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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
34th IAHR Congress, Brisbane, 2011
Co-financed with the support of the European Union ERDF – Atlantic Area Programme
Investing in our common future
Planned Renewable Energy Provision
Considerable Scope
to increase
EU target of 20% carbon reduction by 2020
Wales
Severn Estuary
England
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
Government Short Listed
Proposals
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
Slide – courtesy
of STPG
• Save > 6.8 million
tonnes carbon pa
Barrage Layout (1989 Report)
Key facts:
• 216 x9m
turbines
 17 TWh pa
• 166 sluices
• Ship locks
• Fish pass
Slide – courtesy
of STPG
• Public road &
railway
Tidal Power Generation (Ebb-only)
Slide – courtesy
of STPG
Barrage Operating Schemes
• Ebb Generation
• Flood Generation
• 2 Way Generation
• Pumping to increase energy output
Proposed Operation - Ebb Generation
Slide
courtesy
of STPG
Proposed: One Way Generation
Option 1: Generate over ebb tide only
Level of water inside
impoundment
Alternative: Two Way
Generation
Option 2: Generate over full tide
Rapid filling and
emptying of basin
required at either
end of tidal cycle
Level of water
inside impoundment
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
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
Model Extent
Measured and Predicted Water
Elevations and Current Speeds
Measured and Predicted Sediment and
Bacteria Concentrations
Main Effects of Barrage for Ebbonly generation
• 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 biodiversity of benthic fauna and flora
• Upstream tidal range of 7m is still relatively
large compared to most estuaries world-wide
Water levels without Barrage - High
Water
Water levels with Barrage- Ebb GenerationHigh Water
Water levels with Barrage- Two Way
Generation- High Water
Velocities without Barrage – Mean Ebb
Velocities with Barrage- Ebb Generation–
Mean Ebb
Velocities with Barrage- Two Way
Generation– Mean Ebb
Faecal Bacteria without Barrage - Mean
Flood
Faecal Bacteria with Barrage- Ebb
Generation- Mean Flood
Faecal Bacteria with Barrage- Two Way
Generation- Mean Flood
Conclusions
• 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
• 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
Thank You
[email protected]
Co-financed with the support of the European Union ERDF – Atlantic Area Programme
Investing in our common future