We will be using the Venturi Enhanced Turbine Technology (VETT) in our Electric Bridge that we believe presents the following benefits over a conventional barrage.
A barrage reduces the tidal range so that the high tide is lower and the low tide is considerably higher. The low tide position will be increased by 2m to 3m metres leading to the loss of inter-tidal area (mud-banks) and the loss of feeding grounds for birds. The precise extent of this loss will have to be determined for the Wyre but for the Severn, as an example, it was around 70%. The loss for the Wyre should be less but still significant.
The permanent increase in the low tide height in the river upstream of the barrage combined with the need to hold back the ebb by closing the barrage at high tide to allow the ebb to establish a head so generation can begin mean that ground water drainage from surrounding farms and other land will be significantly affected. Farms already have problems with drainage and this will therefore get worse with a barrage in place.
The EU Habitat Directive requires Compensation for any loss of habitat – i.e. the creation of mud banks in another location to compensate for the loss resulting from a barrage. This is expensive and a site will have to be found, acquired and developed.
The passage of fish is the next concern. Fish would have to pass through large slow moving turbines and the level of damage to fish would need to be determined but is unlikely to be zero. Screening and fish passes are options but do not provide a satisfactory solution. Species with swim bladders are more at risk because of the pressure changes. Extensive feasibility work will be needed. The Wyre is not a major salmon river but there are important migrations and passage of several species.
Sedimentation is also an important issue. A dam or Barrage will significantly change the nature of the river and sedimentation patterns. Barrage turbines are large and slow moving. Sedimentation occurs when water moves at less than half a metre per second. The sedimentation issues would have to be modelled and estimated in the feasibility work.
WTE have therefore decided that it is likely to be impossible to build a barrage because of these environmental impacts. This is reflected by the failure of WTE to attract any level of funding to start or complete the feasibility work for a previously considered barrage scheme.
The VETT Electric Bridge will have rise-and-fall gates on top of the concrete sections. These will be maintained at low head levels during flood and ebb to create a pressure head that drives the turbines. VETT is designed for low head power generation. The gates will be fully down at low tide and high tide – still water. The level of the water in the river basin will only change slightly at low tide and there will be less than a 10% loss of inter-tidal area (mud banks).
Fish can pass through the main Venturi that sees 80% of the flow – it is just fast flowing water with no obstructions or moving parts. This was tested in 2013 with 800 fish from 5 species. The fish were monitored for 48 hours after passage and all were perfectly healthy. Further tests have been conducted in 2016are planned with additional species, the fish were kept for 2 weeks, there was zero mortality and a small % of fish bladder damage but all fish recovered. The 20% flow through the turbines can be screened but fish friendly turbines are also being investigated.
The water flows fast through the Venturi and even faster through the secondary channel with the turbine, far too fast for sedimentation to occur. Sedimentation studies will be needed to evaluate any sedimentation effects either side of the concrete sections but we believe these are relatively minor.