An RAF Chinook helicopter is dropping off 400 tonnes of aggregate to help shore up a reservoir at risk of collapse, threatening to engulf a Derbyshire town.

More than a thousand residents of Whaley Bridge, on the edge of the Peak District, had to leave their homes on Thursday in a mass evacuation. Many have been told they may not be able to return for several days after damage to the dam wall of Toddbrook reservoir, which sits high above the town and contains about 1.3m tonnes of water.

Engineers have been pumping water out of the reservoir after part of a dam wall collapsed following extreme rainfall. They have managed to reduce water levels by 200mm, aided by a mostly dry night in the Goyt valley, which runs from Buxton to Whaley through to Stockport.

The helicopter was sent from RAF Odiham in Hampshire to assist at the reservoir, a Ministry of Defence spokesman said. He told PA Media: “It will drop one-tonne bags of aggregate – a mixture of sand, gravel and stone – into Todd Brook. This is intended to stem the flow of water into the reservoir.”

The helicopter was also being used to shore up the damaged face of the dam on Friday.

The situation remains critical, according to Julie Sharman, the chief operating officer of the Canal and River Trust, which owns Toddbrook.

Asked on BBC Radio 4’s Today programme about the chances of the dam bursting, Sharman said: “We’re in a critical situation here on site. There’s a lot of work going on now dropping these gravel bags in to protect the dam. We’re still at risk and obviously are taking a precautionary approach as we want to maintain public safety. We’re taking it very seriously.

“The operation loading the front face of the dam with the Chinook helicopter is still in progress. It’s going to go on for most of the day here. The good news is that inflows into the reservoir have reduced considerably and we’ve lowered the level of the water by 200mm.”

However, she warned it was not a quick operation and would take longer than a day.

Kem Mehmet, the assistant chief constable of Derbyshire police, said: “The public has been absolutely fantastic. Our officers have explained the situation to them and the residents have taken that advice. And they’ve been directed to alternative accommodation.

Facebook Twitter Pinterest Engineers pumping water from the reservoir. Photograph: Leon Neal/Getty Images

“It’s been very distressing and difficult for the local residents to leave their homes. We want them to get back to their homes as quickly as possible but we can only do that when it’s safe to do so. I can’t really say when that is. We’re constantly monitoring.”

Mehmet said police would help residents return when it was safe. “We’ll do everything we can to support all the residents. They’ve been absolutely fantastic. They’ve really recognised the risk. We’ll be absolutely more than happy and more than willing to ensure they get back into their homes safely,” he told Radio 4.

Whaley Bridge is in a steep-sided valley and many houses are above the reservoir water line, but flood experts fear at least 400 properties could be at direct risk were the dam to burst.

Most residents evacuated on Thursday spent the night with friends or relatives. Care home residents were found places at alternative accommodation and some of those with nowhere to go were put up at the Palace hotel in Buxton.

Late on Thursday night, some people were allowed to return home, including the residents of Cromford Court sheltered accommodation, which is 250 metres from the police cordon stopping vehicles coming in and out of the town.

They included Mandy Barnes, who lives alone in one of the flats. “By 11pm most of us were all back – the council told us it was fine, so most of us chose to return. I didn’t get much sleep, though, what with having such an upside-down day. I dropped off at maybe 1am and then at 4.45am I was woken up by the sound of the helicopter,” she said.

Facebook Twitter Pinterest The RAF helicopter prepares for repair work on Friday morning. Photograph: Leon Neal/Getty Images

Barnes was anxious that her asthma medication runs out on Saturday, knowing that Whaley’s shops, which line the main street, would probably remain shut for the foreseeable future. “I can’t drive so I can’t get out and I am worried about my medication,” she said.

She had spent most of Thursday afternoon at Chapel-en-le-Frith high school, which had been turned into a respite centre. Watching BBC News on the canteen television, she reflected on the extreme weather of the previous week. “I just can’t contemplate that this time last week we were sweltering in 34C heat,” she said.

Peter Willis, 70, was also evacuated from Cromford Court, leaving in such a rush he did not have time to put any socks on. He was set to spend the night in the Palace hotel – “I was looking forward to it, I’ve never stayed there before. I was going to have a good breakfast too” – before residents were told they could return home.

Despite the lure of the Palace, Willis decided to go back to his flat. “I wanted to check that everything was OK. Unfortunately there are vagabonds around and we are quite vulnerable. I was worried about looters but everything was fine. I drove up and down a few times and it was all quiet.”

He had enough food to keep him going for a few days and planned to sit out the drama in his flat until the roads reopen to let him go to the shops. “I feel a bit calmer now,” he said. “Yesterday it was very worrying. I really thought the dam was going to go. I really thought it was going to happen – they wouldn’t have evacuated so many people if they didn’t think it was serious. But now that they are relieving the pressure from the dam by reducing the water I am feeling a bit calmer.”

Facebook Twitter Pinterest A police roadblock near the reservoir. All roads into Whaley Bridge town centre are closed and trains are not stopping there. Photograph: Leon Neal/Getty Images

All roads remain closed into Whaley Bridge town centre and no trains are stopping there or continuing through to Buxton.

Rail lines between Hazel Grove, Marple and Sheffield are closed. Trains are also unable to serve stations between Strines, Chinley and Sheffield and stations between Middlewood and Buxton.

“We don’t have a firm estimate of how long this incident will last; however, the emergency services are advising that the line could remain closed until Sunday 4 August,” said National Rail Enquiries.