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Not a Bore ? Issues of Tidal Power in the Severn Estuary. Agenda University of the West of England 30 th June 2008. 6 - 6.10 pm Introduction and Welcome Jim Longhurst and Eric Albone 6.10- 6.40 pm Presentations Mervyn Bramley, Tom Appleby, Chris Spencer and David Bird

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Not a Bore ? Issues of Tidal Power in the Severn Estuary

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not a bore issues of tidal power in the severn estuary

Not a Bore? Issues of Tidal Power in the Severn Estuary


University of the West of England

30th June 2008

6 - 6.10 pm Introduction and Welcome

Jim Longhurst and Eric Albone

6.10- 6.40 pm Presentations

Mervyn Bramley, Tom Appleby,

Chris Spencer and David Bird

6.40 - 7.20 pm Discussion at Tables.

One key issue for the Feasibility Study

7.20 -7.30 pm Report Back on one key issue for the Feasibility Study

7.30pm Close

severn tidal power not a bore engineering factors and technologies

Severn Tidal Power ‘Not a Bore?’Engineering factors and technologies

Dr Mervyn Bramley

Visiting research fellow

technologies tidal range and tidal stream
Technologies – Tidal range and tidal stream

Tidal stream technology is now at prototype development stage

Low head housed turbines as used in tidal range barrages and lagoons – e.g. La Rance – are production technology

potential tidal energy capture
From tidal range: Power ≈ k1A H2

H = level difference across barrage / lagoon

A = wetted surface area upstream of empoundment

From tidal stream: Power ≈ k2V3

V = mean free-stream tidal current

Potential tidal energy capture
tidal stream array
Prototype stage moving to full-scale demonstration

Small commercial array up to 5 MW

Large array up to 30MW

Potential interference with shipping lanes

Tidal stream array
tidal lagoon swansea
Impoundment by 9 km length embankment

Plan area of 5km2

Mean spring tidal range ≈ 8.5 m

Installed capacity 50MW

Annual energy output 124 GWh

Cardiff Weston barrage ≈ 135 tidal lagoons

Estimated cost ≈ £250m

Tidal lagoon – Swansea
tidal barrage options different sites
Cardiff – Weston Barrage

Location: Lavernock Point to Brean Down

Length: 16 km (10 miles)

Generating capacity: 8.6 GW or 8,600 MW

Annual average output: 17 TWh or 17,000 GWh

Percentage of UK electricity supply: 4.4%

Estimated cost ≈ £15bn

Tidal barrage options – different sites
cardiff weston barrage a mega engineering project
Concrete caissons towed to prepared foundations for sluiceways and turbines

Some embankment

Shipping lock

Connection to national grid

Rail or road possibly, but adds considerably to width

200,000 person years of employment – Site works

Cardiff – Weston barrage A mega engineering project
issues for consideration
Are the technologies sufficiently proven and appropriate to be applied at a significant scale in the Severn Estuary?

Can the environmental impact of a Severn tidal scheme both during and after construction be adequately mitigated?

Can the other uses of the Severn Estuary, in particular ports, commerce and shipping, be accommodated?

Could the UK finance and handle a project of the size of the larger Severn barrage proposals?

Can the carbon embedded in a Severn tidal scheme be adequately offset by the low-carbon power it generates ?

Can we envision a multi-functional scheme and a managed changing environment, or is this our philosopher’s stone?

Issues for consideration
not a bore a barrage of legislation

‘Not a Bore?’A Barrage of Legislation

Law Lecturer


How does the law view the estuary?

  • Law views it as land covered by water
  • Needs consent of the owners of the seabed
  • Otherwise compulsory purchase requirements
rights in the estuary
Rights in the estuary
  • Public right to navigate
  • Public right to fish
  • Private fishing rights
  • Private drainage rights
the effect of human rights
The effect of human rights
  • Compensation for interference with those rights
  • Difficult to assess at this stage
environmental legislation
Environmental legislation
  • Habitats Directive
  • Severn candidate ‘Special Area of Conservation’
  • Requirement to form compensatory habitat for protected features lost to power scheme
not that difficult
Capital engineering costs plus

Compensation costs for people’s rights

Compensation for the environment

Not that difficult?
severn barrage not a bore

Severn Barrage ‘Not a Bore?’

Sediment Distribution and Flood Risk

Sue Marriott and Chris Spencer

current issues

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

Current issues


McLaren et al. 1993

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


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

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?

Future Sea-Level Rise
issues for consideration24
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?

Issues for consideration
severn barrage not a bore the estuarine ecosystem

Severn Barrage ‘Not a Bore?’The Estuarine Ecosystem

Environmental Biologist

estuaries support huge numbers of invertebrates but few species

Edible cockle

(Cerastoderma edule)

Baltic tellin

(Macoma balthica)


(Hediste diversicolor)


(Arenicola marina)

Estuaries support huge numbers of invertebrates – but few species
estuaries support huge numbers of invertebrates but few species28

Mud snails (Hydrobia ulvae) – density may exceed 35,000 m-2

Brown shrimp (Crangon crangon)

estimated population ~10000 million shrimps

Estuaries support huge numbers of invertebrates – but few species
100 fish species have been recorded from the severn estuary bristol channel

1. Sand goby

2. Bass

3. Whiting

100+ fish species have been recorded from the Severn Estuary & Bristol Channel
  • just 10 species contribute to 90% of all fish



European eel

River lamprey

  • Important nursery for many marine species
  • Used as a migratory corridor by some species





Golden plover

(Pluvialis apricaria)

Ringed plover

(Charadrius hiaticula)


(Calidris alpina)


(Calidris canutus)

Invertebrates & fish support an average wading bird population of 88,500 individuals

issues for consideration32
Issues for consideration:
  • How do we balance the need for energy against the potential for environmental damage?
  • The ecosystem will not be destroyed – but it will be changed – for better or worse?
  • How do we put a value on species & biodiversity – are salmon more important than lampreys?
questions for the berr feasibility study to address
The following slides provide a list of questions for the BERR Feasibility Study to address as formulated by the participants at the Not a Bore Science Café Event.

Some 85 people drawn from the Bristol hinterland attended the Science Café event held at the University of the West of England on 30th June 2008.

Following a series of short scene setting presentations participants discussed the implications of tidal power from the Severn estuary.

Arising from these discussions are a list of questions that the BERR Feasibility Study should address.

Questions for the BERR Feasibility Study to Address
questions for the berr feasibility study to address34
Will the evaluation make comparison with the potential impact and input of other low carbon energies such as wind and solar?

Will the SEA evaluate the human costs in the areas following changes arising from an energy capture system in the Severn?

Given that environmental impact is a necessary consequence of whatever system is put in place, rather than compromise why not go for the project that produces the most power and maximises the low carbon energy potential? Will the SEA evaluate such a scenario?

Why is tidal power from the Severn back on the political agenda?

Will the SEA and Feasibility Study consider implications for Human Rights?

How will the proposed schemes affect local communities?

How will a tidal power scheme benefit the South West?

Will the preferred scheme be evaluated against other forms of renewable energy?

Is consideration being given to smaller community based energy schemes?

How will the Feasibility Study ensure that the selected tidal power scheme will not be a white elephant?

Questions for the BERR Feasibility Study to Address
questions for the berr feasibility study to address35
How confident can we be that the Feasibility Study methodology is not biased in favour of any particular type of technology or size of scheme and that it takes account of the cost effectiveness of non-tidal power alternatives to put the study’s conclusions in context?

What are the likely health impacts of a barrage particularly in construction and also through increased risk of flooding.

How are the cultural aspects/damage/change to be costed?

The Severn bore is a unique natural phenomenon – how can a monetary values be put on it?

If a barrage is the preferred option how will sufficient aggregate be sourced to build the structure?

Will the Feasibility Study identify the responsible parties to undertake environmental compensation?

What will be the impact of any scheme on flood risks?  Will the flood scenarios considered actually be worst case?

How will the commercial effect on the Bristol's Docks be taken into account?

Questions for the BERR Feasibility Study to Address
questions for the berr feasibility study to address36
How is the lack of scientific and engineering knowledge being addressed? 

Will ownership of a scheme be considered in the feasibility study?

How certain are the cost estimates? Are allowances made for the legal compensation that will be required?

How seriously are the alternatives to a barrage being considered?

How much energy conservation could the likely cost of a Weston – Lavernock Barrage buy?

What is the payback period for the carbon footprint?

What will be the impacts on the ecology and how certain can we be that the ecological compensation schemes will be able to replace the lost biodiversity?

How confident are we that a tidal power scheme will be future proof? Will a scheme be as efficient in 10 years time as the day it is commissioned?

What effect will the discontinuous operation of a tidal power scheme have on the supply of electricity and on the maintenance of the site?

Questions for the BERR Feasibility Study to Address