Prolonged rainfall over Queensland during the 2010/11 wet season led to unprecedented wide spread flooding. Record flood peaks were recorded by the Bureau of Meteorology at over 100 gauge stations and over 78% of the state was declared a disaster zone. The economic costs associated with the floods are similarly unprecedented, with reduced production in the coal mining industry leading to a loss of $5.7 billion AUD (2.2%) in Queensland's gross state product for the year ending June 2011. During the floods, 85% of mines either restricted or ceased production entirely; by May 2011 the industry had returned to just 75% of its pre-flood output.
After years on a drought footing, many mine sites were designed to collect and store water, and were unable to cope with the volumes of water entering sites. As storage dams were overwhelmed, many mines resorted to diverting water into the mine pits, while other mine pits were directly inundated. Months after the heavy rains eased, much of this water remained in mine pits, and in May 2013 it was estimated that 250 Giga litres (GL) of legacy waters remained in central Queensland coal mines.
This legacy water is generally dealt with by discharging into natural streams, subject to strict environmental conditions. Permission to discharge mine affected water is predominantly dependent on the affect of the discharge on the water quality in the vicinity of the mine and at key downstream locations (such as rivers feeding town water supplies), and the cumulative affect of multiple mines (and other industries) on the Great Barrier Reef, into which the Fitzroy River Basin drains.
Clearly, mine operators must carefully manage the discharge of mine affected waters to minimise the immediate environmental impact, to mitigate against future wet seasons, and to minimise the time legacy waters remain in mine pits. Minimising the time water is resident in mine pits has a two-fold objective, to return to normal operation as quickly as possible and to limit the degradation of the water, as the longer water remains in mine pits the more contaminated it becomes.
We demonstrate the value of coupled weather-streamflow forecasts and optimisation methods to inform the management of water on a mine site, through hindcasts of a stream catchment in the Fitzroy River Basin. The rainfall-runoff model IHACRES is coupled with localised weather forecasts produced using the Weather Research and Forecasting (WRF) model.
We then use mathematical optimisation to find optimal release strategies under a variety of settings, using forecast and actual data in order to demonstrate the power of decision making with high quality forecasts.