SPILLWAY FAILURE which caused last week's near catastrophic embankment scour at Ulley Dam near Rotherham was a repeat of a similar incident at Boltby Dam in North Yorkshire two years ago.
Although investigations into the precise chain of events that caused the Ulley spillway to disintegrate are ongoing, engineers have identified similar design ws to those highlighted at Boltby in 2005.
Boltby's 20m high earth embankment was extensively damaged in June 2005 when a flash flood exceeding a one-in10,000 year event ripped down its spillway channel.
Investigations at the time concluded that the dam could only be safely reopened by lowering the overflow level 8m. Works were completed in March.
One senior water engineer involved in the investigation at Ulley told NCE this week that the two situations appeared similar.
Last week, excess water caused a masonry spillway to fail at the Ulley Dam, built in 1847. This failure allowed the huge volume of escaping water to scour large quantities of earth from the toe of the embankment and potentially undermine the stability of the puddle clay core structure (see page 14).
Boltby is smaller than Ulley, holding back just 121,000m 3 of water compared to Ulley, which contains 582,000m 3. Both were built at around the same time and have overflow spillways running across the downstream embankment The senior engineer told NCE that the 'unusual design' of spillway used on the dams - in which the water is channelled over the potentially vulnerable embankment toe - was awed, because it was unable to contain sufficient volumes of water.
'There are two suggestions at Ulley: either the water exploited a weakness in the spillway or there was too much water, which came out of its channel.'