Letter from Professor Roger Falconer concerning Severn Barrage and Tidal Lagoons

Professor Roger A. FALCONER


  South Glamorgan, 


12th January, 2009


The Editor
Penarth Times

Dear Sirs,


I read with interest the letter from Paul Kinnersley in this week’s Penarth Times, where he states that the tidal lagoon is cheaper and more efficient than a Severn Barrage and he encourages ‘all your readers (to) make their views known urgently to our elected representatives’.  I would be grateful if Mr Kinnersley could first explain how an offshore lagoon could possibly be ‘cheaper and more efficient’ than a barrage, which links to the land.


Basically the barrage and lagoon work on the same principle, in that they create an impoundment, hold back the water and then release the water from within the impoundment when there is a sufficient water level difference either side of the wall (barrage or lagoon).  The amount of energy available for supply is proportional to the water level difference either side of the wall and, in particular, the plan surface area of the impoundment.


The Cardiff to Weston barrage will impound approximately 500 square kilometres of water and will have an impoundment perimeter of approximately 160km and of which 144km of which is already in place from the Welsh and English foreshores from Cardiff and Weston to Gloucester respectively, with closure of the impoundment needing just the 16km wall (barrage).


If the dimensions of the proposed Swansea Bay offshore lagoon are considered typical, the 5 square kilometre impounded lagoon will require a 9 km wall to be built all the way around the impounded waters.  In equating areas only, which I accept is an approximation, we would need typically about 100 lagoons to give the same potential power as a barrage, with each lagoon being 2.5 times the size of Cardiff Bay (which has a surface area of 2 square kilometres).  At low tide the lagoon would expose a 9km wall, of approximate height 12m or more and which would be comparable to the height of a two-storey house.


Whilst I fully accept there are a number of environmental concerns about a barrage, particularly with regard to the loss of intertidal habitats for the bird populations and fish migration, a barrage will also bring a number of benefits which also need to be considered.  For example, it will provide protection against sea level rise, generally reduce flood risk (both upstream and downstream), provide clearer water in much of the estuary with the reduced currents, and create more jobs (estimated – 40,000) and tourist opportunities for Cardiff and the region.


I trust that your readers will consider all of the issues regarding the complexity of renewable tidal power generation in the Severn Estuary, before urgently writing to their elected representatives.


Yours faithfully


Professor Roger Falconer


5 comments on “Letter from Professor Roger Falconer concerning Severn Barrage and Tidal Lagoons

  1. The Severn Barrage issue

    Although the eco-issue are sites specific, in energy terms, arguing for or against particular projects in isolation misses the point-what matters is the overall system.


    If we have 10GW of nuclear (maybe more) and a lot of wind (another 20- 30GW?), plus the 8.6GW barrage, what happens when demand is low- under 20GW will be needed. If wind is delivering at that point and the barrage happens to fire off at full output (which it will only do part of the year – neep tides- and in any case only for 2 hours or so twice every 24 hours) , which power do we forgo?

    We could try to vary the nuclear output- that will stress the plants (so they age fast and may have safety problems) and expensive, since they will need to run 24/7 to payback their huge capital costs. The latter is also of course true of the Barrage and indeed wind. Basically no one wants to be curtailed.

    Economically speaking you would I assume simply curtail the most expensive source. Which looks like the barrage.

    This does seem a waste, if you have built it. Maybe it would make economic sense to invest in storage. Then they would all be freed up.

    The barrage could have some reverse pump storage capacity (e.g. for excess nuclear or wind output) but that wouldn’t help in terms of the barrage output (it can’t store itself!) and would not be very convenient for wind or nuclear, since it couldn’t be done at high tides (you would overtop it) and these occur at varying times of day. Note that this means the barrage could only store when it wasn’t generating, so this option would be no good at dealing with its output conflict with wind and /or nuclear at low demand times.

    Muddying the water a bit (literally as well) the barrage output profile could be improved a bit (an extra hour or so) with two way flow generation, but the costs are high and this may not provide enough advantage (in terms of improving its productivity) to justify curtailing wind and/or nuclear at minimum demand times .

    Basically I feel that it is going to be hard enough balancing the wind output (both when low and when in excess of needs) . Adding a sometimes very large, cyclic but variable size output from the barrage would make it even harder.

    In summary, as far as I can see, without a lot of storage, and unless you want to curtail wind and/or nuclear, the only time you would use the barrage was when

    (a) demand was high
    (b) wind was low
    (c) tides were high

    That’s not going to be often

    So the SDC’s estimate of it only saving 0.92% of UK annual emissions by 2020 was I think too HIGH!!!

    A geographically distributed system of smaller tidal units (barrages, lagoons, tidal current turbines) could have roughly the same capacity but would be much more flexible, offering more nearly continuous output from the network as a whole.

    For example you could have the 1GW Shoots barrage on the Severn (or the 1.3GW Tidal Fence}, and some lagoons there (100MW) together with barrages on the Mersey, Humber, Dee etc (another 2-3GW) and say 2-3 GW of tidal current turbines off Scotland, Wales and Cornwall, plus some more lagoons (e.g. in the Mersey, Thames and the Wash, as have all been proposed). The SDC report says that the Cardiff-Weston barrage would
    have a load factor of 22.5%. I’m would imagine this distributed network could do much better than that, and possibly better than wind at 30%.

    Of course then you might say that it would be wind that was the weaker option- it would be much more variable. So curtail it. But then again wind will no doubt be much cheaper than tidal. So we can’t escape the curtailment issue.

    Pity we don’t have lots of hydro!

    (Prof) Dave Elliott

  2. With respect, Professor Falconer appears to equate cost with wall length. That seems to be an over simplification. Wall depth in far more important in cost than length.
    The Severn scheme is restricted to a single large environmentally sensitive area. Lagoons can be built in many areas of shallow water with far less environmental consequence.

  3. Dear Professor Elliot, my understanding is that the 3 basin tidal lagoon is capable of continuous generation (unlike the barrage which has 2 drifting daily peaks which cannot be shifted).

    Furthermore, as as I understand it, the 3 basin lagoon, can be either turned of completely say during a wind spike, or have the output suddenly increased to deal with a wind fall say.

    And that this curtailment, or boosting, can happen at will, at any stage of the tide.

    If this is true, then it makes the barrage look hopeless.

    The lagoon scheme can of course then fit in with either the nuclear program, the wind program, or both.

    Kind Regards

    C Stevens

  4. I know the Severn Barrage (Cardiff-Weston) has been studied to death and has some big commercial backers, but the other approach that I am proposing with the ‘Severn Tidal Reef’ is that you start with the environmental constraints and shipping interests and design the project around them, rather than consider them at the end of the process. The ‘Reef’ is a totally different concept, so I ask Professor Falconer how he thinks the monolithic barrage still has any advantages over the Minehead -Aberthaw Reef.

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