Severn Barrage Not a Bore

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Severn Barrage Not a Bore?

• Sediment Distribution and Flood Risk
• Sue Marriott and Chris Spencer

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Current issues

• Sediment
• What types of sediments exist in the estuary and
  how are they distributed?
• Where does the sediment come from?
• Is the sediment contaminated?
• Flood risk

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Sediment
McLaren et al. 1993
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Sediments

• How will the barrage impact on sediments?
• Circulation by tidal currents restricted to
  Bristol Channel below barrage
• Fine sediment can settle in lagoon behind barrage
  making water clearer, allowing ultraviolet
  penetration therefore cleaner water plus
  photosynthesis and phytoplankton blooms
• Contaminated sediment from floodplains may build
  up
• Build up of sediment may reduce volume of upper
  estuary

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Flooding

• Flooding
• What contributes to flood risk in the area?
• What will the effects of the barrage be?
• Lagoon will raise water tables so river water may
  not drain away so quickly
• May cause extra deposition of mud in lower
  reaches of rivers due to lower gradients reducing
  capacity
• Increased flood risk in lower reaches and backing
  upstream

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Increased flood risk upstream?
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Future sea-level rise

• Sea-level rise will have impacts on coastal areas
  due to increased erosion and increased flood risk
• Would also cause backing up of river water in
  lower reaches
• Barrage may be opened during high river flows to
  move water out quicker
• Erosion energy will be reduced behind barrage
• Could a barrage help mitigate against future
  sea-level rise?

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Issues for consideration

• Would a barrage result in changes to sediment
  distribution?
• Will changes have impacts in the Bristol Channel?
• What are the implications of raised water tables
  in the surrounding areas?
• What is the likelihood of an increase in
  contamination of estuarine sediment?
• What are the likely impacts on sediment and flood
  risk of other tidal power generation options?
• Costs of extra coastal and flood defences needed
  due to future sea-level rise vs cost of barrage?