2013 – Cotter Dam – recovering from major floods during construction

Peter Buchanan, Damian Nott, Martin Weir and John Dymke

The Bulk Water Alliance (BWA) consisting of ACTEW and ACTEW-AGL, GHD, and John
Holland/Abigroup, was formed to deliver the Enlarged Cotter Dam project in Canberra,
ACT. This project consisted of the construction of an 87 m high RCC dam and two
saddle dams, 15 m and 20 m high, to provide additional capacity to the ACT‘s water
supply system. The project is scheduled to be completed in September 2013.
During construction, the dam site was subject to three significant flood events which
affected the construction program. The March 2012 flood, the largest of the three, also
indirectly caused the formation of longitudinal cracks at the top surface of the RCC, when
the dam had reached about 45 m in height.
This paper first looks at the consequence of the flooding on both the design and
construction of the dam; in particular the modification of the diversion strategy and the
impacts on the final dam arrangement. The risk mitigation strategies put in place,
including the construction of a significantly larger diversion conduit through the partially
completed dam, are also discussed. The paper then focusses on the formation of
longitudinal cracks in the dam; the cause of cracking, analysis of the likely extent of
cracking, and the treatment of the cracks to minimise the risk of any significant long-term
impacts on the safety of the dam.
Finally the paper will discuss lessons learned from constructing the Enlarged Cotter Dam
during a period of above average rainfall.