The water company has submitted plans to Kirklees Council to modify the overflow spillway at Butterley Reservoir at Marsden, near Huddersfield.
The company says the work is vital to ensure the Victorian structure can cope with flooding conditions but critics say other options have not been fully explored and a campaign by villagers to preserve the spillway is gathering momentum.
English Heritage has now stepped in to say the spillway should be preserved.
In a letter to the council’s planning department, Kathryn Gibson, inspector of historic buildings with English Heritage, concluded that Yorkshire Water’s scheme would cause “substantial harm to the listed spillway”, adding: “As such, we advise that listed building consent should be refused unless your authority is satisfied that a fully justified case has been made that demonstrates that the loss is necessary to achieve substantial public benefits and that agreement has been reached on an acceptable design solution.”
Ms Gibson says the spillway, built between 1891 and 1906, has a distinctive stepped base which includes two stepped cascades “reminiscent of the 300-year-old Cascade within the grounds of Chatsworth House, some 40 miles away in Derbyshire”.
Only a small number of spillway structures have been given listed status and the overflow at Butterley is the only spillway of this type to be included on the Heritage List for England, she added.
“Its listing at Grade II demonstrates that it is a nationally important example of Victorian engineering and the quality of materials and detailing reflect civic pride of the then Huddersfield Municipal Borough.”
English Heritage has pointed out that the proposals would involve significant changes to the spillway, including the overlaying of the base with reinforced concrete, demolition of the two stepped cascades and the demolition of the walls and turrets and their replacement with a 184m long concrete wall.
The conservation body had asked Yorkshire Water to consider building a new spillway on the opposite side of the dam but this was rejected by YW at the evaluation stage on the grounds of cost and practicability.
The letter to the council concludes that the proposal should be rejected when it comes before councillors.
Yorkshire Water said it had previously considered eight options, many of which involved constructing a second spillway at the other side of the embankment.
A spokesman said the various options of creating another spillway “raised concerns, as they would involve working on the clay core of the embankment, which is vital to ensure the safety of both our colleagues whilst on site and the local community of Marsden.
“These options would also involve capping off the existing spillway which would stop water running down it, a feature that both customers and English Heritage are keen to keep.
“Replacing the existing spillway is our preferred option as it is the safest one to do and meets with the Reservoirs Act. It also means that the spillway will continue to be used and maintained in the future.”
A campaign to preserve the spillway has been backed by Marsden-born poet Simon Armitage.