The Challenges of Surface Water Management at large Opencast Mines – The Sishen Experience

Sishen Mine is located in the arid Kalahari region of South Africa near the town of Kathu in the Northern Cape Province. Sishen Mine is respected as one of the largest opencast iron ore mines in the world and is faced with large challenges regarding surface water management on the Mine. The Mine is governed by strict environmental legislation, where pollution compliance is not negotiable. The impact of global warming is also a growing reality, where surface water is a scarce resource that must be managed strategically. Linked to this, the impacts of climate change have shown that storm events are of a higher intensity. These higher intensities lead to extreme flooding that can bring mines to a stand-still, with huge financial losses when production is impacted for weeks on end.

To assist Sishen Mine with these challenges, the well-known Canadian programme PCSWMM was used to create a hydrological surface water model for the bigger Sishen mining area. What started off as a very basic concept model to demonstrate surface water compliance, quickly progressed when the model was upgraded to assist the Mine’s overall water balance and design planning to upgrade the drainage systems at the Mine. The model was further developed where interactive 2D flood modelling was done to simulate extreme storm events in the open pit and bigger plant areas.

The surface water model was developed, not only for the Mine’s active areas, but also for the bigger clean water sub-catchments that impacted the mining activities. Various design storm events were modelled to evaluate the impacts on the proposed drainage infrastructure. A separate clean water system was developed to divert clean water away from the impacted mining areas. This clean water drainage system included Life of Mine (LOM) planning, when mine dumps are being rehabilitated and clean runoff from the dumps have to be diverted post closure. Dirty water areas of the plant and pit were isolated and a separate dirty water system was developed where the runoff water is channelled to a pollution control dam. A special 10-year rainfall model was used to run extended time simulations to optimize the pollution control facilities, taking into account the impact of evaporation and return water strategies to effectively reclaim water to the plant. The newly added 2D capability of PCSWMM was used where a digital elevation model was integrated with the 1D surface water models to simulate extreme flood events for the open pit and bigger plant area.

The “what-if” scenario modelling of PCSWMM was used extensively to find solutions in problem areas that were identified during the pre-feasibility studies. This ensured that preliminary design planning could progress to a high level of confidence for the sizing of the drainage infrastructure such as the pollution dams, attenuation dams, canals, sumps and return water pumps. The highly visual 2D models were used where a risk-based approach could be followed to evaluate the critical flood areas in the open pit and plant areas to produce special risk controls and emergency response plans.

PCSWMM has shown to be a powerful tool to understand the many challenges associated with surface water management in opencast mines, such as Sishen Mine.