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34 th IAHR Congress, Brisbane, 2011

Comparison of Hydro-environmental Impacts for Ebb-only and Two-Way Generation for the Severn Barrage Reza Ahmadian, Roger Falconer and Bettina Bockelmann-Evans Hydro-Environmental Research Centre, Cardiff , UK. 34 th IAHR Congress, Brisbane, 2011. Considerable Scope to increase.

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34 th IAHR Congress, Brisbane, 2011

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  1. Comparison of Hydro-environmental Impacts for Ebb-only and Two-Way Generation for the Severn BarrageReza Ahmadian, Roger Falconer and Bettina Bockelmann-EvansHydro-Environmental Research Centre, Cardiff , UK 34th IAHR Congress, Brisbane, 2011

  2. Considerable Scope to increase Planned Renewable Energy Provision EU target of 20% carbon reduction by 2020

  3. Wales Severn Estuary England

  4. Proposed Tidal Devices for Severn • Tidal Stream Turbines - wind type turbines located in water column and energy created directly from tidal stream currents • Tidal Lagoons (OTIs) - enclosed embayment constructed offshore, creating tidal phased head difference - similar to barrage concept • Tidal Barrage - embankment across estuary - ideal for renewable energy with high tidal range and large upstream plan-surface area

  5. Government Short Listed Proposals

  6. Slide – courtesy of STPG Severn Barrage Proposal Site Some key facts: • 2nd highest spring tidal range  14 m • Cardiff to Weston • Length about 16 km • Generate  5% of U.K. electricity • Total cost  £20 bn • Save > 6.8 million tonnes carbon pa

  7. Slide – courtesy of STPG Barrage Layout (1989 Report) Key facts: • 216 x9m turbines  17 TWh pa • 166 sluices • Ship locks • Fish pass • Public road & railway

  8. Slide – courtesy of STPG Tidal Power Generation (Ebb-only)

  9. Ebb Generation Flood Generation 2 Way Generation Pumping to increase energy output Barrage Operating Schemes

  10. Slide courtesy of STPG Proposed Operation - Ebb Generation

  11. Option 1: Generate over ebb tide only Level of water inside impoundment Proposed: One Way Generation

  12. Option 2: Generate over full tide Rapid filling and emptying of basin required at either end of tidal cycle Level of water inside impoundment Alternative: Two Way Generation

  13. Existing Estuarine Environment • Tide Range - 14 m on springs, 7 m on neaps • High tidal currents and large inter-tidal areas • 30 Mt sediment suspended on springs, 4 Mt neaps • Little sunlight penetration through water column • Reduced dissolved oxygen levels • Ecology • Harsh estuarine regime with high currents • Limited aquatic life in water column / bed • Bird numbers per km2 are relatively small

  14. Hydro-environmental Modelling • 1D-2D dynamically linked model • DIVAST – ADI (2D) • FASTER (1D) • Sediment Transport and Water Quality including bacteria and sediment interaction and dynamic decay rate • 200m * 200m rectangular grid (2D) • 240m between 1D cross sections • Barrage modelled as wall with flow through submerged sluice gates and turbines using orifice flow equation

  15. Model Extent

  16. Measured and Predicted Water Elevations and Current Speeds

  17. Measured and Predicted Sediment and Bacteria Concentrations

  18. Spring tide range reduced from 14 m to 7 m Significant loss of upstream inter-tidal habitats Reduced currents up & downstream of barrage Reduced turbidity / suspended sediment levels Increased light penetration through water column - with increased water clarity Increased primary productivity and changed bio-diversity of benthic fauna and flora Upstream tidal range of 7m is still relatively large compared to most estuaries world-wide Main Effects of Barrage for Ebb-only generation

  19. Water levels without Barrage - High Water

  20. Water levels with Barrage- Ebb Generation- High Water

  21. Water levels with Barrage- Two Way Generation- High Water

  22. Velocities without Barrage – Mean Ebb

  23. Velocities with Barrage- Ebb Generation– Mean Ebb

  24. Velocities with Barrage- Two Way Generation– Mean Ebb

  25. Faecal Bacteria without Barrage - Mean Flood

  26. Faecal Bacteria with Barrage- Ebb Generation- Mean Flood

  27. Faecal Bacteria with Barrage- Two Way Generation- Mean Flood

  28. Study of hydro-environmental impacts of Severn Barrage under ebb and two way operation modes Ebb generation Reduction of peak water levels and significant increase of minimum water levels Significant reduction of tidal currents, suspended sediment and bacteria concentrations Conclusions

  29. Two way generation Higher peak water levels and considerably higher low water levels than for ebb tide only Less pronounced reduction in the tidal currents, suspended sediment and bacteria levels Less hydro-environmental impact overall Conclusions

  30. Thank You Bockelmann-Evans@cf.ac.uk

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