Investing in our common future Co-financed with the support of the European Union ERDF – Atlantic Area Programme 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

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

Wales England Severn Estuary

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 Save > 6.8 million tonnes carbon pa 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 Slide – courtesy of STPG

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

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

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

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 km 2 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 Ebb- only 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 bio- diversity 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 Generation- High 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

Investing in our common future Co-financed with the support of the European Union ERDF – Atlantic Area Programme Thank You