Green World Trust

Tidal Energy:
Reef vs Barrage
Tidal Fence
Tidal flow turbines

Severn Estuary: Comparing Tidal Reef with Barrage

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See maps, tables and summaries from the Government consultation paper here

Severn Estuary Tidal Reef
Severn Barrage
Location Good Minehead-Aberthaw reduces flood risk by breaking storm surge before it reaches low-lying areas Poor Brean-Lavernock (Weston-Cardiff) increases flood risk by damming the estuary above low-lying areas
Average output (TW-hrs a year) 20*
7% UK elec
likely appr. 15% higher average output than barrage, also generated over a period of 18+ hours per day, 85% usable 17**
5% UK elec
lower output, generated over a period of 6 hours per day in 2 three-hour blocks, thus only 25% usable
Output Profile
(MW @ optimum)
16+ hours per day

4-5 hrs on, 1-2 hrs off, generating on both ebb and flow tides. Output varies between 5000 MW at spring tides and 2000 MW at neap tides*

8 -12 hours per day

3 hrs on, 9 hrs off, generating on ebb tide only. Estimated 8600 MW output when operating**

Supply Issues Could be constant If a lagoon or tidal coffer dam is included to kick in at the high & low tides, supply could be constant at all times. Problematic Very expensive to provide major backup from other sources for most of the cycle.
Est. Build Costs
(£ bn)
£19 bn* £2 bn less than the barrage, using the same costing methodology. Saving is in weight of concrete, foundations, and installation. £21 bn** Due to massive size and consequent knock-on effects & time involved, Government would have to subsidise it initially.
Environmental Disruption Minimal Tides function; scouring / silting unchanged; habitats are preserved - little overall change. Severe Habitats upstream are changed; clyces will be needed; silting and scouring patterns will change a lot. See Fundy barrage failure
Generation Start Date Soon 2 years after construction start - does not need to be complete to start production Delayed Estimated 7 years to build; expected to start operating 2020 or even 2030**
Longevity Excellent Likely to be upwards of 100 years; no decommissioning costs Uncertain Silting could seriously limit life span & cause other problems - see Fundy barrage failure
Visual Impact Little Very low profile as it rises and falls with the tide Large Massive fixed structure - will cut right across visual skyline.
Shipping and Silting Easy Ships will be able to pass through at all times. Channel will not become silted. Difficult Ships' passage times will be severely restricted. Also, serious silting may occur.
Flood Alleviation Good This "early" barrier can inhibit side-wash seiche effects, storm surges, and be adapted to sea level rise issues if necessary. Bad May cause side-wash that could flood the Somerset levels. No protection from storm surges or sea rise.
Practicality of design Likely to be good Needs funding for modelling R&D. But standard turbines used in new overall design. Little environmental protective work needed. Looks poor Massive funding needed for R&D, environ't protection, and shipping locks. More concrete, more road wear, no railways.
Maintenance Feasible Many small turbines - can easily be repaired at relatively low cost "Golden Handcuff"? There are many unknown questions, due to severe environmental disruption: shipping, silting, river sluices, etc.
Manufacturing Diversity Fairly good Smaller units can be made locally, by competing firms, and can be more easily repaired "Golden Handcuff"? Such a monolithic and disruptive scheme can play into the hands of distant multinationals & monopolies

*   Figures taken from RSPB Atkins 2008 report (output p.7; cost p. 8).
** Figures taken from Severn Tidal Power Consultation

Note the metrics: one gigawatt (1 GW) is a unit of power = 1,000 megawatts (MW) = 1,000,000 KW  (A CANDU nuclear reactor is 900 MW; the Aswan Dam is 2100 MW). One gigawatt is identical in size to 24 x 365 / 1,000 = 8.76 terawatt hours per year (8.76 TWh/yr). MW are used to describe running output (when the tide is flowing through the turbines); TWh/yr are used to describe the total output over a year, which is useful for evaluating sun, wind and tide generators which all have substantial times of nil output. Multiply the GW optimum output by 9 to get it in TWh/yr, to compare with the actual output. But even this does not reveal the cost of providing instant electricity supply backups for intermittent generators' down time.

From the above checklist comparison, there seems no doubt whatsoever which option should be chosen. Smaller schemes such as lagoons should only be considered as supplementary to a major scheme, due to the need for a substantial, sustainable generating capacity to meet increasing demand.

Yet the official Government document seriously downplays the reef. We dispute their reasoning and their figures, and believe they are allowing by far the best scheme to drop while favouring a scheme that is very far from proven to be workable economically or ecologically, and seriously hinders shipping. Green World Trust have no financial interests in all this; we just want to see the best and not find ourselves chained. See the Minehead-Aberthaw reef project Severn Tidal, or a shorter earlier version here. The RSPB Atkins report is excellent, explores the engineering feasibility, and is only 20 pages long.

Tidal coffer dams (see Courtney pdf p.18) are a proven way of utilizing tidal power, with timing that can be chosen to fill in gaps from other sources; this alone makes them economic. Tidal lagoons are free standing structures built either offshore or connected to the shoreline at each end. They operate on similar principles to barrages but have a smaller impact and allow ships through easily. A Tidal Fence is an array of tidal flow turbines; it is less suitable for the Severn estuary as it only really functions economically in fast-flowing tidal races which tend to be small local phenomena (like SeaGen in the Strangford Lough narrows).


SeaGen: the use of tidal flow turbines

Another way of harnessing the power of the ocean, where there are strong currents, is by the use of Tidal Stream turbines. This is an established technology which is being marketed by SeaGen, who have already installed units in Strangford Lough near Belfast, Ireland; and off the Somerset UK coast at Lynmouth. This form may not be suitable for the Severn Estuary but is ideal for many other locations around the British Isles. See the SeaGen website at

Tidal Stream Turbine - suitable for fast current
SeaGen installed and doing well in Strangford Lough, Ireland
Row of tidal stream turbines



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