Aberthaw Tidal Power Station

2021 Outline Design by Edward Grist

(10) Barrage Shipping

  1. Transit Channels and Locks
  2. The Barrage Ship Docks
  1. Transit Channels and Locks

Ships trading from and within the Bristol Channel and the Severn Estuary support an essential part of many profitable commercial enterprises within the UK. The Aberthaw-Minehead barrage must accommodate these ships. They come in many sizes ranging from the small to the very large. The Aberthaw-Minehead design caters for the vessels of all descriptions including those likely to be the maximum ship size in the future.
Large ships travel speedily and most efficiently near to high tide. The Aberthaw-Minehead barrage provides two shipping channels that enable non-stop transit through the barrage for a two-hour period centred on high tide. One of 54m width for ships of up to 50m beam and one of 27m width for ships up to 25m beam.
The unrestricted passage of shipping through the barrage has priority over power generation. Requirements arising from operational protocols for the two transit channels must be met. Only after a design to achieve this has been established can the barrage electricity generation capability be established.
As high tide is reached the water close to the barrage loses its momentum. The aggregate of energy in upstream and downstream counter-flows is zero. Ships and marine life can pass through transit channels unhindered.
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. 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. During this period the largest ships can still navigate the estuary to enter and pass through an appropriately designed transit channel. Ocean going vessels arriving from distant ports and those leaving from the upper estuary require flexibility. A two-hour slot is the minimum acceptable. It is the maximum available.
Bascule - Shipping transit channel - end view
The double bascule design meets all the very onerous demands associated with needing to operate reliably in the Bristol Channel. It is, unlike many other designs, able to cope with all the waves and local currents arising in a shipping channel during the one hour before and after high tide. It can be opened and closed even when storm conditions create very large surface waves. The design can also be used to reduce the risk of upper estuary flooding.
The channel nearest to Aberthaw is 54 metres wide and has an 18 metres deep sill relative to the lowest annual high-water level. It is suitable for the very largest vessels. The transit channel nearest to Minehead is 27 metres wide - half the Aberthaw width. A majority of the vessels using the estuary are able to pass through this narrower channel. Adjacent to each shore is a 24m wide gated lock that makes a 24 hours transit available as an option for many smaller ships.
The bascule arm in the normal open position and the normal closed position
The shipping transit channels have a bascule closure at each end. In addition to the transit function they can form a dock by closing both ends. The trapped water then remains at a practically constant depth. It allows the delivery supplies to the barrage dockside by ship. Large items up to and including the electrical power generation machinery (T-G Boxes) are transferred easily. All plant items required during the barrage construction phase can be offloaded from ships at the dockside, delivered to the required location and installed using barrage cranes.
Weather and tidal currents make a ship passing through at an appropriate speed a matter of 'judgement on the day'. It is inevitable that errors will occur and, consequently, collisions with a bascule closure will happen during the 70-year design life. The bascules are designed and tested to operate alone in the most onerous conditions. This enables the barrage to function for a period on a minimum of three bascules.
The barrage does not require either a high-level bridge (that would mean limiting the height of ship superstructure) or a form of swing bridge.
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 serving each bascule arm extends across the full width of the shipping channel. Use of these chambers requires safety procedures being followed similar to those used by divers.
Each bascule arm is almost balanced about a pivot point by a counterweight in a dry pit at the side of the transit channel. The counterweight arm has within it a large internal cavity filled with ingots of a heavy metal. A driving electric motor controls position and movement. The 'see-saw' between bascule arm and counterweight is biased towards gravity causing the arm to fall to the open position on loss of electric driver power. A manual backup means of closing is provided.
The bascule closure is well proven as a bridge mechanism. It meets the particular needs of the very wide shipping transit channels in the Bristol Chanel. The following comparison table identifies an internationally known bridge where over one hundred and twenty years operating experience has relevance.
Tower Bridge, London uses the bascule principle and has done so since 1894. It is used for opening and closing a roadway crossing above the shipping traffic of the river Thames. It now carries 21st century traffic without restriction. This includes heavy lorries and buses. The demand to open and close quickly is paramount. Failure to operate promptly highlights the vulnerability of the bridge. A serious traffic jam that spreads quickly is the consequence. An 83-degree opening angle was provided to allow the safe passage of the sailing ships of yesteryear with their wide yardarms. For nearly one hundred years hydraulic accumulators supported by steam pumps powered the bascules. In 1976, in response to the rapidly increasing traffic volumes, a change was made to an electrical system.
The comparison table shows that the Aberthaw-Minehead bascule arms are of a much lighter construction than those of the vehicle carrying London design. They are not required to operate so promptly and a second shipping channel is available should a failure occur.
The Bristol Channel design is appropriate for an aggressive region of stormy salt water environment that always has sediments in suspension plus, at times, substantial flotsam. All bascule components are readily accessible for cleaning (sedimentary accumulations, marine life attachments etc.), lubrication, or removal and replacement. Maintenance activities use a pressurised chamber in the barrage when appropriate.
  1. The Barrage Ship Docks

The two ship docks are – nearest Aberthaw 54m wide ; nearest Minehead 27m wide
Raising the two bascule arms forms a stationary waterway, a ship dock, along the length of the shipping transit channel. The level can be adjusted and kept constant at a desired level by appropriate pumping. It is free from local currents, estuary waves and the worst ravages of Atlantic storms. Unloading goods for the barrage is made easy. Delivery of large items by ship rather than by road becomes preferable.
Choosing to use the dock facility when the channel is closed during the four-hour periods allocated for electrical power generation makes economic sense. If required at other times the closure should be restricted to a single shipping transit channel and appropriate advance notice should be given to shipping interests.