Aberthaw Tidal Power Station

2021 Outline Design by Edward Grist

(4) The Past (2013)

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Energy and Climate Change Committee written evidence submitted by
Dr Edward Grist (SEV92 UK Government Report 10 June 2013)

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  1. Essential Requirements and Acceptance Criteria
    1. Wildfowl
    2. Wildfowl and Severn estuary bank life must not be affected by the barrage. The existing upstream water level patterns adjacent to wetlands and river banks should be retained.
      To accomplish this the barrage should include transit channels distributed across the barrage that remain open for a sufficient period at both high tide and low tide for water levels either side of the barrage to level out at values typical of those prior to 2013.
    3. Marine life
    4. Marine life should retain the same freedom to roam or to migrate as enjoyed prior to 2013.
      To accomplish this the barrage transit channels should have at least a cross sectional area (width by depth) that approximates in aggregate to at least that at the combined mouths of the principal rivers entering the Severn estuary above the barrage, namely the Taff, Wye, Usk, Severn and Avon.
    5. Sediments
    6. Sedimentary accumulations, flotsam (including trees) and jetsam must be either removed from the water or, where appropriate, diverted through the barrage transit channels. The access routes and equipment required for this activity must be included in the barrage design.
    7. Shipping
    8. Shipping, pleasure craft and other vessels currently crossing the estuary line—Lavernock Point to Brean Down—should continue to pass through the barrage at the same traffic volumes recorded prior to 2013 with minimal obstruction or delay. Provision should be made to accommodate increases in the size and volume of commercial shipping to meet reasonable forecasts of future growth in maritime trading activity.

      A separate transit channel is required for very large ships. This should permit transit through the barrage for a limited time period at high tide without the need to stop.

      A ship lock for vessels (typically those less than 5,000 dwt tonne) should be included to make possible the option of providing transit through the barrage 24 hours a day.

      Two locks for small craft, one adjacent to each estuary shore should be included. These locks should be designed to make possible the option of transit through the barrage 24 hours a day.
    9. Electricity Generation
    10. Water turbines, generators, pumps and electrical transmission equipment are required that maximise the electrical power that can be generated for transmission to the National Grid. A priority requirement is for the barrage structure to be of a design that restricts the loss of electrical power output resulting from credible events, both accidental and intentional, to less than 20% of the total barrage design electricity generating capacity. The barrage design must enable this 20% loss to be restored within 180 days.
    Fig. 1
    TRANSIT PASSAGES WIDTHS IN THE BARRAGE

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    (Measurements are metre lengths along the barrage)
  2. Tidal Flow Control—Severn Barrage Design Essentials
    1. Protecting Wildfowl
    2. At high-tide and at low-tide the water flows close to the barrage have very little momentum. If sufficient water channels are provided through the barrage and these are distributed sensibly at points across the estuary then, provided such channels are open long enough, the water levels either side of the barrage will equate to estuary levels prior to construction of the barrage.
      The reference design includes ten 3 metre wide channels that extend down to the bed of the estuary and a single channel that is 54 metre wide and 18 metre deep (relative to high water). All these channels are always to be open for two hours at every high tide and low tide. The resulting total of eight hours in each 24 hour 50 minute diurnal cycle is considered adequate to ensure that the existing wildfowl and Severn estuary bank life is unaffected.
    3. Protecting Marine Life
    4. The Bristol channel, extending as it does from the Atlantic ocean to the city of Gloucester, is the natural home to a wide spectrum of marine life. The line between Lavernock Point and Brean Down is crossed by many indigenous species that share the waters with a large variety of migrating and breeding species. They range from those able to swim vigorously to those crawling along the bottom. In 2013 this marine life has the freedom to pass through the line of the barrage at any time of the day or night. Replacing this freedom with the channels that have limited opening times necessarily imposes a restriction.

      Channels that extend down to the estuary bed are considered to be the best way of encouraging the existing marine life to continue and adapt to meet the challenges imposed by the barrage with the least influence on their lifestyle and breeding patterns. The total channel opening time of eight hours evenly distributed in blocks of two hours in each diurnal cycle of 24 hour 50 minute is considered sufficient.
    5. Managing Sedimentary Deposits
    6. The barrage will change patterns of sedimentary accumulation. Ten 3 metre wide channels are more than adequate to provide routes through the barrage through which unacceptable sedimentary accumulations can be periodically scoured. These same passages also provide a place where flotsam and jetsam can be isolated and removed.
    7. Accommodating Large Ships
    8. Ships in the Severn estuary are an essential part of a profitable trading structure within UK commercial markets. The Severn barrage must accommodate these ships. They come in many sizes ranging from small to the very large. The reference design caters for increasing ship sizes to meet future needs. Ships travel speedily and most efficiently when taking advantage of the estuary flows at high tide. A barrage that requires very large vessels to stop as they pass through ship locks at high tide is unacceptable. An alternative is provided in the reference design.

      The submerged double bascule closure permits very large ships through the barrage. A shipping channel 54 metre wide and 18 metre deep sill (relative to high water) meets needs now and in the future. It enables non-stop transit through the barrage—albeit at a very slow speed to avoid the risk of unintentional collision with the barrage structure.

      The design imposes no limitations on the height of ship superstructure. Underwater access tunnels provide for ready inspection and maintenance of the sealing between the bascules and their sills. These tunnels avoid the need for bridging across the channel above the water line.

      The double bascule design can be opened and closed quickly even when storm conditions create large estuary surface waves. It is the most appropriate engineering design and can easily cope with local currents arising in the channel for one hour before and after high tide.

      A 28 metre wide double-gated lock is also provided for ships travelling through the barrage outside the two hour period at high tide. Fig.1 identifies in a single diagram all the transit passages through the barrage.
  3. The Reference Design
  4. The reference design and its associated logic are described so as to form a benchmark against which the merits of any alternative can be judged.
    1. The validity of two hour periods of “unhindered passage”
    2. The presence of a Severn barrage distorts the estuary flow patterns. The magnitude of this distortion is governed by the design choices that interact with tidal flows—essentially the water channels through the barrage provided for power generation, those for the transit of marine craft and those for transit of marine wildlife, sediments and debris.

      Ship locks normally have two-gates which, by sequential operation, facilitate the transfer of vessels between two water levels. Vessels have to stop. This is a time consuming process. In a tidal barrage it is possible for a short period at both high tide and low tide to sail through the barrage with both gates open. Only shallow draught vessels can do this at low tide.

      Consider conditions as high tide is reached. The water close to the barrage loses its momentum. At high tide the aggregate of energy in upstream and downstream counter-flows is zero. The power generation capability for that moment is zero. All gates to ship locks and all sluices in the barrage could be open without loss of potential power generation. Ships could pass through the barrage unhindered. Marine life could also pass freely through the locks and sluices.

      For five minutes before high tide the water level is slightly below the maximum for the particular tide. After high tide it is again slightly lower as the estuary water slowly begins to recede. A tiny amount of electricity could, in theory, be generated. Consider now one hour before and after high tide. The flows through the barrage are stronger and a meaningful amount of electricity generation starts to become a possibility. At one hour either side of high tide the largest of the ships can still navigate the estuary to pass unhindered through an appropriately designed transit channel in the barrage. Ocean going vessels arriving from distant ports and those leaving from ports upstream of the barrage require flexibility. A two hour slot is the minimum acceptable.

      Throughout the two hour period when very large ships can pass through the barrage there is no obstacle to marine life choosing to pass through this channel at the same time. However, to fully meet the needs of marine life more channels are required—ideally smaller in width but tofull estuary depth and distributed across the estuary. Adding such channels allows measures to be taken to prevent marine life from passing through the water turbines and permits the inclusion of high-power jet cleaning to prevent damaging sediments and debris from entering the running clearances, including the shaft seals, of the water turbines.

      The limitations on power generation as high tide is reached also apply at low tide. All the channels passing through the barrage could open once more for a two hour period giving marine life a further opportunity to travel through the barrage. Equally important, this two hour period is sufficient for the estuary levels either side of the barrage to level out and replicate the upstream conditions in the estuary that existed prior to the construction of the barrage.

      The conclusion reached is that opening all the channels one hour before and closing all of them one hour after both high tide and low tide has considerable advantages and is a valid basis for a compromise to meet all the requirements described earlier.

      Fig.2. shows how the unimpeded passage for marine life is accommodated in the tidal cycle.

      Fig.2
      THE TIDAL CYCLE—A COMPROMISE THAT MEETS ALL REQUIREMENTS
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    3. The double bascule channel
    4. The reference design makes use of two independently operated bascules. The hinge of each bascule is situated below the level of the shipping channel sill which, of course, is well below low water. Maintenance of the hinge seal is carried out dry from a chamber in which pressurised air drives out the water prior to and during use. The maintenance chamber for each bascule gate, accessed from adjoining service tunnel, extends across the full width of the shipping channel. Use of these chambers requires safety procedures being followed similar to those used by divers in tourist underwater activity vessels.

      Each bascule is balanced by counterweight arms that extend above the water on each side of the channel. The bascule in operation is fast and certain. Operating the bascules in sequence particular to tidal direction minimises still further the energy to raise or lower them.

      The alternative choice of a vertical hinged “lock gate” design for the 54 metre wide channel could have problems operating against tidal currents and surface waves during storms. Vertical seals in sea water for large heavy “gates” are very difficult to maintain.

      A well recognised use of the bascule principle is Tower Bridge, London. Each of its 30m long counterbalanced bascules has a roadway plus pavements width of 20m and weighs over 1,000 tonnes. It opens and closes again in only five minutes to allow ships to pass through. When open each bascule stands at 86 degrees to the horizontal.

      The Severn barrage bascules require a more modest load carrying capability and are, consequently, of a lighter construction than those of the vehicle carrying Tower Bridge but do have to contend with an aggressive sea water environment. The four hour period of closure at low tide provides ample time to secure the channel as a “stop lock” whilst remedial work is carried out.

      Fig.3 and Fig.4. detail the double bascule and its operating cycle.

      Fig. 3 THE DOUBLE BASCULE CHANNEL—CROSS SECTIONAL VIEW 3566

    5. Security
    6. The barrage design must be such as to enable a 20% loss arising from any credible event to be restored within 180 days. Two credible events are identified.
      1. A large ship driving into the barrage at speed either deliberately or accidentally.
      2. An explosion at any location on the barrage caused either maliciously or accidentally.
      The Severn barrage has to be capable of resisting a collision from any ship in the Bristol Channel both from downstream or from upstream of the structure. A worst case collision scenario is a ship deliberately driven at speed into the barrage—such as a vessel hijacked as part of a terrorist attack. Designated “exclusion areas” and other forms of maritime policing are unlikely to prevent this happening. Bristol docks can accommodate vessels “up to 120,000 tonne dwt”. Cardiff docks can accommodate vessels “up to 20,000 tonne dwt”. Larger ships ply their trade at ports in South Wales downstream of the barrage. Clearly, the barrage must be of a very robust design and incorporate engineering features that enable it to repel collisions from any such vessels whilst at the same time minimising the loss of hydroelectric power generation. At the barrage detailed design stage a plan for dealing with credible events is required that describes how aspects of the implied capability can be demonstrated.

      Fig. 5
      SINGLE & DOUBLE DEFENCE LINES—A COLLISION DAMAGE COMPARISON
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      A strategy for limiting loss to 20% of the electricity generating capacity in a way that can be restored within 180 days can be achieved by adopting the following:
      1. Prevent a breach in the barrage that releases very destructive energy inherent in tidal flows. A single 30m wide structural line that forms a “single defence line” has proved suitable for a barrage in a narrow river estuary. This is not appropriate for Severn barrage. The resulting loss of total power generating capability and the inevitably difficult access to effect a repair, particularly out in the estuary, make very costly and time consuming repairs inevitable. A “double defence line” is essential. These defence lines should be separated by a means of absorbing a significant amount of the energy inflicted by the colliding ship.

        The reference design shows two structural defence lines 100m apart separated by two water filled buffer ponds that straddle the turbine halls provided the ponds are filled to a height of at least 5 metre above an appropriate value for maximum high tide. The natural evaporation loss from the ponds can be met by diverting the waters of the River Axe just upstream of the point where these waters enter the estuary in 2013. Security of this make-up water is ensured at times when drought restrictions prevent its use by topping up with water pumped into the ponds from the estuary.

        Following a collision with a first defence line the retardation produced by buoyancy forces from the buffer ponds together with the resistance offered by the destruction of the turbine hall(s) the second defence line should escape unscathed. Access enabling repair from a collision to be speedily concluded is provided by integrating a passage way into the downstream defence line and a roadway into the upstream line.
      2. Locate power generation units in a number of separate machinery halls so that any credible event disables a maximum of two of them. In a Severn barrage comprising twelve 1,000 metre long halls where each is hydraulically, electrically and security isolated from its neighbours damage to any two halls meets the 20% requirement. A single source accident or malicious act in adjacent halls could produce this worst case scenario. Multiple events involving non-adjacent halls where each has an independent security regime and is at least over 1000 metre away are considered incredible.
      Fig. 6
      BUFFER WATER PONDS—A VITAL SECURITY FEATURE

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      Fig. 7
      THE EFFECT OF BUOYANCY FORCES ARISING FROM A BUFFER POND

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      Fig. 8
      PUMPED STORAGE UTILISING THE BUFFER WATER PONDS
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    7. Pumped storage—A “not-to-be-missed” economic benefit
    8. Buffer ponds are necessary to mitigate the consequences of a ship colliding with the barrage. The ponds provide a golden opportunity to add, at minimal capital cost, a substantial pumped-storage facility. The security requirement is for the pond height never to be less than a fixed amount above the high tide annual maximum provides a particularly advantageous base from which to achieve this. This minimum pond height is shown as 5m in Fig.6.

      Storing water on top of the buffer minimum requirement produces the possibility of generating significant amounts of electrical power at times in the diurnal cycle when tidal power generation from the main water turbines is not possible. The very substantial tidal range particular to the Severn estuary make this possible. It is achieved, as shown in Fig.7, by pumping up to the buffer pond at high tide and recovering the stored energy at low tide. The total pond area above twelve machinery halls is approximately one million square metres. With an average annual tidal range approaching 10m pumped storage more than pays for providing the buffer ponds, an essential security feature.

      A PUBLIC ACCESS BRIDGE OVER THE BARRAGE—AN UNACCEPTABLE SIMPLE AND EASY OPPORTUNITY FOR A TERRORIST

      It is generally agreed that a breach in the barrage that releases the stored energy during a significant tidal difference always leads to catastrophic damage. Estuary currents through the breach quickly erode the sea bed—even where this is concrete—making repair work very, very difficult and time consuming. Following such a breach the claimed 5% of National Grid power generation would be unavailable for a considerable period.

      A public road bridge very obviously provides a means for terrorists to simply and efficiently deliver long term destruction to the power generating capability of the Severn Barrage. At a time of their choosing explosives (bombs or depth charges) can be dropped/ejected from lorries on the bridge—day or night; clear or foggy weather; placid sea or storm force Atlantic generated waves. Lorries with high-lift cranes or conveyor arrangements capable of delivering explosives to previously selected locations are in everyday use by builders merchants. The 10 plus miles length of barrage below the bridge carriageway is indefensible from this threat. To make matters worse, the high level of the bridge carriageway makes unplanned detonations below the bridge of little risk to those carrying out this activity so ensuring a full load can always be delivered.

      It is clear that a public road bridge above a Severn Barrage is totally unacceptable. The generation of power by a barrage does not require a bridge. There is an alternative IF a bridge across the River Severn really is needed. Put it upstream from the Lavernock Point/Brean Down barrage as shown in Fig.A1. Assuming a Severn bridge from Cardiff to North Somerset is really needed then it makes more sense to have one that directly serves the docklands and industrial districts that are to the east of Cardiff. I suggest consideration be given to a Tremorfa—Sand Point bridge. This is some four miles upstream of the proposed Severn Barrage site. At a stroke the real terrorist threat to the Severn Barrage is removed.

      A bridge between Tremorfa and Sand Point would require less than one kilometre of new road infrastructure and about half a kilometre of new rail infrastructure on the Welsh shore. Fig.A1 shows in more detail the potential site access from the M4/A48(M) and from the Cardiff to London & Midlands rail line. This is a very significant saving compared to the recent Lavernock/Brean proposal. The Somerset shore connection to the M5, at 2.5 kilometre, would also be slightly less than a Brean Down alternative. An upstream bridge might seem to pose an additional hazard to wildlife in the Site of Special Scientific Interest (SSSI) along to shoreline near Tremorfa. Such an assessment is false. An anchorage point well inland to first bridge pier easily spans the protected area.

      The presence of bridge piers at points across the estuary might also seem to present a hazard to shipping. The difficulties in estuary navigation for shipping are increased but only marginally. There are already times when visual observation by ships in the estuary is very difficult or impossible. Estuary fogs are the worst example. Finding the narrow “window” of passages for shipping to pass through already imposes a need for extreme vigilance in poor weather and sea conditions. Avoiding widely spaced “mid-stream” piers of an upstream bridge that are outside designated shipping channels adds only marginally to this difficulty.


Fig. A1
A POTENTIALLY MORE APPROPRIATE SITE FOR A PUBLIC TRANSPORT BRIDGE
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