Scour valve replacement at Digley reservoir

Digley reservoir was constructed after the Second World War and completed in 1954. The reservoir has a storage capacity of over three million cubic metres at the top water level. The original pipework arrangement, which included Larner Johnson valves, provided an adequate discharge rate to ensure a minimum of 1·0 m per day drawdown. A statutory inspection of the reservoir was carried out in 2008 in accordance with the Reservoirs Act 1975. Following the inspection, Mott MacDonald Bentley was commissioned by the reservoir owner to undertake works to identify a suitable solution for the scour valve replacement to improve the operability of the valve. The reservoir head of over 40 m and estimated discharge velocity in excess of 10 m/s within the scour pipeline posed limitations to the selection of a suitable valve. The performance of a terminal fixed cone valve retrofitted within the existing stilling basin was modelled as an alternative to an in-line valve replacement. The fixed cone valve has since been installed and it is the first valve of this type for the reservoir owner. This paper describes the valve optioneering process, physical model testing and installation of the fixed cone valve. It also refers to issues working within an operational stilling basin while maintaining provision for the scour facility, and requirements to maintain compensation flow for the duration of the works.