The vast coal reserves in South Africa are largely mined from the surface. Mine plans generally rotate in and out of areas to extract the most profitable coal first. The open pit areas tend to be gradually extended, continuously back-filled and then rehabilitated. Mined areas are rehabilitated by backfilling with spoils, surface sculpting to minimise erosion and re-vegetating with grass.
The challenge was to develop an understanding of the integrated dynamics of water movement at the mine site. Of particular interest was the spatial-temporal distribution of the water balance, with respect to recharge and evapotranspiration.
Water is critical
Water is critical to coal mining in Mpumalanga. The project area receives about 700mm annual rainfall, with some 60% of that falling in short, intense events from November through January. Water is used continuously in operations for washing coal and dust suppression. To accommodate the erratic supply, rainwater must be stored for supply during the dryer months. Yet clean, storm water during the wet season must be safely routed away from mine impacted areas. Rainfall in mining areas must be separated for treatment and safe disposal or re-use. To complicate matters, the coal spoils are acid forming. Thus, infiltration in spoil piles and rehabilitated areas generates acid leachate that must be treated.
A physical water balance
A physical water balance is key to providing clear insight into the complex relationships and feedbacks between rainfall, infiltration, evapotranspiration, groundwater flow, runoff and stream flow.
The water balance is driven by the physical characteristics of the hydrology, the geology, and the mine. Simple, lumped conceptual models that ignore the spatial and temporal relationships of these components are at serious risk of failing.
Using MIKE SHE
MIKE SHE is a fully integrated, physics based, coupled groundwater-surface water modelling framework. MIKE SHE simulates all the necessary components of the hydrologic cycle, including 2D dynamic overland flow and runoff, detailed channel flow hydraulics, unsaturated infiltration, vegetation, and 3D groundwater flow, plus water quality. MIKE SHE is not a software code, but a proven engineering framework with multiple numerical methods for each component of the hydrologic cycle, and a modern graphical user interface.
Understanding the mine water balance
The MIKE SHE models of the different mine areas are able to capture the dynamics and spatial distribution of the of the water balance in a way that earlier models are unable to. The models are able to show how:
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