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Paul Southcott,Suraj Neupane and David Krushka

TasWater owns and operates the water supply in Queenstown on the west coast of Tasmania. Anew water treatment plant was constructed downstream from one of the seven small dams that made up the original supply system, making the remaining six dams redundant.Two of these dams hada very high annual probability of failure and unacceptable societal life and financial risk due to their poor condition.Both dams required urgent attention (upgrade) to retain them as a lasting asset and legacy for the community or decommissioning to create a new ecological legacy.Roaring Meg Dam (6m high with a 9ML capacity) was constructed on Roaring Meg Creek around 1963. The Cutten Street Dam No 3 (10m high with a 2.4ML capacity) was constructed on Reservoir Creek around 1902to supply water to a growing mining community and had been in use since then. From a heritage perspective, the dam had some value as a timber crib and rockfill dam and its historical context as a key factor in the development of the town.There is limited guidance in the ANCOLD (2003) Dam Safety Management Guidelines on decommissioning and a process had to be developed in cooperation with the Regulator in this relatively new area of dam engineering. Detailed design of the decommissioning including diversion work during decommissioning, channel design to align with the original creek to help restore its ecological function and rehabilitation work on the exposed reservoir soils to stabilise them were undertaken. Aboriginal and historic heritage studies, flora & fauna studies and fluvio-geomorphological study at the dam sites were also undertaken to ensure that the decommissioning work did not interfere with the heritage, threatened species and riparian processes. The community were consulted to ensure acceptance of the changes to their town. Dam safety emergency management plans for the decommissioning of these dams was were also prepared. A significant issue in the decommissioning work was frequent and high rainfall due to the location of these dams on the west coast of Tasmania. The entire dam removal work had to be planned within the window of dry weather or very little rainfall. This paper presents the process, activities and lessons learned in successfully decommissioning these dams,to eliminate the unacceptably high risks posed by these dams and to restore the normal riparian processes.The general approach adopted for this project has applicability for other damsandis proposed as a starting point for an ANCOLD practice note in this area