Innovative design and solutions for mine water
Introduction: The Curragh North coal mine is located 200 km west of Rockhampton in central Queensland, Australia. Wesfarmers Limited won the right to develop the coal deposit and contracted PB to design the mine’s civil infrastructure.
The development concept for the mine is an open-cut mine producing up to 7 Mt/a of run-of-mine (ROM) coal over a 25-year mine life. The coal will be transported from the Curragh North Mine along a transportation corridor to the existing Curragh Mine for processing and rail load-out. The mine site is located on an alluvial floodplain of the Mackenzie River where the catchment area is almost 50,000 km2. The natural floodplain in the vicinity of the mine site is subject to relatively infrequent flooding during events in excess of the 1 in 10-year average recurrence interval (ARI) event. The mine site is protected from flood ingress by a 22-km-long perimeter levee, designed to provide the dual functions of external flood protection and management/containment of internal site water. The topographic constraints, the mine layout and the dual function of the levee combine to effectively provide a water storage facility located within an area subject to flooding.
Climate considerations: Water is a valuable, yet limited, resource throughout the region. Competing demands from industry, farming, communities and the environment have led to the preparation of governmental regulation in the form of water allocation schemes. Allocation of water throughout the region must be in accordance with the water supply scheme for the Fitzroy Basin Water Resource Plan (WRP), the Fitzroy Basin Resource Operations Plan (ROP) and the Interim Resource Operations Licence for the Nogoa Mackenzie, all under the Water Act 2000. Due to these regulations, users cannot simply extract water from rivers and streams in an uncontrolled manner. Limited availability of water allocations and the subsequent high costs makes sustainable water management critical to the success of mining projects throughout this region.
Water management system philosophy: The philosophy behind the water management system (WMS) for the Curragh North Mine is to retain and re-use as much on site run-off as possible, inside the levee system. The WMS has been designed to provide a reliable water supply for mining operations, for the entire 25-year mine life
- minimize the volume of run-off entering mine pits, thereby maximizing pit availability for mining operations
- eliminate uncontrolled discharge and limit the frequency and volume of controlled discharge from the project to off-site receiving waters.
Local run-off within the levee system is directed to a series of water storage dams via overland flow paths and drains. These dams act as both sedimentation dams and as water supply dams. The water storage dams are located based on naturally occurring depressions within the mining area. Four dams are located alongside the flood levee around the perimeter of the site. These dams collect site run-off and transfer water to a central dam via pipelines. The central dam is connected by a two-way pipeline to a final void at the existing Curragh Mine. The final void is used as a balancing storage, enabling excess water from Curragh North to be efficiently stored at Curragh and then brought back when needed.
Water management system layout: The water storage dams have been designed to satisfy the dual functions of meeting mine water demands and controlling contaminated run-off. The water storage dams comprise a lower level excavated sump storage component and a higher level overflow storage component. The concept of providing sump storage and overflow storage for the dams was adopted to limit nuisance flooding, by storing run-off from minor events below the general ground level. Maximum excavated depths for the sump storages are typically 4 m. This concept also minimizes losses to evaporation and seepage due to the reduced surface area of the excavated storage.
Water demands: Water demands for the Curragh North project are satisfied by a combination ofclean water captured from site run-off over undisturbed catchment areas; dirtywater captured from site run-off over disturbed catchment areas andsupplemented clean water pumped from the Mackenzie River. Total waterdemands were determined by summing the component demands from dustsuppression, vehicle wash down, potable use and construction requirements.Water demands were calculated for six snap shots through the mine’s 25-yearlife span. The maximum predicted water demand for the mine was calculated as1,300 ML/a and this was expected to occur during years 2-5 of the mine’s life.
Water sources: Rainfall falling within the perimeter levees is captured, stored and used withinthe site as the highest priority water supply. The balance of the waterrequirements is satisfied by a supplementary surface water allocation from theMackenzie River.
Daily water balance modeling: Daily water balance modeling was undertaken to assess the performance of theWMS. The modeling was used to predict the reliability of supply in satisfyingthe estimated water demands, to predict the pit availability and to predict thefrequency and volume of releases from the WMS.