Surveys and temporary works in spillway construction at Windleden Upper reservoir

Windleden Upper reservoir in South Yorkshire is an earth embankment dam located within the Peak District National Park. It was created by impounding a small tributary stream of the River Don. Following recent recommendations made in the interests of safety, a hydraulic model of the spillway was constructed for the probable maximum flood outflow of 40 m³/s. The model showed that the existing spillway did not have sufficient capacity in the tumblebay area, which could pose a risk to reservoir safety. This paper describes the investigation, construction and temporary works required to deliver the project. Comprehensive geotechnical analysis was carried out on the existing tumblebay walls, using finite analysis methods, to calculate their stability during excavation construction works. Cores were taken through the existing walls to establish the construction method and trial pits were carried out to understand where these are founded. Access was a major issue on this site, owing to the narrow width of the embankment and concerns about the stability of the wave wall. Extensive three-dimensional modelling was carried out to provide a solution for a temporary access road during works to the spillway.