Engineers plan to start safety work on an historic Marsden reservoir at the centre of a row.

Yorkshire Water say the work at Butterley spillway is vital to understand how to safeguard Marsden if there were ever to be a major flood caused by a breach at the Butterley Reservoir.

The work will take place from August 19.

Meanwhile conservation watchdog English Heritage has vowed to save the spillway.

They have urged Yorkshire Water to think again over controversial plans to carry out major work on the structure.

The water company insist the work planned for the Victorian reservoir spillway is essential for safety reasons.

Kirklees Council recently granted listed building and planning consent for next week’s investigations.

The exploratory work will include the digging of pits on one of the reservoir’s banks to understand ground conditions, opening a valve at the base of the reservoir, as well as the removal and then reinstatement of small parts of the spillway itself.

Matt Jenkins, Yorkshire Water’s project manager for the work, said: “Regardless of the outcome of our application to make permanent changes to the spillway we need to understand more about the construction of the reservoir and spillway so we can better plan for what might happen in the event of a major flood.

“Opening the valve will give us valuable information about the volume and speed of water which can be released through the pipe at the base of the reservoir and the other investigations will help inform our plan for how to tackle a potential risk to the reservoir before a permanent solution is in place.

“We’ll continue to keep local people informed and have written to residents this week.

“We’ll also keep people updated on the progress of our other planning application for permanent changes to the spillway itself.”

The work is expected to last four weeks.

The plans have attracted a massive amount of opposition, with residents in Marsden leading the fight with a concerted campaign.

The historic spillway, which carries overflow water from Butterley Reservoir, is built of stone in a giant cascade, but Yorkshire Water want to repair it with concrete.

In a letter to the council’s planning department, Kathryn Gibson, inspector of historic buildings with English Heritage, said that Yorkshire Water’s scheme would cause “substantial harm to the listed spillway.”

She added: “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.”

The water company 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.

“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.”