Kirkthorpe Hydropower will be the largest hydro scheme in Yorkshire, and, once commissioned in November the £5.3M facility will have a 500kW capacity – producing around 2.3M.kWh per annum. This is enough renewable electricity to take around 800 homes off the grid.
The scheme comprises a 500kW axial turbine and is being developed on land owned by Wakefield Council on the River Calder 6.4km east of Wakefield. Funding has come from the client, Yorkshire Hydropower, which is a part of Barn Energy, a company which is privately funded via equity shareholders.
The project will protect fish and eels using custom-designed inlet screens. The fish passage at Barn Energy’s previous hydro project at Thrybergh, 48km away, has already aided the recovery of the River Don and seen Atlantic Salmon returning to historic waters upstream for the first time in 150 years. The Thrybergh scheme entered commercial operation in October 2015.
barn energy kirkthorpe aerial shot
The Kaplan turbine being used in the Kirkthorpe plant is highly efficient, achieving around 94% efficiency in extracting energy from the water. Counting losses throughout the system such as from bearings, generators, and belt drives, the overall system achieves around 82% energy efficiency. The efficiency of the turbines means it produces a higher energy output per kW of capacity than solar photovoltaics or wind technology.
Forecast to still be supplying clean energy in a 100 years time, the proven reliable technology has been with us in the modern sense for a century. Austin Flather, director at ANF Consulting says: “What modern technology brings is more in the control of these turbines to make them gain the most from the available water source while maintaining regulation and environmental requirements.”
While all hydro schemes are unique, as all rivers are different, having a team of suppliers that know their role and those of the other team members reduces the learning curve. Those involved in the Thrybergh scheme reconvened for the Kirkthorpe project - the client (Barn Energy), project manager (Cobalt), consultants (ANF Consulting and JNP Group), and main contractor (Eric Wright Civil Engineering).
“These projects have been delivered under NEC Option A and have definitely been administered and delivered in the spirit of the NEC contract. We have worked together with the project team in a collaborative manner to overcome any challenges as the works have progressed,” says Eric Wright Civil Engineering contracts manager Gavin Hulme.
The Kirkthorpe scheme is a lot bigger and more complex than Thrybergh. The weir at Thrybergh was located at a natural bend in the river so the hydro scheme was constructed in a more regular rectangular shaped cofferdam to “cut the corner” of the river and benefit from the head drop across the weir. It involved excavations adjacent to and in the river approximately 7m in depth.
Kirkthorpe is a more complex shape as the water has to be channeled through the hydro scheme via a diversion around a weir and then back into the main flow channel. The works at this site involved excavations up to 10.5m deep in a horse shoe shaped cofferdam. Thrybergh was a 30 week programme, while Kirkthorpe is an ongoing 50 week programme.
Hulme says, “Traditional rectangular cofferdams are much easier to support as there is usually a wall directly opposite and parallel that can be used to prop off and equally distribute the active pressures from the ground. At Kirkthorpe the propping could not do this due to the shape so extensive angled propping and shear connections were needed along with ground reduction and tie back anchors to create a composite temporary works design.”
“Traditional rectangular cofferdams are much easier to support as there is usually a wall directly opposite and parallel that can be used to prop off and equally distribute the active pressures from the ground. At Kirkthorpe the propping could not do this due to the shape so extensive angled propping and shear connections were needed along with ground reduction and tie back anchors to create a composite temporary works design.”
Gavin Hulme, Eric Wright Civil Engineering
The load on the lower frames in the deepest section of the cofferdam was 570kN/m, which is high for proprietary shoring equipment, so Mabey selected Supershaft Plus as the strongest proprietary brace on the market, which allowed maximum spacing of the cross struts. Using proprietary equipment meant a quicker installation and removal time than on-site fabrication of frames with structural steel, which would also have been more expensive.
Mabey regional engineering manager Jon Seddon says: “Early involvement with Eric Wright ensured we only used proprietary equipment in the areas it could offer a real benefit to the project in terms of cost and programme.
“Once formation level was reached and the base slab was cast, we were able to remove the complete lower frame and a large amount of the upper frame. By opting to use our proprietary system rather than structural steel, Eric Wright was able to free up large areas of working room very quickly, using the concrete base as the new lower support to the sheet piles.”
Permanent base slab
Hulme adds, “For the permanent base slab we also used a higher strength concrete in OPC than was required by the permanent works design. This enabled us to remove the lower frames quicker than envisaged and hence streamline the cofferdam programme”.
The final temporary works designs satisfied the requirement to develop a cofferdam support solution that avoided the need for propping at higher level. This was a key early requirement in the design brief from Eric Wright. Mabey worked in conjunction with Eric Wright and temporary works designer Pascoe Consulting proposing different alternatives regarding the construction sequence, so Eric Wright could evaluate the most cost-effective solution. Being involved at the early stage allowed Mabey to be involved in evaluating possible solutions that could quickly be dismissed or progressed.
As part of the desire to reduce the amount of propping, to eliminate clashes and interferences between the cofferdam support and the reinforced concrete works, the use of tie back anchors in lieu of the top two rows of frames was considered.
“The reason for this was that the upper two levels of frames had sufficiently smaller loads that could be accommodated by tie back anchors. The lower two levels of props/frames had significantly higher loads, which would have meant a large number of anchors,” says Hulme.
The reduction in the number of props and frames also means that when the turbine arrives in August it can be lowered into the cofferdam without any temporary propping clashing or affecting it.
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