As part of the Woolgoolga to Ballina Pacific Highway Upgrade project, Transport for NSW (TfNSW) were required to undertake extensive revegetation.
GeoLINK’s team of environmental scientists and engineers were engaged in several revegetation roles including:
From a revegetation perspective, the Woolgoolga to Ballina project was considered an overall success. Several design, planning and documentation system were developed to enable successful revegetation outcomes, including prescriptive quality assurance specifications.
Although this meant some additional requirements on the part of contractors responsible for implementation (including GeoLINK), they were valuable tools for this and future projects. By and large they enabled the successful regeneration of the project. A great result in the face of persistent drought conditions, including the devastating 2019-2020 bushfires.
There are always lessons to be learnt from a project of this calibre and GeoLINK is open to improving how we do things. The GeoLINK team identified vital lessons learnt and key factors in the delivery of the project’s revegetation as we look to apply these to future projects.
Key factors enabling the success of linear infrastructure project revegetation
1. Topsoil Management
Prior to the commencement of construction, contractors undertook topsoil testing to determine:
These recommendations were provided by the soil scientist engaged for the project using knowledge of the local topography, soil properties and environmental conditions. These are essential in maintaining topsoil health for the duration of the project.
Maintaining topsoil health and viability is a crucial consideration in achieving successful revegetation that will inevitably save time and money. Topsoil’s physical, chemical, and biological characteristics must be maintained and where possible, enhanced to achieve optimum results.
Appropriate stockpiling methods are crucial in preserving topsoil characteristics and viability. Stockpiling must be undertaken in a way that prevents compaction and maintains aerobic conditions to the greatest extent possible.
Practically, this means forming stockpiles into long low mounds and limiting the total height to 2 metres with a preferred upper limit of 1.5 metres. To further maintain biological activity, prevent weed colonisation and avoid erosion topsoil, stockpiles should be sown with a sterile cover crop and be placed outside areas of pooling water.
Unfortunately, topsoil management faced a few challenges on the Woolgoolga to Ballina project with
a) Limited space which meant the optimum stockpiling methods could not always be achieved.
b) Conflicting priorities and hurdles with communication meant there were instances where optimum stockpiling could have been achieved but wasn’t. This led to increased requirements for weed suppression, higher rates of amelioration application, slow and stunted vegetation establishment, and the need for multiple re-sowing efforts.
The final step in topsoil management is the placement at the point of use. Key factors to improve revegetation outcomes include:
2. Appropriate seed mixes
The Project involved considerable planning to ensure revegetation of batters and other areas of disturbance reflected the native vegetation immediately adjacent.
Seed mixes included multiple variations of dry sclerophyll, wet sclerophyll, forested wetland, grassy woodland, health land and natives grass vegetation communities along with temporary cover crop, pasture grass and vegetated drainage channel mixes.
Aside from the biodiversity and habitat outcomes, this ensured the vegetation type most adapted to the prevailing conditions was introduced for each length of works. Benefits were gained from using the natural seed bank and seed exchange from adjacent vegetation was capitalised on.
Additionally, soil improvements were achieved via the introduction of nitrogen fixing plants in two different uses:
3. Ameliorate Application
Appropriate topsoil and subsoil amelioration was a key factor in successful seed strike, plant growth and establishment. While amelioration typically involved mineral fertiliser it also relied on the introduction of both calcium and carbon.
The crucial factor for success however was the broad use of microbial inoculants. The introduction of beneficial microbes (bacteria and fungi) into the soil enhances revegetation success by increasing the plant availability of nutrients through symbiosis in the rhizosphere. This improves water infiltration and retention along with increasing soil carbon through atmospheric carbon sequestration.
4. Regular quantitative monitoring
To track and verify revegetation targets, the Woolgoolga to Ballina project opted to use a modified version of the “Landscape Function Analysis” (LFA) revegetation monitoring methodology. This method was originally developed by the CSIRO for monitoring rangeland and mine site rehabilitation.
A key strength of the LFA method is the calculation and output of quantitative data which reduces ambiguity and subjectivity. Numeric scores for a monitoring site’s nutrient cycling, infiltration and stability indicators are calculated with a set target based on a calculated ‘threshold of self-sustainment’ set prior to the commencement of construction.
Once a monitoring site reaches the set target it is deemed as self-sustaining and requires no further monitoring or intervention.
The LFA approach authorised contractors to get a real-time snapshot of the condition of revegetation areas. Additionally, multiple successive monitoring events allowed for the tracking of progress creating early interventions to maintain progress. This is a useful tool in ensuring revegetation is progressing as planned.
Revegetation lessons learnt from the project
Strong planning laid the foundation for success. When the process was followed according to the planning, positive results were achieved in the revegetation of the Woolgoolga to Ballina Pacific Highway Upgrade.
With intense drought experienced over much of the project’s construction period, the outcomes demonstrated the benefits of a well-planned and considered approach to revegetation. While there’s always room for improvement, this project was innovative and is now thought of as a leading example of linear infrastructure project revegetation.
Consistency with the processes in addition to using best practice approach ensures there were key lessons learnt.
The process developed and implemented for the Woolgoolga to Ballina project, provides a great foundation for the revegetation of future linear infrastructure projects.