Aberthaw Tidal Power Station

2021 Outline Design by Edward Grist

(12) Flood Protection including after Climate Change

Contents
  1. Flood Risk (2014)
  2. Upstream Depth Control
  3. Flood Prevention 2014
  4. Surges and Large Waves
  5. Flood Protection in the Future
  1. Flood Risk (2014)

  2. 1770
    Somerset Levels in flood – 2014
    A flood risk occurs when the depth of water in the upper estuary exceeds the level where flood protection measures is not catered for.
    Restricting the upper estuary to the one metre incurs a very small reduction in electrical power generation capability. It restricts shipping but only at or very close to Spring tides. It occasionally lowers the 'naturally occurring' maximum levels in the wildfowl wetlands but only marginally. Perhaps more importantly, it makes flood protection simple, easily applied and cost-effective. The use of the existing Avonmouth measurement point enables the changes brought about by the construction of the Aberthaw-Minehead Tidal Power Station to be numerically quantified by being part of a continuing record.
    The barrage design has to prevent large flows passing over the dockside and across Machine Hall roadways and on into the T-G boxes. The design accommodates simultaneously a 11.7 metre maximum high tide, a one metre tidal surge and two metre maximum half-waves. The barrage is not a public right-of-way. When Flood Protection mode is invoked all barrage personnel are aware that challenging conditions that can give rise to spray and minor water flow are imminent.
  3. Upstream Depth Control

  4. Public access level
    3538
    Upstream depth control & flood protection measures
    Tidal range table
    The Aberthaw - Minehead dockside level is 14.7 metres over Minehead zero low tide
    Tide table heights are vertical distances relative to the datum in navigational charts.
    Should circumstances arise that demand immediate action the Aberthaw-Minehead design enables all the flow paths crossing the barrage to be completely closed within one hour.
  5. Flood Prevention

  6. Flood Prevention measures are initiated following receipt of credible information that forecasts a risk of flooding. A change to the Flood Protection cycle can be formally declared at low tide some 18 hours in advance of the need to commence actions to close all the water paths through the barrage. A public notice of the change can then be issued at least 12 hours in advance of the shut-down. Shipping interests can be separately notified.
    The simplest way of providing flood protection is adopted. This is to prevent an incoming tidal mass of water from entering the upper estuary. All marine life and shipping transit channels are closed. All water turbines are stopped and all isolation valves are closed.
    The closures should last for two hours either side of high tide. For the operating staff this is simple to understand, apply and monitor. The closure always results in the upper estuary remaining about two metres lower than that of the incoming tide. This is effectively an 'empty space' for flood waters to fill. The Flood Protection cycle is repeated until the danger has passed.
    High tide always dominates the magnitude of the depth of water in the upper estuary. Tidal flows are driven on an approximately 12-hour 20-minute long cycle.
    The contribution of waves, surges and river flows, though random and largely weather dependent can be significant at high tide. The maximum upper estuary depth - and hence the quantitative measure of flooding risk - is the sum of all these sources.
    The potential for flooding that results from these events, either singly or together, can easily be controlled by implementing the Flood Protection measures described.
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    The five rivers flowing into the upper Severn estuary
    Five rivers flow into the upper Severn estuary, the Severn, Wye, Taff, Usk and Somerset Avon. Their location is shown above. Additionally, Table 1 lists the gauged daily volume of river flows into the estuary.
    For all tides the difference in water level either side of the barrage is less than ten centimetres. Regularly returning to this base at every low tide ensures continuity for shipping and for the flood prevention measures already installed between the barrage and Gloucester.
    River flows into the upper estuary do not compromise the flood protection logic. Even with all the rivers in full flood it takes nine days to raise the upper estuary by one metre. At every low tide the estuary water on each side of the barrage comes close to levelling out - even in Flood Protection mode.
    3532
    The values give an approximation to 'normal' flows compared to higher flows that are associated with upstream flooding. This is achieved by comparing 'mean flow' and '10% exceedance' values, terms defined by the Centre for Ecology and Hydrology.
    The magnitude of the upper estuary inflows is dominated by the Rivers Severn and Wye. They are always high after precipitation on Plynlimon Fawr, their common source. Fortunately, if needed to assist in making operating decisions on the barrage, a sudden rise in water volumes entering the Severn estuary from these two rivers is reliably characterised of reaching this estuary by the readings at a single measurement station - that at Buildwas close to the Ironbridge gorge on the outskirts of Telford.
    Typically less severe are inflows from the rivers Taff, Usk and Avon. These shorter rivers deliver waters at their confluence with the Severn relatively quickly. Sudden precipitation in the lower catchment areas of the rivers Severn and Wye that are in Shropshire, Hereford, Warwickshire and Gloucestershire reach the estuary on the same timescale.
    An accurate forecast of precipitation levels is always available 12 hours in advance from the Meteorological Office, Wallingford.
  7. Surges and Large Waves

  8. Atlantic barometric pressure fluctuations that are rapid and severe occur regularly, particularly during intense ocean storms. When the direction of the pressure contours aligns with the Bristol Channel and Severn Estuary a rise in tidal level often occurs. A surge can pass through in a matter of hours during which the estuary level can be raised by about one metre. The volume of water in the surge is not important. It is the change in local depth whilst the surge passes through that causes flooding.
    A surge moves up the Bristol Channel at the speed of the atmospheric depression that causes it. This can very quickly introduce a large mass of water into the upper estuary.
    The Meteorological Office, Exeter provides a reliable forecast on the direction and intensity of barometric pressure changes that will induce a storm surge.
    Large waves originating in the Atlantic, occasionally several metres high, enter the upper estuary. The outer walls of the buffer ponds form a tiered, replaceable structure. This contains and manages damage from any cause. The Machine Halls are protected from wave effects by ponds totalling 100 metres in length. The first 40 metres reduce the wave effect to a pulsating mass of water that finds relief on reaching the channel between the ponds. A more quiescent flow, with less severe pulsations, passes through a further 60 metre length to reach the well from which the water turbines draw water. The swell from the estuary waves can never rise to the roadway level in the Machine Hall.
  9. Flood Protection in the Future

  10. The Aberthaw Tidal structural steel barrage design, unlike concrete barrages or concrete lagoons, is easily able to adapt to a significant permanent change in Atlantic Ocean depth in advance of it actually happening.
    Root-tube extensions are used in the barrage Machine Halls. Every crane, generating machine, storage battery and internal roadway is suspended from the Machine Hall steel superstructure. Consequentially, all can readily meet a new height requirement at a later date.
    For an increase in sea level at the barrage of two metres the protection against flooding remains the same but power generated on the ebb tide reduces by about 5%. For an increase of between two and five metres the generating output is much reduced since two-way generation of a commercially meaningful magnitude is not possible. The barrage must become a pumped-storage unit operating only on the incoming tide. The benefit of accepting this limitation on power output (if events prove it is necessary) is considerable. Primary flood protection is maintained for all upstream of the barrage.
    Any significant increase in the depth of water channels is accompanied, over time, by a change in sediment settlement patterns. The flexibility inherent in the structural steel barrage is a great advantage. It is a simple matter to relocate water-turbine generators in the barrage to match a significant change in flow patterns across the barrage.
    The design can accommodate the consequences of an increase in ocean depth. The barrage protects all in the Severn Estuary from up to five metres if necessary. This includes Hinkley Point C PWR nuclear power station.
    Raising a Machine Hall to maintain flood protection when necessary in future years.
    (Applicable in the event of a permanent Atlantic Ocean rise of 2 metre or 5 metre)
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    (i) A design to provide flooding protection in 2018
    3534
    (ii) Implementing a change in advance of need
    3535
    (iii) A design to provide flooding protection in future years
    (Raising Machine Halls up to 5 metre if and when necessary)

    Raising all 15 Machine Halls is a simple matter. The illustration above shows that in addition to Machine Hall components piers and dockside buildings need to be designed with the future possibilities in mind. Dock level offices, shipping channel docks, workshops and bascule gates need to be designed accordingly. The most expensive advance provision is for the raising of the main crane rail where it services shipping.