Upgrading of the bottom outlet facilities at Abberton reservoir

Essex is one of the driest parts of England and there is a gap in the availability and demand for water. The Abberton scheme is being undertaken to work towards alleviating this problem. Part of the scheme involves increasing the storage capacity at Abberton reservoir, by raising the full supply level of the reservoir by 3·2 m. The reservoir is owned by Essex and Suffolk Water who are part of Northumbrian Water Limited. The project is large in scope and there are many interesting aspects including raising the main dam while maintaining an operational reservoir, modification of the existing drawdown facilities, construction of several col dams, modification to the existing B1026 causeway embankment and the upgrading of the offtake pumping station. This paper focuses on the modification of the bottom outlet located in the valve tower, on the upstream slope of the main dam. The paper describes the arrangement and condition of the existing bottom outlet and discusses its capacity, for both existing and future conditions. It reviews the drawdown capacity at the reservoir and compares it to standards commonly applied elsewhere in the UK. A description is given of some of the studies that were undertaken during design development to explore how the drawdown capability might be improved. Construction is currently underway and the paper concludes with a summary of progress to date.