Unexpected inundation of a service spillway on the Ulley Dam between Rotherham and Sheffield during this week's torrential rainfall has been identified as the cause of severe structural damage to the earth embankment.
Scour at the toe of the15m high structure prompted engineers from Rotherham Metropolitan Borough Council to take emergency action to prevent collapse after a 20m long by 6m deep hole was cut into the embankment face. 'One of the spillways was overtopped and (flood water) scoured a hole in the toe of the embankment,' said Jim Claydon, the All Reservoirs Panel Engineer overseeing the dam's stability as part of the emergency response team. 'It's a very friable material. Our initial priority is the emergency stabilisation of the dam embankment,' he added. A fault appeared in the oldest of the three spillways which was exploited by the large volumes of water causing one side to collapse. The water then began to cut deep into the face of the embankment. The two large spillways are intended to cope with high volumes of overflow water while the smaller channel carries lower volumes and also picks up water draining around the structure. Engineers yesterday worked to divert water from the damaged spillway to the secondary spillway so that it may be completely sealed off with concrete later today.At least 11 fire services pumps were emptying 5000litres a minute into the secondary spillway with assistance from 5 12inch diameter (300mm) pumps moving 1500 cubic metres a hour.The water level was still 500mm above the top water level mark this evening after a 650mm drop over night. The aim is to bring the water level to 2m below the top level mark.The emergency services woke around 900 locals early on yesterday morning and evacuated them after fears that the dam might collapse. The M1motorway, which passes within 2km of the dam, was also closed between junctions 32 and 34 until yesterday afternoon as a precaution. Tom Knight, director of Streetpride at RMBC, is directing operations. He told NCE: 'There was a problem with one of the slipways. We are bringing in a huge pumping capacity to get the pressure down and carrying out a temporary repair.' 'We have two deliveries of 500T of stone fill on its way to stabilise the toe,' Claydon said yesterday evening. 'We will be working through the night to place this material using excavators.' He said that the material - a full fines mix of up to 150mm diameter stone - would be placed on a geotextile membrane to separate the fill from the embankment. This would be rolled out over the scour hole.Some 500T was successfully placed last night with another 1500T expected to be complete by the end of the day.Claydon pointed out that the process would have to be carried out with care to ensure that no further destabilisation was caused.