Water management at sites generating excess impacted water can result in high costs and risks if the development concept is advanced without consideration of certain key aspects. SRK developed a conceptual design for surface water management related to a project in Papua New Guinea (PNG) which included large open pits. The region’s mean annual precipitation is approximately 8m.
The primary objective of the surface water management system is to divert clean water, as well as direct contact water, around the open pits to appropriate transfer facilities and, from there, to the water treatment plant. Contact water that is not collected by diversion channels is collected at the pit base and pumped to the transfer facilities.
The surface water management system comprises:
- Clean water diversion channels/ bunds
- Contact water channels
- Drop structures
- Treatment plant transfer points (TPTPs)
- Pumping line from the base of the open pit to the TPTP
The clean water diversion structures collect runoff from the catchments upstream of the open pit and discharges this water to the natural stream system downstream of the pit. Ideally, as much clean water as possible should be diverted; however, due to the steep topography, it was difficult to find suitable sites for the surface water infrastructure. SRK suggested minor modifications to the pits’ layouts to improve water management; clean runoff from approximately 40% of the total catchment area is diverted.
The contact water diversion structures collect runoff that has been in contact with the walls of the open pits, and discharges it to the TPTPs, from where it is sent to the water treatment plant.
Contact water diversion structures (drains) are installed on the pit benches; the minimum bench width allows for the bench width and drain components. Drop structures are installed to direct water from drains on the upper benches to lower bench drains.
Where possible, the contact water diversion structures are placed along mine access ramps. Contact water runoff from approximately 42% of the total catchment area is diverted by gravity towards the TPTPs.
Finally, contact water that cannot be diverted is pumped from the base of the open pit to the TPTPs, i.e. runoff from approximately 18% of the total catchment area.
The conceptual surface water design and analysis resulted in a significant cost saving, as less water needs to be pumped to the TPTPs and a smaller water treatment plant is required.
