Emergency services have been working with high volume pumps (HVPs) to withdraw some of the floodwaters from the reservoir since shortly after midnight, when an alert was issued for evacuation of local people. The reservoir is about 6.5km from the town of Rotherham and is about 130 years old.
However, there has been some erosion on the downstream slope of the south end of the grass embankment, close to where the side steps out as a berm. The water had passed along the overflow after the pump rate from the reservoir exceeded the pooling capacity at the head of the main spill channel, itself relatively small. The HPVs had been pumping the water past the dam’s small sluice spillway and directly into the head pool of the spill channel.
Following the damage, the emergency services reduced the flow rates to keep the discharge within the stepped spill channel, it was reported. But, according to the emergency services the threat to the dam is still being treated as critical.
The approximately 200m long dam is a bermed embankment with a maximum base width estimated at about 100m, narrowing to approximately 25-30m at the ends. The dam sits close to the A618 road from which the emergency services lifted the HVPs into place. Behind the dam is a dog-legged shaped reservoir of approximately 900m in length, narrowing sharply at the upstream sides, which is up to 10m deep in places and has a normal surface area of just over 12ha.
By last night, the impounded water had reached dangerous levels and posed an overtopping risk to the dam, which only has a small side spillway at the south end. The emergency services called on 10 HPVs and the main authority, South Yorkshire, called on support from six neighbouring regions to help tackle the dam and flood threat.
Concerns over the stability of the dam near Sheffield increased after days of torrential rain in central England, causing flooding in nearby cities, towns and roads. Around 300 homes have been evacuated in the Catliffe, Canklow, Whiston and Treeton areas.
As a precaution, part of the nearby M1 motorway had been closed should the dam fail. An elevated stretch of the motorway already has flood waters at an embankment toe and the level would have raised with any rush of water from Ulley. Another area that could be at risk from a breach is the Brinsworth substation, which supplies electricity to Sheffield and surrounding areas.
The dam and reservoir were built in the 1870s to supply drinking water to the town of Rotherham. By the early 1980s the reservoir was no longer needed for water supplies and was kept on standby by the then owner, Yorkshire Water Authority. Ownership passed to Rotherham Metropolitan Borough Council and in 1986 the area was designated Ulley Country Park.
