An RAF helicopter has been drafted in to repair the dam amid fears it could collapse in what police have called an “unprecedented, fast-moving, emergency situation”.
The reservoir contains around 1.3 million tonnes of water.
“The plan is to try and stop the dam breaking, clearly. And so a huge amount of effort is going into that,” Mr Johnson said to a group of residents when he visited the town on Friday evening, describing the barrier as “dodgy but stable”.
“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.”
“We’ve had an exceptional weather event, we must make sure that this dam can cope with it in the future,” he added.
“That will mean a major rebuild, clearly.”
The reservoir’s spillway partially crumbled on Thursday after heavy rain fell in the area.
The RAF Chibook has dropped around 400 tonnes of sandbags in an effort to repair the dam.
But officials said it was unclear how long the crisis would go on for. The reservoir’s water level has dropped by half a metre, but needs to drop by several more metres to be considered safe.
“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,” said Kem Mehmet of Derbyshire Police.
“This is very controlled, I must stress that, because this is still life at risk.”
Police have closed railway lines in the Whaley Bridge area over the risk of potential flooding.
The reservoir is on the northwest 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.
Meanwhile, clean-up operations are under way across parts of the North West hit by heavy rain. Residents in Poynton in Cheshire were also evacuated on Wednesday night.
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