Solid Energy New
Zealand Limited owns and operates the Ohai coal field (Wairaki Coal Mining
Licence, CML) in western Southland. The Morley Stream passes directly through the middle
of the CML and has required diversion in the past to enable open cast mining to
proceed. Solid Energy is evaluating two new open cast pits at Ohai.
Commissioning these new pits would necessitate further diversion of the
Morley
Stream. One of these
potential new mines is adjacent to an abandoned State Coal Mines (State Coal)
pit and there is an opportunity to substantially rehabilitate this old pit as a
corollary to the new mine and the stream diversion project.

Past
Morley
Stream diversions were
undertaken by State Coal and so they predated the Resource Management Act 1991
and the degree of community environmental awareness and involvement that has
become the norm these days. Not surprisingly the design and construction of the
historic stream diversions was based purely upon mining (operational) and
engineering considerations. This led to the stream being diverted in a manner
that achieved the clinical conveyance of design volumes of water from points A
and B.

Solid Energy realised
that the proposed new diversions need to adopt more holistic and environmentally
sustainable design principles while also including appropriate mining and
engineering constraints. This project involved the design of a 1.7 km diversion
of the Morley Stream (median flow of about 1000 litres/second) that would enable
progressive prestripping and development of the new mines and would also address
final rehabilitation (of the stream), geotechnical risks, hydrological and
ecological issues. To address all of these factors the project required a
multidisciplinary approach. Moreover, it required a clear understanding of how
the mining operational constraints could be integrated with the environmental
considerations. A significant aspect of the project was how to take account of
fishery instream habitat needs, the resource consent requirements and most of
all, the involvement of key stakeholders such as the Fish & Game Council,
Department of Conservation, the Iwi and local community in the design process.
This was achieved through coordinated consultation and stakeholder involvement
in a technical design workshop.