Lockyer Creek is a tributary of the Brisbane River and it flows through the Lockyer Valley, the salad bowl of Brisbane. The black earth soil of the valley is fertile but highly erodible, hence creek bank erosion is a major problem for the local government. Recent flood events had caused severe erosion on the banks of Lockyer Creek at Helidon, 100 km west of Brisbane, Australia; the erosion threatens the stability of several houses on its bank, the adjacent highway and railway.

The following is the summary extracted from the report submitted to Gatton Shire Council by the Consultancy firm Water Studies Pty Ltd, for the stabilisation of the banks of the Lockyer Creek at Helidon. The values of different methods are expressed in Australian Dollars.

After surveying the site and studying the hydrology, geology and topography of the region and the site, Water Studies Pty Ltd proposed the following options and recommendations:

Six options are available:

1. Establishment of native vegetation
2. Use of Vetiver grass
3. Placement of rock riprap
4. Construction of gabion basket/rock mattresses
5. Construction of rock groyne and
6. Construction of concrete pile walls

Pro and con for various options

Native vegetation: Native vegetation is preferred but shear stress analysis indicates that native vegetation is not a long-term erosion control measure because failures are expected to occur during flood events.

Vetiver grass: Vetiver grass has a massive, deep and finely structured root system that has been used effectively for riverbank and slope stability throughout the Pacific and Asia. Although it is not native, a sterile variety of vetiver grass has been registered in Queensland as Monto vetiver. Monto vetiver has been used with some success at several locations in Queensland. DNR literature shows that vetiver grass offers much better resistance to erosion and slope stability than native vegetation

Rock riprap: Coarse rocks of 0.8m average in diameter, if installed correctly has the capacity to withstand shear stress higher than that would be expected in this section of the river. It is effective but expensive.

Gabion basket/Rock mattresses: this is wire mesh cages containing well graded rocks. They are effective but very expensive.

Rock groyne: These are walls of rocks placed by excavator against the bank, perpendicular to the flow of the water. They are effective but very expensive and difficult to build

Concrete pile walls: Walls built with contiguous concrete piles which need to be 4m long for zone 1 and 20m long for zone 2,.they are effective but very expensive to build and maintain.

 Recommendations

According to the cost estimate above, the most suitable option to control erosion of the main channel along Zone 1 is Vetiver Grass. Vetiver grass is relatively inexpensive and provides low environmental impacts and medium to long-term sustainability. Vetiver grass cost only about 25% of the next suitable option (rock riprap). Thus it is more likely to be more economical to repair the bank again with vetiver, if it failed, rather than adopting a more expensive option

For Zone 2, dumped riprap appears to be the preferable option. Although it is more expensive than vetiver and native vegetation but results in low environmental impacts and provides long-term sustainability.

Selection of Appropriate Packages

Based on the options identified and other factors discussed above, five packages were short-listed to resolve the issues that have to be addressed.

• Package A involves the protection of the bed and banks of the main channel against further erosion using Vetiver Grass for Zone 1 and a dumped rip rap wall for Zone 2. For this option, the bank is allowed to batter back naturally.

• Package B involves the same bed and bank protection works as for Option 1. Additional bank stabilisation works are undertaken by establishing native vegetation in Zone 1 and constructing a gravity retaining wall in Zone 2.

• Package C involves the re-alignment of the creek in the study area.

• Package D involves the relocation of 6 dwellings on Railway Street. Vetiver Grass bank erosion protection and native vegetation bank stabilisation will be provided in Zone 1.

• Package E involves the relocation of the 6 dwellings on Railway Street. No bank protection will be provided under this option.

Summary of the Impacts and Costs Associated with Selection Packages

* including the cost of relocations of 6 dwellings

Based on costs, benefits and impacts shown in the above table, the most suitable option for the study area appears to be Package D. Due to the high capital cost of providing sufficient erosion protection and bank stabilisation, this option involves the relocation of the affected properties in Zone 2 to an appropriate location in Helidon. In Zone 1, it is recommended that Vetiver Grass be established along the banks of the main channel to provide erosion protection and native vegetation be established behind it to provide long-term bank stability. The total cost of this option is estimated at about $193,500.

ADOPTION

The council has adopted a modified version of package D using only vetiver grass not native species for both bank and channel stabilisation. The bulk of the $193 500 was spent on the relocation of the 6 houses. Vetiver planting is now underway and the actual cost is only $16 830.