Video caption Boris Johnson: "I flew over the dam and it looks pretty scary"

Prime Minister Boris Johnson has visited crews working to repair a damaged reservoir that threatens to flood a Derbyshire town.

Some 1,500 residents of Whaley Bridge have been evacuated in case the dam holding back the 300-million-gallon Toddbrook Reservoir fails.

Mr Johnson flew over the dam and said it looked "dodgy but stable".

Police said some residents would be allowed home due to concerns for animal welfare, but only for 15 minutes.

Part of the reservoir's spillway broke away on Thursday and rescue teams have been trying to drain the reservoir and shore up the dam.

It was damaged after large swathes of the country were battered by heavy rain and floods earlier in the week and has been described as a "critical situation" posing a "risk to life".

Police, the Environment Agency, and the Canal and River Trust, who own the reservoir, have all said there is a "real risk" the dam could collapse.

Video caption An RAF Chinook is dropping 400 tonnes of aggregate to shore up the dam and divert water

Meeting residents at nearby Chapel-en-le-Frith High School, Mr Johnson said: "I flew over the dam and it looks pretty scary. I can see the problem."

The prime minister described the dam as looking "dodgy but stable", but he told residents they would be "properly housed" if the dam burst.

Mr Johnson spent about 45 minutes talking to police officers and different groups of residents in the school gym.

He told one group of evacuees: "The plan is to try and stop the dam breaking, clearly, so a huge amount of effort is going into that.

"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.

"They've got a long way to go. Whatever we do, we'll make sure we rebuild it."

Image copyright Getty Images Image caption Boris Johnson has been meeting emergency teams and residents

Derbyshire Police Assistant Chief Constable Kem Mehmet stressed there was still a risk to life and it was not clear when residents would be able to go home for good.

"There are concerns about animal welfare. We will be putting plans in place for residents to return to their addresses for a very short time, in a very controlled manner, to collect belongings," he said.

"We will be restricting it to one person per household. The plans are being worked through at the minute and the residents will be informed when it's in place."

If the dam is about to fall, police said all the emergency service vehicles will sound their horns three times as a warning and residents would have to get to higher ground.

Image caption Resident Mike Breslin said it was "madness" to build at the base of the dam

Mike Breslin described it as a "crazy situation".

"They should never have built a school and a social club at the bottom of a dam. It's madness," he said.

Eric Baker, who has lived in the town for 30 years, said: "It's shocking really, it's like living next to a ticking bomb. If that goes the devastation will be unimaginable."

An RAF Chinook helicopter has put 400 tonnes of sandbags on the affected part of the dam in an attempt to shore it up, while firefighters are using pumps to remove water.

In the more than 24 hours since the operation began the reservoir has been reduced by half a metre, but Julie Sharman from the Canal and River Trust said it needed to come down "several metres" before it would be considered safe.

Image caption Residents have praised the emergency services battling to save their town

She told a press conference workers were "doing all the things that we should be doing" to lower the water level of the reservoir and protect the dam from further erosion. She added the dam and the infrastructure used to deal with emergencies had been "regularly" inspected.

A specialist panel of engineers on site had been advising the operation and she was "hopeful" the dam could be saved.

However, she said it was still a "critical situation".

Image copyright Derbyshire Fire and Rescue Service Image caption Firefighters are removing about 4.2 million litres of water per hour from the reservoir

It is unclear when the residents will be able to go home but they are being told by police to expect to be gone for days.

A helpline for evacuated residents will be operated by Derbyshire County Council from 09:00 to 22:00 on Saturday. The number will be 01629 533 190.

Although the weather is currently dry, the Met Office has issued a yellow weather warning for thunderstorms in the area starting on Sunday afternoon and continuing into the evening.

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