Experts have warned of “massive flooding” if there is further damage to a dam near the town of Whaley Bridge in Derbyshire.
Thousands have already been evacuated from their homes over fears it may collapse and firefighters are currently pumping tonnes of water from Toddbrook Reservoir after it overflowed following heavy rainfall.
Derbyshire Police said further evacuations were taking place downstream of the reservoir as work continues to ensure the structure of the damaged wall.
A small number of properties in Furness Vale and New Mills – outside Whaley Bridge but inside the flood risk area – are in the process of being evacuated.
An RAF Chinook is being drafted in overnight on Thursday to help prevent the dam from collapsing further.
An expert from Brunel University in London said the damaged spillway of the dam – designed to release water – could become “fully broken” within hours.
This could lead to “massive flooding” following the heavy rainfall.
Dr Mohammed Heidarzadeh, assistant professor and head of coastal engineering and resilience LAB, said: “Due to heavy rainfall in Whaley Bridge area, the spillway is now broken and a big chunk of its concrete structure is damaged.
“There is a possibility that the spillway could then become fully broken in a few hours.
“If the spillway is fully gone, the embankment dam will be washed away very rapidly, which could cause a massive flood.”
He added that a similar situation occurred at the Orovill dam in California in February 2017.
Rebecca Simpson, one of those evacuated from Whaley Bridge alongside her children, told PA: “This morning at 8.30am I walked over the dam with our dog and it was okay, but it quickly became apparent that the amount of water we had yesterday was too much and was causing a big run-off.
“The sluices were opened mid morning but there’s so much water and it’s now currently raining very hard again. There’s nowhere for it to go, the river has already burst its banks and the ground is sodden.
“I don’t think I would class it as scary, lots of experts on hand and it seemed well managed. The scary part will be what happens next as all services — such as electricity, gas and medical services, and the primary school — are in the direct line of the water.”
However, as long as the core of the reservoir is not damaged, the wall “should be okay”, according to another specialist.
“Within the last few years new valves have been placed in the dam to expedite rapid drawdown in emergencies: presumably, this is happening now,” said Professor Roderick Smith, from Imperial College London.
The former chief scientific adviser for the Department for Transport said the reservoir previously had “issues” with an inadequate valve system which has seen been replaced.
Severe flooding in South Yorkshire in 2007 sparked the evacuation of roughly 700 people around Ulley Reservoir, near Rotherham, over fears its walls could burst due to unprecedented rain and apparent “areas of weakness”.