Boris Johnson meets Whaley Bridge residents near collapse fears reservoir dam

Water levels at the Toddbrook Reservoir have been reduced by half a metre but the damage to the 180-year-old structure remains at a “critical level”.

Residents of Whaley Bridge are being temporarily allowed to return home to collect essentials due to “a substantial threat to life” if the dam wall – which the Prime Minister described as “dodgy but stable” – fails.

An RAF Chinook and around 150 firefighters using high-volume pumps appear to have partly stabilised the reservoir’s spillway, which partially crumbled following heavy rain on Thursday.

Meeting locals at the Chapel-en-le-Frith High School, a few miles away from Whaley Bridge and currently being used as an evacuation centre, Mr Johnson called the damage “pretty scary”.

“The plan is to try and stop the dam breaking, clearly. And so a huge amount of effort is going into that,” he told a group of residents on Friday.

“The Chinook’s been over putting in the aggregate and putting in the sandbags to try and stop it bursting. They’re pumping out huge amounts of water.”

Mr Johnson, who arrived by helicopter, said he thought they had to get the level of the water down about eight metres, although there was some discussion with the surrounding officials about whether this was the exact figure.

After speaking with locals and emergency crews at the scene, the Prime Minister told reporters the dam will need a “major rebuild”.

He said: “I was talking to one of the villagers from Whaley Bridge who said that he remembered something like this happening 50 years ago.

“We’ve had an exceptional weather event, we must make sure that this dam can cope with it in the future.

“That will mean a major rebuild, clearly.”

During a multi-agency press conference, Assistant Chief Constable Kem Mehmet of Derbyshire Police asked residents to continue to heed police advice and stay away from the area.

“We will be putting plans in place for residents to return to their home to pick up very vital things they need along with their animal welfare,” he told reporters.

“This is very controlled, I must stress that, because this is still life at risk.”

Numbers returning will be restricted to one person per household, he added, and it was “difficult” to say when people would be allowed to return permanently.

If the dam is about to fail, police said emergency service vehicles will sound their horns three times and a loud hailer will also sound as a warning to residents.

Julie Sharman, chief operating officer for the Canal and River Trust, told the press conference the water level needs to be reduced by “several more” metres, with more pumps being installed on Friday evening.

“This is still a very critical situation,” she said.

“Until we are confident we can control that risk, then our position has to be to protect the public safety and limit access because we don’t want to put people at risk.”

Around 1,000 people were evacuated from the town but most found their own accommodation with family and friends, according to Derbyshire County Council.

Police have closed railway lines in the Whaley Bridge area over the risk of potential flooding.

The reservoir is on the north-west edge of the Peak District National Park and was built in 1831, according to experts, although the Environment Agency records it as being built in 1840-41.

According to a 2011 Environment Agency report on national dam incidents, Toddbrook “has a history of leakage”.