Roe Highway Stage 7 – South St to Kwinana Fwy

Roe 7 Before Aerial 2008

The Brief

In June 2005 Tranen was commissioned by the Roe 7 Alliance (consisting of Main Roads WA, Clough, Henry Walker Eltin, and Maunsell) to undertake the rehabilitation works on Roe Highway Stage 7, connecting Roe Highway to the Kwinana Fwy from South St.  Over 41 ha of revegetation was required, with sections immediately adjacent to high conservation value Banksia woodland supporting a range of plant species including the critically endangered Grand Spider Orchid (Caladenia huegelii).  The site was to be maintained for five years after completion of the initial works program.

Roe 7 After Aerial 2016

Constraints & Issues

This project brief differed from most other road projects. For most other road projects the goal is revegetation / landscaping using larger screening species, some of which may be non-local.  The goal here was re-establishment of pre-existing vegetation communities post-construction to provide buffers for adjacent high-quality conservation reserve.  Given the highly modified topography and soil profiles from batter construction, this created a challenge for the rehabilitation.

Our Management Approach

A combination of direct seeding and seedling planting were selected as the primary revegetation techniques for this project.  In order to achieve a successful outcome with direct seeding erosion management was critical, but the typical measures (thick mulching, matting, etc.) typically inhibit seed germination.  The solution was to apply a thin layer of mulch using a bark blower to barely cover the surface, following dozer tracking to create micro-niches for seed retention.

Although preliminary works commenced in 2005, the majority of the revegetation activities took place in 2006 and 2007.  Subsequent to this targeted infill maintenance works continued until 2012.  A total of 91,000 tubestock were planted and 159 kg of native seed hand-broadcast across the 41 ha of revegetation. 240 larger trees were also installed in strategic locations.

Intensive weed control was also undertaken throughout the project area, and in some of the adjoining bushland reserves (Ken Hurst Park, John Connell Reserve).  This included targeted and careful management around known populations of the critically endangered Grand Spider Orchid (Caladenia huegelii).

The Outcome

Fill batter densities and growth rates were generally higher than cut batters.  The median strips took time to establish due to disturbance by vehicles crossing the median.  Banksia trees on the batters were initially slow to grow due to the modified soil profiles (yellow sand without topsoil, low organic content), but now reach up to 4 m.

The revegetation works ultimately met all the project goals by the end of the five year maintenance period.  Twelve years after initial works commenced the revegetation areas blend seamlessly into their surrounds, and provide a natural buffer from the long term conservation areas.