WET News

8 WET NEWS APRIL2014 The spillway improvements to the Grane Valley Reservoirs involved the use of around 1,300 pre-cast concrete wedge-shaped blocks, each weighing 1.3 tonnes. The use of a wedge block spillway design also reduced the excavation required for the stilling base at the foot of the spillway Images: Suave Air Photos Spillway improvement in the interests of safety • A £4.8M project to improve the overflow and spillway facilities at the Grane Valley Reser- voirs for United Utilities allows the safe routeing of PMF flows, while ena- bling the reservoirs to be returned to full oper- ating capacity. Gavin Hulme spills the beans. TECHKNOW • This is the first major wedge block spillway scheme in the UK • 1,300 pre-cast concrete wedge-shaped blocks have been used, with each weighing 1.3 tonnes • Wedge-shaped 'steps' of the spillway dissipate water energy • Enables smaller stilling base • Minimum excavation of the embankment was required to provide a base for the sections • The wedge block spillway lined the existing masonry spillways • Materials were re-used where possible to reduce costs and waste MOANS & GROANS 1 Business critical programme due to planned Haweswater Aqueduct outage 2 The incumbent overflows and spillway facilities at Ogden IR and Holden Wood were insufficient to meet PMF requirements without overtopping onto the embankments 3 Holden Wood Reservoir needed additional reinforcement T he Grane Valley in Ha- slingden, Lancashire, is home to a cascade of three reservoirs, retained by earth em- bankments. The lowest lying of these, Holden Wood Reservoir, is now used only for compensation releases from the other two res- ervoirs but remains the respon- sibility of United Utilities (UU) for safety and maintenance. Calf Hey and Ogden Impounding Reservoirs (IR), meanwhile, are used to supply raw water to the nearby Hasling- den Grane Water Treatment Works (WTW), with Calf Hey IR sited upstream and to the west of Ogden IR, feeding water into the lower reservoir which then feeds the treatment works. All three facilities were sub- ject to a statutory inspection under section 10 of the Reser- voirs Act (1975) and, while the overflow arrangements at Calf Hey were considered suitable for the predicted probable maxi- mum flood (PMF) flows, flood studies and physical model test- ing demonstrated that the incumbent overflow and spill- way facilities at both Ogden IR and Holden Wood were insuffi- cient to meet PMF requirements without overtopping onto the main earth embankments. Safety concerns This presented a risk of embank- ment failure and UU commis- sioned a spillway improvement scheme in response to the inspec- tion's In the Interests of Safety recommendations. Reductions in operating lev- els to address the PROJECT SPECS • First major wedge block spillway scheme in the UK • 1,300 pre-cast concrete wedge- shaped blocks used, each weighing 1.3 tonnes • Wedge-shaped 'steps' of the spillway dissipate water energy • Enables smaller stilling base VERDICT • EWCE successfully value engineered the scheme to reduce the project cost • Sustainable approach and local supply chain delivered tangible carbon savings • Project delivered on time • Spillway improvements will protect against flooding and embankment erosion ONSITE RESERVOIR MAINTENANCE safety concerns about spillway integrity were required until completion of the spillway improvement programme, and a proposed outage of the Haweswater Aqueduct in Octo- ber 2013 meant that Ogden IR had to be at full capacity by this date. To address this business crit- ical deadline, the improvements at Ogden IR and Holden Wood were carried out concurrently as a single 64-week programme. Wedge block UU decided to utilise a wedge block spillway, which had never before been used on a major impounding reservoir in the UK. The approach had several major advantages in terms of fast tracking project delivery, reducing project costs and mini- mising excavation of the embankment to reduce the risk of embankment instability. The scheme involved the use of approximately 1,300 pre-cast concrete wedge-shaped blocks, each weighing 1.3 tonnes, which were cast to precise specifica- tion off site and delivered to site on an as needed basis. Minimal excavation of the embankment was required to provide a base for the pre-cast concrete sections, which were laid in a staggered joint pattern by the Eric Wright Civil Engi- neering (EWCE) team to maxim- ise the integrity of the spillway. The use of a wedge block spillway design also reduced the excavation required for the still- ing base at the foot of the spill- way as the wedge-shaped steps of the spillway dissipate the

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