Author: Brian Daws, Senior Project Manager Hydro Tasmania.
Rowallan Dam is part of the Mersey Forth Power Development and is located in the head waters of the Mersey River. Rowallan Dam is a 43m high structure comprising two earth and rockfill embankments either side of a free overflow concrete spillway. The dam was built in the 1960s and impounds Lake Rowallan in the upper reaches of the Mersey River. Lake Rowallan is the main storage for the Mersey Forth Power Development, with a ‘High A’ consequence category.
Rowallan Dam’s reinforced concrete spillway walls were built to the standards of the day, but are considered under-designed with respect to modern design criteria. Significant deformation of these wall panels, which retain the embankment fill, have been monitored since construction. A schedule of works has been commenced to address the stability of the spillway walls and undertake a dewatering capacity upgrade. Associated embankment repairs will follow in 2014/15.
The current work involves the installation of 88 post tensioned anchors on the inside of the spillway ranging from 26mm to 40mm in diameter and up to a depth of 11.8m. New reinforced concrete walls have then been constructed against the existing walls which bring the spillway training walls up to modern standards. At the same time it was decided to raise the height of the spillway training walls by several metres. This was the result of new hydraulic modelling which has been conducted and will form part of potential future flood capacity upgrades that are planned for the dam in 2018.
In addition to this, to enhance the operation of the dam a new dewatering outlet has been installed which increases the size of the discharge valve from 1.2m to 1.8m and a capacity increase from 25 to 45 cumecs. This increase is required to assists with the control of lake storage levels which is a critical part of embankment works that are planned to occur in 2014/15.
The current works on the spillway walls was conducted by Hazell Bros and the dewatering outlet works by Pentair Southern Cross. The works commenced in October 2012 and will be completed in June 2013 for an estimated cost of $6.4M.
Please see photos that follow.