Bourkes Gorge Spoil Dump #2 Restoration – Kosciuszko National Park

 Elizabeth MacPhee and Gabriel Wilks

Bourkes Gorge Spoil Dump #2 is one of two large spoil dumps created during construction of the Murray 1 Pressure Tunnel between 1962 and 1966 to carry water from the Geehi Reservoir to the Murray 1 Pipelines.  These pipelines deliver water to the Murray 1 Power Station on the western side of Kosciuszko National Park near the township of Khancoban.  At this site during Scheme construction, approximately 300 000 m³ of unconsolidated rock spoil was removed from the tunnel access point on a rail siding and dumped in the steep valley of a tributary creek flowing to Bogong Creek.

 The site prior to rehabilitation. Bourkes Gorge Spoil Dump #2 was one long unstable rock slope devoid of native vegetation with scrap metal, timber and concrete jutting out along erosion scars. It was too steep to stand on, with a slope height of 60m and an angle of approximately 38⁰. The spoil dump was 150m wide across the valley and extended about 250m upstream, blocking the tributary creek. As a result, an 8m washout scar was left in the southern side of the spoil dump with continual erosion down the creek, eroding particularly during peak flows.

Fauna and vegetation surveys were conducted on and in the surrounding forest. Three fauna species listed as vulnerable under the NSW Threatened Species Conservation Act 1995 (TSC Act) were identified in the surrounding forest – the Yellow-bellied Glider Petaurus australis, Gang-gang Cockatoo Callocephalon fimbriatum and the Eastern False Pipistrelle Falsistrellus tasmaniensis. (Schultz, M unpublished). Habitat requirements for nesting and roosting of these species did not occur within the site.  The Spotted Tree Frog Litoria spenceri is listed as Critically Endangered by the International Union for Conservation of Nature (IUCN) and is also listed in the Federal Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act) and the TSC Act. This species was known to occur in the vicinity of site, but surveys had recorded a dramatically declining population (Hunter & Gillespie 1999). It was not recorded on or around the site during the fauna survey in 2008. Weeds such as Blackberry (Rubus sp.) and willow (Salix spp.) occurred at the top edge of the site and minimal vegetation was found on the majority of the spoil.

Objectives of the restoration works :

• Re-shaping the spoil dump to a more stable slope.
• Constructing an environment that favoured revegetation and habitation of site-indigenous flora and fauna species.
• Integrating the site into the surrounding tall montane forest
• Constructing a channel to enable high water flow events to move across the site without scouring or rendering the site unsafe.
• Slowing high flow events to limit the scour effects on the downstream environment.
• Safely managing contamination or general construction waste found at the site

Treatments. Works were undertaken from December 2009 to April 2010. Stabilisation works consisted of reshaping the spoil dump and lining 300m of artificial creek line. The resulting land form was planted with 50,000 tubestock in 2010 – 2011 and had specific management practices applied to minimise potential impact on the Spotted Tree Frog.

Plant species used in the revegetation had to be sourced from plants as cuttings, seed or division from the surrounding environment, capable of being commercially propagated due to the number required, and robust enough to withstand the more extreme conditions found on site than in surrounding forest. One rare species Bertya findalyii was found colonising the edge of the site and so was incorporated into the planting list.

Results.

Erosion Control.  The rehabilitation of the Bourkes Gorge #2 spoil dump resulted in 43,300 m³ of rock soil being re-shaped to reduce slope and direct water flow, reducing the potential for surficial erosion and mass slumping. Slope angles were reduced from around 38ْ to between 26ْ and 30ْ  (URS, 2009). Around 560M³ of concrete reinforced with structural synthetic fibre and on site rock was used to form the water channel. In the three years since rehabilitation, there have been two major flood events in the region – October 2010 and March 2012. The Jagungal weather station in Kosciuszko NP recorded 6, 12 and 24 hour duration rainfall intensities exceeding the 100 year Average Recurrence Interval (ARI) intensity. There was no evidence of erosion or slumping at Bourkes Gorge Site following these events.

Revegetation.  Assessment of the vegetation was done two years after planting by Greening Australia Capital Region using BioMetric (http://www.environment.nsw.gov.au/papers/BioMetricOpManualV3-1.pdf).  This monitoring has shown outstanding survival and growth rates – with 35% cover by 19 native species, with virtually nil weed. (Species are listed in Table 1 ).

Lesson learned: Rock spoil in high altitude, steep conditions with no organic matter in a compacted and unstable condition will not naturally revegetate, even if left for a fifty year period.  Applying site appropriate techniques such as re-shaping for stability, allowing for water flow, moving compacted rock to create air pockets and allow water infiltration, and adding the essential ingredients of organic matter, nutrients and plant material can trigger successful site revegetation. Covering the ground with a layer of organic matter such as rice straw ameliorates temperature extremes on site, allowing young seedlings to survive and flourish.

Acknowledgements. Thanks are extended to the restoration team at Kosciuszko National Park, including the many contractors who participated.  We also thank Nicki Taws and Angela Calliess (Greening Australia Capital Region) who undertook the formal vegetation monitoring.

After earthworks, planting niches are filled with compost

Main slope at Bourkes Gorge #2 spoil dump

Liz MacPhee pictured at Bourkes spoil dump one year after planting

Table 1. Vegetation data recorded on a 50m transect approximately 2 years after treatment.  (Data from Greening Australia Vegetation Monitoring Former Snowy-Hydro Sites Kosciuszko National Park).