LONDON (AP) — A British military helicopter dropped sandbags Friday to shore up a reservoir wall as emergency services worked frantically to prevent a rain-damaged dam from collapsing.

Engineers said they remain "very concerned" about the integrity of the 19th-century Toddbrook Reservoir, which contains around 1.3 million metric tons (1.5 million (U.S tons) of water.

As the Royal Air Force Chinook helicopter made continuous runs to dump the sandbags against the reservoir wall, about 150 firefighters used pumps to bring down the water level, lowering it around 8 inches (20 centimeters) overnight.

"It is a critical situation at this point in time," said Julie Sharman, chief operating officer of the Canal and River Trust, which runs the reservoir. "And until we're beyond that critical situation, the risk is a material risk and that's why we've taken the action we have."

Hundreds of people were evacuated from the town of Whaley Bridge on Thursday with police officers going door-to-door to notify residents. People in the town, located 175 miles (280 kilometers) northwest of London, were advised to stay with friends and family elsewhere if possible, and to take pets and several days of essential medications with them.

A heat wave last week has been followed by heavy rains in many parts of the U.K., causing flash flooding that has inundated homes, roads and train lines. Railway lines near the Whaley Bridge area have been closed.

Prime Minister Boris Johnson ordered Environment Secretary Theresa Villiers to chair a meeting of the government's emergency committee, COBRA, to discuss the situation.

Some residents in the Whaley Bridge area were shocked by the sudden turn of events.

"I've lived in Whaley for the best part of 45 years and I've never seen water flood over the dam like that, ever, nor thought that we could possibly be at risk in this way," Carolyn Whittle said.

The Environment Agency has 10 flood alerts, six flood warnings, and one severe flood warning in place in England.

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