Author: Bob Wark.
The town of Carnarvon is protected from major flooding from the Gascoyne River by an extensive levee bank system. However, the surrounding horticultural district and the Kingsford area have minimal flood protection and are prone to severe flood damage. Major flooding has occurred regularly in recent decades, most recently in December 2010/January 2011. This flood was the highest recorded and caused extensive damage throughout the region. The flood damage bill was estimated to be approximately $90 million.
This flood damage estimate does not include the wider regional impacts as well as the flow-on effects throughout the Carnarvon and the Western Australian community and transport losses to major mining traffic through to the Pilbara. There are major sections of the North West Coastal Highway in the Carnarvon area significantly affected by flooding and its level of serviceability is critical as it is the main coastal route that services the major mining and resource sectors in the Pilbara.
Carnarvon’s horticultural industry comprises 2000 hectares that are zoned intensive horticulture with up to 1200 hectares planted at any one time. There are 170 plantations that play an important role in the stability of the Gascoyne economy providing 60-70% of Perth’s winter fruit and vegetables and it is valued at $80M per year.
The Department of Water has led a State Government Inter Agency Working Group to develop and implement a plan to provide flood mitigation benefits to the Carnarvon horticultural district. The chosen flood mitigation works comprises of five levees with a total length of ~17km.
Main Roads WA managed the design and project managed the construction of the works on behalf of the Department of Regional Development (State funding body), Department of Infrastructure and Regional Development (Commonwealth funding body), Regional Department of Lands (asset manager and owner on behalf of the Western Australian State Government), Department of Water (designer) and Shire of Carnarvon (project beneficiary). Construction was via a Program Alliance Arrangement with each of the five levees forming a discrete Work Package.
Funding for the Project has been provided by:
· $46.84 million – State Government with $45 million being sourced through Royalties for Regions.
· $2.0 million of this funding is quarantined for the ongoing maintenance of the new levees
· $15 million – Federal Government through its Regional Development Australia Fund
Due to the urgency to commence construction as early as possible, ongoing design reviews, funding uncertainties and outstanding approvals a Program Alliance approach was chosen. A major benefit of this approach is the ability to vary the project scope, in particular to reflect any funding constraints. The participants in the construction Program Alliance, known as Browns Range Alliance (BRA), are Main Roads WA and MACA Civil Pty Ltd (MACA).
The project consists of the construction of five levees, as five discrete Work Packages, adjacent to Carnarvon’s horticultural district (refer Figure 1). The levees are:
|Six Mile Creek Levee – 0.9km long & ~2.5m high
|Lawson Street Levee – 4.8km long & ~4.0m high
|Nickol Bay Levee – 5.4km long & ~5.0m high
|South River Road Levee – 4.6km long & ~1.5m high
|McGlades Road Levee – 1.6km long & ~2.5m high
The project commenced in late 2012 and has been progressing well, with the first 3 work packages reaching practical completion. However while the river flow in Photograph 4 was a welcome relief to the irrigators who had been in a drought since the floods of 2010/2011, work on the last work package (McGlades Road Levee) has been delayed due to the disruption of access across the river. However, the complete project remains on schedule to be complete by mid-2014.