People in the U.K. town of Whaley Bridge were bracing for more rain Sunday and hoping efforts by engineers will keep the nearby Toddbrook Reservoir dam from collapsing.

Workers were into their third day on Saturday of trying to prevent the dam from crumbling and flooding the town in central England with more than one million tonnes of water.

A majority of the town’s 6,500 residents left their homes on Thursday after heavy rain caused some of the concrete slabs on the dam’s dry front to slide off.

Royal Air Force helicopters have since been dropping sacks of ballast onto the outer slope of the dam. Specialist contractors have been placing concrete grouting around the bags to bind them together.

By Saturday, pumps had reduced the water level by 35 per cent, but authorities were warning that pressure on the 180-year-old dam remains severe.

Images from the operation showed personnel hanging out of the side of the Chinook, directing the bags into the hole to fill gaps — like a craftsman smoothing plaster. Hundreds of one-tonne bags have been dropped in the last few days, said Daniel Greenhalgh of the Canal & River Trust, which manages the dam.

Sand bags sit outside shops in the town on Saturday. (Vickie Flores/EPA-EFE)

“This is still a critical situation,” Greenhalgh told the BBC. “We need to draw the water down significantly.”

Firefighters have brought in large pumps to pull water out of the reservoir and into the River Goyt.

“We are hoping the weather will be kind to us,” Derbyshire Deputy Chief Fire Officer Gavin Tomlinson said. “But everyone is working as hard as possible to get ahead of the curve and remove as much water as possible today, overnight and into tomorrow, to minimize the impact of any bad weather that does materialize.”

 Britain’s meteorological agency says “torrential downpours and hail” are possible across northern England on Sunday. That could hamper emergency efforts to further reduce the water level in the reservoir.

Brief visits after homes evacuated 

Some residents of the town, 280 kilometres north of London, briefly returned to their homes on Saturday to collect essential or treasured items. Authorities allowed one family member per household to return home for 15 minutes. The government says it may be several days before residents can return to their homes permanently.

A picture shows the deserted centre of the town of Whaley Bridge in northern England on Sunday. Emergency workers are racing to lower water levels behind a damaged dam near the town as forecasters warn more rainy weather is on the way. (Oli Scarff/AFP/Getty Images)

Among the residents going home quickly was Tracey Coleman, who lives just outside the area marked for closure. She left in haste with her family on Thursday, taking two dogs, a cat, a tortoise and a neighbour’s dog.

“We just took some medication, the animals and a bit of food,” she said.

Coleman said she had been walking in the park Thursday morning when she heard what she thought was thunder.

“Now I think I must have heard the dam breaking,” she said.