ARTICLE : OWHIRO BAY QUARRY

 September 2002 – Restoration of Owhiro Bay Quarry

In early 2000 Wellington City Council took ownership of Owhiro Bay Quarry from Milburn New Zealand.   The site had been quarried for 80 years and on closure comprised:
• A number of old quarry workings now sheared and faulted with pockets of weak rock and soil / overburden above
• Over 40,000 cubic metres of overburden that needed to be stabilised and relocated on site
• Several bunds on site made from overburden and sludge

Most of the site was devoid of vegetation because of quarrying and the harshness of extreme southerly storms.

The restoration of the site started in 2001 following major recontouring of the site and stabilisation of the overburden into a series of batters.   Initially major plantings of coastal natives such as Phormium cookianum, Acaena pallida, Coprosma acerosa “Red Rocks”, Coprosma propinqua, Disphyma australe, Isolepsis nodosa, Olearia solandri, Poa cita, Sophora microphylla ‘South Coast form’ took place on some of the least modified sites.   However, the size of the site and the need to revegetate large areas of overburden required a method that could restore large areas cost effectively and stabilise the slopes to prevent erosion and water run-off.

Rural Supply Technologies were commissioned in late 2001 to develop a rehabilitation plan for the site using hydroseeding technology.   This Palmerston North based company has been involved in mine site restoration, and has worked closely with Landcare Research to develop methods to revegetate difficult sites.

The project will take place is three phases:
1. Stabilising the site using quick-growing exotic species
2. Trial hydroseeding techniques using native coastal species
3. Replacing the exotics by hydroseeding natives on the stabilised slopes

The batters comprise a mixture of small rock particles with no organic matter.   Initially (in 2000) they had a very high pH of about 8, however more soil tests taken in early 2002 showed the pH was down to 7.1, similar to the existing coastal vegetation at the base of the cliffs.   The tests also showed that the fertility of the batters was becoming similar to that of the surrounding vegetation.   This is good news as little adjustment was required to enable plants to establish.   The most important thing was to establish the bacteria and fungi that are present in the natural soils, on the new batters.

The first stage took place in autumn 2002 and involved hydroseeding two hectares of re-contoured batters with a seed mix comprising an annual grass and clover.   The annual ryegrass will die off after a year.   Some will reseed but because of the site it is expected that it will not be a problem during stage three

(establishing natives).   The other species established was white clover.   This will fix some nitrogen into the system and help the grass to survive.   As the plants die the nutrients in them will be recycled back onto the site.   The seed was applied in a slurry consisting of a mixture of New Zealand wood fibre, glue, starter fertiliser, starter bacteria and fungi.   The aim was to establish 100 seeds per square metre on the site.   Of these only 40% are likely to survive to maturity.

In June 2002 grass and clover had established well in spite of the extreme conditions, and gave the batters a green tinge when seen from a distance.

The second stage will begin in September on a sloping fan of quarry waste rock about 70 m long (across the slope) by 18 m wide (down the slope).   The area has a scattering of self-seeded plants, mostly flat weeds, ground herbs, and some succulents.

A trial will be established to find out the best way of establishing natives on the site.   Treatments will include:

1. Surface Condition

Half of the trial area will be scarified by machine to improve surface conditions for seed germination and seedling establishment.   This scarifying will have the effect of ‘pitting’ the surface of the substrate, thus presenting a diversity of niches in which vegetation may establish.   The other half will be left smooth.

2. Two Hydroseeding Treatments

a. A low fertility hydroseeding mix, including some local topsoil / leaf litter as an inoculum for mycorrhizae, with native seeds and vegetative material only, applied to half of the plots.   Seeds of local provenance adapted to the coastal conditions will be used especially Flax (Phormium cookianium) Other plants used will include Taupata (Coprosma repens), tauhinu (Ozothamnus leptophyllus), Manuka (Leptospermum scoparium), kanuka (Kunzea ericoides),and Carex sp.
b. The native seed mix plus the mix of low fertility exotic grasses and legumes, as used for stage one.

The trial will start in September and will be monitored over the summer months with a final assessment of vegetation cover in February / March 2003.

Stage three will begin in autumn 2003 with hydroseeding of the existing batters using the technique found to be most successful in stage two.   The aim will be to establish 2 to 3 plants per square metre.

For more information on the project please contact the Project Manager, Jonathan Bussell, Berhampore Nursery Manager. Jonathan.bussell@wcc.govt.nz

Mike Oates
Manager Natural and Botanic Areas
Parks and Gardens Business Unit
Wellington City