I understand the wildlife trusts position but if its that bad for the waders wont they fly 3 miles south to the Dee estuary ? That assumes that the proposed barrage doesn't operate sympathetically to the environment and focuses on energy output and return on investment.
What about the birds already on the Dee estuary?
I know some migratory song birds are declining due to trapping (e.g.in Cyprus) but for waders which come in from the north I guess habitat loss will be a big concern.
(I have wondered whether a cluster of barrage lagooons could be emptied and filled in a sequence which would keep some part of the mud available in turn for birds all the time - so maybe an improvement for them - but I don't know if their prey inthe mud would be able to cope with the false or erratic tides they experienced?)
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Thats my point though, if the turbines are mounted and run using 1 meter differential head on the ebb and the flood then the slack water period of one hour at the top and bottom of the tide will still be of the same duration but the time they occur will be offset by the time taken for the 1 meter differential to dissipate (ebb) accumulate (flood). Slack water is around 1 hour either side of the tides and therefore the 1 hour of mudflat/ sand bank availability will always be there. Barrages can only sensibly operate within the context of the tidal range available, they can chose to produce more power for a shorter period of time using a greater differential head, or generate for a longer period but at a lower power output, either way they get an enforced shut down for around an hour at the top and bottom of the tide for slack water meaning the same duration of availability for the birds.
Sedimentation will be much the same as it is without a barrage as sufficient turbine positions are provided to maintain the main channel and auxiliary channel velocities and in the even of storms the birds get the advantage that their habitat is able to be protected from the worst of the wind driven tide ensuring their habitat isn't washed away.
As with everything it would the operation of the plant and output would need to be balanced against the habitat, however in this case what's good for the habitat usually means its good for the long term operation of the barrage.
Hi Moxi. Assuming this will operate as per the tidal lagoon / Severn Barage ideas, then you will get 4 cycles per day, two in, and two out. With a 3.5hr generation window, followed by a 2.5hr period.
Instead of the area refilling slowly and steadily, it will remain low, till the tide has risen outside. Once peak differential has been reached, the area will fill, and generate for 3.5hrs. Obviously it will fill faster than normal, but after that longer low period.
From full, instead of emptying slowly as the tide goes out, it will stay full till the tide has gone out, then be emptied (faster than otherwise) and generate again for ~3.5hr.
If I'm picturing this correctly, as the tide is coming in, the mudflats will remain available longer. As the tide goes out, the flats will be unavailable (flooded) for longer. But the two balance each other I suppose. Perhaps the birds will adapt?
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That’s an example of profit before all, as I said at the outset you can work with nature if you compromise.
Moxi
Hi. Sorry, I don't understand. I thought it sounded pretty good, and might not have too much of a negative impact on the local fauna. I appreciate that having a slack period of 2.5hrs is longer, and keeps the top of the flooded area, flooded for longer. But it also keeps the lower mudflats available for longer when the barrage area is empty.
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I’m dredging back to my university environmental credentials here but if I remember correctly if you extend the low tide period you increase the drainage of the mud flats forcing the fauna to migrate closer to the land water interface to maintain the correct environment. Since the low water level is the same under a barrage as it is naturally the nett effect is that the viable corridor for mud/ sand dependant fauna is reduced creating stress and temporarily increasing availability for waders feeding in a congested area - every probe of the beak being a winning meal if you like.
Now add on to that the fact that the other end of the Severn barrage strategy creating a prolonged high water hold which means slower water velocities and hence increased sedimentation. Most of the sedimentation would be estuarine and riverine alluvials and so sand banks reefs and the general estuary bed would be subject to a build up of clay silts changing the balance of fauna and flora and affecting fish nurseries and waders food types.
The change in tidal levels and periods also means a change on dissolved oxygen and anaerobic zones which will both create changes.
My personal feeling is that birds fish and other fauna and flora would adapt but why risk that if you can get a more constant lower power output over a longer period?
I assume, given the high generation cost estimates for these barages and lagoons, that they simply wouldn't be economical at lower levels of generation, hence the need to maximise the head. Looking at the River Mersey, it has a tidal range of 4m (neap tide) to 10m (spring tide).
I assume it's this large tidal range, helped by the fact that the Mersey has a narrow point, with a large area behind*, that makes it potentially viable for leccy generation, v's other types like wind/PV with storage.
*Just below the Mersey, we see the more typical look of the Dee estuary, with a wide opening at the sea, and a narrowing area behind.
Have you heard of the idea of using the 'split' in Scotland, to generate leccy via tidal flow, going through Loch Ness and Loch Lochy? I don't think this was ever that serious, and assume it would have a massive environmental impact.
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Mart I genuinely believe it’s payback on investment that’s stuffing barrage development.
No individual wants to invest in something that pays back over its 120 year lifespan. It’s the realm of the state investment.
If you need a quick payback then the operating parameters you mention give a shorter payback not because they generate more power but because they use fewer larger generating plant located in the mid channel. So the opex and capex is less, and combined with the environment getting a bit of a kicking all ensures the investor gets paid before they die.
Now imagine the same barrage equipped with small uniformly distributed bi directional hydro turbines all operating over the full tidal range except flood and ebb slack water with a 1 m driving head. You get the same relative power because it’s the same volume of water spinning magnets inside copper coils. Lots of small coils vs a few big coils if you like. There are some efficiency losses for the smaller units but overall generator availability offsets these compared with fewer but larger units.
Multiple flow paths gives a water velocity and stream pattern analogous to the estuary without a barrage for a lower but more sustained power output. Capex and opex are similar as although you have more generators handling, maintenance and loss generation potential are vastly reduced.
