And if the consultation says that "they have had significant environmental modelling done on the effects of flow on the barrage" I've 30 years experience hydraulic flow modelling.
It is not an exact science.
As I am sure we have all seen before with Environmental Statements / Impact Assessments etc.
Depending on the assumptions you give me as my starting data, the limits of the scope of my study and the amount you pay me I could come up with ten different answers.
Any answers on this thread are my own personal opinion, were not commissioned by any company or body and are solely and utterly my own personal thoughts.
For those unaware there are quite a few different ways to utilise tidal power and I personally think from an environment point of view (not just birdlife) that a barrage on an Ecosystem like the Mersey is the worst option.
Conversely - a "lagoon like the proposed Severn Barrage" does have quite an environmental impact, but mitigation can be put in much more manageably for when the lagoon drains between tides - e.g. mudflats, managed vegetation at the verges).
No one has yet built a barrage such as this at the Mersey and had enough data after to prove "no loss" (see later there is one in France which provide "Ecosystem Changes"). The Mersey is quite unique having a very high tidal range 10.5m (Severn 12m? is a bit higher and a few places in Canada 14m? and China have higher ranges). The results cannot be predicted as water hydrology and ecology are complex systems.
For example:
The tide will be delayed going up the estuary as it is left to build up to "high tide" on the downstream (ocean side).
Once high enough over the turbine inlets the tide will be let into the estuary. This will be water from the "top" of the high tide. It won't be scouring the Irish sea floor and bringing sediment and nutrient in, that will have settled out whilst the tide built up at the barrage.
After water levels equal (if you indeed do let the full height of the high tide into the estuary, some scheme don't for a) flood protection and b) the high tide has passed on the Ocean side so the estuary side never catches up) you then open the turbines to flow the other way................
.............only you need to have let the ocean side water fall below the turbine outlet for there to be a static head to let the ocean water flow back out.
The gates then cose to stop the flow.
And in the meantime river water has flowed down the Mersey reducing the salinity of what was left in the the Estuary by dilution with clean water and possible most days some "Brown Mersey Trout" released from Eccles WWT).
So the salinity in the estuary slowly reduces over time, you don't get the sea sediment and plankton, nutrients, small creatures etc in and the estuary moves over time to approach a freshwater body. So do the Redshank, Gulls, Oystercatcher, Shelduck etc etc stay or move on.
Just think of a world where the tide is restricted where it no longer floods Hale Marsh, Norton Marsh, moves so slowly it doesn't scavenge enough river bottom to expose new nutrients at each tide and there are no longer any waders at Hale Head, Halfway House, Frodsham Score becomes a silted up sandbank.......
Here is a National Geographic link with the pertinent section copied.....feel free to further read other sources, I've tried to find the one that explains the issues best in Laymans terms - hopefully the Public Consultation will be "above board" about how well their plans are developed and how accurately they can predict the future.
In the meanwhile - get the Cheshire / Mersey Bird records into the formal bodies that will be used for Environmental Assessments.
I'll admit a conflict of interest here - I have worked on tidal turbines - looking at the aspect of making the pressure drop over the turbine so low it doesn't affect / burst swim bladders on fish. The blades don't chop marine animals up as peeps might think, it is the pressure wave (you need a difference in pressure to make the blade turn!).
Once the consultation is out I'll see if ther is any more info I can provide to help give a balanced view on these schemes.
Barrage Another type of tidal energy generator uses a large dam called a barrage. With a barrage, water can spill over the top or through turbines in the dam because the dam is low. Barrages can be constructed across tidal rivers, bays, and estuaries.
Turbines inside the barrage harness the power of tides the same way a river dam harnesses the power of a river. The barrage gates are open as the tide rises. At high tide, the barrage gates close, creating a pool, or tidal lagoon. The water is then released through the barrage's turbines, creating energy at a rate that can be controlled by engineers.
The environmental impact of a barrage system can be quite significant. The land in the tidal range is completely disrupted. The change in water level in the tidal lagoon might harm plant and animal life. The salinity inside the tidal lagoon lowers, which changes the organisms that are able to live there. As with dams across rivers, fish are blocked into or out of the tidal lagoon. Turbines move quickly in barrages, and marine animals can be caught in the blades. With their food source limited, birds might find different places to migrate.
A barrage is a much more expensive tidal energy generator than a single turbine. Although there are no fuel costs, barrages involve more construction and more machines. Unlike single turbines, barrages also require constant supervision to adjust power output.
The tidal power plant at the Rance River estuary in Brittany, France, uses a barrage. It was built in 1966 and is still functioning. The plant uses two sources of energy: tidal energy from the English Channel and river current energy from the Rance River. The barrage has led to an increased level of silt in the habitat. Native aquatic plants suffocate in silt, and a flatfish called plaice is now extinct in the area. Other organisms, such as cuttlefish, a relative of squids, now thrive in the Rance estuary. Cuttlefish prefer cloudy, silty ecosystems.
Public consultation opening on 1st Oct for the tidal barrage between Birkenhead and Liverpool - will reduce tidal ranges in the Mersey estuary, mudflat expanse etc and provide a physical barrier for birds flying low to come up the estuary.
