On the night of 11 March 1864 the newly built Dale Dyke Dam above Bradfield collapsed, sending an estimated 700 million gallons of water down the Loxley and Don valleys through Malin Bridge, Hillsborough, Neepsend and the Wicker. Around 240 people died and thousands of homes and works were destroyed. It remains one of the worst dam failures in British history.
Wikipedia: Great Sheffield Flood