Queen Mary and King George V emergency draw down schemes

Thames Water owns a large number of non-impounding reservoirs. These have unusually long embankments with narrow clay cores and gravel shoulders, which could be susceptible to piping failure and erosion. They are typically located in urban areas. Thames Water has therefore reviewed its policy regarding emergency draw down capacity and started an improvement programme on the most critical reservoirs. The Queen Mary and King George V reservoirs are two of these critical non-impounding reservoirs with continuous embankment lengths of 6·3 km and 6·5 km and storage volumes of 30 Mm³ and 12 Mm³ respectively. Following the statutory inspection of these reservoirs, a safety recommendation was made that required a significant increase to the existing emergency draw down capacity of each reservoir. This required Queen Mary reservoir to achieve 0·75 m emergency draw down from top water level within 24 h and King George V reservoir to achieve 1 m draw down in 24 h. Studies instigated as a result looked at various options of achieving the draw down, with the final solution involving the design and construction of twin siphon pipes fitted with submerged discharge valves. It is believed that a number of UK reservoirs may require additional emergency draw down capacity over the next few years, as measures in the interest of safety, defined within the terms of the Reservoirs Act 1975. This paper looks at the options considered before arriving at the final solution and discusses challenges in the design and construction of the schemes.