GRFMA Lower Gawler River Works

 

The Lower Gawler River Works were initially scoped to be the development of a parallel flow path including the maintenance of levees, and where necessary, the removal of rubbish and debri from the river channel. During 2006 this work was identified as Strategy 6d, the development of a parallel flow path from just downstream of the Old Port Wakefield Road and the construction and reconstruction of approximately 25 kilometres of levees.

Following the receipt of the Department for Transport Energy and Infrastructure Hydrological Study of the Gawler River Catchment – March 2007 report, a Mapping of the Floodplain project has been undertaken to identify the parts of the floodplain that will remain at risk of flooding, post North Para and South Para works, in a 100 ARI flood event.

Extensive levees have been constructed adjacent to the Gawler River most of which are on private land and most of whch are not constructed to a 'fit for purpose' standard. While these levees provide local protection in the smaller floods Stage One of the Mapping Project shows that they will be totally inefective in a 1 in 100 year flood.

A Roles and Responsibilities Fact Sheet was prepared that provides information about responsibilities to maintain these levees and the Gawler River waterway. (The Fact Sheet titled ‘GRFMA Summer 06-07 Responsibilities.pdf’ can be downloaded from The Scheme page)

The Lower Gawler River Flood Mitigation - Detailed Design Part 1 Report - AWE November 2008 made the following conclusions:

Strategy 6d was a flood mitigation strategy that was developed in 2005 before the hydrology of the North and South Para had been reviewed (refer DTEI, 2007). At that time the 100 ARI mitigated flood at Gawler was estimated to be approximately 150 m3s-1. The hydrological review outlined in DTEI, 2007 resulted in a substantial increase in this flow rate to 643 m3s-1. The revised hydrology and subsequent floodplain mapping resulted in breakouts occurring at Gawler and hence well upstream of Virginia Park. This causes floodplain flows to occur outside the levee system proposed for Strategy 6d and hence the effectiveness of the strategy is much reduced.

It is therefore considered that Strategy 6d on its own has become obsolete in light of the revised catchment hydrology and there would need to be a range of additional structural flood mitigation measures to be put in place before Strategy 6d would be effective.

Other components of this investigation and design work remain valid irrespective of the changed hydrologic modelling regime and could be implemented or relied on.

The flood modelling process undertaken during this investigation examined the effects of vegetation clearance of the Gawler River channel. It was found that vegetation clearance would have a limited effect in increasing channel capacity and was not a viable solution for the flooding issues associated with the Gawler River (including the lower reaches of the Gawler River). Channel geometry, slope and stream sinuosity were found to be more significant

A copy of the Part 1 report can be downloaded from this page. Appendix C of that report an Aboriginal and Cultural Heritage Survey of the Lower Gawler River Flood Mitigation and Riparian Rehabilitation Works, Virgina, SA - Kaurna Report has been abridged to comply with the provisions of the Aboriginal Heritage Act 1988.

GRFMA LGR Flood Mitigation Design Part 1 v2 web 20081128(5436 kb)

After Hours Contact Call 08 8522 9211
 
43 High Street, Gawler East SA 5118
Tel: 08 8522 9211
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