The Telfer Gold Mine, owned and operated by Newcrest Mining and located in Western Australia, is currently being expanded. A new underground sublevel cave development is proposed along with an appreciable expansion of the open pit. The Telfer area is generally subject to low rainfall, however, during the wet season cyclonic events can result in sizable rain events, which have the potential to cause substantial inflow. In addition to this, during the first five years (nominal) of the mine's life, there is the potential for drainage paths between the pit and the underground mine via decommissioned underground workings which lie directly under the pit. After this initial five year period the pit floor will be mined out below the old workings and will therefore no longer connect to the underground, however cave breakthrough to the surface may start to occur thus providing another water path into the underground mine. To arrive at an optimum, safe solution for dewatering the mine that successfully addressed the above items, a rigorous risk analysis was included as an integral part of the design process. This involved the determination of the probability of events occurring and the consequences of these occurrences. This was compared against the cost of mitigation for the various events. This paper outlines the process by which a safe concept addressing all the issues relating to dewatering the proposed mine has been arrived at. This process included: Establishing the pit catchment areas. These were based on historical run-off data and took into account practical mitigation works._x000D_
Establishing an Average Recurrence Interval (ARI) based on reference material and historical rainfall data._x000D_
Utilising a standard approach based on Australian Rainfall and Run-off (ARR) to calculate pumping and storage requirements for the various catchments._x000D_
Analysing the 20 years of recorded rainfall in the Telfer area to establish the duration, magnitude and sequence of events. This analysis assisted in determining the surface run-off coefficient._x000D_
Establishing potential water storage areas and determining the risks associated with using particular storage options._x000D_
Establishing the percolation rate through the cave zone. A degree of uncertainty regarding the percolation rate in the cave zone required that the consequences of using various percolation rates were considered to aid in establishing the rate which was adopted._x000D_
Establishing preliminary concepts and determining the most economically viable concept._x000D_
Conducting a risk assessment based on the principles outlined by Australian Rainfall and Run-off (ARR) to confirm that the appropriate Average Recurrence Interval (ARI) had been selected. A risk assessment was also conducted in accordance with client requirements._x000D_
Establishing the optimum pump station sizes and storage capacities._x000D_
Establishing the appropriate numbers of duty and standby equipment._x000D_
By using the foregoing process, which included a number of stages of risk analyses, an optimum dewatering system concept was produced.