Revegetation of highway projects is typically undertaken with planting out and seeding of large areas of disturbed land. This can be an expensive and sometimes not entirely successful process. Therefore, as the Glenugie upgrade is located almost entirely within a dense native forest, the use of an unconventional and innovative revegetation technique was proposed. This approach adopted a natural process of regeneration harnessing the resources that are inherent in and adjacent to the road corridor.
The natural regeneration process developed for the Glenugie project was designed to mitigate environmental impacts on land disturbed by road construction works using an environmentally sustainable, time-efficient and cost-effective process. The approach aims to restore native ecosystems by recreating conditions conducive to the recruitment and successful establishment of indigenous trees, shrubs and groundcovers.
A critical component of the approach is the recovery and reuse of site materials, including seed, soil and mulched material from cleared vegetation. By carefully stripping and stockpiling site topsoils and mulched vegetation, we were able to reconstruct the natural soil profile to preserve not just the fertility and seedbank of the existing topsoil, but the depth and formation of subsoils required to support the plant communities that are characteristic of the locality. The mulched site vegetation provided not only a valuable additional seed source but, by blending with the topsoil, an essential and highly effective erosion protection measure in all areas with exposed soil, especially on steep batters.
Glenugie is rapidly becoming the exemplar project for the Roads and Maritime Services to be adopted in future projects where native forests adjoin road corridors. It has been successful not only in the establishment of indigenous local tree, shrub and groundcover species, but also very successful in restricting the growth of weeds, maintaining soil stability and therefore protecting water quality in the adjoining watercourses. Not only did it cost less to construct than the traditional revegetation process, but it has minimised the ongoing maintenance works and the associated costs. As a result, RMS has received no critical comments from agencies or the public on this project, only compliments on the success of the site revegetation.
The Glenugie upgrade, which is approximately 7 km long, is located about 14 km south of Grafton and 68 km north of Coffs Harbour. It forms part of the larger Woolgoolga-to-Ballina Pacific Highway upgrade on the NSW North Coast. The project involved the upgrade of the existing two-lane highway to a four-lane divided highway. The Glenugie project is unusual in that it runs almost entirely within the dense Glenugie State Forest, which provided the opportunity of working with the natural forest ecosystem processes. The enclosed nature of the forest means there is an abundance of local seed, allowing for native forest regeneration and establishment to occur naturally. There is also the added benefit of a limited extent of weed species in the area.
The development of an alternative revegetation process came about from a close collaboration between the landscape architects and the soil scientists. This collaboration was substantially assisted by the support and enthusiasm of the Glenugie Upgrade Alliance: RMS, Arupand Macmahon Holdings Ltd.
The Alliance team embraced the alternative revegetation approach and set in place a process for the successful implementation of the revegetation works from the outset of construction through to the completion of the works.
The approach of encouraging natural processes to rehabilitate the site progressively during the construction process required very careful control and management of all of the site materials. Utilising the indigenous seedbank that occurs in the stripped topsoil and mulched vegetation and incorporating the wood mulch into the soil as a stormwater erosion control and weed inhibitor required these materials to be separately stockpiled and protected throughout the construction process to avoid damaging this fragile resource.
This approach to regenerating disturbed sites demonstrates the powerful and beneficial processes of natural regeneration within forest / woodland ecosystems. The benefits of this approach are:
- reuse of the available site resources without the need to import new materials from outside the site
- use of mulched timber as an extremely effective erosion control on batters
- natural re-establishment of local indigenous vegetation communities
- dramatic reduction in need for nursery-grown plants and sowing with imported seed
- the need to plant only in areas immediately adjacent to creek lines and water quality basins
- reduced establishment and ongoing maintenance costs
- minimal manual work
- dramatic reduction of the usual proliferation of invasive weed species through the landscape
- establishment of a long-term and robust landscape with significant species diversity
- use of onsite materials, resulting in a smaller carbon footprint.
Innovation – demonstration of a new direction in the profession
The natural regeneration process involves the use of shredded mulch collected from on-site clearance operations and site topsoil stripped with its existing litter and trash layer to form a highly erosion-resistant topsoil protection material that maximises indigenous regeneration and obviates the need for conventional planting and sowing processes.
Application of best-practice techniques – clarity of purpose and method
The regeneration process can be broadly described as follows:
- Merchantable timber is logged, leaving as much trash, bark and loppings as possible in situ.
- The larger loppings, tops and logs are shredded to produce longitudinal splinters of varying size. The resulting mulch knits together and maximises the potential for erosion resistance. Fine stick, leaf and twig material remain on the ground to be stripped with the topsoil. The mulch is placed in windrows to the outside of the cleared area.
- The A horizon topsoil is stripped together with the organic leaf litter (O horizon) and remaining trash containing seedbank and plant propagules all mixed together.
- The soil/trash mix is placed to the outside of the cleared area, but inside the mulch windrows.
- Mulch windrows along the outside corridor form a very effective silt fence substitute for erosion control.
- Cut-and-fill operations are conducted and soil analysis is undertaken on new batter surfaces to confirm latent soil conditions.
- Lime and gypsum are added to the subgrade, as required, to form a new subsoil, roughened and keyed, before topsoil is placed.
- A number of trials are undertaken to determine under what circumstances cover crops, mulching or additional trash incorporation might be needed to achieve adequate resistance to erosion.
- The stockpiled mulch and soil/organic mix are then mixed together in ratios determined by trial tests.
- The topsoil and treated shredded wood mixture are pushed over the slopes to obtain a depth 150–200 mm. The soil/mulch mix is deeper at the bottom of fill slopes to improve slope stability.
- A cover crop is applied at low rates to assist in erosion control.
- Repair work and periodic application of additional mulch or mulch/soil mix are undertaken to maintain mulch depths during establishment, as required.
- The success of regeneration, seed bank establishment and erosion resistance is assessed at 6, 12 and 24 months.
Demonstrates the fusion of practice and theory
A number of trials were initially set up to test the probability of soil stability and germination of plant species. Results proved to be highly successful, as documented in a revegetation monitoring record, which intends to record the progress over a 2-year period. The natural regeneration process developed for this project in 2010 and 2011 is being used as a best-practice approach for other similar projects on the Pacific Highway and elsewhere in NSW.
Appropriateness to function
The landscape revegetation area for this project was approximately 42 hectares, which is a challenging amount of revegetation works with restricted timeframes and strict budgetary limits. The natural revegetation process proved to be time and cost efficient as well as highly successful in establishing the endemic plant species onto the site and reducing weed invasion to an absolute minimum which in turn significantly reduces maintenance requirements and associated costs.
The mulch/topsoil mix proves highly effective at preventing topsoil loss through a 250 mm rainfall event on 1:3 batters solving a long-standing problem for roadworks revegetation.
Effectively responds to, and extends, intention of the brief
Our brief was to undertake the landscape works associated with the revegetation of the highway upgrade. Due to the particular context of the project in its forest setting and potential for application of natural regeneration techniques, SMM and SESL collaborated to develop a tailored landscape revegetation strategy for this project.
Quality of outcomes
With valued support and input from the Alliance team, a natural regeneration technique was developed and implemented. On-site trails were undertaken and proposed methods were refined to achieve a tangible best-practice technique for natural regeneration. SMM produced an “Urban design and landscape concept design” report, a natural regeneration brochure and detailed design and documentation.
Role and influence of the landscape architect within the team
The alternative revegetation process came about through a synthesis of landscape architecture and soil science. SMM as the landscape architects with a comprehensive understanding and experience in large infrastructure projects were able to collaborate closely with SESL in the development of the alternative landscape revegetation strategy, merging the scientific procedures with the practical requirements of the construction team on site. The initial landscape strategy was put forward to the construction Alliance and RMS as the client which resulted in a number of trials being undertaken in order to test the viability of the proposed strategy.
In terms of regeneration of plant species, SMM and SESL ensured that the client, design team and construction team understood that the outcome for the new upgraded section of highway at the time of project completion would be very different from conventional methods. Instead of lush green vegetated roadside batters and evidence of new tree planting, there would be gradual growth over a longer period of time as plant species started to regenerate naturally from the adjacent forest seed bank and site topsoil.
Incorporates effective community engagement
The project is located on the edge of a State Forest with significant areas of natural vegetation with a distinctive landscape character that is of importance to the local community and travellers passing through the area. One of the key objectives of the project was to maintain this distinctive landscape character through minimising the construction footprint and therefore minimum impact on users of the highway and the adjoining private properties.
SMM in collaboration with SESL and the construction Alliance compiled a brochure to outline the purpose and process of natural regeneration and its benefits. This brochure was used as an informative tool for all members of the project including management, engineering disciplines, construction and maintenance teams and as an information tool for the local community.
Fosters broad support and understanding of Landscape Architecture
This project required an in depth understanding of the site constraints and a close working relationship with the project team, specialists advisors and stakeholders in order to foster broad support for the proposed regeneration process.
Raises the awareness and understanding in the community of environmental/cultural values and processes
The natural regeneration process developed for this project had not been used for this scale of infrastructure project before in NSW but was adapted from successful work commenced by SESL and Mark Blanche of EDAW on the smaller scale Hunter Economic Zone project. The local community and indeed the greater project team’s understanding of the process and outcome for the project was guided by the landscape architectural team. A brochure was compiled to outline the purpose and process of natural regeneration and its benefits. This brochure was used as an informative tool for all members of the project including management, engineering disciplines, construction and maintenance teams.
The approach to regenerating disturbed sites aims to increase awareness of the powerful and beneficial process for natural regeneration within forest / woodland ecosystems. Benefits of this approach are:
- reuse of all available site resources within the project site
- natural re-establishment of local vegetation communities over time
- dramatic reduction of areas of new planting and sowing, saving time and money
- significant reduction of establishment and ongoing maintenance costs
- reduced need for conventional seeding processes owing to the topsoil and O horizon recovery process
- significant reduction of erosion and subsequent impacts on water quality
- establishment of a robust landscape of facsimile natural soil profiles for the long term
- better ESD outcome and a smaller carbon footprint for the whole project over conventional methods.
The Australian Landscape Principles
Values the existing landscape
The value of the existing landscape for this project was unprecedented in terms of the native seedbank available for regeneration of native vegetation. Through the natural regeneration process, existing site resources were recovered and used to stabilise and revegetation the new embankments required as part of the highway upgrade works. Overtime, the forest would regenerate along the new highway alignment and restore the sense of enclosure that the forest provided for the highway prior to the upgrade. Respect for and replication of the natural soil profile was paramount.
Protects, enhances and regenerates
This project retained and protected the maximum area of native vegetation, and reduced the negative impact on Glenugie State Forest as much as possible. Site materials were recovered and reused in rehabilitating areas of disturbed land so that the existing forest could regenerate up to the highway boundaries over time.
Design with respect
The unique context of this project required design with respect for the existing landscape and forest ecosystem that has a high biodiversity due to its location where the Tropical and Temperate Zones overlap. This ecosystem includes threatened flora and fauna species that are of local and state significance.
The natural regeneration process is designed to manage the extreme rainfall events with highly effective mulching techniques to reduce erosion on disturbed landforms.
Designs for the future
The approach seeks to restore native ecosystems by recreating conditions conducive to the recruitment and survival of native plants. A critical component of the approach is the recovery and reuse of site materials. By reconstructing the natural soil profile we preserve not just the fertility and seedbank of the existing topsoil but the depth and formation of subsoils required to support the plant communities that are characteristic of the locality. Invasive weed species are restricted to a very small proportion of the site comprising less than 10% of the ground cover.
Embraces responsive design
The design response for this project was to firstly acknowledge the unique characteristics of the site. Once this appreciation was understood by the design and construction teams, a review and assessment of the potential for utilising the sites natural resources was collectively undertaken. The collaborative team approach was an essential ingredient in the project’s success.
Revegetation Monitoring Record July 2011 – January 2013
SESL Australia is grateful to Spackman Mossop Michaels for their kind permission to reproduce this information.