2012 – Enlarged Cotter Dam Diversion Gate Design – The Challenge and the Test

Frank Nitzsche

This paper highlights the importance of hydraulic diversion control structures during construction of large dams and the value of allocating sufficient resources during project planning and implementation.
The design of the diversion gate for construction of the Enlarged Cotter Dam presented various challenges, including operation for up to 38m head for discharge into a 3m diameter conduit and the need to serve as an upstream concrete form during eventual diversion closure.
The short duration of operation allowed acceptance of increased level of operational risk and a higher level of design uncertainty. The design used generally accepted gate design methods, but no hydraulic modelling. The hydrodynamic forces were estimated using published data. After installation, a 1 in 100 AEP flood event resulted in the gate being subjected to 90% of its design head while operating in conditions close to the maximum design down-pull force. Attempts to raise the gate succeeded only after increasing the hydraulic pressure above the design value.
Keywords: Guard gate design, outlet works, dam, construction.