Revegetation of
Pacific Highway
Spackman Mossop & Michael
2013
Glenugie, Australia
Awards: 2013 Australian Institute of Landscape Architects - Land Management in Landscape.
SESL’s soil scientists were engaged to design and implement the landscape works associated with a 7km section of highway at Glenugie on the NSW north coast.
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.
Innovative design & collaboration
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 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. 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 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. 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 between all parties was an essential ingredient in the project’s success.
Resource Recovery & Sustainability
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. By carefully stripping and stockpiling site topsoils and mulched vegetation, whilst utilising the indigenous seedbank within the topsoil, we were able to reconstruct the natural soil profile. This preserved 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 recycled wood mulch was reintroduced into the soils as well to act as a stormwater erosion control and weed inhibitor. Through the creation of this blend, SESL created a highly effective erosion protection measure in all areas with exposed soil, especially on steep batters.
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
Trialling & Project Implementation
A number of trials were initially set up to test the probability of soil stability and germination of plant species. 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. 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. 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.
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 logs are shredded to produce splinters of varying size. The resulting mulch knits together and maximises the potential for erosion resistance. Finer stick materials 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
REGENERATION MONITORING RECORD: JULY 2011 – JANUARY 2013
Long-term Regeneration & Outcome
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. 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. 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.
The mulch/topsoil mix proved highly effective at preventing topsoil loss through a 250 mm rainfall event on 1:3 batters solving a long-standing problem for roadworks revegetation. Invasive weed species were restricted to a very small proportion of the site comprising less than 10% of the ground cover. 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.