access to the membrane and to the fixed structures incorporated into the barrier, such as turbine housings, a shipping lock and sluices is provided by light rail supported on the props, stanchions or struts, the rolling stock being protected from falling in the event of any derailment by travelling in a tubular metal protective cage.
18. 18. Inflated tubular gaskets may be used to secure the bottom of the flexible membrane into a slotted tunnel in the seabed the membrane being wrapped around the gasket.
19. 19. An inflated gasket may be used to effect a watertight seal at the bottom edge of the rigid membrane to a smooth, machined slot on the seabed and adjacent edges of the rigid panels to each other.
20. 20. As an alternative method of fixing to the seabed, a manufactured surface of metal or other material to be fixed to the seabed with fixings with a water tight sealant such as epoxy resin glue and the margin of the membrane dam may be held against this structure by fixings with an inflated gasket intervening in order to have two reliable manufactured surfaces forming the water tight joint.Dependent Claims.1. Such a membrane can be used to create a lagoon completely detached from land.2. Such a lagoon can bc used to generate electricity in a fashion similar to that advocated here across a sea inlet.3. Such a lagoon could store fresh water in a way similar to the current function of a terrestrial water supply reservoir as part of a national water supply grid.4. Water may be pumped to such lagoons by ducts from upper courses of rivers in flood.Amendments to the claims have been filed as follows Claims 1. The use of a membrane composed ot'panels held on a supporting structure and given rigidity thereby to dam a sea inlet or at sea away from land for the purpose of generating electricity or for storage of water for supply or for flood control. thejunctions respectively between the margins at the side of each panel to its adjacent partner andior at the bottom of each panel to a track on the seabed being sealed against water leaks by means of an inflatable tubular gasket.2. The use of a membrane as claimed in claim 1 in which the membrane concerned is either flexible, changing its contour with the phase of the tide, or rigid composed for metal, plastic or concrete.3. The use of a membrane as claimed in claim I in which whether flexible or rigid the membrane is fixed to the seabed and to the necessary fixed structures along its route such as a turbine housing and a lock.4. The use ofa membrane as claimed in claim I in which junctions of the membrane with these structures and with the seabed are protected against leakage of seawater by a skirt in continuity with the membrane on seaward and landward side held down by fixings to the seabed and by ballast or concrete bars deposited on it or by an inflated gasket held between to dam margin and the smoothed seabed.5. The use of a membrane as claimed in claim 1 in which the top of the flexible version of such a membrane is held above water level by a tape or cable on its margin which is in turn suspended from a cable attached to supporting props.6. The use of a membrane as claimed in claim 1 in which the props supporting the flexible membrane are either metal tripods. tetrapods or single stanchions of metal or concrete fixed into the seabed, with or without a stabilizing strut.7. The use of a membrane as claimed in claim 1 in which sections of flexible membrane are joined at their reinforced edges, the joint being overlaid by a skirt or patch on each side, the edge of which is tethered to the receiving surface by ties at pre-manufactured fixation points on the membrane's surface.8. The use of a membrane as claimed in claim I in which the material of the flexible membrane is ol' 0 plastic or other manufactured material adherent to and overlying a strong mesh of cords on both sides with fixing points affixed to each surface in manufacture at which the watertight integrity of the membrane is not breached.r 9. The use of a membrane as claimed in claim 1 in which an alternative material for the flexible membrane is a woven canvas of material resistant to the marine environment, 10. The use of a membrane as claimed in claim t in which the top and bottom and the side margins of the membrane are reinforced by tape or cable enfolded in the margin or incorporated into it allowing attachments to hold it securely.1 I. The use oF a membrane as claimed in claim I in which the rigid version oF the membrane is of metal plate coated to protect it from the marine environment.12, The use of a membrane as claimed in claim 1 in which the surface of the rigid version is held by a frame in jointed sections by welding, bolts or rivets.13, The use of a membrane as claimed in claim tin which the rigid membrane is fixed to the seabed, the Junction protected against leaks by skirts of flexible membrane similar to that described above, attached to the seabed by fixings and pressed down against the seabed by deposited ballast and/or concrete bars until no further leakage is detected.14. The use of a membrane as claimed in claim tin which the rigid membrane is supported in the upright position by struts from the seabed to its top edge on either side or both sides with or without the additional support of a zigzag configuration %vhen seen from above.15, The use of a membrane as claimed in claim 1 in which joints in the rigid version are protected by seals similar to the skirts described above.16, The use ola membrane as claimed in claim I in which protection from the impact olwaves is provided by a linear array of devices for the generation of electricity by the kinetic energy of waves, leaving a calmer band of sea to the lee of the array, on the membrane's seaward side.17. The use of a membrane as claimed in claim 1 in which access to the membrane and to the fixed structures incorporated into the bawler, such as turbine housings, a shipping lock and sluices is provided by light rail supported on the props. stanchions or struts the rolling stock being protected from falling in the event of any derailment by travelling in a tubular metal protective cage.18. The use of a membrane as claimed in claim 1 in which inflated tubular gaskets may be used to secure the bottom of the flexible membrane into a slotted tunnel in the seabed the membrane being wrapped around the gasket.19. The use of a membrane as claimed in claim I in which an inflated gasket may be used to effect a watertight seal at the bottom edge of the rigid membrane to a smooth, machined slot on the seabed and adjacent edges of the rigid panels to each other.20. The use of a membrane as claimed in claim 1 in which as an alternative method of fixing to the seabed, a manuflictured surface of metal or other material to be fixed to the seabed with fixings with a water tight sealant such as epoxy resin glue and the margin of the membrane dam may be held against this structure by fixings with an inflated gasket intervening in order to have two reliable manufactured surFaces Forming the water tight joint.Dependent Claims.1. The use of a membrane as claimed in claim 1 in which such a membrane can be used to create a lagoon completely detached ftom land.2. The use of a membrane as claimed in claim 1 in which such a lagoon can be used to generate electricity in a fashion similar to that advocated here across a sea inlet.3. The use of a membrane as claimed in claim 1 in which such a lagoon could store fresh water in a way similar to the current function of a terrestrial water supply reservoir as part of a national water supply grid, 4. The use of a membrane as claimed in claim 1 in which water may be pumped to such lagoons by ducts ftom upper courses of rivers in flood. C?) r