Colac Basin No. 5

Barwon Water required assistance to undertake the detailed design and documentation for “Basin No. 5”, a 450 ML storage to be located upstream of Colac’s Water Treatment Plant (WTP). Colac Basin No. 5 will increase security of supply capacity for Barwon Water while reducing energy consumption by positioning the basin such that raw water will gravitate from the upstream source to the basin and WTP. The construction value of Colac Basin No. 5 was approximately $5 million.

There were several constraints at the site identified for Colac Basin No. 5, including:

Close collaboration with Barwon Water and Coffey Geosciences was undertaken to overcome these constraints and develop an innovative solution which would maintain functional requirements, meet customer preferences and maintain long term constructability and operability.

To mitigate groundwater problems, an underdrain system was developed and a sloped storage base was adopted to allow construction above the groundwater table. A permeable HDPE liner was used to avoid cracking when the storage operates at lower levels and a geosynthetic clay liner was used to avoid excessive seepage into the underlying sand lenses.

The storage outlet included a multi-level draw-off system to allow operators to draw raw water from the best quality “band” and inlet/outlet pipe-work was configured to allow the WTP operators to draw water from the new and/or existing basins while filling either or both of the storages.

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