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 the 15m 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 (ood 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.
As NCE went to press it remained unclear precisely why so much water had been forced down the smaller spillway. It is known that a wall built hard against the embankment at the spillway inlet collapsed during the ooding but it was unclear whether this was the cause or as a result of the inundation.
The larger spillway is intended to cope with high volumes of overflow water, taking the water away from the embankment toe along a stepped channel. The smaller channel carries lower volumes and also picks up water draining around the structure.
Claydon said that when engineers arrived on site early on Tuesday morning the water was 0.5m above the top of the small spillway. He said investigations into the cause of the inundation would continue but only after the dam had been stabilised.
The emergency services woke around 900 locals early on Tuesday morning and evacuated them after fears that the dam might collapse. The M1 motorway, which passes within 2km of the dam, was also closed between junctions 32 and 34 until Tuesday 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.' Fire Brigade teams have been using these pumps to lower the water level in the reservoir by up to 2m to reduce the load on the structure. The immediate priority was then to reinforce the embankment.
'We have two deliveries of 500T of stone fill on its way to stabilise the toe, ' Claydon said on Tuesday as NCE went to press. 'We will be working through the night to place this material using excavators.' He said that the material - a full nes 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.
Claydon pointed out that the process would have to be carried out with care to ensure that no further destabiilisation was caused. He added that residents would be allowed back into their properties as soon as he was satisfied that the dam was safe but could not give a timetable.
Ulley dam - background:
15m high, earth embankment with puddle clay core;
582,000m 3 capacity reservoir Designed by Lawson and Mansergh and built in 1874 Owned by Rotherham MBC