Author: Gregg Barker.
TasWater (formerly Southern Water) is currently commissioning the new Duckhole Rivulet Dam which is located in south-eastern Tasmania, approximately 20 km north of Hobart. The purpose of the dam is to store recycled water for use by irrigators in the Clarence Recycled Water Scheme. The capacity of the storage is 1049 ML at full supply level.
The works are owned and operated by TasWater, designed by Entura and were constructed by Andrew Walter Constructions. Construction occurred from October 2012 to April 2013.
The dam is a 14.1 m high earthfill dam with a chimney filter and downstream horizontal filter blanket. The dam is founded wholly on dolerite. Earthfill, rip rap and transition materials were sourced from the site and filter materials were imported.
The dam is supplied via gravity by a 600 mm diameter transfer pipeline. The same pipeline is used for filling the dam during the non-irrigation season and for releases from the dam during the irrigation season. The pipeline bifurcates to twin 450 mm diameter conduits along the dam foundations which are fully encased in reinforced concrete. There are twin inlets/outlets within the storage at different levels for operational flexibility. All controls for the outlet works are located downstream. The works are currently operated locally but provision has been made for the installation of a control system to permit remote operation.
The works were also subject to stringent environmental and cultural heritage permit conditions, and these needed to be incorporated into the design and construction of the works. One of these conditions was to have no discharge from the works for a 1:10 AEP flood event. In order to achieve this, a system of bypass channels were constructed around the storage to divert much of the catchment run-off and the spillway crest level was raised 350 mm above full supply level to provide a buffer storage.
The spillway is located on a saddle just beyond the right abutment. The design spillway flow is only 37.2 m3/s (1:10,000 AEP flood event); however the spillway was designed to be 65 m wide to minimise flood rise and maximise storage capacity. The design ensures that frequent flood events will not result in backwater across the road at the upstream end of the storage. The spillway is designed to operate very infrequently and is lined for much of its length with reinforced topsoil as a cost-effective option.
First filling commenced in late June/early July 2013 and the works appear to be performing very well. The site successfully withstood a significant rainfall event while much of the grass was still establishing. The storage level is currently at around 50%