WET News

AUGUST 2015 WET NEWS 9 projEcT SpEcS • Installation of a new grille at the draw off inlet • Removal of the timber plug from the Neptune shaft, and replace with a penstock operable from the dam crest • Installation of a new outlet pipe • Installation of an energy dissipation chamber • Installation of a new outlet headwall and channel T he Canal and River Trust (CRT) is responsible for 72 'large raised reservoirs' in England. These reservoirs have an average age of 195 years, ranging from Pebley Reservoir (238 years old), to the 123-year old Winterburn Reservoir. Pebley Reservoir, situated between Sheffield and Worksop to the east of the M1 motorway in north Derbyshire, was built in 1776 to supply water to the Chesterfield Canal via a feed into Harthill Reservoir. It is classified as a Category A (where a breach could endan- ger lives in a community) reser- voir as defined in the 3rd edition of Floods and Reservoir Safety. The reservoir embank- ment dam reaches a maximum height of 8.5m and has an average crest width of 4m along its 160m length. The reservoir impounds 280,000m 3 of water, which was confirmed by a hydrographic survey in 2008. Water control is via a spillway channel at the eastern end of the dam and via a low level timber and stone box culvert from a Neptune Sha— near the centre of the dam. The Reservoirs Act 1975 requires a Section 10 inspection of the reservoir every ten years, and the 2010 inspection Neptune sha improvements auger well at Pebley reservoir • Inspection of the Canal and River Trust's Pebley Reservoir led to improvements in the interests of safety. ONSITE ASSeT mANAGemeNT Construction of the coffer dam begins at the inlet channel contained recommendations in the interests of safety. These included the requirement to repair or replace a timber and stone box culvert within four years. Following the Section 10 inspection, the Trust's frame- work consultant, Hyder, and contractor, Kier Services, were contracted to develop an appropriate design solution and subsequent implementa- tion on site. The inlet structure of the outlet consists of a circular 840mm diameter brick barrel inlet culvert upstream of the 'Neptune' sha— with a sub- merged protective grille and approach channel inside the reservoir at low level. The Neptune sha— is located within the crest of the embank- ment and comprises a 1.04m diameter round brickwork / masonry sha—, approximately 5m deep. Within the sha—, flow had been controlled histori- cally with an oak plug valve with a worm and wheel gear, driving the rack and pinion gearing located in a kiosk at the top of the sha—. Downstream of the Neptune sha— there is a timber and stone box draw off culvert (300mm x 460mm) that runs to the base of the downstream TEchkNoW • A 610mm ID pipe was inserted within the culvert to give a more robust solution • The existing grille was replaced with a new Fibre Reinforced Polymer • An auger was the most efficient way of installing the new outlet pipe with minimal disruption to the embankment • The FRP grille is submerged, so UV degradation is expected to be minimal NEED To kNoW 1 The reservoir was drawn down to 10% between June and September in before construction could start 2 The maximum flow to be accommodated through the auger pipe / liner was 400l/s 3 The reservoir can continue to operate successfully into its third century of use ThE VErDIcT • The scheme was completed at a capital cost of £350,000, and commissioned in January 2013. It has successfully replaced the existing timber plug and timber and stone culvert outlet structure, with a new inlet grille, penstock, 355mm diameter auger outlet pipe, energy dissipa- tion chamber and outlet channel.

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