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In February and March 2016, VerdErg Renewable Energy completed a second fish testing programme, in collaboration with Fishtek Consulting Ltd to acquire further audited experimental evidence of VETT’s safe fish passage.

This programme follows on from the previous trials, conducted in 2013, to establish the operating conditions of VETT required for safe fish passage. The latest programme was conducted with fish species and pressure ratios which had not been previously tested.

It was established in Fishtek’s Fisheries Risk Assessment that VETT posed no risk of shear stress or mechanical strike on fish passing through the system. However it was concluded that further due diligence should be obtained on the potential effect of barotrauma that fish could experience during passage through the primary flow path and venturi (the turbine in the secondary flow path will always be screened).

A scope for the second phase of trials was then developed in conjunction with Fishtek and the EA’s National Fisheries Team to ensure the test conditions and parameters were appropriately captured and assessed.

There were four key aspects that formed the basis of the methodology:

  1. Firstly, the assessment of fish survivability at the upper VETT operating range, with heads (pressures) up to 3.5 m.

  2. Physoclistic fish (fish with a closed swim bladder with gas exchange occurring through the blood in adjacent capillaries) are more sensitive to rapid pressure fluxes so these species were extensively tested to qualify how their swim bladders would react.

  3. The existence of fish in the water column varies with species and feeding behaviour. Buoyancy and movement in the water column is dictated by the inflation of the swim bladder for physoclistic fish and physostomous fish (fish with a pneumatic duct connecting the swim bladder and gut).
    Depth acclimatised fish are more sensitive to barotrauma so surface and depth acclimatised fish up to 3.0 m were also assessed.

  4. Finally, an assessment of post-passage swim behaviour was also conducted to identify the occurrence of any disorientation that could result in fish being more vulnerable to predation at the outfall.

A full scale, fully instrumented Co-axial VETT test rig was commissioned in HR Wallingford’s Froude Modelling Hall to replicate a VETT operating in a river. Two submersible pumps were used to create a flow rate of up to 500 m /s. Over 1,200 fish were tested comprising of European eel, Atlantic salmon smolts, Brook lamprey, Perch and Bream.

These species represented a range of physostomous, physoclistic and native coarse fish, some with conservation designations due to their vulnerable populations.


VerdErg is pleased to report that the results demonstrate zero mortality and 100% scheme passage rate (SPR) for all species tested.

SPRbarotrauma considers long term or permanent injuries associated with barotrauma. SPR                was 100% for physostomes (eel, lamprey, salmon, and bream) and above 99% for physoclists (perch) at the set maximum operating level.Not only is VETT able to achieve low-cost, reliable hydropower, it is able to do so whilst preserving the natural fish populations in our rivers. This body of work has proven VETT’s ability to facilitate downstream fish passage and contribute to the EA’s work in creating fish highways required for migration, feeding and the completion of their life cycles.



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