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19. Ingwe develops innovative solution to stormwater run-off during mine site rehabilitation
When one of our Ingwe coal mines in South Africa ceases production, the Mine Closure Operations (MCO) team is responsible for rehabilitation of the mine site. A major issue during rehabilitation can be the drainage of stormwater run-off from discard dumps. Conventional concrete drainage structures are not environmentally friendly and, at times of high rainfall, can be easily damaged. As an alternative, the MCO team has developed the concept of spiral contour drains. This solution is not only environmentally more responsible but also costs less and is less subject to failure.
With conventional stormwater drainage on a discard dump, the water is collected in chutes that direct it down a steep slope to the bottom of the dump. The chutes are typically constructed from concrete materials. As it rushes towards the bottom of the dump, the water can reach high velocities and sometimes gain sufficient energy to seriously damage the chute. At the bottom of the dump, specialised structures are required to dissipate the energy of the water to prevent excessive soil erosion.
There are other problems associated with this type of drainage. The concrete structures are costly to erect. As well as being subject to damage from the rushing water, they can fail due to differential sagging of the coal discard material. They are difficult and costly to remove once the dump is rehabilitated; and, if they remain on-site, they can impede access to the area.
Realising the need for a different approach, the MCO team developed a solution based on the spiral contour principle. The contour drain starts at the top of the dump and winds downwards in a gently spiralling manner until it reaches ground level, where the water is dissipated into the natural watercourse. The use of concrete structures is minimised and sometimes eliminated.
There are numerous benefits. The final product is more environmentally responsible due to the minimal use or absence of concrete structures. The cost is substantially lower, the risk of failure is vastly reduced, and all areas of the dump are accessible from between the spiralling contours.
The first spiral contour drain was constructed at South Witbank Colliery on a small discard dump covering approximately 5 hectares. The drain spirals down to the bottom of the dump, where the remaining energy of the water is dissipated in a delta-shaped outlet.
The second spiral contour drain was also built at South Witbank on a dump covering 8 hectares. This time a double spiral contour was installed, the first draining clockwise and the second anti-clockwise.
The double spiral contour system was also applied on the 65-hectare Ermelo Mines Services, but with both contours draining anti-clockwise. In this case, because of the steepness of the topography and the huge volume of water to be drained in the event of a heavy rainstorm, some concrete blocks are used to reinforce the outlets at the bottom of the dump.
The latest mine site where the principle has been implemented is Transvaal Navigation Collieries. Building on experience gained at the other sites, the aim was to install the minimum amount of contours to enhance accessibility. Farm machinery can operate more effectively, allowing for easier management of vegetation until closure of the mine. This facilitates future conversion to grazing on a sustainable level.
Because of the varying topography of this site, the drain outlets that dissipate the energy of the water were designed with different methods. On one side, which is very steep, the drain discharges at ground level into a concrete structure. On the less sheer side, two contours drain into a delta, so the energy of the water is released over a wide area and into the natural watercourse. Provided the topography allows for it, the ideal is to dissipate the water into such a delta-shaped outlet, as it costs less and is more environmentally responsible.
Based on these projects, spiral contour drainage has been shown to be a successful system for draining run-off stormwater drainage from discard dumps; and the MCO team has adopted the principle in preference to conventional drainage structures.
