A LEAKING reservoir in Yorkshire was this week being drained to allow defects to be inspected and repaired.
A rapid increase in seepage rates last January flooded a drainage and monitoring system in the dam at Winscar near Holmfirth, forcing water through the ground and prompting the decision to drain.
Inspections have continued during draining. 'The leakage rate decreased when we reached 22m below full level. We concluded the source was a defect in the asphaltic concrete membrane, ' said Yorkshire Water reservoir engineer Jim Claydon.
The asphaltic concrete dam built in 1975 suffered leakage problems when movement occurred after the reservoir's first filling. Patch repairs were carried out up to last December.
The 520m long, 53m high rock fill dam has a membrane of two layers of asphaltic concrete 80mm and 40mm thick. The reservoir has a capacity of 8.3Mm 3and covers 47ha.
Asphaltic concrete contains 12% bitumen instead of cement and consists of limestone graded from dust to 12.5mm aggregate.
Layers should be heat bonded to provide a seal.
The Winscar material is nearing the end of its design life of around 30 years. 'Inspections have revealed localised defects and ageing of the asphaltic concrete which has become more brittle, ' said Claydon. Membrane layers have also delaminated in some areas.
He said that the stability of the dam was not threatened and a contract to repair the entire structure rather than continue patching will be let and completed by the end of the year.
Claydon said the reservoir contains no fish life and that Yorkshire Water is liaising with the Environment Agency over discharges to the River Don.