2001 – Upgrading the Forth River Dams: A Case Study on Risk Based Decision Making

RL Herweynen

The Forth River dams consisting of Cethana (110m), Devils Gate (84m) and Paloona (43m), were constructed between 1964 – 1971. The Population at Risk (PAR) downstream of this cascade system is significant in the event of hypothetical dam failure.

By 1990 a Generalised Method had been fully developed for estimating extreme rainfalls for South East Australia. Using these extreme rainfall estimates, flood estimates were updated for all dams owned by Hydro Tasmania. These estimates indicated that the spillway capacity of the three Forth River dams no longer complied with current practice.

The risk position of these Forth River dams did not comply with the ANCOLD risk based criteria, indicating that some level of upgrade should be considered to reduce the risk associated with flooding. Given the risk position, considerable priority was placed on resolving this issue.

Due to physical constraints within the Cethana Dam site area, it was difficult to upgrade to a “Standards Based” level of upgrade without very high expenditure and imposing additional risk arising from major dam modifications. Instead the ALARP (As Low As Reasonably Practicable) principle was adopted to determine an appropriate level of upgrade, which did not preclude upgrading to a higher standard, should this be necessary at some time in the future.

The spillway upgrade for the three Forth River dams was approved in 1999 and detailed design has commenced with completion of construction planned for 2003. This paper will include discussions on the decision making process, communicating complex dam safety issues to senior management and some interesting details of the design.