A Risk-based Approach to Water Management at Rotowaro Open Cast Coal Mine

The Rotowaro Opencast Coal Mine, located near Huntly in the Waikato, is one of New Zealand’s largest mining operations. Water management at the Rotowaro site is vital to the operation, the two key issues at the site being: water in the opencast mine pits interfering with coal winning operations; and the requirement to protect the local environment by the collection, conveyance and treatment of site water. Solid Energy uses a modelling and risk-based approach to plan the water management infrastructure at the site.

An innovative modelling approach has been applied that uses the US Environmental Protection Agency’s Stormwater Management Model (SWMM) modelling package to simulate rainfall-runoff processes and the performance of the water management system (sumps, pumps, treatment). The model is run for a long-term rainfall record, which assesses the theoretical performance of the water management system for the rainfall events that have been experienced at the site over the last 25 years. The use of real rainfall records has been found to be important at Rotowaro as different parts of the water management system are sensitive to different types of events, e.g. sumps are at risk from short duration events, whereas pits are most at risk from long duration wet weather periods (not catered for by standard hydrological methods). The results are presented in a way that shows the risk of flooding of pits/sumps for different potential management options.

This methodology has been applied to the planning of water infrastructure to support the Awaroa 4 pit (maximum footprint 250 ha). Key considerations have been the provision of sufficient pumping capacity so that flooding of the pit from runoff is managed at a level that avoids unacceptable durations of interruption to coal winning, while at the same time designing sufficient storage outside the pit so that overflows to the local environment are eliminated.