Flood studies and physical model tests demonstrated that the original overflow and spillway facilities at the Ogden Impounding Reservoir were unable to pass the Probable Maximum Flood (PMF) without overtopping onto the main earth embankment with a consequent risk of failure due to the destructive action of out-of-channel flows.
The main purpose of the project was to improve the overflow and spillway facilities at Holden Wood and Ogden Reservoirs allowing PMF flows to be safely routed through the Grane Valley cascade and return the reservoirs to their normal operating levels.
PaSCoE were commissioned by Eric Wright Civil Engineering to provide both permanent and temporary works design.
The main works at Ogden IR comprised the construction of a wedge block spillway on the downstream shoulder towards the centre of the dam. The wedge block spillway is the largest of its type in the UK and is intended to dissipate the energy of the flow by introducing a long cascade. The wedge blocks are design for both hydraulic and structural stability. The underlying embankment has to be stable under all flow and seepage conditions. Filter layers beneath the wedge blocks act to retain the embankment soils however they also possess the potential to provide a preferential failure plane of sliding.
To counteract the potential destabilising sliding force a piled toe restraint was designed by PaSCoE. The laterally loaded piles were designed to Eurocode 7 and their capacity verified by insitu testing.