Spillway investigation and emergency repairs at Lower Carno reservoir

Lower Carno reservoir sits above Ebbw Vale in South Wales and provides raw water for supply to the local district. The reservoir was inspected in 2015 under the Reservoirs Act, 1975, resulting in various measures in the interests of safety. One of these measures was to take action to minimise uplift pressure beneath the masonry spillway chute. Investigations carried out in 2018 assessed the condition of the concrete beneath the masonry chute and identified the source of water upwelling in the channel. The concrete bedding beneath the channel floor had completely disintegrated, although the wall backing concrete was relatively intact. Reviewing the hydraulic loading on the structure identified that, even at moderate flows, the risk of plucking of masonry blocks and erosion of the embankment mitre posed a significant and credible threat to the safety of the reservoir. Hence the Undertaker, in conjunction with the qualified civil engineer (QCE), decided to drawdown the reservoir so that the spillway did not operate until works to safeguard the structure had been implemented. The paper describes the investigations, reservoir drawdown, dam safety management plan, key aspects of the spillway design and construction and wider implications for masonry chutes located on embankment dams.