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.
Part of the temporary works involved the design of a 5m high sheet pile wall to retain the existing soft to firm Clays to facilitate the construction of the new stilling basin reinforced concrete channels. The design was complicated by the need for the sheet pile wall to restrain a concrete thrust block for the new water main located immediately behind the sheet piles.
In addition a means of conveying the continuous scour tunnel water flows was required to the front of the sheet piles. A number of methods were investigated and a cost effective innovative solution was devised using visqueen and plywood to form an open channel of sufficient cross sectional area and hydraulic capacity.