PC Geoff Marshall was off-duty on August 1 last year when news emerged that the reservoir might give way - meaning complete catastrophe for people below in Whaley Bridge.
The officer - part of a specialist search team with experience of working at heights - told how he and other members were asked to ‘go and save the dam’.
PC Marshall was told by the inspector on duty the situation was ‘extremely dangerous’ and ‘the dam could go while we’re there’.
Geoff’s terrifying task was to place sandbags - 400 by the time the rescue was complete - at the top of the dam.
Before he descended to the wall where he would be passed the sandbags from volunteers above he was told by engineer David Brown ‘you will have between 30 seconds and two minutes to get off the dam if it collapses’.
However the officer said: “I was on a big overhang of concrete that could just drop and the chances were that if it went I would go with it.
“I was on top thinking ‘this is not good’ as people were passing me the sandbags then putting them around the big open gap.W
The 55-year-old police officer told how moments before he climbed down to the wall Derbyshire Police’s Gold Command had asked engineers at the scene for more time to assess the risks.
However they were warned by engineers ‘if you want to wait then don’t do it at all’.
PC Marshall added: “So we decided to do it.
“About halfway through a big burst of water came through the wall so everyone evacuated as quickly as they could, then we went back down and finished the job.”
Speaking about the terrifying minutes on top of the wall Geoff said: “It’s a strange experience - I just knew I had a job to do quickly and to get off.
“I was thinking ‘this is not a good place to be’ but it was more about getting myself off the wall and in the other direction.
“I think afterwards I went to the rail and said something like ‘blimey, I’m glad I’m off’.”
During the perilous operation Geoff and other volunteers including mountain rescue teams laid the bags in an L-formation around the hole - preventing a bow wave and more water flowing in.
By placing the sandbags when they did Geoff and other volunteers were able to direct the water away from the hole.
They were told by engineer David Brown afterwards: “You’ve saved the dam”.
PC Marshall added: “It was a phenomenal effort by the whole team but afterwards we were so cold and wet all we could think about was getting a hot drink.
“It was a career-defining moment for all of us and we did something absolutely brilliant.
“Afterwards my wife called me an idiot for doing it.”