The task of civil engineers in ensuring the careful management of mine-impacted water around mine waste facilities is being made more challenging by changing rainfall patterns.
Tailings dams and waste rock dumps – as well as slag dumps at smelting operations – are among the main mine residue deposits (MRDs) that must comply with regulatory requirements GN R704 of the National Water Act. A key aspect of this regulation is that water contaminated by, flowing from, or following through the MRD must be effectively separated from the clean water upstream of the MRDs.
To achieve this, the engineering de-sign needs to include adequate drainage and storage infrastructure, which is appropriately sized for the flows that are anticipated. In the past, an accurate prediction of flow levels could be calculated from the planned site conditions, combined with the historical rainfall data for that region. The sizes required for the water channels and pollution control dams are modelled, providing the basis for design parameters such as widths, depths and lining.
VARIABLE RAINFALL EVENTS
Changes to climatic conditions are leading to greater variability of rainfall patterns. In many areas, storm events are becoming more intense, while the duration and frequency of these events is also changing. This in turn is making it challenging to identify and apply appropriate levels of safety when designing the dimensions of water channels and dams on mines. At the same time, some older mines have found that their existing infrastructure for water management may not cater adequately for the growing intensity of rainfall events.
Mining companies are now applying the concept of probable maximum flood (PMF) in their design processes for mine infrastructure. This is a clear recognition that the approach to water management is changing, and that a much higher margin of safety is called for, which in turn increases the size of these water management controls. A significant impact of this engineering shift is the added cost that is expected with these larger structures with greater capacity for flow and storage. Managing the PMF could raise the capacity and size of a water channel, for instance, by anything from three to six times – adding considerably to excavation, concrete, and other costs.
