Aberthaw Tidal Power Station

2021 Outline Design by Edward Grist

(13) Barrage Marine Life

  1. Transit Channels
  2. Migration Aid - Salmon
  1. Transit Channels

    The principal aim is to maintain as near as practicable the freedom of marine life, both fauna and flora, to continue to migrate along the Bristol Channel and the River Severn estuary. The safe passage of marine life through the barrage has priority over power generation. Requirements arising from operational protocols for transit channels must be met. Only after a design to achieve this has been established can the barrage electricity generation capability be established.
    The estuary bed varies significantly in depth from Aberthaw to Minehead. To provide 'unhindered passage' to all forms of marine life by offering a route through the water turbines is not a practical possibility. The location of water turbine intakes at the level above the estuary bed sufficient to prevent unacceptable ingress of aggressive sediments and other flotsam but deep enough to prevent air entrainment and cavitation problems excludes the 'bottom crawlers and many of the air breathing residents of the Severn estuary.
    Coarse screens can never prevent some creatures passing through and onto the rotating blades. Selecting machines that minimise the mortality rate or the severity of damage to marine life that bypass such access prevention measures is very welcome but on its own is not enough. Expecting all marine life to pass safely through the barrage water turbines is a non-starter.
    Note : Atlantic salmon migrating through the estuary to river tributaries during September to February are a special case. Transit for them is made possible. 24/7. On the ebb flow this is achieved by using a Shipping Channel bascule arm to form a salmon leap. Their ability to easily leap up weirs of 1.5 metres in height against the direction of a flow with a crest of over 10 centimetres has been well demonstrated.
    Marine life transit channel layout
    The transit channel bascule closure
    This pivot box design is physically adaptable to meet a changing estuary bed contour. However, a bottom pivoted bascule design could be substituted if greater flows became necessary to assist in upstream level reduction.
    Aberthaw-Minehead Tidal Power Station provides numerous marine life transit channels each with the bascule closure. The channels are designed to provide a through route outside the periods of tidal power generation. Fourteen channels, each of four metre width, are at locations approximately 850m apart spread across the barrage. They all extend from the free estuary surface down to the existing estuary floor.
    The bascule closure has the following important advantages: -
    1. All parts are at the surface when the channels are open giving ready access for maintenance including the removal of seaweeds, molluscs and limpet like creatures. This is a simple matter either by hosepipe or manually.
    2. On loss of electrical supply to the driving motors the counterbalance weight chosen ensures that the bascule arm always falls to the 'closed' position.
    With fourteen marine life transit channels a malfunction or damage to just one poses no problems to other aspects of barrage operation. In the event of a channel not being available for service the channel must be closed. The four-metre wide channel is then isolated by 'stop lock' beams until an opportune time for repair arrives. The marine life transit channel can be converted into a 'stop-lock' for this purpose.
    The water currents flowing through the transit channel will erode the estuary bed particularly near the entrance and exit to each channel. 'Mini dykes' are provided to prevent excessive erosion but provision for regular inspection needs to be made.
    Marine life could be exposed to predation when it emerges from a lengthy relatively narrow channel. The Aberthaw-Minehead design includes two 'fish dispersal eyehole' several metres from each channel end. This allows those able to swim through an eyehole the possibility of fanning out under the lower buffer pond and emerging some distance away from the transit channel.
    Four metres is the maximum allowable channel width. This is determined by the need to avoid a complete breach of the barrage following a collision by a large ship that could not be immediately contained using the measures provided. A ship with a beam greater than four metres could have considerable momentum, particularly if travelling under power. Such a vessel crashing into concrete walls four metres or less apart would come to a rest before the end of the 220 metres long transit channel. The channel could then be isolated using the remaining 'stop lock'. A greater channel width risks craft reaching and destroying the far stop lock. A significant breach of the barrage could follow. Repair of such a breach is very difficult and time consuming. Power generation may not be possible under such conditions
    The marine life transit channel is crossed by overhead cranes that run through the Machine Halls. Infrequently, perhaps once every three months, a large item such as T-G box may pass through. This requires the 20 metres high doors to be opened for a period.
    A solid floor covers the transit channel between the Machine Halls. When Machine Hall doors are open visual observation from marine life in the transit channel is prevented. Human activity should be in the background and pass largely unnoticed.
  2. Migration Aid - Salmon

    A salmon leaping a Severn weir
    A considerable effort has been made in recent years to encourage the historic migration of salmon through the Severn estuary and on to the upper reaches of the various tributary rivers following their journey across the Atlantic Ocean. Many weirs and other 'obstructions' built across the rivers in the nineteenth century have now been removed or negated as an obstacle. The Aberthaw-Minehead barrage design seeks to support this progress by meeting the special needs of returning salmon in the September to February period.
    The two-hour opening of the marine life and shipping transit channels centred on each high-tide and each low-tide meets the needs of most fish. It does not impose a restriction on the lives of crabs, lobsters, eels and the like. Atlantic salmon are the exception. When making a migratory return to the place of their birth these salmon are intolerant of delays. The possibility of salmon finding no way through the barrage and returning to sea after failing to spawn must be avoided. However, meeting their special need is easy. To do this the operating height of the bascule arms in both shipping channels is positioned as shown. On a falling tide as each arm is lowered to form a weir up which the salmon can leap during an ebb tide. On a rising tide salmon swim in the direction of the water that is moving upstream through the transit channel. They pass over the weir and drop into the upper estuary.
    Salmon are naturally gifted and have the ability to leap 1.5 metre and more with ease against a modest flow. This is comparable with the level difference at which Aberthaw-Minehead barrage electrical power can be generated.
    Salmon migration through the shipping & marine life channels
    A minimum height above the weir sills of 20 centimetre is provided at all times to ensure safe landing. To match the falling tide without restricting the barrage electrical power generation capability the weir height is lowered in stepped increments - one every 60 minutes is suggested.
    Water flowing 20-centimetre-deep over a 54 metres wide rectangular weir does so at a rate of less than 10 cubic metre per second. The velocity is less than one metre per second. Compared to the normal combined river flows into the upper estuary of about 250 cubic metres per second and tidal movements many times greater still shows 'salmon weirs' to be insignificant and have no effect on electrical power generation.