The owner of the Toddbrook Reservoir in Derbyshire has said it is still too early to say how the reservoir came close to collapse and pledged that a full investigation will follow the emergency response.
Transport Network contacted environment department Defra about a wider national review of older dams and was told it was too early for government to comment on this.
Work to reduce water levels in the reservoir continued on Monday morning with the aim of allowing residents back into the nearby town of Whaley Bridge.
The spillway the reservoir was damaged last week after heavy rain and has been at risk of collapse. Whaley Bridge, which lies immediately to the east of the reservoir dam was evacuated.
The Canal and River Trust, which owns the reservoir, said the damage to the spillway – a structure that allows floodwater to pass over, through or around a reservoir when it is full – occurred after a period of very heavy rainfall last Wednesday (31 July).
It said that it was ‘still too early to say how this has happened and a full investigation will follow the emergency response’ but that the intense rainfall ‘is thought to be a significant contributory factor’.
The Trust added: ‘Our engineers are on site and will commence with surveying the dam once it is safe to do so.’
In a statement, the Trust said the last detailed expert inspection of the reservoir ‘took place by highly trained reservoir engineers in November 2018’. It said this was an independent 10-year inspection, with the reservoir also inspected annually and the dam visually inspected twice weekly by Canal and River Trust operatives. It said the reservoir was built in 1831.
Professor Roderick Smith of Imperial College London told ITV News: ‘Extreme weather events mean that there is increasing unease about the safety of older dams: particularly the need to release excess water safely and easily.’
The Trust said on Sunday evening that it was continuing to stabilise the spillway by pouring grout around the 525 one tonne bags of aggregate that were dropped by an RAF Chinook helicopter to increase the force applied to the dam.
The Environment Agency said it was continuing to monitor the flood risk closely as water was both released from and diverted away from the reservoir.
Derbyshire Police said on Sunday that once a level has been reached and maintained that allows engineers to view the damage to the wall ‘a decision will then be able to be made as to the situation with regards to residents and businesses being able to re-enter Whaley Bridge’.
The BBC said on Monday morning that the reservoir is 48% full and said to be 'relatively stable'.
Derbyshire deputy chief fire officer Gavin Tomlinson said: 'If the calculations are right we should be able to get below the level of the damage on the dam wall which will allow the engineers and the specialists to assess it.
'As soon as we get the water level down to a safe level, which is around 25% of the contents of the dam, then the emergency phase is over and then the contractors can look at the repairing of the dam wall.'