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.

To provide temporary access across the existing combined spillway trapezoidal open channel required the provision of a 16m long steel bridge. The capacity of the bridge for the proposed construction loading was checked and bridge abutments, which comprised filled manhole rings, were designed by PaSCoE. The proposed settlement of the abutments was analysed in order to maintain a clearance between the underside of the bridge beams and the top of the channel walls.

A temporary crane pad of reinforced unbound aggregate was designed and certified by PaSCoE to enable the bridge to be lifted into position using a 500t mobile telescopic crane.